What happens if motor rotor is shorted to ground

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philly

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What would happen if an induction motors rotor is somehow shorted to ground or has an extremely low insulation reading to ground.

Just guessing I would think that this connection to ground would somehow shunt the resistance of the rotor, and it would decrease and as a result of decreased rotor resistane the motors torque would decrease. This is just a guess of the performance effects of this happening.

What would happen to the motors current? Would this be the same as shorting the secondary of a transformer, which would create and overload on the primary dependent upon the turns ratio? Would this be the same on a motor with the turns ratio being "a" and therefore cause an overcurrent condition in the stator?
 
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philly:

Your question was incomplete. Are you asking about about a typical induction motor with low resistance slugs of copper or aluminum in the rotor, or a wound rotor. The wound rotor will have slip rings for connection of an external adjustable resistor.

I believe Ham's response was with the assumption of a wound rotor.

Note: wound rotor 3 phase motors may have 3 phase windings on the rotor and apparently in some cases the rotor is used for input power and the stator used with the variable load resistance.

The windings in a squirrel-cage, slugs of conductive material, are not insulated from the core material and a squirrel-cage type is inherently grounded thru the the rotor bearings.

See Bailey and Gault 1951 Chapter 12, p 175-177.

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One ground on an induction motor rotor does nothing. There is no place for current to flow.

If the rotor shaft is grounded through the bearing and bearing housings, currents can be induced in the rotor that flow through the bearings causing arcing damage to the bearings. The amount of power consumed by the motor will go up but I'm thinking the increase will be so small in comparision to the rest of the losses that it would be very hard to measure it.
 
rcwilson said:
One ground on an induction motor rotor does nothing. There is no place for current to flow.

If the rotor shaft is grounded through the bearing and bearing housings, currents can be induced in the rotor that flow through the bearings causing arcing damage to the bearings. The amount of power consumed by the motor will go up but I'm thinking the increase will be so small in comparision to the rest of the losses that it would be very hard to measure it.
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rcwilson said:
One ground on an induction motor rotor does nothing. There is no place for current to flow.

If the rotor shaft is grounded through the bearing and bearing housings, currents can be induced in the rotor that flow through the bearings causing arcing damage to the bearings. The amount of power consumed by the motor will go up but I'm thinking the increase will be so small in comparision to the rest of the losses that it would be very hard to measure it.
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I guess for the sake of my question I was referring more to a wound rotor motor.

So you are saying that on a wound rotor motor if I connected one of the rotor leads to ground nothing would happen because there is no path back to the rotor for current to flow? (Rotor is ungrounded)

However if the rotor is somehow grouunded through the shaft and housing then current would flow from the rotor cables through ground and back to the rotor through the housing and shaft? You are also saying that this current will be negligable compared to the other losses in the motor and wont really reflect on the primary? Is this due to the fact a full load the voltage on the rotor is quite small?
 
philly said:
I guess for the sake of my question I was referring more to a wound rotor motor.

So you are saying that on a wound rotor motor if I connected one of the rotor leads to ground nothing would happen because there is no path back to the rotor for current to flow? (Rotor is ungrounded)

However if the rotor is somehow grounded through the shaft and housing then current would flow from the rotor cables through ground and back to the rotor through the housing and shaft? You are also saying that this current will be negligible compared to the other losses in the motor and wont really reflect on the primary? Is this due to the fact a full load the voltage on the rotor is quite small?
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If I may be so bold, I think Bob was responding to your question assuming it was NOT a Wound Rotor Induction Motor. You never specified up to that point, but WRIMs are comparatively rare forms of Induction Motors, the most common one we generally refer to as an "induction motor" is a Squirrel Cage Induction Motor (SCIM). In an SCIM, there are no windings in the rotor at all. The current that is induced into the rotor to make it spin is done so onto a "cage" of aluminum or copper bars that are pressed or cast into slots in the steel core and connected by end-rings. So "grounding" the current carrying parts of a rotor cage is only possible with the destruction of the rotor itself and operation is a moot discussion. What I believe Bob was referring to was that in the SCIM rotor itself, the steel mass is either isolated (no connection to the frame and therefore ground) or is grounded through the one bearing. If you ground only the one bearing, you allow any stray voltages created on the rotor to flow safely to ground. But if you ground BOTH ends, you create the possibility of current flowing THROUGH the ground loop, and that can be bad for the bearings.

