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Friday, October 20, 2017
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induction Motor V/Hz Ratio Is Low - Effect?
Induction motor run at a lower than designed V/hz ratio...what happens?
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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.

1 out of 1 members thought this post was helpful...

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..

>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.

==========You are correct. It was defaulted to SVC and not set to
V/Hz. This was one of, but not the only problem. After changing it to V/Hz. The 320 volts jumped up to 460 volt, but problems continued. More below

>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.

============== a bad connection was not the problem. One of the 2 motors behind this drive was smoked, and replaced, 3 times. Sure there could have been a bad connection somewhere 1 time, or even 2, but not 3. Good point to check this, but alas it was not the problem in this situation.

>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.

======== All of these things you describe about a current phase imbalance are possible, but I never saw it during the times I checked it. I put clamp on ammeter on all 3 phases and saw the same amount of current in each leg.

I have been speaking extensively with AB and my supplier about this. We are beginning to think the problem is not the drive, but the other components in the circuit.

The drive output goes to a line reactor. Then after the reactor, it splits into 2 parallel circuits. Each circuit has (in order, after the line reactor) an MCP (specifically 140M-C2N-B10) and then an OL (193-T1AA80). After leaving the OL, there is a 50ft run of cable to the motor. All connections from the moment it leaves the drive output are made with inverter duty cable and the shield and ground's continuity are maintained the whole time.

It seems as if **maybe** the MCP or the OL (or both) cannot handle the PWM. If they cannot handle it, perhaps they then present a strange impedance (too high? too low?) on the circuit. I have now destroyed 1 MCP and 3 OLs, in addition to the 3 smoked motors.
My vendor suggested a standing wave problem at the motor.

I have ripped open 2 of the 3 smoke motors (the 3rd got tossed in garbage and I never had a chance to look at it). 1 of the motors had black/burnt windings: a sign of a current/heat problem. Not a standing wave problem. I was *told* by my vendor that a motor whose winding insulation has failed due to standing wave issue would show a blue color to. The other motor's windings looked as copper and a clean new penny. No visible signs of damage at all. However, 2 of the phases measured only a couple ohm phase to phase and a 3rd measured 55ohm. Clearly its a gonner.

We have now just ordered inverter duty motors to replace the original ones and will be done with this mess. However, the other VFD, which is running only a single 1/3HP motor, has had no problems whatsoever. Nor have the MCP and OL in between the motor and the line reactor. IF the problem was that the MCP and/or OL couldn't handle PWM, I would have seen a problem by now. But no such thing has happened. Knowing what I know now, I have no idea why my vendor recommended the MCP and OL for that circuit. It's just a single VFD running a single motor. Sure the line reactor is needed for helping the regular duty motor survive the dv/dt, but why the MCP/OL? VFD should handle that, right?

it makes me wonder if its a problem lying in wait. We've already had to go and replace stuff at this customer 3 times. To put the new inverter duty motors in we'll have to go back for a 4th. If I smoke another component it'll be a 5th. Its beginning to look ridiculous...