Medium Voltage Motors Phase Rotation Test


by Royherd27
Tags: medium voltage, motors, phase sequence
Royherd27
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#1
Sep12-13, 11:15 AM
P: 11
Is it possible to check the phase rotation sequence of a Medium Voltage Motor (Up to 6 kV) with one instrument?

Does the shaft need to be moving while it is being tested?

If instead of one, there are two motors electrically connected in parallel, with a common source feeder. will it be possible to test them from the source side before it is separated to each motor?
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Baluncore
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#2
Sep14-13, 06:48 AM
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Quote Quote by Royherd27
Is it possible to check the phase rotation sequence of a Medium Voltage Motor (Up to 6 kV) with one instrument?
I think not. Fundamentally the phase rotation direction is dependent on which end of the motor shaft you look at. No single instrument can know that.

If you apply low voltage AC to one phase, you may be able to infer a sequence from the sign and magnitude of the voltage induced in other phases. You may need more than three phases to apply that method. As above it cannot give you direction of rotation, only a relative sequence.

I see no way of separating the sequence of two motors in parallel without isolating one at the time.
Windadct
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#3
Sep16-13, 08:05 AM
P: 535
I do not know of a MV phase rotation test, since this typically takes 3 connections - would be very dangerious.

You want to test the rotation before you disconnect them? Or looking to confirm the reconnect is correct after service?

Often with motors the best test is to mechanically de-couple the load - then energize the motor and check rotation - for example if a motor is being installed or returned to duty after service..

Royherd27
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#4
Sep16-13, 08:49 AM
P: 11

Medium Voltage Motors Phase Rotation Test


Thanks Baluncore, and Windadct for your comments.
Megger has a portable portable instrument for low voltage
http://www.megger.com/us/products/Pr...71&Description

However I am still not sure if it might be used for Medium Voltage.
Good thing you don't need to energize the motor to verify what rotation it has.


The big challenge thta I have Windadct is thta I can't d ecouple the motor...

Thanks again.

Regards
jim hardy
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#5
Sep16-13, 09:01 AM
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You're speaking of a test on the motor before making electrical connection to its leads ?

I've watched handheld meters like this on small motors.
http://www.fluke.com/fluke/auen/elec....htm?PID=56153



You have to turn the shaft a little by hand. I'm not sure how you'd do that on a several thousand horsepower motor.

Our electricians always 'bumped' for rotation before coupling the shaft.
Royherd27
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#6
Sep16-13, 09:54 AM
P: 11
Megger States:

"The Motor and Phase Rotation Tester permits the electrical contractor or industrial maintenance electrician to permanently connect and tape the terminals of the motor being installed, without having to first energize the motor by a temporary hookup from a power source, if available, to determine motor rotation. Therefore, the test set eliminates the need for temporary connections that can be time consuming, costly and quite hazardous, particularly where many large, high-voltage motors are involved".

Can it be done in Medium Voltage?
Parallel?
Windadct
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#7
Sep16-13, 01:29 PM
P: 535
I do not know what megger means by "High Voltage" - they should know better.... haha

It is the MV issue that is the problem, connecting any test instrument to live MV is just dangerous. You may also be able to touch base with the motor manufacturer - they may have a "trick" - or can guide you in a way to visually confirm the lead to winding sequence. The bump method is the only thing that comes to mind for V > 600VAC.

If they are induction motors - they will be difficult to get a reading on without being energized, as well as turning them mechanically enough - etc.

There are also companies that specialize in testing / commissioning that may be of assistance, and properly trained to work on the voltage levels and have the equipment- etc.
Baluncore
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#8
Sep16-13, 05:17 PM
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“High Voltage” is a regulation classification. Anything below 50V is deemed “Low Voltage” and has relaxed wiring rules.
jim hardy
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#9
Sep16-13, 05:50 PM
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Place an analog ohmmeter across the winding of an induction motor , like say your electric fan. Give the shaft a turn and watch the needle briefly deflect.

The DC current from the ohmmeter is making a small magnetic field in the motor. When you rotate the shaft rotor current flows and induces small counter-emf in the windings, hence the small dip of the meter needle .


Handheld phase checkers like that Fluke and Megger use that effect to figure out phase sequence on a 3 phase motor.
I never saw one used on a really big motor though, probably because it'd be impractical to turn the shaft by hand. But i'd bet somebody sells a gizmmo for making that check.


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