Year: 2024

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Smart use of the upper and lower temperature alarm points of the temperature control table to achieve temperature range control

The user went to a hardware and electrical store to buy a thermometer for automatic temperature control of the material tank in the production workshop. The control requirements put forward by the user are: when the temperature of the material tank is as low as 25 º C, start the boiler fan to heat it up; when the temperature of the material tank rises to 32 º C, stop the fan. The temperature is not required to be precisely controlled at one point, but it can be roughly maintained between 25 º C and 32 º C. In this way, the fan does not need to be started frequently and run for a long time, and the power saving effect will be very good. It is said that after searching several electrical and mechanical stores, no suitable temperature control meter can be found. If it can be solved here, it doesn’t matter if the price is a little higher, and five or six units can be purchased at once.

The father and son run the hardware and electrical store. The father is an old electrician, and the son is a student of the Department of Mechanical and Electrical Engineering, with top grades. The father and son were excited and decided to make this deal. The temperature controllers in the store generally have the following functions, see the wiring diagram below:

Figure 1: Temperature control meter wiring and three sets of contact output status diagram

From the wiring diagram, the temperature controller has three groups of output control contacts. KA0 is the control temperature output. If its normally closed point is used as the control output, the contact closing and opening action area is near the temperature setting point. For example, if the setting point is 25 º C, when the measured temperature rises to 25 º C, the normally closed contact is disconnected, and when the measured temperature drops to 23 º C, the contact is closed. This control method belongs to “point” control. The action of the relay tracks a temperature setting point. Although in order to avoid frequent switching at one point, there is often a temperature hysteresis value in the middle, such as 2 º C between 23 º C and 25 º C. Some models have this hysteresis fixed, while others are adjustable, but the temperature difference adjustment range is not too large. Obviously, the control output contact of KA0 cannot meet the control requirements put forward by users.

Let’s look at the output contact of KA1, which is the lower limit control output. It is also output based on a “temperature point”. When the measured temperature reaches the preset lower limit, KA1 will act. As long as it is within the lower limit, KA1 will maintain the output. Only when the measured temperature is higher than the lower limit setting point, KA1 will lose power and the contact will be released. The control requirements cannot be achieved by relying solely on the control contacts of KA1. The output contact of KA2 is the upper limit setting point. Its control principle is the same as that of KA1. It can also be regarded as a “point” control and cannot complete the control task independently.

My son suddenly had an idea: Is it possible to combine the lower limit and upper limit control outputs to meet the control requirements proposed by the user?

Dad nodded approvingly: Okay, I thought so too, let’s try to analyze it.

See the following control state diagram of KA1 and KA2 output combination and user control requirement diagram:

Figure 2: Upper and lower limit control states and user required control characteristic diagram

My son said: By connecting the normally closed points of the two relays KA1 and KA2 in series, the fan can be powered on and operated in the area where the two relays are not operating. That is, the fans in sections a and b in the figure can be powered on and operated. Doesn’t this meet the requirements?

Dad said: It seems not to work. The fan is also powered in sections c and d. The fan is powered and running most of the time. The fan stops only when it is outside the upper and lower limits. It still doesn’t work to use the two contacts directly. The user’s requirements are shown in the right figure of Figure 2. Only the thick line segments (a, b/1a, 1b) in the right figure are the time for the fan to run. When the detected temperature reaches the lower limit, the fan runs, and when it reaches the upper limit, the fan stops; then the temperature drops and reaches the lower limit again, the fan starts running again. The fan does not run at one “point”, but only runs on “segments” a and b, achieving a good power saving effect.

My son said: I didn’t notice the c and d segments in the left figure. It’s not possible. But we can use the upper and lower limit contact outputs and add an external control circuit to achieve it. This additional circuit should not be difficult to make.

The father wanted to test his son’s level, so he said: How about this, let’s both make an external control circuit, and see who’s circuit is simpler and more reasonable, and then we’ll use their circuit, okay?

The son knew that his father wanted to test his ability and it was also a small challenge for him. He thought that since he had worked on some complicated electromechanical control circuits, this small circuit should be no problem for him. So he readily agreed.

It seems to be a simple thing to think about, but in practice, this small function is not so easy to achieve. It seems that it cannot be completed with two additional relays, and the circuit is too complicated to use three relays. According to my father’s idea, it should be possible to achieve it with two additional relays. It seems that it cannot complete the task if one contact of KA1 and KA2 is used.

The son spent half a day drawing several diagrams, optimizing and simplifying the circuit, and finally succeeded through wiring tests. However, the analysis from the control principle was a bit confusing. The father frowned and thought about it at first, but suddenly grabbed a pen and drew a wiring diagram in no time. Without wiring tests, he announced that the circuit would definitely work.

The control circuit diagram made by the father and son is as follows:

In the figure, KA1 and KA2 are the upper and lower limit signal output relays inside the temperature control meter, KA3 and KA4 are external relays, and KA3 provides control signal output. The control circuit designed by my son uses the normally open and normally closed contacts of the upper limit relay, the normally closed contacts of the lower limit relay, and two sets of contacts of KA4. The control process is as follows: After the equipment is powered on, the normally closed contact circuit of KA2 and KA4 provides power to the KA3 coil, KA3 is energized, the fan runs, and the discharge tank begins to heat up; when the temperature reaches the upper limit, the normally closed point of KA2 is disconnected, the normally open point is connected, KA3 loses power, and at the same time KA4 is energized for self-protection, the fan stops, and the discharge tank begins to cool down; due to the self-protection effect of KA4, when the temperature drops below the upper limit, KA4 maintains an energized state through the normally closed point of KA1 and its own self-protection contact, and KA3 maintains a de-energized state, and the temperature of the discharge tank continues to drop until it reaches the lower limit of the temperature, the KA1 lower limit relay is activated, the KA4 self-protection circuit is disconnected, KA4 loses power, KA3 is energized again, and the fan runs.

The control circuit designed by my father seems to be simpler. The circuit is clear in principle and is more convenient for analyzing the control process: when the temperature of the unloading tank reaches the lower limit, KA3 is energized and forms a self-protection (self-locking) circuit through the normally closed point of KA4 and the normally open point of KA3, and the fan runs; when the temperature reaches the upper limit, the normally open point of the upper limit relay KA2 is connected, KA4 is energized, and while disconnecting the self-locking circuit of KA3, it forms its own self-locking circuit through the normally closed point of KA3 and the normally open point of KA3. KA4 remains energized, KA3 remains de-energized, and the fan stops; when the temperature drops to the lower limit of the temperature, the relay KA1 is activated, KA3 is activated, and while disconnecting the self-locking circuit of KA4, KA3 forms its own self-locking circuit, and the fan starts running again.

The focus of control requirements is to meet two conditions:

1. Once KA3 and KA4 are powered, they can be self-protected (self-locked);

2. When two relays are powered on for self-protection, the self-protection circuit of relay B must be disconnected to make it lose power. The same is true in reverse.

This control method is not conventional temperature point control, but temperature range control, which can be regarded as a special application.

Both the son’s and the father’s circuits can accomplish the task, and use the same number of contacts, but the father’s circuit is easier to understand and more classic. The son’s circuit also accomplishes the task well, which is rare.

Dad said: I thought of these two conditions, and based on these conditions, I formed this circuit in the computer. It was a waste of effort before the logical relationship of the circuit was clear.

Although the son has figured out the circuit, it seems that his father is still better in terms of circuit routing and principle analysis. His father’s circuit is more “smooth”, while my own circuit is a bit tortuous. When you make a circuit yourself, you should be better at analyzing the logical relationship and take a “smooth” path to make the circuit more optimized and reasonable.

The fan control wiring diagram is as follows:

In the figure, KA1 and KA2 are the upper and lower limit signal output relays inside the temperature control meter, KA3 and KA4 are external relays, and KA3 provides control signal output. The control circuit designed by my son uses the normally open and normally closed contacts of the upper limit relay, the normally closed contacts of the lower limit relay, and two sets of contacts of KA4. The control process is as follows: After the equipment is powered on, the normally closed contact circuit of KA2 and KA4 provides power to the KA3 coil, KA3 is energized, the fan runs, and the discharge tank begins to heat up; when the temperature reaches the upper limit, the normally closed point of KA2 is disconnected, the normally open point is connected, KA3 loses power, and at the same time KA4 is energized for self-protection, the fan stops, and the discharge tank begins to cool down; due to the self-protection effect of KA4, when the temperature drops below the upper limit, KA4 maintains an energized state through the normally closed point of KA1 and its own self-protection contact, and KA3 maintains a de-energized state, and the temperature of the discharge tank continues to drop until it reaches the lower limit of the temperature, the KA1 lower limit relay is activated, the KA4 self-protection circuit is disconnected, KA4 loses power, KA3 is energized again, and the fan runs.

The control circuit designed by my father seems to be simpler. The circuit is clear in principle and is more convenient for analyzing the control process: when the temperature of the unloading tank reaches the lower limit, KA3 is energized and forms a self-protection (self-locking) circuit through the normally closed point of KA4 and the normally open point of KA3, and the fan runs; when the temperature reaches the upper limit, the normally open point of the upper limit relay KA2 is connected, KA4 is energized, and while disconnecting the self-locking circuit of KA3, it forms its own self-locking circuit through the normally closed point of KA3 and the normally open point of KA3. KA4 remains energized, KA3 remains de-energized, and the fan stops; when the temperature drops to the lower limit of the temperature, the relay KA1 is activated, KA3 is activated, and while disconnecting the self-locking circuit of KA4, KA3 forms its own self-locking circuit, and the fan starts running again.

