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How to Solve Overvoltage of Variable Frequency Drive?

Variable frequency drive often encounters various problems in debugging and application, and overvoltage phenomenon is the most common.
Once the overvoltage occurs, over-voltage protection function of the variable frequency drive will acts to stop the operation of VFD, so as to prevent internal circuit damage. As a result, the equipment would be unable to work normally. Therefore, measures must be taken to eliminate overvoltage and prevent the occurrence of faults. Because the applications of VFD and motor are different, the causes of overvoltage are also different, countermeasures should be taken according to the specific situation correspondingly.

Causes for Overvoltage of VFD
1. Overvoltage occurred in disconnecting the transformer. According to the theory of forming cut-off overvoltage, when the transformer is disconnected, the current in the transformer inductance can not change abruptly, and the stored magnetic field energy forms an oscillation between the transformer excitation inductance and the ground capacitance, thus leading to overvoltage.

2. Overvoltage generated by transformer switching on with load. In practical test, load-free transformer has detected an overvoltage of several times of the power supply voltage. Its physical principle is: the load-free transformer can be equivalent to the parallel connection of an excitation inductance and the equivalent capacitance of transformer itself. If the neutral point of transformer is not grounded and the switch is non-periodic closing (one phase or two phases firstly close), high overvoltage occurs due to the oscillation of feeder capacitance, transformer ground capacitance, longitudinal capacitance and transformer inductance, particularly when the neutral point voltage of transformer is rather high.

Although transformers are basically switched on with loads, overvoltage will also occur when transformers are switched on with loads, differently, it is smaller than that of load-free.

Commutation overvoltage of rectifier elements will not only damage the components, but also produce electromagnetic interference.

Treatments of VFD Overvoltage
1. For disconnection overvoltage of phase shift transformer for variable frequency drive, better result can be obtained by using the overvoltage absorption circuit, which is composed of capacitance absorption network and zinc oxide arrester.

2. For overvoltage generated by transformer switching on with load, switches with good periodicity can be selected, or good resistance-capacitance absorption circuit or active suppressor technology scheme can be adopted, or transformer with electrostatic shielding measures can be adopted. However, it will also be quite difficult to make electrostatic shielding layer for high-power transformer.

3. For overvoltage generated by commutation of rectifier elements, the reverse voltage withstand value of rectifier elements should be sufficient, and measures for absorption circuit and continuation circuit shall be proper, otherwise the rectifier may be broken down by overvoltage.

4. As the overvoltage of VFD is basically generated when the transformer is switched on or off, measures should be taken from the transformer to suppress the overvoltage of VFD. Measures are:
  • Increase the transformer’s excitation inductance and ground capacitance. No-load current reduces with increasing excitation inductance, which will increase transformer cost.
  • Increase the ground capacitance of transformer: it is easy to analyze on principle, but in fact, it is impossible to produce transformers with arbitrary insulation mode or high insulation level due to the transformer structure and material limitations, so it’s also quite difficult to increase the ground capacitance C of transformer.

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