difference between rotor & stator of 3 ph induction motor

I have a three phase slip ring induction motor and i have 6 terminals from the machine.

There are no markings on the terminal. By measuring the resistances & indutances between 6 terminals i have managed to group them into two each containing 3 terminals.

Now i not able find out which is rotor & stator terminal.

Is there a way to figure out this.( i am not able to trace the wires)

What will happen if i give the supply to the rotor and short the stator???

I think the motor should run (theritically)...

Plz help me with this

 

[electricpete]

rotor and stator windings are different in many ways. You can't switch them. I suspect if you apply full voltage to the rotor you are pretty much shorting out your power supply and you will get extremely high current which will trip or damage the motor in a very short time.

 

[jbartos]

Suggestion: What about lifting brushes from the slip-rings?

 

Thanx

The motor i am talking about is part of huge system details about which are not to released..

I have access only to the external terminals and there is no way i could reach the motor with out disassembling the entire system which is a clostly affair.

Elecctricpete you had mentioned that the rotor & stator windings are different in many ways. Can you please
elabrate on that. It could help me to identify the terminals.

Thanx

 

[electricpete]

If you compare the two sets of terminals, you should find a much lower inductance on the rotor side (between rotor terminals) than on the stator side.

Tho get between terminals on the stator side, current will have to flow through multiple series turns. To get between terminals on the rotor side would require much fewer times through. Inductance would be lower on the stator due to fewer turns. I wouldn't trust a resistance measurement due to unknown contact resistance.

 

[electricuwe]

As you assume the motor will run if you short the stator and connect the supply to the rotor.

 

[jbartos]

Suggestion: The following procedure may be feasible unless there are again some hidden obstacles, namely:
1. Select a single or three phase low voltage power supply.
2. Have it sensitively protected, e.g. 1 amp fuse or 15 Amp or lower circuit breaker.
3. Apply voltage supply to two conductors (if single phase supply) or to three conductors (if three phase supply and two sets of three conductors of the motor are distinguishable).
4. Measure voltages on the remaining free conductors.
5. If the voltage is higher than the supply voltage, this will indicate stator-winding conductors, if lower that will indicate the wound rotor-winding conductors.
The reason behind it is this: Normally, the rotor voltage is lower than the stator voltage. Essentially, you are dealing with a step-down transformer from the stator to the rotor, which is good to remember. E.g. the motor may have the stator rated for 3kV and rotor for 500V to 650V or so.
This may prove to be a simple and more accurate approach than to measure impedances.

 

[electricpete]

jbartos' approach sounds reasonable. Inductance measurement is also easy.

I believe the practical aspects of your question are answered. I'm still thinking about the academic aspect (what happens if you connect the supply to the wrong terminals). I still believe you will likely trip the power supply or damage the motor if you attempt to apply full stator voltage to the rotor. My reasoning is as follows:

The number of series turns in a stator circuit is at least 10, and typically closer to 100. (It is the product of turns-per-coil times coils per circuit). I believe the rotor turns from terminal to terminal would be approx 1. The no-load (excitation) current drawn is limited by the magnetizing inductance, which is proportional to turns-squared. If we conservatively assume that the rotor has 1/10 turns as the stator, then the inductance goes to 1/100 and the current increases by 100. Even more significant, when the core enters saturation (and it certainly will), the current increases even further (Possibly to the point where resistance starts to become limiting). Now remember also that resistive losses change with I^2 giving at least 100^2=10,000 times.

OK, there are a few things to consider on the other side of the ledger.... rotor resistance is lower than stator resistance and rotor cooling is more effective than stator once rotating. Even so, I believe that the enormous difference in magnetizing current would likely prove damaging. Bottom line... don't try this at home.

 

[electricuwe]

I do not agree to jbartos and electricpete last posts. In a wound rotor induction motor nominal voltages of both windings are often choosen equal, resulting in nearly equal number of turns. I do not know a way to distinguish both windings by measurement.

 

[electricpete]

Thanks E.U. I didn't know that. I guess the purpose of multi-turn rotor is to limit the current thru the slip rings?