The focus of control requirements is to meet two conditions:

1. Once KA3 and KA4 are powered, they can be self-protected (self-locked);

2. When two relays are powered on for self-protection, the self-protection circuit of relay B must be disconnected to make it lose power. The same is true in reverse.

This control method is not conventional temperature point control, but temperature range control, which can be regarded as a special application.

Both the son’s and the father’s circuits can accomplish the task, and use the same number of contacts, but the father’s circuit is easier to understand and more classic. The son’s circuit also accomplishes the task well, which is rare.

Dad said: I thought of these two conditions, and based on these conditions, I formed this circuit in the computer. It was a waste of effort before the logical relationship of the circuit was clear.

Although the son has figured out the circuit, it seems that his father is still better in terms of circuit routing and principle analysis. His father’s circuit is more “smooth”, while my own circuit is a bit tortuous. When you make a circuit yourself, you should be better at analyzing the logical relationship and take a “smooth” path to make the circuit more optimized and reasonable.

The fan control wiring diagram is as follows:

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Charging control circuit of VFDs energy storage capacitor

The main circuit of the inverter is a voltage-type, AC-DC energy conversion inverter. Since there is a large-capacity capacitor energy storage circuit between the rectifier and inverter circuits, and the voltage across the capacitor cannot change suddenly, in the initial power-on stage, the capacitor device is equivalent to a “short circuit”, which will form a huge surge charging current, which will cause a large current impact on the rectifier module and cause it to be damaged, and will also cause the air circuit breaker connected to the inverter power supply end to trip due to overcurrent.

The conventional processing method is to connect a current limiting resistor and a charging contactor (relay) in series between the rectifier and the capacitor energy storage circuit. The control of the capacitor charging process is as follows:

When the inverter is powered on, the charging resistor first limits the current of the capacitor to suppress the maximum charging current. As the charging process extends, the charging voltage gradually builds up on the capacitor. When the voltage amplitude reaches about 80% of 530V, two control processes occur. One is that the switching power supply circuit of the inverter starts to oscillate. The 24V output of the switching power supply directly drives the charging relay, or the relay connects the coil power supply circuit of the charging contactor. The charging contactor (relay) is closed. When the charging current limiting resistor is short-circuited, the inverter enters the standby working state. After a certain voltage is established on the capacitor, its charging current amplitude is greatly reduced, and the closing/switching current of the charging contactor is not too large. After that, the power supply of the energy storage capacitor circuit and the inverter circuit is supplied by the closed contactor contacts, and the charging resistor is short-circuited by the normally open contact of the contactor. The second is that as the charging voltage on the capacitor is established, the switching power supply starts to oscillate, and the CPU detects the voltage amplitude signal sent by the DC circuit voltage detection circuit, determines that the charging process of the energy storage capacitor has been completed, and outputs a charging contactor action instruction. The charging contactor is powered on and closed, and the capacitor charging process ends.

The common main circuit forms of the inverter and the charging contactor control circuit are shown in the figure below:

Figure 1: Common types of inverter main circuits
Figure 2: Control circuit of charging contactor

For some inverters and high-power inverters, the rectifier circuit often adopts a three-phase half-controlled bridge circuit, that is, the lower three arms of the three-phase rectifier bridge are rectifier diodes, and the upper three arms use three unidirectional thyristors, using thyristors as “contactless switches” to replace charging contactors. This saves installation space and improves circuit reliability. The circuit form is shown in the figure below:

Figure 3: Control circuit of charging contactor

Although the charging contactor is omitted, the working principle is the same, but the control circuit is different. During the power-on period of the inverter, D1 ∽ D6 is used for rectification, and R is used for current limiting to charge C1 and C2. When the charging process is close to the end, the CPU outputs the opening instruction of the three thyristors SCR1 ∽ SCR3, and the control circuit forces the three thyristors to conduct. The power-on pre-charging circuit composed of D1, D2, D3, and R is used. SCR1 ∽ SCR3 and D4, D5, and D6 form a three-phase rectifier bridge. At this time, the thyristor is in a fully conductive state, which is equivalent to a rectifier diode.

The opening of the thyristor requires two conditions: 1. The positive voltage between the anode and the cathode; 2. A trigger current loop is formed between K and G. The circuit is connected to the three terminals of the AC input power supply to provide unidirectional controlled rectification. During the three positive half-wave voltages of the three-phase AC power, if the trigger current is formed at the same time, the three thyristors can be opened. The first condition has been formed naturally, and the second condition is sufficient to control its opening.

To put it simply, as long as a trigger current (pulse or DC) is provided to the thyristor during the period when the thyristor is subjected to a forward voltage – when the AC voltage passes through zero, the thyristor can be turned on well during the positive half-wave of the AC power and rectify the input AC voltage (just like a diode). The simplest trigger circuit is to introduce a resistor from the anode to the G pole, and during the positive half-wave of the AC power (after the zero point), the trigger current is synchronously introduced to the thyristor to turn on the thyristor. For example, the main circuit form of Dongyuan 300kW inverter is the same as Figure 3, and the trigger circuit is relatively simple:

Figure 4: SCR trigger control circuit 1

Figure 4 is one of the thyristor trigger circuits. The other two trigger circuits are the same. The components R45, C30, C31 and other components connected in parallel at the anode and cathode of the two thyristors are peak voltage absorption networks to provide overvoltage protection for the thyristors. The KA2 contact, D15, R44, and 24R form a trigger current path. The function of D15 is to rectify the input voltage half-wave to prevent the thyristors G and K from being subjected to the impact of reverse trigger voltage/current. R44 and 24R are current limiting resistors to limit the peak trigger current and protect the safety of the thyristors. R43 is a noise elimination resistor to increase the reliability of the thyristor operation.

When the CPU issues a thyristor on command, relay KA2 is powered on and closed, a positive half-wave voltage is input, rectified by D15, limited by R44 and 24R, flows into the G pole of the thyristor, and flows out from the K pole, forming a trigger current path, and the thyristor is turned on. The thyristor in the circuit is not in the voltage regulation working area, the conduction angle is the largest, and it is in the “full conduction state”, just like a switch device, only in the two states of conduction and cutoff, without the third state of phase shift (voltage regulation). This is where attention needs to be paid. Therefore, the control circuit is different from the conventional phase shift control circuit and is relatively simple.

A slightly more complex thyristor control circuit, such as the trigger circuit of the Delta 37kW inverter thyristor, is shown in the figure below:

Figure 5: SCR trigger control circuit 2

After rectification and filtering by an independent power supply winding of the switching power supply, it is used as the power supply for the thyristor trigger circuit. The control circuit consists of the NE555 time base circuit, the DPH2, DQ22, DQ3 trigger pulse on/off circuit, and the D and R three-way trigger flow circuit. After the switching power supply works, the NE555 time base circuit is connected to a multi-resonance oscillator and is powered on. Whether the oscillation pulse output from pin 3 is sent to the three trigger circuits of the subsequent stage depends on the command control of the CPU. The command signal of the CPU is introduced to the input side of the photocoupler DPH2 through the 24th pin of the control wiring terminal DJ8. When the transistor on the output side of the optocoupler is turned on, the pulse signal of the NE555 oscillator is sent to the D and R trigger circuit loop of the subsequent stage through the transistors DQ22 and DQ3. After the CPU issues the thyristor opening command, the three devices DPH2, DQ22, and DQ3 are always in the on state, and the trigger pulse is always added to the G and K of the three thyristors. The peak trigger current is about 100mA.

In addition, in the Panasonic and Fuji small power inverter models, another form of main circuit structure is used to complete the initial charging control of the main circuit capacitor. This is the internal circuit structure diagram of the 7MBR35SD120 integrated power module. The circuit is shown in Figure 6:

The difference of the circuit is that a controllable device is added after the three-phase rectifier bridge. A charging resistor must be connected in parallel to the terminals 21 and 26. After a certain charging voltage is established on the main circuit capacitor, a trigger current is input from terminals 25 and 26, then the thyristor is turned on and the inverter enters the standby working state.

The control circuit is generally powered by an independent 24V winding of the switching transformer to obtain control power with a “floating ground”. The control circuit is mostly an oscillation circuit, which increases the pulse trigger current of the thyristor device. The oscillation circuit is not a conventional phase-shift trigger circuit, but provides high-frequency/density random trigger pulses to put the thyristor in a fully conductive state. The thyristor here, under the action of high-density trigger impulses, is like a switch “turned to the on position”. There is no actual surveying circuit on hand for the trigger circuit of this model, so we can only draw a simple diagram based on the circuit structure for reference.

Figure 6: 7MBR35SD120 integrated power module
Figure 7: thyristor trigger circuit of 7MBR35SD120S module
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Why should we add an input reactor to the fragile small power VFDs

A factory has a 5.5 kW submersible pump. To facilitate water volume adjustment and energy-saving operation, an electrician proposed using a VFD to drive the pump.

