Phase rotation check for large motors 2

[electricpete]

Phase rotation meters are used to check the phase rotation of a motor by spinning the rotor and checking deflection of a needle. That helps ensure the motor is wired up correctly before it is connected to the power system for initial run.

1 - What principle does a phase rotation meter work on? As I understand it, when you spin the rotor, you get an oscillating meter indication. The direction of initial deflection is used to decode direction of rotation. I'm having a hard time imagining what type of circuitry developes that meter signal.

2 - Our real problem as I'm told (havent' watched myself) is that it is hard to do a phase rotation tests on very LARGE motors. The reason is that it's tough to suddenly start the rotor spinning (due to high inertia). If you slowly start it spinning, you just get an oscillating needle with no initial direction for a reference.

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[electricpete]

Some thoughts:

1 - What if I spun the motor up without leads connected and then suddenly connected the leads? Would that work?

2 - Seems like it could be done with a multi-channel digital O-scope? Just monitor A-B and B-C (and C-A if desired...redundant) and figure out which is leading which?

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[Skogsgurra]

Right. I use a recorder. There's always enough remanent magnetism left to produce a reliable indication. Like this one.

An oscilloscope could, of course, also be used.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[edison123]

pete

Doesn't the motor direction of rotation (DOR) depend as much on the supply sequence as on the winding sequence ?

I don't understand how checking the sequence on a motor off-line alone will ensure you have DOR you want.

 

[waross]

Hi edison123.
There is no problem determining the supply phase rotation.The issue is determining the phase rotation of large motors which do not indicate well on the instruments that are normally used to determine the phase rotation of smaller motors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Yes it does. But we know the supply sequence. We have a fairly consistent known phase sequence accross the plant on the power cables to the motor terminal boxes. C/B/A rotation. A labled with black, B with Red, C with Orange.

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[electricpete]

I did a trial run on a small 460 volt motor that was readily accessible in our plant shop. Used a Fluke 199B 2-channel scopemeter: one channel T1-T2, the other T2-T3 (neutral was not accessible but not needed imo). Having two channels made life a little trickier, but it was still relatively easy to recgonize which channel was leading looking for the small 120 degree difference vs the big 240 deg difference.

Next step is our large motor that I am interested in. I need some setup to do that (this motor needs an oil lift system running to rotate by hand).

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[edison123]

Can't you have a adjustable test bed with your standard supply sequence so that you can do a bump test or even a vibration check for all your incoming MV motors?

My $ 0.02.

 

[electricpete]

The effort to set up a test run is a lot more than the effort to rotate while watching with O-scope, so from that perspective the O-scope is preferred. But performing a test run before installing into the plant could have some other advantages in checking for damage from shipping etc to identify before it goes into the plant. It's something I'll think about.

Our capability to set up power for a shop run is probably limited to 460v supply with no ability to vary voltage. I think that would be ok for reduced voltage unloaded dol start/run of 4kv motor... do you think so? What about 13.2kv motor in general ?

Unfortunately the motor I am most interested in right now is a 13.2kv motor with large flywheel. I am pretty sure it would take forever to accelrate at 460vac and probably wouldn't be good for the motor (it takes 25 seconds to accelerate when powered from 13.2kv!).

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[edison123]

I have had strange results trying to run MV motors with LV supply (415 V). While some 3.3 KV & 6.6 KV motors with a low as 300 to 600 KW would hardly get up to full speed, other motors with higher KW have gone to full speed at 415 V. Most recent case was 3 x 6.6 KV, 3 MW, 4 pole machines with white metal bearings, which to my complete surprise, got up to full speed of 1490 RPM beautifully.

My recommendation - from your availability of only LV supply for the test bed - would be to have a 0-460 V dimmerstat feeding into a 460 V / 13200 V step up trafo, which in turn feeds your motor. This would ensure a low starting current and smooth acceleration. Of course, you need to ensure that step-up trafo output sequence matches with your other MV supplies. I am rigging one up for our shop right now and am adding parallel capacitors on the LV side to still reduce my intake current from my supply.