Squirrel cage induction motors - rotor issues

[Willr90]

Hello all,

I am currently involved in a diagnostic project for an induction motor with suspected rotor stator contact. all critical dimensions have been checked but the question has been raised by none electrical observers:

• Does the location of the rotor bars effect the magnetic center line?

The explanation that has been given why it doesn't is as follows:

"A three-phase induction motor has a stator winding which is supplied by a 3 phase alternating voltage and has balanced 3 phase currents in the winding cable. The current produces an mmf (magnetomotive force) that creates a rotating field of constant magnitude. This field causes the shaft to rotate at synchronous speed. The rotating magnetic field has a constant magnitude but the axis of direction rotates in space.

The rotating field caused by the current in the stator winding cable induces emf in the rotor by a transformer action. The rotor is a closed set of conductors consisting of copper rotor bars that are brazed or electrically welded to the copper short circuiting end rings at both ends. This results in an emfs in the rotor causing a rotor current. The interaction of the rotor and stator currents cause a rotating magnetic field producing forces/torque seen on the rotor bars causing the shaft to rotate.

The rotor bars key function is to act as a connector between the two short circuit rings similar to two pieces of wire connected together to form a circuit.
Conclusion:
1) Position of rotor bars not important since it’s a current carrying conductor.
2) The bars must be brazed properly to withstand rated amps. "

I remain unconvinced that the position of the rotor bars will not effect the rotation of the shaft. can someone please either confirm or deny this explanation. one thing to add is that this is a 350kW machine.

Thank you,

A concerned Engineer.

 

[zoki85]

The rotor slots are generally skewed by some small angle with the shaft in order to reduce harmonics, produce more uniform torque etc. Have you considered these effects?

 

[Averagesupernova]

The position of the rotor bars most certainly affect the rotation of the shaft, but I am not sure they do in a capacity that affects your concerns. Zoki85 pointed out the uniform torque by skewing the bars which will also reduce audible hum which one would assume is caused by reduction of vibration within the rotor. Technically I suppose this is motion. I think positioning and size of the bars has more to do with torque and ability to come up to speed. Torque under load of course will be related to slip. Typically when the copper bars are buried deeper within the iron core of the rotor the motor will have less starting torque. This is because the circuit that the copper within the rotor forms will have a higher inductive reactance, which of course will see a frequency of 50/60 hertz at zero RPM, and prevent higher relative rotor currents. An induction motor won't have much torque if anything impedes rotor current. At synchronous speed the rotor circuit only sees a few hertz.
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Keep us posted. I am curious to hear results.

 

[Willr90]

Thank you for the prompt responses!

The rotor bars of this unit are not skewed and it is based on a previous design from the 1990's that had no known issues.
The reason the question has been raised is because when the motor is run up to full speed it makes an unusually loud hum and on strip and inspection we found the bearings have been in contact with the shaft.

When we received the rotor it was not to drawing and due to the need for haste the lamination's were ground back but I am afraid that they may have been ground unevenly (rotor bars are showing at one end of shaft).

to illustrate my point about what i think may have happened to the rotor bars i have done the following: (wildly exaggerated to make point, in reality this difference will be a few thou)

Would this make the rotor rotate unevenly?

Thanks.

 

[Averagesupernova]

I doubt that a few thousandths would make a difference. The thought that a person could have bars buried the in the core the way you have in the attachment has never crossed my mind. I assume the rotor was put in a lathe and turned down to reduce the diameter? One would assume that the copper was centered about the shaft when manufactured. So that tells me when rotor was turned down it was not centered in the lathe. This alone would make it out of balance. No wonder it makes noise and you are having bearing issues.

 

[zoki85]

to illustrate my point about what i think may have happened to the rotor bars i have done the following: (wildly exaggerated to make point, in reality this difference will be a few thou)

View attachment 76221

Would this make the rotor rotate unevenly?

Thanks.

That would make an axial asymmetry of electromagnetic situation in the machine and would result in increase of loss, harmonics and vibrations.

 

[jim hardy]

Does it hum when running unloaded? Slip is really low unloaded,so rotor current isn't much.
That would make me look to stator. Possibly a phase miswired internally?

A rotor anomaly ought to cause variation in phase current at slip frequency --- as the anomaly passes under a phase winding the current is affected, for example open rotor bar gives a dip. Have you analog meters that you could watch?

Just guessing, old jim

Btw... Bearings contacted shaft? I don't know what you mean...
Shaft currents in a machine that size could wreck bearings quickly.
Tthey come about from magnetic unbalance, as if rotor not centered in frame.
I assume one bearing pedestal is insulated?