Unbalanced Rotor Currents ? 1

[edison123]

We recently overhauled & tested a 600 HP slipring motor with following nameplate:

Output - 600 HP Speed - 990 RPM (50 Hz)
Stator Volts - 3300 V Rotor Volts - 720 V
Stator Current - 96 A Rotor Current - 390 A

During open shaft run, with the rotor energized at 400 V, we found unbalanced and hunting rotor currents of 70-75, 85-95 and 44-46 Amps. Suspecting rotor winding problem, I did a surge comparison test but it did not reveal any rotor winding problem (no phase unbalance or reversed connections etc.)

On open shaft run with stator energized at 400 V, the stator currents were equal and steady at 5.5 Amps.

The open shaft start up with stator energized was quicker and smoother as compared to start up with rotor energized (when it should have been the other way about) thus confirming that the unbalanced rotor currents were producing negative sequence torque resulting in net lower starting torque when rotor is energized.

Apparently both the stator & rotor were rewound some 12 years back and it was running without any problem. The motor came to me for conversion of sleeve bearings to anti-friction type.

What could cause these unbalanced and swinging rotor currents ? If the rotor winding is wrongly connected (reversed connections or unbalanced winding etc.) should not the surge test reveal it ?

I welcome any opinions on this strange phenomenon.

 

[jbartos]

Comment on edison123 (Electrical) Jan 29, 2004 marked ///\\Wound rotor motors can be run with rotor energized and stator shorted and it behaves exactly the same way if it is energized other way about.
///Yes, somewhat. WRIM is not designed to be powered from the rotor and deliver the shaft rated HP. There an energy flow problem.\\ No load currents (with stator or rotor energized) are always equal for balanced voltages. I have tested many WRIM's with rotor energized without any problem. This is the first time I am seeing a rotor, which passed a surge test, showing unbalanced no-load rotor currents.
///The reverse tests will always be somewhat off since the rotor side is energetically rated less than the stator side. Also, the rotor source of electromagnetic field is different from the stator source because of the air gap geometries. Perhaps, a motor simulation might reveal more about the differences.\\