Situation: I have 5 unit SIEMEN VSD, all the time running 3 unit VSD, each of the incoming ampere carries R=33A, Y=35A, B=41A to control pressure A, but when running 4 unit VSD, you will notice 2 or 3 out of 4 unit one of the phase carries 0A (R=40A, Y=42A, B=0A) to control pressure A. However output secondary all carries balance load so no harm to the motor. Question is why this thing able happen? Is it due to load distribution? Is it due to voltage fluctuation affect the firing of power electronic component? Is it due to harmonic voltage high incure unstability of firing of power electronic component?|||Most variable frequency drives (VFD) have just a passive 3-phase rectifier in the input section. Each phase of the rectifier conducts for only a short time when the voltage is near the peak of the sine wave. The input current is often imbalanced due to slight voltage imbalance and/or differences in supply line phase impedances. When the VFD is lightly loaded, the current imbalance is most noticeable. However, one phase current dropping completely dropping to zero would indicate a serious supply voltage imbalance, a bad connection or a bad circuit breaker or contactor somewhere in the supply wiring.
If the VFD has an active input section such as a controlled rectifier, the problem could be due to a problem with firing.
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