If, however, you say now that your question was referring to a WRIM, then Bob's response would not apply. In a WRIM, you are using the rotor windings to allow for changes in the resistance and therefore the strength of the magnetic field in the rotor to change. This then changes the amount of torque produced by the rotor in relation to the relative frequency difference between the rotor and staor; the stator's frequency being fixed by the line supply and the rotor frequency varying with speed. So through the slip rings, you add or subtract external resistance to change the dynamics of the motor's torque / speed relationship. If you allow one of those leads coming off the slip rings to go to ground, the reaction would be dependent on where you were in the torque/speed curve for that given amount of resistance applied, but no matter what, the result will be fairly disastrous. Rotor current can be as much as stator current at some point, so other than a small amount of added impedance, the ground fault current from a grounded rotor winding can potentially be as high as it would be in a grounded stator.
 
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philly said:
I guess for the sake of my question I was referring more to a wound rotor motor.
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For a wound rotor motor on a fixed speed application then one might reasonably assume a short circuited rotor in which case a short to anything might not matter.
I'd add more but I'm not terribly well.
Temperature 102F and a anxious wife.
 
jraef- Great response. You were correct. I assumed the motor was a squirrel cage induction motor, not a wound rotor.

A single ground on a wound rotor circuit would not have any effect, unless there was another ground somewhere else in the circuit. I don't recall seeing ground detectors or an intentional ground connection on the wound rotor units I've worked on. (There weren't that many and it has been a long time.)

On generators and synchronous motors, a ground can cause vibration problems by shorting out some of the field and affecting the symmetry of the air gap flux. Could the same thing happen on a wound rotor?
 
rcwilson said:
...A single ground on a wound rotor circuit would not have any effect, unless there was another ground somewhere else in the circuit. ...
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I guess I don't understand this part of what you are saying, maybe I'm being thick here. In a WRIM, the rotor current is induced by the stator current. So it is essentially a rotating transformer with whatever turns ratio exists between stator and rotor, usually something between 1.33:1 and 2:1. So if you ground one pole, isn't that going to have the same effect as grounding one phase of a 3 phase transformer secondary?
 
Jraef said:
If I may be so bold, I think Bob was responding to your question assuming it was NOT a Wound Rotor Induction Motor. You never specified up to that point, but WRIMs are comparatively rare forms of Induction Motors, the most common one we generally refer to as an "induction motor" is a Squirrel Cage Induction Motor (SCIM). In an SCIM, there are no windings in the rotor at all. The current that is induced into the rotor to make it spin is done so onto a "cage" of aluminum or copper bars that are pressed or cast into slots in the steel core and connected by end-rings. So "grounding" the current carrying parts of a rotor cage is only possible with the destruction of the rotor itself and operation is a moot discussion. What I believe Bob was referring to was that in the SCIM rotor itself, the steel mass is either isolated (no connection to the frame and therefore ground) or is grounded through the one bearing. If you ground only the one bearing, you allow any stray voltages created on the rotor to flow safely to ground. But if you ground BOTH ends, you create the possibility of current flowing THROUGH the ground loop, and that can be bad for the bearings.
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O.k. so for a SCIM there is usually one end of the rotor grounded through a bearing to help dissipate stray voltage. So then if another part of the rotor became grounded, then how much current would flow through this ground loop? What effects would this current have?

Jraef said:
If you allow one of those leads coming off the slip rings to go to ground, the reaction would be dependent on where you were in the torque/speed curve for that given amount of resistance applied, but no matter what, the result will be fairly disastrous. Rotor current can be as much as stator current at some point, so other than a small amount of added impedance, the ground fault current from a grounded rotor winding can potentially be as high as it would be in a grounded stator.
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If there was a single ground fault on one of the cables I would think that as rcwilson stated there would be no current unless another part of the rotor was grounded. This is simply due to the fact that that there is no return path for the current? Your example of the secondary transformer L-G fault makes sense, but with the rotor not having a ground reference then I agree with rcwilson about no current flowing?

Now if there was a second ground somewhere and current did flow then would there be a great deal of current? Even though at full speed the slip voltage would be very small I guess the rotor resistance and the fault resistance would be very low an thus allow a great deal of current to flow into fault. Would this fault simply transer to primay stator windings as overload an trip the overload? If not detected by overload what performance effects would this fault have on the motor?

This subject came up in regards to an arc flash rating on the slip ring shorting contactor enclosure. I dont believe there is much voltage present on the rotor at full speed however as mentioned above resistance is small so there still may be a good deal of fault current.

I dont think we would be concerned with L-L faults on the rotor cables since after all we are shorting these cables intentionall during running operation.
 
Jraef said:
I guess I don't understand this part of what you are saying, maybe I'm being thick here. In a WRIM, the rotor current is induced by the stator current. So it is essentially a rotating transformer with whatever turns ratio exists between stator and rotor, usually something between 1.33:1 and 2:1. So if you ground one pole, isn't that going to have the same effect as grounding one phase of a 3 phase transformer secondary?
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I wouldn't think this would be the case without any ground reference, or neutral reference on the rotor as was stated above.
 
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