The task was taken on by Mr. Zhang, a friend of the electrician. For safety, Mr. Zhang selected a 7.5 kW VFD from a reputable brand. This brand’s VFDs are widely used in various industries such as plastics, chemicals, and wood processing without significant issues, indicating decent quality. The VFD control box was installed in the boiler room, with power sourced from the workshop distribution panel. The three-phase voltage balance was excellent, maintaining within 380V ±5%. During the trial run, the VFD did not need to operate at full speed, running around 30Hz with the working current at half of the VFD’s rated current. Mr. Zhang was confident that the VFD would operate safely for a long time.

However, three days later, the factory’s electrician called Mr. Zhang, reporting that the VFD had stopped, and the control panel was unresponsive, indicating no power. Surprised, Mr. Zhang visited the site and confirmed the issue. Assuming a quality problem with the VFD, he replaced it with another 7.5 kW VFD from the supplier and sent the defective one for repair.

This time, the VFD failed after just a day and a half. Frustrated, Mr. Zhang called the supplier to complain and replaced the VFD with another brand. He again checked the operating voltage and current, which were similar to the initial installation, indicating no issues with the pump or power supply. Mr. Zhang concluded that the first batch of VFDs might have had quality defects and hoped the new brand would solve the problem.

Unexpectedly, the newly installed VFD also failed within hours, with the factory’s electrician calling Mr. Zhang again. The recurring issue led to reprimands from the factory boss to the electrician, who then passed the blame to Mr. Zhang. Baffled, Mr. Zhang inspected the three faulty VFDs. He found that two had open circuits between the R and P1 (external brake resistor terminal), possibly due to damaged charging resistors, which indicated rectifier circuit issues. The third VFD had a short circuit between R and S, suggesting a failed rectifier module. However, the inverter modules in all three VFDs were intact. The damages seemed to result from power surges, not load-related issues, as the three-phase supply voltage and input current were normal. Having worked with VFDs for several years, Mr. Zhang found this problem unprecedented.

In desperation, Mr. Zhang called his friend, electrician Mr. Li, for advice. Mr. Li suggested installing a three-phase input reactor before the VFD’s power terminals to solve the problem.

Mr. Zhang inquired about the cause of the failures. Mr. Li explained three potential reasons related to the power supply:

1. The submersible pump continued to operate after work hours for employee showers, meaning a 630 kVA transformer supplied a 7.5 kW VFD, causing a significant capacity disparity. The VFD’s input current contained high harmonic components, generating large rectifier inrush currents that damaged the rectifier module and charging resistor during startup.

2. The workshop distribution panel might have parallel capacitor compensation cabinets. The start and stop of large motors (above 100 kW) and the switching currents of capacitors created harmful voltage spikes and inrush currents, impacting the VFD.

3. The same power line might have other large VFDs, soft starters, or DC speed controllers. The nonlinear rectifier currents from these devices severely distorted the power supply waveform, increasing harmful harmonics and deteriorating power quality.

Mr. Li noted that input reactors, often depicted in VFD wiring diagrams, are frequently omitted during installation to save costs, leading to such issues.

Mr. Zhang didn’t have ready-made three-phase reactors and needed an immediate solution. Mr. Li suggested using XD1 series chokes from old capacitor compensation cabinets as reactors, which could suppress inrush currents effectively. Mr. Zhang contacted several suppliers but learned that most manufacturers had stopped producing these chokes.

Under pressure, Mr. Zhang reached out to Mr. Li again, demanding a solution. Mr. Li, while having lunch, suggested using current transformers (CTs), which Mr. Zhang likely had. Any CT, regardless of size, with a rated current of 5A, could be used. CTs with more winding turns (e.g., 50/5) would offer better inrush current suppression and filtering but might have a higher voltage drop. Conversely, CTs with fewer turns (e.g., 250/5) would have a smaller voltage drop but less effective smoothing. Depending on the VFD’s rated current, Mr. Li recommended using three CTs per phase or two if the running current was around 7A. CTs would provide better inductance and performance than XD1 chokes.

After installing CT-based “input reactors,” the VFD’s input current became stable, reducing harmonics and voltage spikes, ensuring safer operation.

Months later, Mr. Zhang checked with the factory electrician, who confirmed that the VFD had been operating normally. Mr. Zhang realized the VFD failures were due to power supply issues, not the VFD quality. He felt vindicated and teased the electrician about owing him a drink for resolving the issue.

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inverter driver ICs and methods to clear OC alarms

1. The difference between various types of driver ICs:

The core component of the inverter drive circuit is the driver IC, and the commonly used models are TLP250, A3120, PC923, PC929, A316J, etc. The driver IC is essentially a type of optocoupler device. The purpose of using optocouplers is to achieve isolation of different power supply circuits on the input and output sides of the coupling, and to ensure that the output side has a certain power driving capability, which has both electrical isolation and power amplification.

Ordinary four-wire end optocoupler devices, such as PC817, have an internal circuit consisting of an input light-emitting diode and an output photosensitive transistor. After the input current (typical application value 5-10mA) is passed through the input side , the output side transistor generates excited photoelectrons and conducts. This type of optocoupler is mainly used for the transmission of switching signals, such as the digital signal control terminal of the inverter.

As an optocoupler device of the driver IC, its structure is slightly more complicated than that of PC817. The output stage is mostly composed of emitter output to the complementary amplifier, such as TLP250, A3120, PC923, etc. The output stage is composed of two-stage emitter complementary circuits, V1 and V2. When V1 is turned on, the VCC positive supply voltage is added to the gate-emitter junction of the IGBT through the output pins 6 and 7, providing the driving current for the IGBT to turn on. If the gate-emitter junction of the IGBT is regarded as a capacitor, the V1 conduction provides the charging current of the gate-emitter junction capacitor of the IGBT to turn it on; and when V2 is turned on, the output pins 6 and 7 are pulled to the GND ground level or the negative supply voltage, providing a charge discharge channel for the gate-emitter junction capacitor of the driven IGBT, so that it is quickly cut off. During work, V1 and V2 are alternately turned on to implement the on and off control of the IGBT. It should be noted that the power supply for this drive circuit often uses a positive and negative dual power supply of +15V and -7.5V to enhance its control ability.

PC929 adds IGBT protection circuit, also known as IGBT conduction tube voltage drop detection circuit, based on the circuit structure of TLP250, A3120, PC923, etc., which is mainly responsible for the rapid protection of IGBT from overcurrent and short circuit. As we all know, in the U, V, W output circuit of the inverter, two or three current transformers are connected in series (the current signal is collected by the Hall element and processed by the amplification circuit), and the output signals are processed into analog and switch signals by the back-stage circuit respectively, and sent to the CPU for current display, output control, automatic speed control and current limiting control and overload protection during startup and operation. However, the current transformer circuit often has a large time constant and cannot implement μs -level rapid protection for IGBT. In fact, the implementation of overload and short circuit protection for IGBT depends on driver ICs such as PC929 and A316J to a certain extent.

Because the tube voltage drop when the IGBT is turned on actually reflects the working state of the IGBT, the overcurrent detection of the IGBT can be implemented by using the IGBT’s energy-conducting tube voltage drop signal, which can effectively implement the rapid overcurrent protection of the IGBT. The IGBT tube is an organic combination of a two-stage device and a field-effect device. The collector and the emitter form an output current path and have a certain conduction internal resistance. When the IGBT works within the rated current, the normal conduction tube voltage drop should be less than 3V. When the overcurrent is nearly 2 times, the overload current forms a large voltage drop on its conduction internal resistance, causing the tube voltage drop to rise to about 7V. Since the overload capacity of electronic devices is poor, the overload time allowed is short, and the faster the protection action, the better. Detecting the IGBT’s conduction tube voltage drop signal to cut off and protect the IGBT has become the most effective and widely used method in the inverter drive circuit.

Compared with PC923, PC929 (pictured above) has an additional IGBT protection circuit. Pin 9 is connected to the C and E poles of the driven IGBT together with the external components to detect the IGBT conduction tube voltage drop signal. When the tube voltage drop signal rises to above 7V due to overcurrent, the internal triode V3 of pin 8 is turned on, and the OC and SC signals are isolated by the external optical coupler and sent to the CPU. The inverter reports the OC signal and shuts down for protection. (For details, please refer to the article “Repair of PC923 and PC929 drive circuits”)

However, it can be seen from the circuit structure of PC929 that V3 of the OC alarm circuit is directly electrically connected to the output side circuit, which is the strong current side. The OC signal can only be sent to the CPU through optocoupler isolation. Assuming that the optocoupler that transmits the OC signal is also integrated into the driver IC, the OC signal output pin of the driver IC can be directly connected to the CPU pin. Then the A316J in the driver IC can complete this task.

The internal circuit structure of A316 is shown in the figure below. The PWM pulse signal from the CPU is input through pins 1 and 2, isolated by the internal photocoupler LED1, the interface circuit and the power output circuit, and output from pin 11 to drive the IGBT; the external components at pins 14 and 16 and the internal circuit constitute the IGBT tube voltage drop detection circuit. When the IGBT protection circuit is activated, the LED2 photocoupler transmits the OC signal to the input side, which is output from pin 6 and sent to the CPU.

Compared with other driver ICs, A316J has the following two features:

（1） The input side is not a light-emitting diode, but a digital gate circuit. It does not need to draw a large original drive circuit from the signal source. The input impedance is high and it is easy to connect directly to the CPU pin, eliminating the intermediate drive circuit link.

