VFD Output Filter?

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TwoBlocked

TwoBlocked

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Bradford County, PA
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Industrial Electrician
I am pretty sure of my diagnosis and remedy, but want to check with you fine folks in case I am missing something. A 25HP 3-ph 480VAC VFD driven motor driving an axial fan was tripping out the VFD on overcurrent. Meggerd fine, but had uneven resistance phase to phase. disconnected all the leads. Each lead read fine by itself, but some were shorted to others. Obviously an internal short, but why, and what to do about it.

The length of the motor leads are over 100ft and the rule of thumb I am used to is to anything over 100ft needs an output filter on the VFD to avoid voltage spikes shorting out the motor windings. There are four of these unit at the facility and because of the configuration of the equipment, It is necessary to bring in a large crane to replace one of these motors. The one that went bad has the longest motor leads.

So do you fine folks think it is likely that an VFD output filter would have prevented this fault. Do you think all the VFDs feeding these motors should get VFD output filters?
 
I have installed VFDs on a lot of motors that were standard motors and with lead lengths in excess of 100' without any type of output filter. We have not seen issues. In many cases it took 50+ feet of wire to even get out of the MCC room.
 
I am an expert in VFD applications. Here’s what might be important to know:

Line/load reactors are used commonly in many VFD applications. LINE reactors are used upstream of the drive. LOAD reactors are used on the output side of the drive. Most likely what is happening here is you need to move the reactor to the line side.
 
herding_cats said:
I am an expert in VFD applications. Here’s what might be important to know:

Line/load reactors are used commonly in many VFD applications. LINE reactors are used upstream of the drive. LOAD reactors are used on the output side of the drive. Most likely what is happening here is you need to move the reactor to the line side.
Click to expand...
That's certainly contrary to what I had learned, but I'm definitely not an expert.
 
LINE reactors mostly resolve apparent power issues (high amperage) that can fault trip drives. LOAD reactors are designed to deal with harmonic distortion. If the drive is tripping because of high amperage, a LINE reactor might resolve it. If not, go to the next size drive.
 
Ok so it single phased? The OP had megged the windings and states that they were fine. You want to see about 2-3 ohms phase to phase.
 
Yeah megging does not tell you everything.

I would assume that the motor was old and was not inverter rated?
 
Just got back from a cabin with family but not with cell service.

Thanks for all the responses. To be clear, there were shorts between windings. No windings were open, no windings were grounded when meggered. Might have been shorts within individual windings, but I do not have equipment that could confirm that.

I did not check the nameplate of the bad motor, but it was a newer installation and always had a VFD. The replacement has "F" rated insulation.

So to repeat my original question: "So do you fine folks think it is likely that an VFD output filter would have prevented this fault. Do you think all the VFDs feeding these motors should get VFD output filters?"
 
IMHO and I'm not an authority, but the presence of a properly-sized load reactor should lessen the chance of this particular fault occurring, and if these are specialty motors a reactor may be good insurance against faults.

As for the second question, it's hard to say without knowing what "these motors" actually are.
 
Load reactors are always a good thing. Here's my analysis:

The motor was not inverter duty, but many times non-inverter duty motors are run on drives just fine. One thing though, on a non-inverter duty motor, you must never go below 20Hz. Also depending on the quality of the winding, insulation, something called "laminar squeal" will damage the windings, but it's at a different rate on different motors, at what speed you run them, and how many start/stops are made.

When you start a drive on a motor load, the high pitched "eeeeeeeeeeeeee" you hear is the windings vibrating causing laminar squeal. Eventually the tiny wires will crack. Depends on the build of the motor for when it does.

What makes a motor "inverter duty" is really about the insulation, how its coated and what they did to prevent laminar squeal. Also, the way the front (shaft side) bearing is grounded. Really good inverter duty motors only squeal for a moment right at start-up.
 
As far as output filters, those are necessary to control non-fundemental harmonics. The 5th and 7th are the ones to watch closely. The only way to test it is with a really high-end data logger. I really like the AEMC data loggers (PEL 102, 103). I don't think an output filter would have helped here much, but I never tested it for THD so....?
 
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