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Induction Motors


shan

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Hi! Friends

Recently i tried to energise an three phase induction motor stator before assembling the rotor. Nothing abnormal happened to it. I only observed that the stator got magnetised. I failed to measure the current. I did not keep it for long for more observation. I discussed this with my colleages who were not present there, who say this is not possible and the stator winding should burn out. No one could justify the statement. What i know is if the stator is energised without the rotor then it will be like an open circuit and will draw magnetising current. To burn the winding there should be excess current more than rated which is possible only if there is no back emf in the winding or if the insulation fails due to stator core heating up due to excessive flux on saturation. Both these things are not possible as there will be back emf developed, and the core will not saturate as it is its rated voltage. Even if we look at the equivalent circuit and we eliminate the rotor resistance we remain with just stator resistance and inductance and the magnetising circuit. So the current should not be more than the no load current which the winding should carry normally.

My question is ...... am i right??? If i am wrong please correct me and let me know what exactly will happen to the winding if the stator is connected to rated supply without the rotor. I will be happy if i could get an answer in detail.

Regards

Shan

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Hello Shan

 

Welcome to the forum

 

If you energize the stator without the rotor fitted, you will draw magnetizing current, but the magnetizing current will be very different to that when the rotor is fitted and spinning at synchronous speed.

The stator without the rotor fitted effectively has a very large air gap compared to when the rotor is fitted.

This increased airgap will greatly reduce the inductance which will result in a much higher current.

 

If you leave a stator energized at rated voltage with no rotor fitted, the excessive current could result in excessive copper losses in the winding and insulation failure.

 

Best regards,

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  • 4 weeks later...

Gidday, Next time, if you are worried about the wind (after a mega and resistance check) simply put variable dc supply through windings (with clip-on set up) and go around inside of stator with a compass to confirm poles. I had the same question as you after memorizing lenzs law.

 

Clayts :)

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  • 3 weeks later...

Hi Shan,

Energizing the stator on it's own (even at much lower than rated voltage) will result in excessive current & overheating in a very short time.

 

We use this method (in a controlled manner) for pre-warming smaller stator windings prior to epoxy impregnation.

We also used to use AC low voltage on newly wound stators to confirm the integrity of the winding connections, however we rely more on surge testing these days.

 

A DC polarity test as Clayts describes is a reliable method of checking correct stator polarity.

 

On series connected AC stators polarity can also be checked by using a prufex tester (provided the stator has 6 leads out). Simply connect a temporary Y, then connect 1 phase lead to the Y point and move the prufex tester around the circumference of the stator. The prufex will show an amount of shorts around the core equal to stator poles. Repeat this sequence on all 3 leads to check phase polarity.

 

*Please note the method will not work on stators having parallel connections*

 

Regards

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