AC Drives FAQ (part 3): Variable-Frequency Drives and Safety-Interlock Circuits

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What are the general rules for safeguarding machinery incorporating motors driven by VFDs (variable frequency drives)? For example, should the contactors be on the input or the output?

A contactor on the input of the VFD rather than the output is preferred, although on can make the case for one on the output.

Putting a contactor on the output of the VFD will assure immediate removal of voltage from the motor which is what you want. On the other hand, some drives are easily damaged by switching on their output and that it's possible that the motor could be reconnected to the output of a drive that was operating above zero frequency and this could also damage the drive. (Effectively a direct (non-soft) or full-voltage start on the output of a VFD)

Placing the contactor on the input of the drive would shut off all power to the motor, but there could be a small delay due to the energy stored in the capacitors in the VFD power supply. This would be safe, would not damage the drive and would prevent direct-online switching on the output of the VFD. The drive would always start from zero when the contactor was re-closed. You may have to reset an under-voltage trip on the drive, but that may be automated if required. You may also use soft starts between the VFD and the motor.

There are other limits of installing a contactor on the input side, including possible damage to the DC bus capacitor pre-charge circuit. The pre-charge circuit will be designed for a certain amount of operations/hour. Is it acceptable contactor(s) to use late-make/early-break auxiliary contact on the output, wired back to the drive?

The early-break contact should break around 500ms before the main legs to be effective. Typically-available 10ms early-break aux-contact isolators are ineffective as the residual motor field will not have decayed, resulting in an inductive kick 10ms later when the 3 main legs open. Pffff! goes the VFD output! It is unclear in this situation what a suitable isolator could be. The only practical solution is a lockable isolator, the expectation being that the person with the key understands the need for care.

If one is expecting a lot of switching on the input, then this could be a problem. If an early-break contact on the contactor is used to panic-stop the drive first, then that may be okay. It depends on the timing and also how the drive responds to this sort of treatment. Some are more tolerant than others. Consult the manufacturers of the drive.

In the case of an emergency, nothing can stop the motor faster than the drive itself (most drives having an E-Stop input anyway). How the VFD handles the emergency condition is usually a programmable function: ramp down at max or coast. This assumes, of course, that the drive is undamaged, one would be wise to tie in a line or load contactor to the NO (normally open) output of the drive-status relay. If it's an emergency, break it on the load side (drive output). Nobody at the inquiry is going to sympathize with your concerns to pamper the thyristors when weighed against the potential risk to human life.

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