（2） After the OC signal fault occurs, there is a fault lock function, which blocks the transmission of the pulse signal. Only after a low-level reset signal is input from pin 5, can the fault lock state be released and the pulse transmission path be opened. Therefore, A316J also has a fault reset function.

In this sense, A316 is an integrated driver IC with the most complete control functions.

2. Methods for removing OC fault alarm

During maintenance, we often need to disconnect the drive circuit from the main circuit for separate inspection. At this time, the OC fault alarm function of the drive circuit will cause us a little trouble. When the drive board is powered on separately, the operation panel often reports an OC fault, making it impossible for us to detect whether the drive circuit is normal.

It is necessary to take corresponding measures to release the OC fault alarm function of the drive circuit to facilitate the inspection of whether the drive circuit can transmit six-channel drive pulses normally.

In the drive circuit, PC923 and PC929 often appear in pairs, and PC929 is responsible for detecting the voltage drop of the lower three-arm IGBT conduction tube, and reports the OC signal to the CPU when a fault occurs. When the drive board is disconnected from the main circuit IGBT, it is equivalent to the IGBT open circuit. Once the start signal is sent, PC929 will send an OC signal to the CPU.

pulse trigger terminals E1 – E6 (dashed lines in the figure below) to artificially create the false appearance of “normal opening” of the three-arm IGBT in the inverter circuit. PC929 no longer returns OC signal to the CPU, so the CPU outputs six pulse signals to six driver ICs, so that you can check whether the drive circuit is normal.

The A316J driver IC can also detect the voltage drop of the upper three-arm IGBT and report the OC signal. See the figure below:

By short-circuiting the trigger terminals P and E1, the “conditions” for turning on the IGBT are artificially created, thus releasing the OC fault detection and alarm function of A316J.

The method of removing the OC alarm function of the three-arm IGBT drive circuit of A316J is the same as above. See the figure below:

The driving circuit of the U-phase lower arm IGBT can short-circuit the U and E2 contact terminals, thereby releasing the IGBT tube voltage drop detection and OC signal alarm functions.

There are two points to note:

（1）For the detection of the voltage drop of the lower three arms or the lower three arms IGBT tubes separately, the trigger terminals of the upper three arms or the lower three arms can be short-circuited to release the OC alarm function. For the detection of the voltage drop of all six IGBT tubes, only the trigger terminals of the upper three arms IGBT can be short-circuited, and the OC alarm of the lower three arms can only be released by using a fine needle to pick up the 6th pin of A316; after checking that there is no problem with the upper three-arm drive circuit, remove the short-circuit line, pick up the 6th pin of the upper three-arm drive IC, short-circuit the trigger terminal of the lower three-arm drive circuit, and check whether the lower three-arm drive circuit is normal;

（2）If the trigger terminal has two pins, only G2 and E2, you should find the cathode of diode D61 from the 14 peripheral circuit of A314J, short-circuit it with E2, and release the OC alarm function.

Most of the maintenance of the inverter is the maintenance of the drive circuit. After the drive circuit is repaired, the power-on test of the inverter circuit must be cautious! ! You should use measures such as series light bulbs, 2A fuses, and DC low voltage. After confirming that the inverter circuit is fault-free, restore the 530V DC power supply.

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Application of S87C196MH (MC) microcontroller in inverter mainboard

1. Pinout of the 80-pin S87C196MH (MC) microcontroller in SMD package:

2. Introduction to the structure and functions of the S87C196MH (MC) microcontroller:

The controller is a 16-bit microcontroller produced by Intel. It is widely used in inverter products due to its powerful functions and high versatility.

The internal circuit includes arithmetic logic unit (RLU), registers, internal A/D converter, PWM generator, event processing array (EPA), three-phase to complement SPWM output generator, watchdog, clock and interrupt control circuits, etc. The internal structure schematic is as follows:

 The S87C196MH (MC) microcontroller uses CHMOS technology, has an operating temperature of -40 º C–85 º C, supports 16KB EPROM, and when the crystal oscillation frequency is 16MHz, it only takes 1.75 μs to complete 16-bit by 16-bit multiplication . It is suitable for the rapidity requirements of the control system. There are 7 I/O ports, and each port pin is multifunctional.

The register array has 512B, which is divided into low 256B and high 256B. The low 256B can be used as 256 accumulators during ALU operation, and the high 256B is used as register RAM. The high 256B can also be switched to 256B with accumulator function through unique window technology. The microcontroller has 13 10-bit/8-bit high-speed A/D converters inside, and the conversion time can be set between 1.39-40.2 μs . The A/D can also be used as a programmable comparator to generate an interrupt when the input crosses a threshold level.

The event processing array (EPA) mainly performs input and output functions. In input mode, the EPA monitors the changes in the input pin signal and records its time value when the event occurs. This process is called capture. In output mode, when the timer matches a stored time value, the output pin is set, cleared or triggered. Both capture and compare events can generate normal service processes or interrupts. There are 4 capture/compare modules and 4 compare modules.

EPA also contains two 16-bit bidirectional timer/counters T1 and T2. T1 can be timed according to an external clock source. In this working mode, EPA can directly process two pulse signals with a 90 ° phase difference output by the position sensor (such as a photoelectric encoder) to monitor the speed and direction of the motor.

External event processing server (PTS). The controller has two types of interrupt systems: programmable interrupt controller and PTS. The programmable interrupt can be set to PTS interrupt service mode. PTS has several micro-instruction coded hardware interrupt service processes, which can work in parallel with the CPU and can complete data block transfer, process multi-channel A/D conversion, control serial communication and other functions.

The S87C196MH (MC) microcontroller has a three-phase complementary SPWM waveform generator built in, which directly outputs six SPWM signals through the P6 port. The driving current can reach 20 mA and the driving frequency can reach 8MHz. Each SPWM signal can be independently programmed and the dead zone interlock time can be set.

3. Application of S87C196MH (MC) in INVT inverter motherboard:

（1） Power supply, clock, reset, etc. Pins that meet the basic working conditions of the microcontroller.

（2） Pins for processing digital and analog signals at the control terminals. The start, stop and speed control of the inverter, as well as the monitoring of the working status, are all carried out through the control terminals. The input and output signals of the control terminals directly enter the microcontroller pins.

The output of the analog signal of the inverter control terminal actually comes from the PWM0 (P6 I/O port) pin of the microcontroller. The actual output is a width-modulated pulse signal, which is converted into an analog voltage signal by the subsequent circuit.

（3）Switching control signal, control of charging contactor (relay control), control of cooling fan and reset control of drive circuit (release its fault lock state).

  （5）Processing of various detection and protection signals:

（6） Processing of other functional pins

Some pins are connected to ground, +5V, or +5V via a pull-up resistor according to their functions. Some pins are left unused.

    The specific functions of each pin of the S87C196MH (MC) microcontroller have been explained in detail above. In troubleshooting, the cause of the fault can also be determined and the fault location can be found based on the level status of the relevant pins.

For example, if the signal input of a certain terminal of the inverter is invalid, measure whether there is a 0-5V voltage change on the corresponding pin of the CPU , and then quickly determine whether it is a terminal input circuit fault or a CPU fault.

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A quick inspection method for PLC control circuits

A set of hydraulic processing equipment in a factory suddenly broke down. A solenoid valve could not be powered, causing the hydraulic system to not work properly. After receiving a notice from the user requesting on-site maintenance, an intern from a mechanical and electrical technical school and an undergraduate majoring in automation set out to do the work.

The hydraulic station is controlled by 8 solenoid valves, which are powered by 24V DC and automatically controlled by PLC according to the process flow. Two interns circled the hydraulic system and PLC electric control cabinet for several times, and measured the normal resistance of the solenoid valve (labeled 8YV) coil, which was about tens of Ω . They judged that the solenoid valve was good, and asked the operator to operate the electric control cabinet. The other seven solenoid valves could work normally. However, the two ends of the 8YV coil could not get voltage and could not be attracted. The two interns roughly checked the circuit. The output contact of the PLC drives the intermediate relay KA8, which controls the action of the solenoid valve. The circuit is as shown below:

The circuit was basically clear, but the two students were confused about whether the fault was a PLC problem, an intermediate relay problem, or a circuit problem. In other words, how to check the PLC peripheral circuit fault when the machine was stopped. They wanted to measure the fault during the operation of the equipment, but they did not understand the equipment control process and were afraid of damaging the equipment, so they did not dare to operate. They were not sure whether it was a PLC or a PLC peripheral circuit problem. They did not know how to check it when the machine was stopped. After tinkering for an hour or two, they did not find any results and felt that their self-confidence had suffered a not-so-small blow.

The factory owner was a little anxious. If the repair was not done, the customer order would not be completed. He quickly called two veteran electricians and asked them to repair the equipment in a short time. The veteran electrician, Master Zhang, came up and scolded the two students: If the solenoid valve does not work, there are only two wires, just follow them and find the problem. How can it not be repaired? ! He asked the two students, have you learned PLC, have you learned control circuits? Both of them answered that they had learned PLC and control circuits. The two felt that they had learned well at that time, and one of them was in the top three in the class, a top student. The student said that he understood what he learned at that time, from the control principle to the circuit, everything was OK. How come it is useless all of a sudden? It is exactly: feeling high-energy in learning, but very low-energy in practice; academic performance is not bad, but it is not good when it comes to reality.

