Brake unit circuit diagram and repair ideas

When the speed of the load motor exceeds the output speed of the frequency inverter due to inertia or some other reason, the motor enters the “dynamic” state from the “electric” state, causing the motor to temporarily become a generator. The reverse generated energy of a load motor, also known as regenerative energy.

Some special machinery, such as mining elevators, winches, high-speed elevators, etc., when the electric motor decelerates, brakes, or lowers a heavy load (ordinary large inertia loads, deceleration and parking process), due to the potential energy and potential energy of the mechanical system, the actual speed of the frequency inverter can exceed the given speed of the frequency inverter. The phase of the induced current in the motor winding is ahead of the induced voltage, resulting in capacitive current, The diodes connected in parallel at both ends of the IGBT in the inverter circuit of the frequency converter and the energy storage capacitors in the DC circuit precisely provide a path for this capacitive current. The electric motor generates excitation electromotive force due to capacitive excitation current, which self excites and generates electricity, returning energy to the power supply. This is the process by which an electric motor converts mechanical potential energy into electrical energy and feeds it back to the power grid.

This regenerative energy is rectified by diodes parallel to the inverter circuit of the frequency inverter and fed into the DC circuit of the frequency converter, causing the voltage of the DC circuit to rise from around 530V to 600-700V or even higher. Especially during the process of decelerating and stopping under high inertia loads, it occurs more frequently. This sharp increase in voltage may cause significant voltage and current surges or even damage to the energy storage capacitor and inverter module of the inverter main circuit. Therefore, the braking unit and braking resistor (also known as the braking unit and braking resistor) are often essential components or preferred auxiliary components of the frequency converter. In low-power frequency converters, the braking unit is often integrated into the power module, and the braking resistor is also installed inside the body. But for high-power frequency converters, braking units and braking resistors are selected according to the load operation situation. The CDBR-4030C braking unit is one of the auxiliary configurations of the frequency inverter.

Regardless of the specific circuit, we can first imagine it from the control principle. The so-called braking unit is an electronic switch (IGBT module) that, when turned on, connects the braking resistor (RB) to the DC circuit of the frequency inverter to quickly consume the reverse power generation energy of the motor (converted into heat and dissipated in the ambient air), in order to maintain the voltage of the DC circuit within the allowable value. There is a DC voltage detection circuit that outputs a brake action signal to control the on and off of electronic switches. In terms of performance, when the DC circuit voltage of the frequency converter rises to a certain value (such as 660V or 680V), the switch is turned on to connect the braking resistor RB to the circuit until the voltage drops below 620V (or 620V), and then the switch is turned off, which is also feasible. Anyway, the braking unit has RB’s current limiting function and there is no risk of burning out. If its performance is further optimized, a voltage/frequency (or voltage/pulse width) conversion circuit will be controlled by a voltage detection circuit to control the on/off of the IGBT module in the braking unit. When the voltage of the DC circuit is high, the working frequency of the braking unit is high or the conduction cycle is long. When the voltage is low, the opposite is true. This type of pulse braking has much better performance than direct on/off braking. In addition, with the overcurrent protection and heat dissipation treatment of the IGBT module, this should be a high-performance braking unit circuit.

The CDBR-4030C braking unit is not very optimized in terms of structure and performance, but the actual application effect is still acceptable. The internal electronic switch is a dual tube IGBT module, and the gate and emitter of the upper tube are not used for short circuiting. Only the lower tube is used, which is somewhat wasteful. A single tube IGBT module can be used. The protective circuit is a combination of electronic circuits and mechanical trip circuits. The manufacturer has modified the internal structure of the QF0 air circuit breaker, changing it from leakage trip to trip when the module overheats. Temperature detection and action control are composed of a temperature relay, Q4, and KA1. When the module temperature rises to 75 º C, KA1 action triggers a trip, QF1 trips, and the power supply of the braking unit is turned off, thereby protecting the IGBT module from being burned out due to overcurrent or overheating to a certain extent.
The power supply of the detection circuit (as shown in the figure below) is obtained by reducing the power resistance, stabilizing the voltage with a voltage regulator, and filtering the capacitor, providing a 15V DC power supply.
The faults of the braking unit mainly occur in the control power supply circuit, manifested as open circuit of the step-down resistor, breakdown of the voltage regulator, etc; In addition, due to the introduction of 530V DC high voltage in the DC circuit of the frequency converter, the insulation of the circuit board decreases due to moisture, resulting in high voltage discharge and burning of copper foil strips in large areas of the circuit, as well as short circuits in the integrated blocks of the control circuit. Due to the fact that all circuit boards are coated with black protective paint, the connection and direction of the copper foil strips cannot be clearly seen, which also brings some inconvenience to maintenance.

