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Thread starter
Willr90
Start date
Dec 4,Tags
In summary, the question of whether the location of the rotor bars affects the magnetic center line in a three-phase induction motor has been raised. The explanation given is that the position of the rotor bars does not affect the rotation of the shaft since they are only current carrying conductors and must be brazed properly to withstand rated amps. However, the engineer remains unconvinced and suggests that the position and size of the bars may have an impact on torque and the ability to come up to speed. The rotor bars of this particular motor are not skewed and have been ground unevenly, potentially causing an axial asymmetry in the electromagnetic situation and resulting in increased loss, harmonics, and vibrations. The engineer also raises concerns about the bearings being in contact
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:The explanation that has been given why it doesn't is as follows: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?
Science Advisor
Gold Member
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
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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 '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.
Science Advisor
Gold Member
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
Willr90 said:
View attachment
Would this make the rotor rotate unevenly?
Thanks.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.
That would make an axial asymmetry of electromagnetic situation in the machine and would result in increase of loss, harmonics and vibrations.
Science Advisor
Gold Member
Dearly Missed
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?
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