Formula for finding the resistance of an inductive 3 phase load

These are purely motor dependant, and as far as I know, the cheap method used by your techs are the best.

ie the phase to phase and phase to ground resistances to know the health of the windings.

Winding resistances are not accurately calculatable per se by the motor ratings.

However some other tests (Motor Circuit Analysers) are also there but they wre to know the impedances and air gap etc balance and are a bit costly.

SB,

Point well taken. Thank you.

But... as I was showing a couple of my guys how to test windings this morning, one of them asked me "How do you know what the resistance should be"?

I told him "I don't, It just needs to be consistent".

To which he responded "Why not?"

I told him I'd bet I knew where to find out... which led me here.

It's not terribly important for what we're doing, but would be nice to pull off if I could figure out how to calculate expected resistance.

Thanks,

B

Dear all,

If you do not mind my awful hand writing, try this.

http://www.akihito-shigeno.com/files/EES_D102_103.zip

Regards

Akihito,

It looks like you're on the right track.

My thought was that if one could calculate current by knowing inductive reactance, impedance, etc, then it should be possible to do the reverse when V, I, PF, f, are known values.

Thank you for your response. I look forward to studying what you sent en depth.

B

The problem with motors (or for that matter many others too with multiple winding) is that the inductances will be including the mutuals (between pri and secondary as well as between primaries) and the effective resistance will be the result of the power transmitted+power loss+ actual resistive losses (again pri and secondary)

So under this circumstances, though motor/ transformers may have an equivalent circuit, but the equivalent are not exactly equal.

Let me give an example- even in locked rotor condition when the motor is going to work like a shorted transformer, the iron and other losses which are not exactly the primary I²R loss will reflect as it and will be difficult to distinguish (except on DC mode) and the reactances , since can not be measured by DC, will not be distinguishable.

And based on rating V, I, CosΦ definitely these can not be calculated.

So practically what is checked are the phase balances, since under any given circumstances, the 3 phases will behave equally.

Dear all,

With Mr. Sb comment, I modified my calculation.

Please access the following,

http://www.akihito-shigeno.com/files/EES_D102_104.zip

since the calculation result is not compared with real test data, I need to study how much my calculation is close to real data.

Regards

I don't know what size motor you're talking about but you should find 3 low value (< 1ohm) resistors with good heat coefficiency which don't change in value at elevated temperatures.

Put these resistors in series with each phase to measure the voltage across each resistors and by using ohms law you can work out the current in each phase.

Make sure you pre measure the value of each resistor and note those readings down so you can get a more accurate current values after calculations it is therefore, very important to make sure you get as accurate voltage readings as possible!

Then the (V supply voltage - resistor voltage)/I calculated current = impedance (XL)/phase

As long as the resistors can handle the current without over heating the results will be pretty accurate for most practical purposes.

This is an ancient method and it shouldn't let you down ever unless you have the right equipment to this otherwise complex job.