induction motor

pallavi · Feb 18, 2012

hi people, i have a question dats bothering me...

Say i have a induction motor. the rotor needs to be taken out for repair. now if someone gives ac supply to the stator without the rotor, what will happen and why ?

My thinking...Basically since we r giving 3 ph ac supply a rotating magnetic field is produced. but since there is no roto mag field there is no interaction and hence no torque is produced. but losses do occur. so this should not be allowed.

If any one has any other explanation please let's discuss

LadderLogic · Feb 18, 2012

http://youtu.be/cIf9iPskgJs

pallavi · Feb 18, 2012

@ ladderlogic

i noe rotor will rotate..but what happens to stator...my query is why is it not recommended to give supply to stator with rotor removed? is the answer only about losses or there is some other problem also...please explain

Mickey · Feb 18, 2012

What is the purpose of this question?

Why would you do this?

Is this a school assignment?

pallavi · Feb 18, 2012

just thought...also in industry one must be facing such situation i guess

Steve Bailey · Feb 18, 2012

I'm not sure why anyone would be facing the situation. While you are removing the rotor, the power to the motor must be turned off and locked out. Why would anyone want to restore power with the rotor removed?

pallavi · Feb 18, 2012

people rather than pondering on why such situation wil arise we shud be discussing what will happen ?

dploof23 · Feb 18, 2012

Steve Bailey said:
I'm not sure why anyone would be facing the situation. While you are removing the rotor, the power to the motor must be turned off and locked out. Why would anyone want to restore power with the rotor removed?

I STRONGLY second that.

If you want to discuss what will happen my guess is eventually if you maintain those type of maintenance practices someone will get hurt or worse.

-Dave

jamied6 · Feb 18, 2012

If you apply full voltage to the stator with no rotor installed you will burn out the stator windings. I do not know why you would want to power the stator with the rotor removed, unless you are working at a motor shop and doing design or rewind tests.

tlbates · Feb 18, 2012

Think of an induction motor as being a rotary transformer -- The stationary (usually) stator windings as being the primary, and the rotatating (usually) rotor as being the secondary. When the rotor is removed, there is no primary to secondary coupling, hence no transfer of power. Stator impedance increases drastically, and current lowers. However, magnetizing and other loss currents still flow as they would if this were a stationary transformer with no load on the secondary. I have seen an application where a reactive vs resistive load was needed for testing, and they fabricated a device which could slowly insert a locked rotor deeper and deeper into the energized stator bore to increase the inductive load. Now to keep this forum related, it may have been PLC controlled :>).

leitmotif · Feb 18, 2012

jamied6 said:
If you apply full voltage to the stator with no rotor installed you will burn out the stator windings. I do not know why you would want to power the stator with the rotor removed, unless you are working at a motor shop and doing design or rewind tests.

This and other comments are good.

The answer to your question is there would still be a rotating magnetic field.

I have a video that shows a ball bearing running around inside an energized stator. This is one of several tricks of trade in motor shops. Video will not load here -tried and failed.

The other purpose of energizing a stator with no rotor is for demagnetizing tools.

In all cases care must be taken to limit current and preventing burning out windings.

Dan Bentler

Mickey · Feb 18, 2012

leitmotif said:
I have a video that shows a ball bearing running around inside an energized stator. This is one of several tricks of trade in motor shops. Video will not load here -tried and failed.

Dan Bentler

This one?

http://www.youtube.com/watch?v=zxUkJcWshlI

leitmotif · Feb 18, 2012

Mickey said:
This one?

http://www.youtube.com/watch?v=zxUkJcWshlI

Basically yes
BUT
mine is better, bit slower in speed, closer with the camera, and bigger ball. Made to increase degree and speed of student comprehension.

Statement made with a healthy degree of "snobbishness"

IF I can upload it as WMV file will be happy to do so.

Dan Bentler

iant · Feb 18, 2012

other than high risk to yourself (electrocution etc)
or possible Stator Damage.
There is absolutely no reason to supply power to a motor stator
with the rotor removed.
Should you consider that there is a need then good luck to you.

rejoe.koshy · Feb 19, 2012

pallavi,
You are raising a question which no pratical significance.No body would ever switch on the power of a stator with the rotor removed.
However, to answer your question- the stator windings will possibly burnt out.
Thats because with rotor removed the reluctance of the magnetic circuit for stator increases (air being the medium), resulting which less flux is produced & this in turn reduces the impedance of the stator electrical circuit drastically.This causes the current to shoot & burn the windings.

induction motor

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