Winding resistance tolerances in three phase motors

[ewandingwall]

Hi guys,

Is there a standard tolerance percentage that should be followed when checking the winding resistance of each coil on a three phase electrical motor?

The general rule of thumb I followed when measuring winding resistance was that each phase winding had to be identical or close to identical, the resistance would

vary on each motor due to the cross sectional area of the winding.

I've searched the web and there is varying opinions from +/- 5% and +/- 10%.

I just want to know what your thoughts are? The IEC to my knowledge haven't posted anything regarding this.

Thanks,

Ewan

 

[Lucien Nunes]

I suppose it depends what you are testing for. If it's acceptance testing then you need to know what the manufacturer intended. If not then I don't set much store by the readings, it's quite possible to get shorted turns that can't be detected by a simple DC resistance check at all. 10% would satisfy me, either the windings are OK or the fault is not going to be revealed by a resistance test.

 

[Marvo]

I don't remember coming across any IEC or NEMA sepcs on this but the winding resistances should be pretty close, anything over a 5% descrepency would make me curious. I'd say that especially with commercial 3-phase motors, once you get over a few horsepower the winding resistances are going to be pretty low and a normal multimeter may struggle with accuracy. In the real world I'd be much more interested in the on-load running currents.

 

[MotorJim]

I am researching the same question. The only thing I have found in writing thus far is 3-5% variation in phase resistance for form wound stators (up to 15Kv operating voltage) according to IEEE STD -1415.