Induction Motor or Generator EMC/EMI Testing?

[BigMotorGuy]

Do DOL induction machines in general generate EMI levels which need to be considered? Does NEMA or IEEE address induction machines in regard to electrical noise? I realize that if they are VFD driven, this can be an issue due to the drive itself, but wondering if DOL applications need to be tested?

 

[KllrWolf]

A DOL induction motor should not cause any distortions on the power line except for changes in power factor or voltage level, which would be constant with a constant load. If the load is capable of running the motor during operation, you might get some power generated back if it spins it over synch speed. A quick search of NEMA MG1 only showed a few references to EMI, but those were about motor operation, not as external. The only external concern seemed to be with definite purpose generators.

 

[Skogsgurra]

There is one case in Grenoble, where an induction motor generates EMI that interferes with air traffic control signals. This particular motor runs off line and neutral with a capacitor to create the missing phase.
It is known that such a configuration can cause EDM, which results in HF interference.
The reason can be seen in frame 39 in

http://www.gke.org/presentationer/files/GE Bearings and their electric environment w O and E.pdf

The EDM can be seen in the green voltage across the bearing with its very fast breakdowns. So, there may be an EMI issue, but it seems highly unlikely that it manifests itself in a DOL three-phase application.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[BigMotorGuy]

Skoggsgura.........good info in the presentation. KllrWolf, I tend to agree that a standard 3 phase DOL induction machine will not generate any EMI except maybe right at the grid frequency. I have had some experience with small universal motors and due to the brush commutation, you would get significant EMI at high frequencies. but with not switching, I would think that generally you get little or not EMI.

 

[electricpete]

We had a bizarre case where noise in sensitive instrumentation (ion chamber detector) was highly correlated to operation of a particular 13.2kv motor. Specifically:

Immediately after starting the motor, the instrument noise began. So we manually monitored motor partial discharge on the motor (measured from coupling capacitor r/c filter output using Iris equipment), and found it was high (when it had never been before), but also a very unusual “pattern” (the pattern considers when in time the discharges occur in relation to power frequency voltage phase). We monitored partial discharge over a period of time and it went back to normal in the same timeframe that the instrumentation noise went away.

Several months went by. Instrumentation showed no problems, motor partial discharge measurements normal.

Then problem came back on instrumentation. We checked motor partial discharge and again high.

Correlation was undeniable. We were scratching our heads. How did the motor cause the instrument noise? We have suspicion the power cables for the motor pass near the detector cables.

Shortly thereafter while instrument noise was still present (and high motor partial discharge indicated), we also got intermittent undervoltage alarms associated with the motor (pt’s to feed this alarm are switched in when the motor is started).

Voltage measurements at PT secondary showed unusual/unbalanced results (measured using fluke... did not capture any waveform). So the PT drawer was opened for troubleshooting and fuses racked out. At that moment the instrumentation noise went away and the next check of motor partial discharge indicated normal.

Fuse was replaced but not saved. Fuse holders will be inspected at the next outage. No instrument noise or unusual motor partial discharge indiations since then.

I think we had an intermittent open circuit in the fuse or the fuseholder causing arcing. EMI from that arcing was coupled either along the power cable shielding to both the motor partial discharge detection circuit and to the unrelated ion chamber instrumentation circuit.

Although the source of the EMI was not at the motor in this case, I'm inclined to think it could be in other cases of medium voltage motors (it would not be a normal condition, but when there is a problem with a connection or with insulation).


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(2B)+(2B)' ?

 

[electricpete]

We have suspicion the power cables for the motor pass near the detector cables.

They would be in separate raceways (separation of power and signal cables). But possibly raceways close to each other.

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(2B)+(2B)' ?