Polyphase Testing

[Col]

Just swotting up on three phase systems and was wondering what sort of tests you could run for checking out trafos and motors. Can only think of:

Motors:
1) Testing resistance between wires to check for broken/damaged coils
2) Testing resistance between wires and chassis for shorts

Trafos:
1) As above
2) Megging between primary and secondary
3) No load test for overheating

Can anyone point me in the direction of any others?

[Martin Wareham]

Megging between all conductors (on high voltage motors and TXFMs) and chassis.

There are also dynamic tests, i.e. without disconnecting anything. I’ve seen a clamp on current meter used to good effect, to highlight which phase of a transformer was faulty when the high voltage motor was powered up.

Is the motor (three phase synchronous AC) running in the right direction, if not swap a phase.

[Col]

Hi Martin, thanks for the reply.

I assume you don’t mean megging between phase conductors? Surely that would be practically nothing anyway.

Good call on the clamp meter though *calls RS*

[Martin Wareham]

Aye, you don’t need to meg all the conductors to chassis if they are connected together electrically, just one from each phase or winding.

[ROVRatt]

On a 3 phase motor a megger test between any connection point and the chassis will give the reading of the worst set of windings, as they’re all connected together. If its above the minimum go ahead and use the motor. If not, disconnect all the coils in the connector box and determine which one is down. Also, ensure that the megger is on the correct voltage setting for the motor. Its no use testing a 3000 volt motor using a 500 volt megger.

Motors also require a vibration analysis test prior to being sent to a system to determine bearing faults, rotor/shaft imbalances and end plate misalignment. The rotor should turn freely by hand as well. A test run should have been conducted to determine amperage drawn is not excessive and overheating of any components is not present.

Transformers require megger testing at the right megger settings for primary and secondary to chassis and between primary and secondary. A load test should be applied before sending it to the system to check for current draw, overheating and excessive humming which could mean loose windings which will cause it to vibrate to pieces eventually.

[Scott Beveridge]

There is also testing the resistance variation between the coils or the temperature of each particular phase under load using a laser thermometer.

[Author]

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