I would like to measure inductance of 3 phase motor to identify whether all windings are equal in turns (rewound motor). Using a LCR meter to measure inductance of 3 phase motor would be an ideal way? What would be the normal inductance of a 50 HP, 460V, 60Hz motor? Is there any other method to identify turns are equal?

Thanks you
Shree

 

Shree, a simple test is to compare the 3 stator windings:

Connect an LCR on each phase. Start with, say, A-B. Then while slowly turning the rotor one reveloution, in small increments and record the LCR measurement for each increment.

Repeat for B-C, and C-A. Then plot the readings and compare the three sets of data! A pattern will be revealed that can be analyzed!

Contact me if you need additional help.

Regards, Phil Corso (cepsicon [at] aol.com)

 

Hi Fellows;

I took the readings with LCR meter as described above. The motor specification as under siemens motor with two pole 50 HP, 0.89 PF, 2960 RPM, 400 volt, three phase, 50 Hz.
<pre>
The readings as under
Phase A-B B-C A-c
0 degree 6.14mH 7.38 6.79
90 7.30 6.20 7.11
180 6.22 7.41 6.77
270 7.16 6.16 7.20</pre>
All readings in mH.Then i calculate average of each and calculate XL=1/2*PI*Frequency*L

I calculate 0.475 Ohm.

Am i right or wrong?
Thanks
Take care bye

 

What frequency did you measure those at? They seem a little low. Iron core inductors don't read well at high frequencies.

Regards,
cww

 

Sir measurement is done at 1 KHz 100 micro Amp.
Now guide me.
Thanks

 

Hamad... I recommended you slowly turn the rotor one revolution, making measurements in small increments.

Four measurements per phase is inadequate to detect a phase unbalance, unless severe. BTW, inductive reactance is calculated as 2 x Pi x F x L, or about 314 x L Ohms, if F = 50 Hz.

Phil Corso

 

Obviously something is wrong because your magnetizing current would be something like 200A. I'd try calculating the Xl and Z from the power factor for starters. I don't have all the 3 phase formulae handy as this isn't something I do often, but if this is a good motor, something about your inductance measuring setup is not right. Do the calcs and perhaps the motor inductance is beyond the range of your inductance meter? A bad motor can show readings like that due to leakage inductance with a shorted winding.

Regards,
cww

 

Sir,
Motor is healthy. I measure the inductance windings are connected in star. actually i want to calculate the reactive power Q as i read q=I*I*X

and apparent power S=sqrt (P sqr + Q sqr)

P= true power. and PF=p/s

How can i achieve this task
Thanks

 

Curt... because the motor is not running during the test, the measured impedance should be that associated with locked-rotor, starting inrush, or break-away current. That is, some 4-8 times nominal depending on its kVA/Hp constant.

Hamad... I'm not sure about what you want to calculate. All parameters not listed on motor nameplate should be available from Siemens!

Phil Corso

 

I could be wrong, but what you are trying to do can get pretty complex with mutual inductance and lots of other factors, like variable coupling through the rotor and non-linearity with increase in current. And as Phil pointed out your formula was wrong. I'm not sure you can get there by any reasonably simple means. Part of what I'm saying is that the working values may be quite different from static bench readings. The other part is that there's a lot going on in a running motor.

Regards,
cww

 

Yeah, I kinda got that. But my poor mind boggles trying to think how to model the (running) motor to get where he wants to go. I think even the motor folks do this empirically.

Regards
cww

 

There is an easy way if you just want to know if the turns count is the same for each motor coil. If you put an ac voltage thru any one winding, perhaps with a light bulb in series with a 220v test supply, or i often use a cvr trafo with a resistor. Then measure the voltage impresses on that winding. Then read the volts on each of the other five coils. Any variation in volts of each coil is in direct proportion to turns difference.

In practice motor coils are wound by weight of the copper for windings. Not often by exactly counting turns,though theoretically they would be equal.