3-phase Motor – High Winding Resistance and Overcurrent Fault?

[elektrotechnik]

Hi all,

I am testing a variety of 3-phase (wye) motors. In my testing, one of my motors is triggering an overcurrent fault. I checked the resistance of the windings (between 1 and 2, 2 and 3, and 1 and 3) and the winding resistance is actually higher than the specification. The winding resistance is also higher than the other motors that I tested.

I’d be violating Ohm’s Law if I said the overcurrent fault was triggered by the high winding resistance. Could an overcurrent fault actually be triggered by this high winding resistance causing some other issue? Is there anything else on the motor that I should be measuring or comparing to the other motors?

 

[barry]

The impedance of the winding is greater than just the DC resistance and maybe you've got a defective winding. When the motor is running there is less current flowing than when it's stalled (lock rotor current). How much difference in resistance is there between motors? Maybe the motor was wound for 240V and you're running it at 480V?

 

[chuckey]

One shorted turn in the winding will change the DC resistance by .5 % but will change the AC impedance by 1000%.
Pass some low AC voltage into the coils and measure the current.
Frank