Tag: 7200MA

Received a 7200GA-41kVA frequency converter for repair, which is a lightning fault. After replacing the damaged input rectifier module, switch tube, and shunt tube of the switching power supply, the operation panel screen displayed normally, indicating that the problem is not significant.

The negative pressure and optocoupler drive input signals of the six channels were tested to be “normal”. During the assembly test of the whole machine, the OC jumped immediately upon power on, but the operation could be started after resetting. The screen frequency output was normal, but there was no three-phase voltage output at the U, V, and W terminals during the actual test. The local driving IC adopts optocouplers PC923 and PC929, which cooperate with SN0357 to return OC signals.
Check the power amplifier circuit on the output side of the driving IC and the detection circuit of the IGBT tube, and there are no abnormalities. When testing the pulse input pin of PC923, it doesn’t feel right. Why is the level of pin 3 high and pin 2 low? Is it because the driver power supply is reversed? 2. Three pins are the input circuit of the photodiode, two pins are the anode of the photodiode, and three pins are the cathode of the photodiode. According to common sense, the two pins are usually powered by+5V and then stabilized to provide an excitation power of about 4V, while the three pins are connected to the pulse output terminal of the CPU. Low level output is effective, that is, when outputting, current is pulled in from the three pins of PC923 to make the diode conductive. When there is a trigger pulse input and the frequency is low, the voltage of pin 3 oscillates up and down to 3V. As the frequency increases, the voltage of pin 3 gradually stabilizes at around 3V. When there is no output, pin 3 is a high level of around 4V (equal to the level value of pin 2).
The current detection results are as follows: when no running command is input, pin 3 is a high level of 0.5V, and pin 2 is a low level close to 0V; When entering the run command, pin 3 drops to 0.2V and there is a change in high and low levels, indicating that the CPU pulse has reached PC923. At the beginning of the maintenance, I took a detour and only paid attention to the changes in high and low levels, without paying attention to the magnitude of the voltage value. Obviously, the loss of 2-pin power supply voltage prevents the IGBT transistor from receiving excitation pulses, resulting in no output voltage from the frequency converter.
Check that the 2-pin power supply is a simple series connected voltage regulator with a transistor and a voltage regulator. The base bias resistance of the transistor is open, causing the supply voltage to be zero. After replacing the bias resistor, measure the voltage on pins 2 and 3 of PC923 to return to normal. After receiving the operation command, the frequency converter has output from the U, V, and W terminals.
Further investigate the reason for the OC jump upon power on. Measure the SN0357 optocoupler device that transmits the OC signal, and the voltage value of the two pins on the input side is zero, indicating that it did not input the OC signal. However, measure the voltage value of the two pins on the output side of the three optocouplers to be 0.5V! But since there is no OC signal input, the voltage between the two pins should be 5V (one pin is connected to a 5V ground level), and there is only one possibility, that is, the 5V pull-up resistance of the signal output pin has changed or is open circuit. At this point, the CPU mistakenly believes that it has received the OC signal returned by the drive circuit, and therefore gives an alarm. Try connecting a 10k resistor between the signal output pin and the 5V power supply. Start up and test the signal output pin to be 5V. Repeat the power supply several times and no longer experience OC faults.

The above two faults actually come from one reason: power loss. The input pin of the pulse signal and the output pin of the OC signal are both directly connected to the CPU pin. When the pull-up high level disappears, the CPU pin only has a low level of 0.5V left. This level is not enough to drive the optocoupler to send out the trigger signal of the inverter module, and it also leads to the detection of low levels and the jumping of the OC fault code when powered on. And 0.5V is also the critical level for detecting signals such as OC, so after performing a reset operation, it can start running again.
Note:
The motherboards of various series of Dongyuan frequency converters have good replacement characteristics, and after replacement, only the corresponding capacity value of the frequency converter needs to be readjusted. After changing its capacity value, the relevant parameters for checking the rated current value of the motor are also automatically modified.
The capacity labeling of Dongyuan frequency converters is mostly not based on kW, but on kVA. For example, for a 22kW capacity, it is labeled as 41kVA with a rated current of around 48A. In the relevant manual, the capacity is set by comparing the HP horsepower with the rated current. Moreover, the setting of horsepower size is not simply expressed in numerical terms, but also mixed with English letters to indicate the size of horsepower value (actually using hexadecimal notation). When initially adjusting its capacity value, it is often difficult for Master Zhang Er to understand and adjust it, and the parameters cannot be written in.
The corresponding relationship between kW, KVA, and HP values is not clear to most users. I don’t know what the Dongyuan people were thinking about doing this. 1HP=0.75kW, indicating that the horsepower is less than the kW value; KW is the active power value, which can be approximated as the actual power value. That is, a 22kW frequency converter can drive a 22kW motor, and the power capacity is sufficient; And kVA is the apparent power value, which means that when there is such power, but it drives some loads such as motors/inductive loads, it cannot reach the marked output value because the motor load requires reactive power consumption, and kVA is the sum of active power and reactive power. The kVA value is virtual (not discussed here) and can be selected based on the rated current value.

The comparison table between the motor capacity HP and the set value is as follows:
HP: Set value:
25 (18.5kW) 29
30 (22kW) 2A
40 (30kW) 2B
50 (37kW) 2C
60 (45kW) 2D
75 (55kW) 2E
100 (75kW) 2F
125 (90kW) 30
150 (110kW) 31
175 (130kW) 32
215 (160kW) 33