VFD (Variable-frequency Drive) typical main circuit introduction

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

1. The AC-DC transformation part

1. VD1~VD6 form a three-phase rectifier bridge to convert AC to 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 withstand voltage, 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 CF1 and CF2. Since the characteristics of the two sets of capacitors cannot be exactly the same, a voltage divider resistor RC1 and RC2 with equal resistances are connected in parallel on each set of capacitor sets.

3. Current limiting resistor RL and switch SL

The role of RL: VFD (Variable-frequency Drive) has a relatively large inrush current at the moment when 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 reduces the charging current of capacitor CF. ***Within a certain range.

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

4. Power indicator HL

Function: In addition to being a VFD (Variable-frequency Drive) power-on indicator, it is also used as an indicator of whether the VFD (Variable-frequency Drive) has power after the VFD (Variable-frequency Drive) is powered off (disconnecting and other operations can only be performed after the light is off. ).

2. Energy consumption circuit part

1. Braking resistor RB

VFD (Variable-frequency Drive) will be in the state of regenerative braking when the frequency drops, and the fed back electric energy will be stored in the capacitor CF, causing the DC voltage to rise continuously, 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 externally connected, and all have external terminals (such as DB+, DB-).

2. Brake 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. Direct-to-AC conversion part

1. Inverter tubes V1~V6

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

2. Freewheeling diode VD7～VD12

Function: (1) The motor is an inductive load, and there is a reactive component in its current, 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 returned to the DC circuit;

(3) During the inverter process of V1~V6, the two inverter tubes of the same bridge arm are constantly on and off. In this commutation process, VD7-VD12 are also required to provide access.

4. Buffer circuit

At the moment when the inverter tube is turned on and judged, the rate of change of its 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, and its 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 on to off, the voltage between the collector C and the emitter E will rise rapidly 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 of the inverter tube from conduction to cut-off, so as to prevent the inverter from being damaged.

2. R01～R06

At the moment when the inverter tubes V1~V6 are turned on to off, the voltage charged by C01~C06 (equal to UD) will discharge V1~V6. The initial value of this discharge current is very large, and superimposed on the load current, resulting in damage to the inverter tube. The function of R01~R06 is the discharge current of C01~C06 when the inverter tube is turned on.

3. VD01～VD06

The connection of R01~R06 will affect the effect of reducing the voltage growth rate of C01~C06 when V1~V6 is turned off. After VD01～VD06 is connected, R01～R06 will not work during the shutdown process of V1～V6; and the discharge current of C01～C06 will be forced to flow through R01～R06 during the connection process of V1～V6.