Master Zhang said, “Let me fix it while you watch. How difficult is it?” Master Zhang found a wire and introduced the 24V voltage directly to the two control leads of 8YV. He saw that the 8YV solenoid valve still did not work, and the lead was indeed broken. Master Zhang said, “Replace the wires. Replace the two leads of the solenoid valve, and it will be fixed. It’s very simple!”

The control line from the electric control cabinet was introduced into the workshop hydraulic station through dozens of meters of buried iron pipes. Master Zhang said, I know, there is a joint in the control line, it must be a bad contact. To repair it quickly, re-route two control leads of the 8YV solenoid valve, and solve the problem in ten minutes! A student asked: Do I need to measure it again? Master Zhang said, there is no need to measure, didn’t I just try it? In addition, two wires were pulled from the control terminal of the electric control cabinet and directly connected to 8YA. The power-on test, just as Master Zhang said, within ten minutes, the fault was eliminated.

At this time, the two students’ interest in learning was aroused, and they asked the old electrician Master Li: If you were asked to repair this fault, what would you do? Master Li was gentle and explained slowly: Master Zhang’s repair method is based on experience, and his inspection method is also more accurate and direct. He is familiar with the circuit of this equipment and can often solve the problem in a few minutes. You can’t reach this level in a short time. I rely on multimeter measurement, and after several inspection methods and steps, I find out the fault and solve the problem. My method is slower, but more universal and can be used anywhere. In fact, my method is more professional and classic.

What method do you use? The two students are getting impatient.

Master Li said: According to this fault situation, I should first turn off the control power supply of the electric control cabinet and measure the resistance of the two-wire terminal of 8YA from the control terminal. It is tens of Ω , indicating that the lead wire and the solenoid valve coil are basically normal. In fact, this measurement also found that the control lead wire of 8YA was broken, and the fault can be repaired by replacing the lead wire.

Student asked: If the lead wire and coil are good, why doesn’t 8YV work?

Master Li said: Next, we should check the peripheral control circuit at the PLC output end.

Master Li said: Find the two output control terminals YO and COM0 of the PLC. In fact, there is a normally open contact of a relay inside the two terminals. To put it simply, the PLC control terminal is a small switch. When the switch is closed, relay KA8 is energized, connecting the power supply of the 8YV coil and the solenoid valve is activated, right?

We can first short-circuit the two terminals YO and COM0 to observe whether KA8 works. If KA8 works normally but 8YV does not work, and then short-circuit the normally open control contacts of KA8, 8YA works normally, it means that KA8 is broken and the normally open contacts are in poor contact. If the two terminals YO and COM0 are short-circuited, the solenoid valve 8YV works normally, which means that the PLC output peripheral circuit is good. If the terminals YO and COM0 are short-circuited and KA8 does not work, check the 24V control power supply and leads.

Of course, on the electrical control cabinet side, you can short-circuit the YO and COM0 terminals and measure the 8YV terminal to see if there is 24V voltage to determine whether the PLC peripheral control circuit is normal.

In this way, by short-circuiting the two switch points, even if the fault of the PLC peripheral control circuit is exposed, it is not necessary to check the fault during the operation of the equipment. You have to find a way to change the normal state of the circuit, make it move, and expose the fault.

What if the PLC peripheral control circuit is fine, but the 8YV still doesn’t work? Two students asked.

Master Li explained slowly as usual: This may be a problem with the PLC and PLC input signal.

How to check it? The student asked again.

Master Li said: We need to let the operator try it out.

During operation, please pay attention. If the indicator light of the YO terminal can light up, but KA8 does not attract, it means that the contact of the internal relay of the PLC is poor. The PLC should be repaired or replaced. As shown in the circuit above, when the output indicator light of the Y0 terminal is on, you can measure that the voltage between the two terminals of YO and COM0 is 0V, indicating that the internal switch is connected; when the light is off, the voltage is 24V, indicating that the internal switch is disconnected. If the output indicator light of the Y0 terminal is on, but the voltage is still 24V, the contact of the internal relay of the PLC must be damaged.

During the operation, the output indicator light of the Y0 terminal is always off. When the PLC program is running normally (such as other controls are normal in this example), it may be that the signal of the PLC input terminal is not input normally, or the state is wrong, and the conditions of YO output cannot be met. At this time, you can use a laptop to call up the PLC internal program, monitor the program running, find out the conditions that meet the Y0 output and various conditions, and then find out which input signal is abnormal, such as poor contact of the normally closed point of the external limit switch, etc., and the fault can be eliminated.

In the figure above, XO, T1, X3, X4, and T5 can all be called conditional contacts of YO. Let’s ignore T5 for now. For the control of YO, only when the XO (input condition) and T1 (after the delay time is up) contacts are connected, and the X3 and X4 contacts remain static, can YO meet the conditions for powering on. Find out which of these four contacts is not met, find the cause of the fault, and then repair it. If you check that the status of the X0, X3, and X4 contacts are all correct, and look at the PLC input terminal status indicator, X0 is on, and X3 and X4 are off, then three of the four conditions have been met, and only the T1 condition is not met. Further search for the control program segment for the T1 coil in the program, and then check the conditions for meeting the power-on condition of T1, step by step, and find the cause of the fault.

The external input signal of PLC, such as limit switch, may be installed in a place that is not easy for us to touch. Its closing and opening are not easy to operate. At this time, you can also take the method of short-circuiting YO and COM0 terminals, artificially short-circuiting the PLC input terminals, forming an input signal, satisfying the YO power-on condition, and finding out the fault. For example, short-circuit X0 and COM terminals to see if the input indicator light is on and whether the output is moving. If the input terminal status indicator light is not on, check the input terminal 24V power supply is normal, then the internal input circuit of PLC is broken.

During the maintenance process, do not forget to communicate with the operator and electrician in a timely manner. They are familiar with the operation process. You may have searched for a long time, but they may not be able to tell you the fault with just one word of reminder. For example, they mentioned that YO can only act after Y1 output delays for 10 seconds, which makes you understand that YO can only act after Y1 acts. As a result, the problem is found in the control circuit of Y1. This can greatly improve the maintenance efficiency.

When Master Li finished speaking, the two students were convinced and felt that this job was not that difficult. They were full of confidence. They learned things that they could not learn in school from this maintenance example and from the two old electricians. They felt that they had gained a lot. They felt that if the teachers who led the internship class could teach like Master Li, they would be very lucky.

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Alpha ALPHA2000 inverter circuit schematic and maintenance analysis

1.《Alpha ALPHA2000 18.5kW inverter》Switching power supply circuit diagram

The 530V power supply of the switching power supply is introduced from the CNPN terminal. The P terminal is added to the drain of the switching tube K2225 through the main winding of the switching transformer, and then to the N terminal through the emitter current sampling resistor, forming the input power supply path of the switching power supply. One path is added to the 6th pin of the oscillation board of the switching power supply (functional block diagram, including L3845P and other auxiliary components, see the second figure of the driving circuit for the specific circuit) through two 150kΩ2W resistors and the LED1 power startup circuit, providing excitation energy for the starting of the switching power supply. After the power supply starts oscillating, the self-powered winding of the switching transformer generates a DC power supply through rectification and filtering, which is input to pin 6 of the oscillation board as the power supply for the internal oscillation chip; pin 5 of the oscillation board is connected to the power supply N terminal, and pins 6 and 5 are the power supply pins of the oscillation board; the current sampling signal formed on the source current sampling resistor R39 of the switching tube is input to pin 3 of the oscillation board as a current control signal and a power supply stop protection signal. When an abnormal overcurrent occurs, the internal circuit forces the power supply to stop oscillating; the PWM controlled (variable pulse duty cycle) pulse is output from pin 4 of the oscillation board and connected to the gate of the switching tube to control the conduction and cutoff of the switching tube, so that the switching transformerinputs electrical energy and stores energy, and transmits the energy input by the power supply to the secondary load circuit in the form of electromagnetic transmission. The output voltage feedback signal of the switching power supply is input from pins 1 and 2 of the oscillation board, that is, the DC -15V voltage rectified and filtered by D48 and E28 outputs, which not only provides power for the CPU mainboard control circuit, but also serves as a feedback signal input to the oscillation board. Compared with the internal circuit reference voltage signal, an error voltage output is generated to control the PWM wave generator inside the oscillation board, so that the pulse duty cycle of the 4-pin input changes, adjusts the energy storage of the switching transformer winding, and maintains the output voltage unchanged.

The oscillation and voltage regulation of the switching power supply is to adjust the output voltage by changing the pulse width or period, which is called time proportion control. It is divided into two control modes: PWM (width modulation) and PFM (frequency modulation). The U3842, U3844, and U3845 series power supply chips are dedicated PWM controllers.

