Medium Voltage Motors Phase Rotation Test

  1. 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?
  2. jcsd
  3. Baluncore

    Baluncore 3,216
    Science Advisor

    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.
  4. 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..
  5. Thanks Baluncore, and Windadct for your comments.
    Megger has a portable portable instrument for low voltage

    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.

  6. jim hardy

    jim hardy 5,450
    Science Advisor
    Gold Member
    2014 Award

  7. 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?
  8. 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.
  9. Baluncore

    Baluncore 3,216
    Science Advisor

    “High Voltage” is a regulation classification. Anything below 50V is deemed “Low Voltage” and has relaxed wiring rules.
  10. jim hardy

    jim hardy 5,450
    Science Advisor
    Gold Member
    2014 Award

    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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