11kV motor: Unequal winding resistanmce 1

[RRaghunath]

We have a motor which tripped on protection operation.
The winding resistance of motor when measured indicates R&Y phases - 99 m.Ohm and W-phase - 198m.Ohm.
The Insulation Resistance values are OK.
Any expert opinion what could have caused the resistance in two windings to be half that of the other winding.
The motor is yet to be opened.

Thanks in anticipation.

 

[waross]

You would see that if the motor was delta connected when you measured and one phase was open.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

If it is a wye connected with two parallel paths per phase and if the readings are phase resistances, one parallel path of W phase got opened (may be due to turn-to-turn short in the coil(s) or jumper lead got cut)

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[RRaghunath]

Thanks edison, I too guess so. The motor is 11kV rated and Star connected.
I will post again when the motor is opened / more info is available.
Thanks.

 

[edison123]

If the winding was internally delta connected and you had only three leads coming out of the winding and if one phase was open, then Bill (waross) is also right since the winding then becomes open delta with one reading showing the twice the value of other two readings.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[waross]

With an open parallel path on a wye connection the ratio of resistances will not be 1:2. The ratio will be 2:3.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Bill

It will be 1:2 if we are measuring individual phase resistances.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[waross]

Yes edison123 you are correct.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

Just to clarify,

in the subject motor case, all the six leads from the windings are available. In fact, the Star formation is done externally in the cable box.

 

[edison123]

raghun

In that case, my first post would apply.

Please post your findings here after you open the motor.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *