I have a 4160, 4 pole, 500 HP wound rotor motor that has had the slip rings shorted. The motor is driving a horizontal, split case centrifugal pump. At times the motor will not start, rotate the shaft by hand a little and it will then start. I performed a rotor influence test by measuring inductance across the stator and rotating the shaft a few degres for each measurement. On a standard AC induction motor the test results produce a nice 3 phase sine wave with 120 degrees of seperation. This motor produced a sine wave that has 0 degrees of seperation. There is a little anaomly in the influence test that looks like a possible bad coil in the windings. I have seen similar faults in other motors that sart ok, they just run hot or have some vibration. The stator windings tested good with the All Test Pro 4 and 31. The motor showed no signs of trouble before having the rings shorted. I did not get a chance to test the rotr windings before they were shorted.

I have other wound rotor motors that have had the slip rings shorted, all driving similar pumps. they all have been operating for years without incident. The other motors are 6 and 8 pole. I have yet to check the rotor influence on these motors. Will try to do that this week.

My question is, What would keep my 4 pole motor from starting? I know starting torque is reduced when shorting the rings, but a centrifugal pump doesnt take much torque to get moving. I can turn the assembled machine by hand. Is the starting problem due to the motor being a 4 pole?

The motor shop that shorted the rings said that sometimes they just dont work. For me, that is an unacceptable answer. They said that they could add resistance to the rotor windings, but it would take so much the motor would burn up. After thinking about that statement overnight i dont know how that would be.

Any help on this sublect would be greatly appreciated. checkk out the attached rotor influence test results.

Rotor_Test.pdf (22 Kb, 24 downloads) Rotor Test pdf

Slip ring motors converted to induction motors by shorting the rings are subject to "cogging" or "dead spots" as a result of unfortunate rotor and stator slot combinations. Also converted slip ring motors have notoriously low starting torques so sometimes they won't start if the load is at all difficult to start such as dry sleeve bearings or a conveyor.

A wound rotor induction motor with secondary shorted has a number of things working against it. It generally has lower starting torque than a standard induction motor. It also has higher inrush current which can create voltage droop (depending on the power system) which in turn further reduce the starting torque. Finally, the rotor of a wound rotor motor is more "delicate" than the rotor of an induction motor since it has insulation which can overheat long before squirrel cage motor hits any temperatre limits.

In summary from above, a wound rotor motor with windings shorted is less suited for D-O-L starting than a standard induction motor. But where are slip ring motors typically installed? In high starting torque applications, because when used with resistor they give very high starting torque. If you take that same high starting torque application and put in the worst possible motor (a slip ring motor with rings shorted), you are asking for trouble. (By the way, what is the load?).

Where to go from here? Installing properly sized rotor resistors during start will certainly boost starting torque and start accelerating the motor faster. But the resistors should be bypassed once the motor is up to speed or else efficiency will be horrible.

It is also possible that rotor windings are already damaged from unsuccessful starts. (Probably even likely, if the motor has stalled and not tripped). I'm not sure what your RIC says. But if you can get access to remove the shorting jumpers for testing rotor winding resistance, or better yet winding inductance, that would be the most direct check.

quote:

Originally posted by jim2f:
I performed a rotor influence test by measuring inductance across the stator and rotating the shaft a few degres for each measurement.

The designer of the wound motor rotor has much less freedom to choose the combination of the stator and rotor slot number. Unlike in the case of the squirrel cage motor he has to choose number of slots both on the rotor and the stator that are divisible by 6. As a result there are always some slots on the rotor and the stator that are almost exactly aligned in any position of the rotor versus stator. That is the fact that affects the inductance readings. But the designer of the motor knows, that if you start the motor with the resistors in the rotor circuit, the combination of the rotor-stator slots is totally irrelevant. In fact, it is possible to choose the resistor in such a way that the starting torque will reach the maximum torque - wow! It is possible only with the wound rotor motor! The designer did not know that somebody would short out the sliprings!
Your gadget made something as a sine-wave from something that should have been a saw-tooth. Your numbers alternate between 22 and 27 most of the time. In order to get more realistic picture, you would have to vary the angle in much smaller increments and use some much more sophisticated instrument to measure the inductance.
If you want to learn more about the inductance of the wound rotor motors, check IEEE 112. You will find out that the variation of the inductance (with the angle of rotation) is a very well known phenomenon.If you draw an unproven conclusions from the variation only, you will likely screw up.
jank