Typical main circuit of VFD (Variable-frequency Drive)

When analyzing VFD (Variable-frequency Drive) faults, sometimes if you know the circuit principle of VFD (Variable-frequency Drive), you can better analyze the cause of the fault.

1. AC-DC transformation part

1. VD1~VD6 form a three-phase rectifier bridge to convert AC into DC. If the three-phase line voltage is UL, the rectified DC voltage UD is: UD=1.35UL

2. Function of filter capacitor CF:

(1) Filter out the voltage ripple after full-wave rectification;

(2) When the load changes, keep the DC voltage balanced.

Because of the capacitance and voltage resistance, the filter circuit is usually composed of several capacitors connected in parallel to form a group, and two capacitor groups connected in series. As shown in the figure CF1 and CF2. Since the characteristics of the two sets of capacitors cannot be exactly the same, a voltage dividing resistor RC1 and RC2 with equal resistances are connected in parallel to each capacitor set.

3. Current limiting resistor RL and switch SL

Function of RL: VFD (Variable-frequency Drive) has a relatively large impulse current immediately after the gate is closed. Its function is that within a period of time after closing the gate, the current flows through RL, and the inrush current is specified, and the charging current of the capacitor CF is specified at Within a certain range.

SL function: When CF is charged to a certain voltage, SL closes and short-circuit RL. Some VFDs (Variable-frequency Drives) use thyristors instead (as shown by the dotted line).

4. Power indicator HL

Function: In addition to serving as a power-on indicator for the VFD (Variable-frequency Drive), it also serves as an indicator for whether the VFD (Variable-frequency Drive) has power after the VFD (Variable-frequency Drive) is powered off (unplugging and other operations can only be performed after the light goes out. ).

2. Energy consumption circuit part

1. Braking resistor RB

VFD (Variable-frequency Drive) will be in a regenerative braking state during the frequency decrease process, and the feedback electric energy will be stored in the capacitor CF, causing the DC voltage to continue to rise, even to a very dangerous level. The role of RB is to consume this part of the feedback energy. Some VFD (Variable-frequency Drive) resistors are external and have external terminals (such as DB+, DB-).

2. Braking unit VB

It is composed of GTR or IGBT and its driving circuit. Its function is to provide a path for the discharge current IB to flow through RB.

3. Orthogonal-to-orthogonal transformation part

1. Inverter tube V1~V6

Form an inverter bridge to invert the DC power rectified by VD1~VD6 into AC power. This is the core part of VFD (Variable-frequency Drive). For commonly used inverter tubes, see: "Commonly used inverter tubes for VFD (Variable-frequency Drive)".

2. Freewheeling diodes VD7~VD12

Function: (1) The motor is an inductive load, and its current has a reactive component, which provides a "channel" for the reactive current to return to the DC power supply;

(2) When the frequency drops and the motor is in regenerative braking state, the regenerative current is rectified by VD7~VD12 and then returned to the DC circuit;

(3) During the inversion process of V1 ~ V6, the two inverter tubes of the same bridge arm are constantly in the on and off state. In this commutation process, VD7~VD12 also need to provide paths.

4. Buffer circuit

When the inverter tube is turned on and judged, the rate of change of voltage and current is relatively large, and the entire inverter tube may be damaged. Therefore, a buffer circuit is connected next to each inverter tube, whose function is to slow down the rate of change of voltage and current.

1. C01～C06

Every time the inverter tubes V1~V6 are judged from conduction to cutoff, the voltage between the collector C and the emitter E will quickly rise from 0V to the DC voltage UD. Excessive voltage growth rate will cause damage to the inverter tube. The function of C01~C06 is to reduce the excessive voltage growth rate when the inverter tube is turned on and off to prevent damage to the inverter.

2. R01～R06

When the inverter tubes V1~V6 turn from conduction to cutoff, the voltage charged by C01~C06 (equal to UD) discharges V1~V6. The initial value of this discharge current is very large, and is superimposed on the load current, causing damage to the inverter tube. The function of R01~R06 is the discharge current of C01~C06 at the moment when the specific inverter tube is turned on.

3. VD01～VD06

The access of R01~R06 will affect the effect of C01~C06 in reducing the voltage growth rate when V1~V6 is turned off. After VD01~VD06 is connected, R01~R06 is disabled during the turn-off process of V1~V6; and during the turn-on process of V1~V6, the discharge current of C01~C06 is forced to flow through R01~R06