3 Phase motor windings Resistance Calculations ?

[TailSpin]

Hi All

Is there a calculation for working out an unknown resistance on a 3 Phase Motor ?

The rating plate has 15kW / 29 amps . Trying to remember from my collage days but it has been a while.

Regards

Tailspin

 

[pmac10]

just curious,but why do you want to know? i would say the winding resistance of a 15kw motor would be close to zero while on small motors you would get much higher, if checking look for 3 very similar readings. cant think of a calculation other than R=P/I^2 but dont think that would work for 3 phase where P=SQ RT3*V*A*PF, so in summary, sorry!, i dont know

 

[telectrix]

me neither. would just expect a fairly low ohms reading on each winding, but would expect them to be almost identical. load is mainly inductive.

 

[TailSpin]

The motor in question is one that a work collage had to investigate. It had winding readings as follow : 4.02 / 2.62 / 3.18 ohms . I know the resistance readings should be closer together. He did manage to record some current readings of : 5.1 / 5.6 / 6.1 Amps. The motor was running freely with no other apparent problems. So I was convinced it was a winding issue . Insulation resistance check carried out with no problem. Which is why I thought it would be nice if the resistance could be roughly calculated.

Regards

Tailspin

 

[pmac10]

just checked the electrodroid app on my phone which contains a resistivity and temp coefficent table. the resistivity of copper at 20 degrees C is 1.67*10^-8 ohms per meter, so now all you have to do is unwind the winding and get a tape measure on it
i agree your ohm readings are a bit off but your running current check seems ok,
what make are the instruments and when where the batteries last changed?
sorry couldnt be of more help.

 

[Canoeboy]

Is the motor 6 wires ? Connected in star or delta ?

Could be a loose connection, disconnect the links check all terminals, measure the resistance of each individual winding (preferred cold) and they should be near enough the same on a cheapo meter. There may be a small difference depending on how the windings are laid down. For a 15Kw you will be down in the less than 5 Ohms range.

With a lift the windings will be the same resistance near enough. 3 Layer wound then the windings will have a small difference between the three.

 

[TailSpin]

The motor I believe is wired in star , and the meter used was the Fluke Insulation resistance tester which has a low resistance range.

Thanks All for your thoughts , motor now sent to be sorted by motor specialist .

I did like the idea of running out the windings and measuring them


Regards
Tailspin