We have the output damaged on a VSD driving a 1200kW cyclone pump on a mine. After three weeks of operation the drive did not restart after it was switched off. The power unit on the white phase had burn marks on and the fuses driving the power unit from the dc bus were open. Can this of been caused by back emf from the motor when stopped? The motor turns backward when stopped from the water header pressure that comes back down after the motor stopped. PS: There is no contactor between the output of the drive and the motor - only at the input.

Very probable. The SC current of a 1200 kW motor is substantial and the inverter usually has free-wheeling diodes that will conduct when a voltage is impressed from the motor side. Cannot the control be arranged so that the motor slows down under power until all water has left the tube? There is a check valve, I hope.

I am assuming an induction motor and a standard PWM VFD. If you have a synchronous motor and some other VFD, there are other options to get the VFD damaged. A WRIM motor with a cascade control adds even more possibilities. Which type of motor and drive do you have?

The problem with slurry applications is that once you turn off the drive, the slurry keeps going and pulls the pump with it, creating that potential regen situation. The same thing happens on oil well screw pumps. Turning off the output of the VFD should be OK even though, because without excitation the motor field will collapse in a very short time, maybe a second or 2 at the most. The caveat to that is if there are capacitors on the load side. For example if, for some reason, someone has connected surge capacitors to that motor. It used to be a very common practice on large MV motors, but you must NEVER have capacitors on the down stream side of a VFD unless they are part of a very specifically engineered filter system. Then of course there is the possibility that there ARE filter caps on the output side, which can unintentionally allow regen in your application. I once heard of an installation where capacitance on long output cables created enough energy to damage components as well, so that's something else to investigate if none of the other issues apply.

But leaving the VFD output on to decel (ramp down) and then providing someplace for the regen energy to go should solve the problem regardless.

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