Inductance of motor

electronicsLearner77

Jul 2, 2015
306
I want to know how to measure the inductance of the synchronous motor, I googled and found an LCR meter can be used. Is it correct? One other question is, i can measure the Line-Line inductance because they are only accessible points (R,Y,B). Can i assume that if L_line is the line inductance, then the phase inductance will be L_line/2? Similarly for Resistance will it be R_Line/2?

TCSC47

Mar 7, 2016
125
I'm not an expert on induction motors but I have a Peak Atlas LCR40 that is excellent for measuring any R, L or C and it does it automatically. Cost about £100. Its main job for me is to help me identify surface mount components when I am constructing a printed circuit. It would measure the inductance of a motor in exactly the same way as for any inductor.

However, the frequency at which the LCR40 measures the inductance will be automatically selected by the instrument to be 1KHz, 15KHz or 200KHz depending on the inductance that it sees. The magnetic properties of the iron core will change with frequency of the measuring current, and I suppose ideally you would want to measure the inductance at mains frequency. I don't know off hand if this would make a great difference to what is measured. How accurate do you need your measurement to be?.

I found these that might (or might not) help.
https://www.eng-tips.com/faqs.cfm?fid=1781
https://www.thesnellgroup.com/featured-tips/the-importance-of-measuring-inductance

electronicsLearner77

Jul 2, 2015
306
However, the frequency at which the LCR40 measures the inductance will be automatically selected by the instrument to be 1KHz, 15KHz or 200KHz depending on the inductance that it sees
I am little confused here, the inductance is fixed independent of frequency, but the impedance is dependent on the frequency by jwL. So, the meter is measuring the inductance or the impedance?

TCSC47

Mar 7, 2016
125
Some apologies. I am trying to recollect my college stuff so if I get this wrong, I need somebody else here to put me right.

A current flowing through a coil creates a magnetic field. If the current is changing, then there is a changing magnetic field generated across the coil generating it. This changing field then generates an EMF in the opposite direction to the pd causing the current to flow in the first place. So this opposes the current flow. This is basically what inductance is.
So if something happens to affect the magnetic field it will affect the inductance. Iron cores increase magnetism because the individual iron atom magnets line up with the magnetic field. But with an alternating magnetic field the iron atoms have to alternate at the same frequency. Depending on the iron / steel alloy, the individual iron atoms have different limits as to the speed at which they can follow the magnetic field and so they increase the magnetic field by different amounts at increasing frequency and there is a limit for all alloys anyway. Hence you have different iron alloys for mains transformers at 50 / 60 Hz and for switching power supplies at 20KHz or so, and then only air cored for high radio frequencies.
This is why I said I was not sure if the LCR40 would measure the inductance accurately if it had to do it at 1KHz. My gut feeling is that it would be ok, but I don't know how accurate you need your measurement or for what purpose.

To summarise, iron cores are needed to increase the inductance of an inductor, but decrease in effect as the frequency goes up.

TCSC47

Mar 7, 2016
125
---------One other question is, i can measure the Line-Line inductance because they are only accessible points (R,Y,B). Can i assume that if L_line is the line inductance, then the phase inductance will be L_line/2? Similarly for Resistance will it be R_Line/2?
Certainly resistance will be Rline / 2 if you measure it across two phases. But I'm still thinking about Lline.

TCSC47

Mar 7, 2016
125
I am little confused here, the inductance is fixed independent of frequency, but the impedance is dependent on the frequency by jwL. So, the meter is measuring the inductance or the impedance?
Ignoring the effect of the iron core, yes, inductance is independent of frequency for a perfect inductor. And the meter will measure inductance not impedance, although the LCR40 or similar will also tell you the DC resistance of the coil.

so, VL = L dI/dt
Where VL is the pd across the inductor, L is inductance in henries, dI/dt is the rate of change of current through the inductor in Amps / second
So, when a current changes through an inductor, there is a voltage produced across the inductor. The relationship between the rate of change of the current dI/dt, and the induced voltage VL is what we call the inductance L. For a perfect inductor that does not change its properties with the rate of change of current, then L is a constant. The units of inductance are henries.

Now XL = 2 π f L (or as you have said, XL = jwL )
Where XL is the reactance of the inductor in ohms. f is frequency in Hz. Reactance, as the units indicate, is the "resistance" of the inductor to the AC current flowing through it, and for instance, if you had a pure inductor, you could use ohm's law to calculate the current that would flow for a given voltage across the inductor. I = V / XL

But of course the current due to the inductance is 90 deg phase shifted to the voltage, and the current due to the DC resistance is in phase with the voltage, so that leads to all sorts of other considerations and some vector maths. That is why we have to think about reactance being different from inductance.

I have hit a wall here in not being able to use a full set of maths notation in this forum, so to cut a long story short, if you haven't wandered off for a cup of coffee already, the overall impedance of an inductor is the vector sum of XL and R. And we can use this to calculate the overall current with Ohm's Law.

Hope this has helped.

TCSC47

Mar 7, 2016
125
I am probably over thinking this. If your problem is a works problem, just spend their money and get yourself a simple LCR meter and play with it. At least your boss will see you doing something.

If it is a home problem, do the same. LCR meters are very useful in experimenting and learning.

Replies
4
Views
209
Replies
1
Views
687
Replies
0
Views
885
Replies
4
Views
2K
Replies
5
Views
1K