The switching transformer generates +11V power supply through rectification and filtering of the self-powered winding, and outputs 5V DC voltage through the 78L05C chip 8-pin voltage regulator to supply power to the input side of the U14-A7840 linear optocoupler. The U14 and LF513 two-stage amplifiers form the voltage detection circuit of the DC circuit. The DC voltage introduced from the P and N terminals is fed through a resistor divider network such as a 220kΩ 2W resistor and R54A to obtain a voltage value below 1V, which is then fed into the 2nd pin of A7840 through a 51Ω resistor . The voltage sampling signal output from the 7th and 8th pins of A7840 is fed into the input end of the differential amplifier (with an amplification factor of 1, which is actually a voltage follower for isolation and impedance transformation) composed of the subsequent LF353 through R55 and R56. The 1st pin of LF353 outputs a voltage signal that changes with the DC loop voltage, which is then compensated and adjusted by a semi-variable resistor and input into the subsequent circuit as a VPN (marked by the manufacturer on the circuit board, meaning the voltage sampling signal at the P and N terminals of the DC loop). When the input three-phase voltage is 380V and the DC circuit is about 530V, the VPN point voltage is about 3.8V .

by parallel rectification of D49 and D50 and 220μF50V current filtering is used as 1. Power supply for the cooling fan. The fan is put into operation when the inverter is powered on and there is no control; 2. Power supply for the control relay REL1 of the charging contactor coil. When the switch tube of the 1-pin of the wiring terminal CNN1 is turned on, REL1 is energized to control the charging contactor to be attracted; 3. Power supply for the module temperature detection circuit. The temperature sensor is a normally closed contact thermal relay, which is connected in series to the power supply branch on the input side of the photocoupler. When the module temperature rises abnormally, the temperature relay is activated, the normally closed point is disconnected, and the photocoupler outputs an OH overheat signal to the 14-pin of the CPU; 4. As the 24V power supply for the inverter control terminal. Other power supplies are omitted here.

The circuit on the lower side of the figure is a three-phase output voltage abnormality or three-phase output violent fluctuation or three-phase output phase loss detection circuit. This circuit form seems familiar to us. This circuit form of three-phase output voltage detection circuit has also been used in other inverters. At first glance, this is a typical instrument small signal amplifier, but it is not the case upon closer inspection. Due to the effect of the two input ends of the amplifier parallel diodes bidirectional clipping clamping, it means that the circuit no longer undertakes the task of amplifying small signals, but plays the role of shaping and synthesizing the input U and W two-phase signals into V-phase signals. Just like the principle of holography, if we look at it from the perspective of holographic theory, the U and W two-phase voltage signals must contain the information of the third phase V-phase voltage. Similarly, the synthesized V-phase signal contains the information of the U and V two-phase signals (in fact, the synthesized signal can be considered as the information of the three-phase voltage signal). I guess the subsequent circuit – the CPU’s internal hardware and software circuits is a circuit similar to a frequency meter, which counts the voltage wave heads input to the circuit. When it detects that a half-wave (or full-wave) of the three-phase output is missing, it determines that the output is missing a phase, and a fault alarm is given and the machine is shut down for protection.

2.《Alpha ALPHA2000 18.5kW inverter》 drive circuit, oscillation board circuit diagram

To check the fault of the driver IC or IGBT module, you must know the principle of the A316J (HCPL-316J) protection circuit and the corresponding output state (level value) of the key functional pins. Let’s first look at how the internal and peripheral fault protection circuits of the A316J work. The figure shows the two-way drive circuit, and the remaining space is filled in here with the circuit inside the oscillator board of the switching power supply circuit, which is also mentioned in passing.

Combined with the above figure (a typical circuit in the A316J related information, it is easy to see the connection between the fault protection circuit and the IGBT), let’s talk about the action flow of the protection circuit. Readers, please first understand the power supply circuit and pulse output circuit on the input and output sides. This part depends on you to read it yourself. The space is limited.

Pin 14 of A316J is the module OC fault (overcurrent fault) detection input terminal, which is connected to the C pole of IGBT through an external 100 Ω resistor and an embedded diode, and pin 16 is connected to the emitter of IGBT. Pins 14 and 16 of A316J and the external circuit constitute the IGBT tube voltage drop detection circuit. Under normal working conditions, during the transmission of the drive pulse, the IGBT tube is reliably turned on, and the external diode of pin 14 is forwardly turned on due to the conduction of the IGBT tube, and the voltage value of pin 14 is embedded in the low (zero) level value of the voltage of pin 16. The internal fault detection circuit does not work, and the pulse transmission channel is unblocked. The internal circuit of pin 11 normally outputs the inverter pulse signal, and the module OC alarm signal output pin 6 also outputs a high-level signal under normal conditions; when the load current is abnormal, the inverter pulse signal path is poor, or the IGBT itself fails, causing the IGBT tube voltage drop to be greater than 7V, the external diode of pin 14 is reversely cut off, and a high-level signal appears between pins 14 and 16. The internal fault detection circuit is activated. On the one hand, the transmission of the pulse channel is blocked through the internal circuit, and on the other hand, the fault protection circuit is activated, so that pin 6 outputs a low-level module OC fault signal and sends it to the previous CPU circuit. When the IGBT has an overcurrent, the drive voltage output by pin 11 drops, causing the IGBT to softly shut down to avoid sudden shutdown caused by overvoltage caused by lead inductance and damage to the IGBT. When the internal IC power supply detection circuit detects that the input power supply voltage is higher than the internal reference voltage value and there is no overcurrent signal input, the pulse transmission channel works normally. If an undervoltage signal is detected, regardless of whether the pulse channel is transmitting normally, the internal fault protection circuit will be activated to turn off the upper tube (transmitting the driving voltage) of the push-pull output stage inside the A316J, and at the same time turn on the lower tube (transmitting the cut-off voltage) to softly turn off the IGBT.

After the module OC fault protection circuit is activated, the 6th pin of A316J will be locked in the low-level fault signal output state until a low-level reset signal is input from the 5th pin. An external circuit must output a fault reset signal to release the circuit lock state. The 14th pin is connected to an external embedded diode, and its working parameters have special requirements, reverse withstand voltage ≥ 1000V, turn-on time = 75ns. If ordinary diodes are used instead, the detection and protection function will be lost due to the charge storage effect of the junction capacitor, causing damage to the IGBT.

The oscillator board of the switching power supply is connected to the power supply/driver board through six pins. The -15V DC voltage output by the secondary winding rectification is input to the voltage stabilization control circuit of the oscillator board through pins 1 and 2. The voltage stabilization circuit consists of U3 (8-pin SMD TL431) reference voltage source and U2 (181349D) photocoupler. The change of -15V output voltage causes the change of input current of the light-emitting diode inside U2. The output internal resistance of the phototransistor on the output side of U2 changes accordingly due to the different amount of light received. This change signal is input to pin 2 of U1 (3845). The internal circuit of U2 controls the duty cycle of the output pulse of pin 6 of U2, and outputs the driving pulse to the gate of the switch tube through pin 4 of the oscillator board, controlling the on/off time ratio of the switch tube, thereby maintaining the output voltage of the secondary winding of the switching transformer within a certain value. The working current signal of the switching circuit is introduced into pin 3 of U1 through the R7 and C7 loop, and the internal circuit implements current closed-loop control and overcurrent protection.

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the control terminal circuit of Inverter CPU motherboard

1. Digital control terminal circuit:

The operation control terminal of the frequency converter receives, outputs and processes two types of signals: digital and analog. Analog signals are generally used to input frequency instructions of 0-5V (10V) to determine the output speed of the frequency converter; output analog voltage signals corresponding to the output (given) frequency for external frequency meters to display the operating frequency, etc. The input signals of digital signals are used for the start/stop and fault reset of the frequency converter; the output switch signals are used for external relays and indicator lights to monitor the fault or operating status of the frequency converter, etc.

The following figure (Figure 1) is a digital control terminal circuit, 3 is a digital signal common terminal, 5, 6, 7, 8, 9 are digital signal input terminals. When any of the signal input terminals is connected to the common terminal 3, it can form an input path of the optocoupler, thereby transmitting the control signal to the CPU. The benefit of using optocouplers is that in addition to achieving electrical isolation, it also improves the anti-interference performance. The signals of the inverter control terminal, both input and output, are directly from the CPU pins. Pins 13 to 17 of the CPU are the switch control signal input pins. Pin 22 outputs the switch signal, and the signal content (operation, fault, etc.) can generally be set by parameters, also known as the programmable output terminal. The output signal provides a forward base bias current for the control switch tube TR3 through R36. The conduction of TR3 directly drives the RY1 relay, and the switch signal is output from the three terminals A, B, and C for the external indicator light or relay to indicate the working status of the inverter.

The output from the 76th pin of the CPU is a pulse signal representing the output speed, which drives the transistor TR2 through R27, and TR2 drives the optical coupler PC2. The pulse signal output by PC2 is filtered into a smooth DC voltage by R75 and C28, and is added to the subsequent two-stage voltage follower for current amplification, and then outputs a 0-10V (frequency output) analog signal from the control terminal 4. This circuit can actually be regarded as a simple analog/digital conversion circuit. Although the terminal output is an analog voltage, the CPU output is still a digital (pulse) signal.

For digital (switching) signals, at any time, the input and output potentials of the circuit are both in two states: 0 (ground potential of the power supply) and 1 (positive terminal potential of the power supply).

Figure 1: Panasonic VFO 220V 0.4kW inverter CPU motherboard circuit 2: digital control terminal circuit

The following figure ( Figure 2 ) is still part of the control terminal circuit. Because the four bidirectional analog switch circuits of IC4 (BU4066BCK) are used for unified input and output processing, the digital/analog circuits are drawn together for ease of reading. IC4 can transmit digital and analog signals. It contains four sets of bidirectional switches. Each set of switches has an independent on/off control terminal CTL. When the terminal is at a low level, the switch is in the off state, and the I/O and O/I are in a high impedance state; when the CTL terminal is at a high level, the switch is in the on state. The purpose of using a bidirectional analog switch is to make the transmission of the signal controlled by the CPU instruction, thereby realizing programmable input and output control. The circuit switches the input of two input signals and switches the output of two digital signals.

