Single Phase Test 1

[wrsharper]

I want to perform a single phase test on a 500HP 480VAC 3550RPM twelve tap star/delta motor in order to check the condition of the squirrel cage rotor.

I have a 15KVA three phase delta/delta step down transformer for 480VAC to 240VAC. The delta secondary has a center tap (stinger leg). I intend to excite the primary with 208VAC. I think that the center tapped secondary should provide 52VAC. I would use the 52VAC to excite one winding of the motor. The motor will be disconnected from its normal source of power and I will force the contactors into a star configuration for the test. I will slowly rotate the motor shaft one revolution while monitoring excitation current. If current deviation is less than 5% then my rotor is in good shape. This test will only take app. 30 seconds.

Any problems with this concept?

 

[electricpete]

I have never run the test but I have read some guidelines.

One guideline is 10 - 20% volts. It seems like you are shooting too low. I would apply in delta or if you want to use the star, then use the full transformer secondary rather than "stinger".

Higher voltage will be more sensitive to faults. I think as long as you stay below 25% and don't energize longer than a minute there is not much concern for overheating.

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[wrsharper]

I just made the final connections to the transformer to check my secondary voltages with 208 three phase excition on the primary. I am not working with the motor yet and only finding a voltage source.

Between X1-X2-X3 (normal three phase output) I have 106VAC as measured by a Fluke true rms voltmeter.
Between X1 and X6 I have 53VAC.
Between X2 and X6 I have 93VAC.
Between X3 and X6 I have 53VAC.

Would you recommend applying 106VAC to the motor for the single phase test? I do have the 93VAC to use and I have a little wiggle room on the primary taps and can lower the voltages by 5%.

How much current would you anticipate when the motor is connected. Right now I have the transformer primary connected to a three phase 15A breaker.

 

[wrsharper]

I am missing point about configuring the motor in star or delta for the test. With the secondary voltages what would you choose for use with either star or delta. What would be the better motor configuration to use, star or delta?

 

[wrsharper]

Thank you electricpete. I had searched this site for information about this test and found some good information but since I was in uncharted waters I wanted a little moral support.

I will now give the results of the test and how I performed it. This data may prove helpful for someone else. As I stated before I had a 15KVA delta/delta stepdown transformer 480V to 240V. It was sitting loose on the floor and only ten feet away from the motor.

I powered it with three phase 208V and the power source needed is 40A. I tried connecting in delta with 53VAC but the volts was pulled down to 18VAC and current was 125A. This was off the stinger and the transformer did not have enough soup. I then tried connecting in star with 106VAC. The volts pulled down to 77VAC and current was 85A. I rotated the rotor slowly and current did not vary at all. This proves to me that the rotor is in good shape. Thanks again to this forum.