The circuit consists of an LM393 integrated operational amplifier, a CD4081BE four input and gate circuit, and a 7555 (NE555) time base circuit. The control principle is briefly described as follows:

The DC circuit voltage of the frequency converter introduced by the P and N terminals is divided by the R1 to R7 resistor network and input to the 2 pins of LM339. The 3 pins of LM339 are connected to the set voltage after further voltage stabilization and RP1 adjustment through 15V control power supply. This voltage value is the set voltage of the braking action point. LED1 also serves as a power indicator light. As LM393 is an open collector output operational amplifier circuit, the output terminals of the two amplifiers are connected with pull-up resistors R13 and R14 to provide high-level output during braking action. The first stage amplification circuit is a hysteresis voltage comparator (sometimes also known as a hysteresis comparator), where D1 and R10 are connected to form a positive feedback circuit, providing a certain hysteresis voltage to make the set point voltage fluctuate with the output, avoiding frequent output fluctuations caused by comparing at one point. The second stage amplifier is a typical voltage comparator connection. In essence, the operational amplifier is used here as a switching circuit, without a linear amplification link, but as a switching output. The two-stage amplification circuit forms a phase inversion process for the signal, so that when the output voltage is higher than the set voltage, the circuit has a high-level output.
When LM393 is static, it is a high level output. This high level is superimposed on pin 3 of LM393 through D1 and R10, which “boosts” the voltage value of the braking action set point. When the input voltage of pin 2 (such as 660V DC circuit voltage between P and N) is higher than the voltage of pin 3, pin 1 changes from high level to low level; After the second stage of phase inversion processing, output a high-level signal to pin 1 of CD4081BE. Meanwhile, due to the low level of pin 1 of LM393, pin 3 also dropped from the raised voltage value to the set value. In this way, when the braking unit acts and connects the braking resistor between P and N, the voltage of P and N starts to fall from 660V and continues to fall until the voltage of pin 2 (580V between P and N) is lower than the set voltage value of pin 3. The circuit flips and the braking signal stops outputting, avoiding the unstable output caused by frequent circuit actions at 660V voltage.

The time base circuit 7555 is connected to a typical multi harmonic oscillator and outputs a pulse frequency voltage with a fixed duty cycle. In the LM393 voltage sampling circuit, the braking action signal is output – pin 1 of CD4081BE is a high level, and the high-level component of the rectangular pulse voltage output by the time base circuit 7555 is combined with the high-level signal of LM393, causing pin 3 of CD4081BE to generate a positive voltage pulse output. This pulse is then processed by the master/slave conversion switch, the second stage, and the gate switch circuit. After power amplification by Q1 and Q2 complementary voltage followers, it drives the electronic switch IGBT module.

When the master/slave control switch is turned to the upper end, this machine acts as the master, implements braking action, and transmits braking commands to other slaves through terminals OUT+and OUT -; When the master/slave control switch is turned to the lower end, this machine acts as a slave and receives braking signals from the main unit through terminals IN+and IN -. The signal is input into pin 6 of CD4081BE through optocoupler U5, and braking action is carried out based on the signal from the main unit.
The part of the circuit marked “What is the intention of this circuit” on the blueprint, let’s start from the circuit itself and try to understand the designer’s original intention. If my analysis is incorrect, I hope readers can correct it. Under normal conditions, when implementing a braking action, it can be seen that the braking signal output by U2 is a rectangular pulse sequence signal (this signal is added to pin 1 of U4), and the signal added to pin 2 of U4 through a step-down resistor at the PB terminal is exactly an inverted rectangular pulse sequence signal. At any moment, one of pins 1 and 2 of U4 is always a high level. For the “high out of low” characteristic of the OR gate, pin 3 of U4 always outputs a low level, Q3 is in the cut-off state, and the circuit implements normal braking action; Assuming that the output module has been continuously connected or has been broken down, the signal from the PB terminal to pin 2 of U4 is a DC low level, which is in phase or phase with the pulse signal from pin 1, resulting in an output of “two low and one high”. By driving Q3 through U8, the output signal of pin 3 of U2 is short circuited to ground, causing pin 8 of U2 to also be at a low level until pins 1 and 2 of U4 are completely locked to ground (low) level, and Q3 continues to enter a fully conductive state, completely blocking the braking signal output by U2. Power must be cut off to lift this blockade. But this protective blockade seems powerless and beyond the reach of the module itself in transient overcurrent conditions or faults in the Q1 and Q2 drive circuits themselves.