Figure 2 Panasonic VFO 220V 0.4kW inverter CPU motherboard circuit 3: digital/analog signal control terminal circuit

For the switching control of the two input analog signals, in order to facilitate the principle analysis, the analog signal path in the circuit of Figure 2 is redrawn as follows:

Panel frequency command circuit: VR1 is a potentiometer on the operation display panel, which can adjust the inverter output frequency from the operation panel. The adjustment voltage of VR1 is input to the 61st pin of the CPU through the R90 and C11 anti-interference circuits, and after internal A/D conversion, it is output from the 17th pin, through the IC4 bidirectional analog switch, and connected to the 11th pin of IC4, and then input to the 79th and 80th pins of the CPU;

Terminal frequency command circuit: The control terminal 2 inputs the variable voltage signal of the center arm of the external potentiometer, which is first converted by IC5 (93321N) into a U/F signal and then converted into a frequency signal proportional to the input voltage. It is then sent to another set of bidirectional analog switches inside IC4 through the PC1 (TLP759) photocoupler and output from pin 11.

The two frequency command signals are controlled by the level states of the CPU’s 39 and 40 pins, and further controlled by the user’s control intention. The control parameters can be set. When the 40 pin is high and the 39 pin is low, the CPU inputs the frequency command from the operation panel and determines the output frequency accordingly; otherwise, the control terminal is connected to an external potentiometer (or other analog voltage control signal, such as an instrument output signal, etc.) to adjust the output frequency.

Control terminals 10 and 11 are open collectors, and the switch signal output can realize programmable output. The user sets the output content through parameters, or the terminal is connected during operation, or the terminal is connected during failure. Under the control of the CPU’s 41-pin output instruction, when the analog switch 1 and 2 are connected, the photocoupler PC8 is driven by TR8 and turned on, and the terminals 10 and 11 are in a low-resistance state, outputting an operation signal; when the analog switch 3 and 4 are connected by the CPU’s 40-pin output instruction, the terminals 10 and 11 are also in a conducting state, but the output at this time is the inverter fault signal.

Similarly, the content of the output signal depends on the user’s intention – the CPU’s instructions. The two sets of analog switches play the role of a two-throw switch. Different output contents are switched according to the user’s intention.

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Function diagram of common IC pins in inverter circuits

illustrate:

From the perspective of application maintenance, it is enough to master the pin functions of some IC devices, so as to measure the voltage (level) status of some pins and judge whether the IC is in normal working condition. It is too late and unnecessary to care about what kind of circuit is inside the IC. For example, for the single-chip microcomputer circuit, it is enough to focus on detecting the voltage (level) status of the power supply, reset, crystal oscillator, control signal, and input signal terminals. For digital (including optocoupler) circuits, in general, knowing the pin functions of the device, it is possible to measure and judge the quality of the IC based on the logical relationship between the input and output terminals. As for the analog circuit, half of it is used to process switch signals in the inverter circuit, such as voltage comparators, etc., and it is as convenient to detect and judge as digital circuits. Some analog circuits that process analog signals can be measured based on the obvious changes in dynamic and static voltages, which is not too difficult.

Therefore, as long as you know two points, 1: What type of chip is the IC, digital or analog circuit? 2: The pin function, is the pin an input, output or power supply pin? Then you can implement the measurement. The commonly used IC pin function diagrams of inverters are concentrated in the appendix, so you don’t have to spend a lot of time to consult the relevant manuals.

1. CPU (microcontroller) chip and peripheral IC circuit pin function diagram:

（1）CPU chip-MB90F562B SMD package 64 pins, widely used:

（2） CPU chip – S87C196MH SMD package 80 pins, widely used:

（3） CPU chip – MN18992MDY-6 plastic package dual in-line plug, 64 pins, used for Panasonic’s early DV551 and DV561 models:

（4） CPU chip – HD6404733037F SMD package 80 pins, widely used:

（5） Memory pin function diagram:

（6） RS485 communication module pin function diagram:

（2）Commonly used operational amplifier pin function diagram:

LF353 dual op amp circuit LM393 dual op amp (open collector output) TL072 quad op amp circuit

Operational amplifiers are mostly used in current and voltage detection circuits to process analog signals and convert them into switch signals – alarm and shutdown protection signals. Open collector output types are mostly used in voltage comparator circuits. Operational amplifiers have good interchangeability. For example, LF347, LM324, and TL072 can all be directly replaced. When the pin arrangement is inconsistent, the terminal wiring can also be replaced.

Open collector output operational amplifiers must be replaced with amplifiers of the same type.

3. Commonly used digital IC circuit pin function diagram:

BU4066 Quad Bidirectional Analog Switch 74LS244 Octal Buffer/Line Driver/Line Receiver (Tri-State)

Digital integrated circuits are also divided into several types according to different materials and manufacturing processes. However, they are mainly TTL (transistor-transistor logic) integrated circuits and CMOS (complementary metal oxide semiconductor logic) integrated circuits. The 74 series TTL circuits and the 4000 series CMOS circuits are the most widely used and have the largest number of applications. TTL circuits consume slightly more power, but have higher operating frequencies and stronger output current capabilities, and the power supply voltage is 5V; CMOS circuit characteristics are the opposite. There is a wider power supply voltage range (3.0-18V ) . Under 5V power supply conditions, the same type of circuits can be interchanged. Under different power supply conditions, if the TTL circuit is damaged, you can consider replacing it with a CMOS circuit.

4. Commonly used driver ICs:

TLP250 and HCPL3120 can be directly replaced, and the output pins can be changed to replace PC923. PC923 and PC923 are often used in combination, while A4504 and MC33153 are also used in combination. The combination of the two completes the function of PC929.

5. Commonly used photocouplers:

Optocouplers are used in the control terminal circuit of the inverter, voltage sampling and isolation of the switching power supply, etc. As long as it is a four-wire terminal component, it can often be replaced by PC817. Linear optocouplers cannot be replaced by ordinary optocouplers, and it is best to replace them with original model devices.

6. Switching power supply oscillation chip:

The pin functions of UC3842 and UC3844 are the same, and both have 8-pin and 14-pin packages. UC3842 and UC3843 can be interchanged; UC3844 and UC4845 can be interchanged.

7. Commonly used power (inverter) modules:

The packaging form and size of the power module are consistent, and the rated current value of the replacement module should be equal to or greater than the original damaged module.

Intelligent power modules should be replaced strictly according to the original model.

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616G3-55kW Yaskawa inverter main circuit diagram and inverter maintenance ideas

1. Main circuit of “616G3-55kW Yaskawa inverter”

The structures of the main circuits of all inverters are similar or even identical. The main circuits of Yaskawa inverters and Taiwan Teco inverters are exactly the same. Later I observed that the control panels of the two machines are the same, and the control panel and parameter settings are also similar. It is found that the two machines are similar or even identical from hardware to software, which will bring a lot of convenience to installation, debugging and maintenance. As long as there is a technical data reference at hand, you can debug and repair the two devices.

When we open the shells of these two high-power inverters and check the main circuits, the six large rectangular box-shaped objects installed above the inverter modules (in parallel with the modules) will first arouse our interest – the model number MS1250D225P is connected in parallel with the upper arm IGBT tube of each phase, and the model number MS1250D225N is connected in parallel with the lower arm IGBT tube. To use a phrase from the Internet: What on earth is this? What is the purpose of installing it here?

Generally speaking, anything connected in parallel to the IGBT tube, whether it is a capacitor or a resistor-capacitor network, is set up to protect the IGBT tube. That is, when the tube is cut off, the energy formed by the reverse voltage is quickly consumed, providing a reverse current path to protect the IGBT tube from (in fact, it makes it bear less) the impact of reverse voltage. As we all know, whether it is a bipolar or field effect device, there is often a certain amount of surplus in the forward voltage, but the ability to withstand reverse voltage is extremely fragile. Therefore, the things connected in parallel to the IGBT tube can be said to complete this task of consuming reverse voltage.

It should be noted that the internal circuits of MS1250D225P and MS1250D225N were not opened for verification. After the module was damaged, these two devices were often intact, so it was not convenient to disassemble them after they were damaged. The internal circuit in the above figure was drawn based on measurement and speculation, and is only a reference for readers. I have searched a lot of information and searched on the Internet, but I have not found any information about this component. It is obviously easy to mislead to analyze the principle based on the speculation of the circuit. Therefore, the analysis of its principle is temporarily omitted.

However, after connecting such components in parallel to the module, it will bring us new experience in maintenance. See below.

According to the conventional maintenance method, after replacing the damaged module, before conducting the power-on test, we must cut off the P point in the above figure, connect two 25W (or 40W) bulbs in series, and then power on. In this way, if the inverter module circuit or drive circuit is abnormal, causing the two IGBT tubes of the upper and lower arms to short-circuit the DC power supply, the expensive IGBT module will not be damaged due to the current limiting effect of the bulb. Other brands of inverters connect small-capacity capacitors of the picofarad level in parallel at both ends of the tube. After powering on or starting the inverter, as long as the U, V, and W output terminals are unloaded, the bulb will not light up. But the performance of Yaskawa inverters in maintenance is different. Connect a bulb in series at point P, power on, and the bulb does not light up. It is correct, I breathed a sigh of relief; press the operation panel to start the inverter, and the bulb becomes bright! It is broken, the output module has a short circuit! This is my first judgment. Check the module and drive circuit during power outage, and there is no abnormality. Looking back at the circuit structure, after removing the MS1250D225P and MS1250D225N, the light bulb did not light up after starting the inverter. The three-phase voltage of the no-load output was normal. These two components and the external 10 Ω 80W resistor provided a current path of about 100 mA, making the 25W bulb bright. Sacrificing tens of watts of power consumption in exchange for higher safety of the IGBT tube is the characteristic of the module protection circuit of the Yaskawa inverter.

