High Voltages

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aharrigan

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At a job site there are two cooling tower pump motors. in the past year one has failed twice and the other once.
Input voltage to VFD is a steady 480-482. At the disconnect with it being off the voltage reads are puzzeling. With a Fluke
meter the one that has the u shape to go around the wire the voltage reads 480 volts. With a better Fluke and a Greenlee
the voltage reads 530 -570 volts.

It seems the cheaper meter reads 480 while the better reads higher. Is there an explanation for this and could the VFD not be
set up properly for these particular motors ? The VFD'S are ABB'S
 
The meter with the U shape sounds like a Fluke T-5. Find a meter with a Low Z voltage scale. That will present a small load across the source to be measured. You indicate the source is off, but electronic devices always leak a small amount. This small amount of current can cause a false reading in meters with high impedance voltage inputs.

Fluke makes meters that are specially designed to make measurements on energized drives, others probably do as well.

A Fluke SV225 can be used on high impedance meters.
http://www.testpath.com/Items/Stray-Voltage-Adapter-116-091.htm?gclid=CMP2rLDNmrQCFQq0nQodB0MAWQ


Between this & the post by ZOG you should have some ideas to try.
 
Are you measuring the input voltage to the drive or the output voltage.

Output voltage of a 480 volt drive will have short duration high voltage peaks maybe even up to or exceeding 1000 volts. This is hard on motor insulation and is compounded by long length leads between the drive and the motor. If the motor is not wound with spike resistant wire, it is even harder on the windings. Depending on your meter it may see this it may not, some will see it differently than others. A true RMS meter will give you what the windings are effectively seeing for voltage, but the spikes are still working on the insulation.

Line reactor on the drive output helps reduce the effects.
 
aharrigan said:
At a job site there are two cooling tower pump motors. in the past year one has failed twice and the other once.
Click to expand...

aharrigan, I am with Kwired and think you may be barking up the wrong tree.... did you get a detailed report from the motor rpr house why the failed? often a good rpr house can give you a pretty good guess what caused a failure.

often when multiple motors fail on vfds the issue is the insulation and pwm voltage spikes Kwired mentions. were the motors vfd rated? are there output reactors on them? what size are they? are they using shielded vfd cable? and often very important, how long are the motor leads from vfd to motors? long leads can lead to standing waves that can increase the normal 1000-1500v spikes to much higher.
 
mike_kilroy said:
aharrigan, I am with Kwired and think you may be barking up the wrong tree.... did you get a detailed report from the motor rpr house why the failed? often a good rpr house can give you a pretty good guess what caused a failure.

often when multiple motors fail on vfds the issue is the insulation and pwm voltage spikes Kwired mentions. were the motors vfd rated? are there output reactors on them? what size are they? are they using shielded vfd cable? and often very important, how long are the motor leads from vfd to motors? long leads can lead to standing waves that can increase the normal 1000-1500v spikes to much higher.
Click to expand...

and I would add that motors running on VFDs (especially ones not designed for VFD) tend to run hotter as the normal cooling system from their fans are not enough to do the job. it is often recommended that additional cooling be provided to motors running on VFDs
 
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