VFD tripping on instantaneous overcurrent

[Electric-Light]

The new information of the hot motor makes me also suspect the motor is failing. Perhaps it would be best to put the spare motor in and send the old one old to be rewound.

I don't necessarily agree. Motor shops don't usually don't have the same variety of materials or capability as a production facility. Its harder to tightly fit thicker winding on a premium efficiency motors and often are often less efficient than factory specs.

There was a study somewhere that compared new premium efficiency vs rewound one. The latter did not score as well on an efficiency test.

 

[petersonra]

I don't necessarily agree. Motor shops don't usually don't have the same variety of materials or capability as a production facility. Its harder to tightly fit thicker winding on a premium efficiency motors and often are often less efficient than factory specs.

There was a study somewhere that compared new premium efficiency vs rewound one. The latter did not score as well on an efficiency test.

if that is an issue, then the only real option if it turns out the motor is the problem is to replace it.

But keep in mind a motor running that low on the curve is not real efficient in the first place.

 

[IslandSparky]

Is a class F motor VFD rated? Don't have my cheat-sheet with me.

You should stick a power quality meter on both the input and output of the VFD and see what is happening. Not to harp on the capacitors, but they are the easiest way to cause this type of tripping-- they can drive down AC voltage instantaneously, which causes a drop in the drive's DC bus, which then causes the instantaneous over current. The other thing it can do is blow the diodes... They can be anywhere in the system, not just on the one VFD circuit.

Does the VFD modulate much, or generally run at consistent speeds? What %? What is your harmonic profile on the output? Are the trip lens over-heating the windings?

Can just be a failing motor, but you usually see that at as a ground fault first.

 

[mxslick]

Is a class F motor VFD rated? Don't have my cheat-sheet with me.

You should stick a power quality meter on both the input and output of the VFD and see what is happening. Not to harp on the capacitors, but they are the easiest way to cause this type of tripping-- they can drive down AC voltage instantaneously, which causes a drop in the drive's DC bus, which then causes the instantaneous over current. The other thing it can do is blow the diodes... They can be anywhere in the system, not just on the one VFD circuit.

Does the VFD modulate much, or generally run at consistent speeds? What %? What is your harmonic profile on the output? Are the trip lens over-heating the windings?

Can just be a failing motor, but you usually see that at as a ground fault first.

The motor is inverter (VFD) rated...

The motor insulation class is Class F which I believe is good for about 155degC (Motor is inverter duty rated).

Capacitors are not an issue as according to the OP there are none in use...

...verified that there are line reactors on the input to the drive and there are no power factor capacitors on the system.

The problem is the motor is going bad...plain and simple.

But short circuits in the windings tend to not be fleeting, they either happen or not.

In most cases but not all. Again, I have seen firsthand motors with intermittent ground faults/shorts in the windings that will run for quite some time before it escalates enough to burn out the motor and/or trip the drive or OCP.

philly, if anyone is near that motor during it's operation, ask them if it ever makes any strange noises or "surges" or "sags" in speed intermittently. That would be a sign of the kind of issue I am talking about.

 

[petersonra]

I am still leaning toward ripping out the motor and putting in the spare, even if just to see if the problem goes away.

I would not completely discount some intermittant issue within the VFD itself. I had an end user with one where there was a small flaw in the power section circuit board that exhibited similar issues. The end user replaced the drive (in this case a small one) after replacing the motor and sent the drive back to the manufacturer who found a small defect in the circuit board. The manufacturer was decent enough to repair it N/C, even though it was well out of warranty. But it took like a year to resolve it.

 

[philly]

You should stick a power quality meter on both the input and output of the VFD and see what is happening. Not to harp on the capacitors, but they are the easiest way to cause this type of tripping-- they can drive down AC voltage instantaneously, which causes a drop in the drive's DC bus, which then causes the instantaneous over current. The other thing it can do is blow the diodes... They can be anywhere in the system, not just on the one VFD circuit..

Although there are no capacitors on the system I am interested to learn further how capacitors can drive down the AC voltage instantly causing the DC bus to drop and therefore instantaneous overcurrent. Can you please explain?

Does the VFD modulate much, or generally run at consistent speeds? What %? What is your harmonic profile on the output? Are the trip lens over-heating the windings?

The VFD runs at pretty much consistant speeds. I believe the drive usually runs between 80-100% but I am still verifying. I'll have to look at the harmonic profile. Can this be done with a power quality meter on the output of the drive? I'm not sure what you are referring to as the trip lens?

Can just be a failing motor, but you usually see that at as a ground fault first.

I agree that this usually occurs as a ground fault first. As I mentioned system is ungrounded although there is ground fault detection package on system. So any real fault occuring must be a L-L fault that may be intermittent. I would think a L-L fault would be high enough to cause permenant damage but maybe not. We are most likely going to replace the motor next chance we get.

I am still trying to determine the source of overheating in the motor. Recent temp readings with an IR scan showed 165deg F on the outside which is about 70deg C. This value although on the outside is not anywhere near the 155deg C rating of the motor insulation.

The motor also has a nameplate frequency of 30-60Hz which I believe the manufacturer intended not to run this motor below 30Hz for cooling purposes. However since this is a fan and a varialbe torque load manufacturer said motor can be safely run down to 10% speed with adequte cooling. We dont typically operate at speeds below 30Hz but it is something I am keeping in mind when looking into this issue.