After the inverter is started without load, due to the relationship between components such as MS1250D225P and MS1250D225N, the inverter circuit itself forms a certain circuit path, which is not caused by the inverter module failure. This machine is a special case. The existence of a circuit path does not necessarily mean that the module is damaged. Observe which components provide the current path? When fresh experience solidifies into a fixed mindset, misjudgment of faults is inevitable.

The control power of the whole machine is obtained from a multi-tap transformer at the bottom of the figure. The short-circuit wires of sockets 3CN and 4CN are different, and the level of input voltage can be adjusted to ensure the accuracy of the secondary winding AC220V voltage. The cooling fan uses an AC220V power supply, which is filtered and used as the input of the switching power supply. When repairing the driver board alone, the terminals 2 and 3 of the fan; 3 and 4 of the contactor terminals; 14CN, 15CN, and 16CN must be short-circuited, and fault signals such as undervoltage (FU/LU), overheating (OH), and fan failure (FAN) must be manually eliminated, so that the CPU can output six pulse signals, which is convenient for checking the drive circuit.

2. Drive/protection circuit of “616G3-55kW Yaskawa inverter”

“616G3-55kW Yaskawa Inverter” drive circuit/protection circuit diagram

The types of drive circuits are similar. The most common ones we see are drive detection circuit (protection circuit) is also integrated. Although you can find circuit information about A3316J, etc., and you can see the internal unit block circuit diagram and the introduction of the circuit principle, you always feel a little confused about the specific composition of its protection circuit – the protection circuit inside the IC is indeed invisible and intangible. It happens that this circuit is a detection and protection circuits composed of ICs such as PC929 and A316J, in which the module fault circuit composed of discrete components, which makes it easier to understand the detection and protection action process. Slightly redrawing the pulse and protection circuit in the above figure, you can see how the IGBT tube voltage drop detection circuit implements the protection action on the module:

Circuit principle: The PWM pulse signal from the CPU pin is isolated and amplified by the U2 optocoupler and then sent to the module protection circuit. Under normal conditions, this pulse signal is amplified by the push-pull power amplifier circuit of Q2 and Q3 to directly drive the IGBT module. It is generally believed that the IGBT module is a voltage-type drive module, which is biased. The input gate-emitter junction capacitance of the IGBT tube just needs a transient large inrush current! This is why Q2 and Q3 are used for power amplification. The introduction resistor of the drive signal is also a 5 Ω 8W power resistor. In this sense, in essence, the IGBT module is still a current-type drive device. This is the author’s opinion, is it correct? When the current output capacity of the drive circuit is insufficient, the three-phase output current will be intermittent, the motor will vibrate, and a rumbling sound will be emitted. The principle of the pulse processing circuit can be found in other figures. The focus here is on how the protection circuit works.

When the inverter does not receive the start signal, the output pins 7 and 8 of U2 are cut-off negative voltages. If the 0V power line is used as the reference point, the voltage of pins 7 and 8 is about -9.5V (ignoring the saturation voltage drop of the internal tube). This negative voltage is introduced to the base of Q2 and Q3 through R13 and R3. Q2 is cut off due to reverse bias, Q3 is turned on due to positive bias, and the gate bias of the IGBT module is negative and is in the cut-off state. Resistors R1 and R2 divide +15V and negative -9.5V to obtain a 3V level. D9 is a voltage regulator with a breakdown voltage value of 9V. The voltage division value of R1 and R2 is not enough to cause it to break down, so Q3 has no bias current and is in the cut-off state. There is no input current to the photocoupler U1, so no fault signals such as GF (grounding) and OC (overload, short circuit) are returned to the CPU. When the CPU sends a drive pulse, the 7th and 8th pins of U2 become positive pulse voltages with a peak value of 15V, and the positive pole of D1 rises to +15V at this time. At this time,two situations occur: In one case, the module is good, and the IGBT tube is quickly turned on under the drive of the positive excitation pulse. It can be considered that the P and E points are short-circuited instantly. The negative terminal potential of D1 is instantly pulled to 0V, and the negative terminal potential of D2 is also pulled to below 1V. Because it does not reach the breakdown value of D2, Q3 still has no base bias current and is cut off; In another case, the module has caused the operating current to be too large due to abnormal load, or the IGBT tube is not turned on well due to the poor driving circuit itself such as Q3. The negative terminal of D1 is high potential and is cut off, and +15V causes D2 to break down through R1, Q3 is biased and turned on, and the positive pulse voltage of the base of Q2 is pulled to zero level, and the IGBT module loses the pulse and is cut off. At the same time, the conduction of Q3 generates the input current of U1, and U1 sends the module fault signal to the CPU. It can be seen that the protection circuit of this circuit first cuts off the driving pulse of the IGBT tube and sends out the module fault signal at the same time. The protection is timely and fast.

3. Drive/FU circuit of 616G3-55kW Yaskawa inverter

“616G3-55kW Yaskawa Inverter” Drive/FU Circuit Diagram

The protection circuit of the drive circuit implements protection actions and sends OC signals according to the tube voltage drop of the IGBT tube during the sending of the excitation pulse. According to the information, the on-state voltage drop of the IGBT module is about 3V under normal (rated current) conditions. When the tube voltage drop reaches 7V or more, it means that the current flowing through the IGBT module has exceeded 180% to 200% of Ie. At this time, the protection action is of course the faster the better. The purpose of setting up this protection circuit is to make up for the slow protection action of the subsequent current detection circuit such as the current transformer. The current detection circuit inevitably uses a large capacity filter capacitor, which makes the circuit have a certain time constant and slow response. The tube voltage drop detection circuit of the IGBT can be called a fast protection action circuit because of its rapid response, just like a fast action unit, which is used to deal with emergency events. The treatment of slight overcurrent and current limiting regulation is still implemented by the current detection circuit of the current transformer circuit.

The drive circuit is also equipped with a fuse detection circuit. In general, a fast-blow fuse is connected in series at the P end of the DC circuit to implement module protection. However, this machine circuit has a fuse connected in series at each phase output module. The inverters produced by each manufacturer generally have the following trends: early products are inevitably crude, old and heavy, and their user control functions are not perfect, but there is a large margin in their production and selection of materials; they are conservative in protection performance, but they do not hesitate to add components that seem redundant now to ensure the reliability of the protection circuit. The early products of Yaskawa inverters were no exception. With the advancement of product technology and the fierce market competition, the functions of inverters have been improved, while the costs have been reduced, and there is even a suspicion of cutting corners. The operating reliability of inverters has also been compromised, and domestic inverters should take this as a warning.

The detection circuit of the three-way fuse blown is to compare the 0V line of the lower three-arm drive power supply with the N line of the main DC circuit to determine whether the fuse is normal. Under normal conditions, the 0V line and the N line of the drive power supply are connected through the fuse and are of equal potential. That is, the E pole of the lower three-arm IGBT tube is connected to the N line of the main DC circuit. Therefore, the base bias of the transistors Q4, Q19, and Q28 is zero. All three tubes are cut off. When the fuse of any phase output module is disconnected, the N line and the 0V line of the phase drive power supply produce a huge potential difference, and the transistor is turned on by positive bias. The three optical couplers Q5, Q20, and Q29 are connected into an OR gate circuit. The input signal of any optical coupler will be transmitted to the same output point, and the fuse signal of the fast insurance will be transmitted to the CPU, so that the CPU reports the FU (fuse) broken signal and refuses to accept the start signal.

There is also an interesting problem of the order of fault signal alarm of Yaskawa inverter. For example, when the power is on, a warning of the fault code is given for overheating, undervoltage, overcurrent, fan failure, fuse failure, etc., and the startup operation is refused; during the startup, the module fault detected by the module protection circuit is warned with the GF (ground fault) code. When the module fault is detected during operation, the OC (operating overcurrent, load short circuit, etc.) fault code signal is reported. The same signal output by the IGBT tube voltage drop detection circuit is output at different times (one is during the startup process, and the other is during the operation process), but the inverter reports two different fault codes (GF: ground fault; OC: overload or short circuit fault). Similarly, in the current and voltage detection circuits, the same means are sometimes used. The overcurrent or overvoltage signal reported by the same protection circuit may report different fault codes or take different treatment measures due to the different working states of the inverter (starting or running). All this depends on the ideas of the software designer. The control ideas of each manufacturer’s inverter must be similar. Pay attention to the relevant characteristics of the inverter fault alarm to facilitate efficient diagnosis of the fault.

When analyzing the protection circuit, you need to look at the main circuit and the drive (protection) circuit in two or three parts. Many drawings are broken down into individual unit circuits. Readers must strengthen their ability to read drawings in a comprehensive and coherent manner. This is a piece of advice I give you.