I have a single 1HP VFD running two 1/3HP induction motors.
Supply voltage to drive 460, motors rated for 460v/60hz/3450rpm (2 pole).
For reasons unknown, when given a speed command of 60hz, the drive was only putting out 320vac.
The drives people at both my vendor and the manufacturer were both stumped to why. The motors seemed to be running fine for many days and the shipment day for the machine had arrived....so out the door it went.
After a few days at the customer, the motor burned up one of its coils. The motor is rated 0.7FLA, and due to low loading, it never actually pulls that much. In fact, when putting out 320vac the drive was also only putting out 0.6A to run both motors. 0.6A total, not per motor. Even if it was 0.6A to just one motor (which it wasn't because they were both spinning furiously) how would that cook the motor?
The OL to the motor never opened up (presumably because the current never exceeded the 0.8A setting) so how did the motor get ruined?
I've searched the net for V/Hz ratio info. All anyone says is that its important. I've read what happens if the V/Hz ratio is too high, but can't find a treatment of what happens if its too low....
Anyone with any insight/experience on this?
320/460 = 69% are you sure you don't have the drive configured for a different maximum frequency? I seem to recall something like that happening to me when I set up a drive to run a 400 Hz motor.
Post some details about your drive and how you set it up. For example, one thing that comes to mind is that your VFD came from the factory defaulted as Sensorless Vector Control and you didn't change it to V/Hz control. You cannot run two motors from one drive in SVC, it confuses the drive and it doesn't know what to do.
To answer your other question, the V/Hz ratio is what allows the motor to develop rated torque. Too high of a ratio, i.e. more V for the given Hz, will saturate the motor windings. The motor will draw more current, but do less work with it, so that extra current is becoming heat in the windings that the motor is not designed to deal with and it fails. Too low of a ratio, meaning not enough V for a given Hz, and the motor loses torque, at the SQUARE of the voltage change. So at 320V on a 460V motor you are at 69% V, so your torque drops to 48% of rated FLT. If the torque required by the load doesn't change, the motor slows down and either stalls, or goes into high slip in an ATTEMPT to try to get to rated speed, so it will pull more current in that attempt and overload.
All that said, if the drive never tripped off, I doubt that was it. Most likely if not the SVC issue, you had a bad connection somewhere and one phase was attempting to draw high current. But the VFD was programmed for a current limit, so it artificially lowered the overall torque in an attempt to maintain that current limit. But that then created a severe current imbalance on the leg with the loose connection, because although current was flowing, it wasn't all getting to all 3 phases of the motor. That current imbalance then created what's called Negative Sequence Current in the motor, which then itself creates negative torque pulsations. So your motor begins essentially "fighting itself" and the result is heat. But since the VFD was in current limit, the current it sees is rock steady, because that's what it's controlling, but it's no longer seeing an accurate picture of what's actually going on in the motor. So the motor over heats and burns up without that ever showing up as an over load situation as measured by current. It's something unique to VFDs when Current Limit is deployed and is a good reason to add a thermistor or RTD to measure actual motor winding heat if using that feature. Most (good) VFDs can accommodate a thermistor input for that reason..