LCR meter

<crossposted to sci.electronics.basics and alt.machines.cnc>

I bought a digital handheld LCR meter thinking I could measure inductance
of motors to see if the motor is bad. The motors are DC-servo with brushes,
3-phase AC servos (220volt), and stepper motors. The meter generates both
1khz and 120 hertz and the display shows the henrie, millihenries or
microhenries and also a "Q" value (don't know what that is). Is there a way
to figure out from a motor nameplate what my meter should read if the motor
is good? AC motor's nameplates tell how many poles, voltage rating, amp
rating, and horsepower. A DC motor's nameplate tells voltage, amp rating,
and torque. The meter also can show resistance in ohms at either 1khz or
120hz.
ANY info about using this meter and/or checking motors to see if they're bad
is GREATLY appreciated.
Thanks in advance,
Bart

 

The Q of an inductor is the measure of its quality. The more 'perfect'
the inductor, the higher its Q value.

Best

Daveb

 

Q is a measure of the sharpness of tuning and the "temporal
response" of a system as I recall it.
Example: A tuned RF circuit: How "sharp" is the
frequency response curve? A higher Q will be "sharper" with
a narrower "band".

HTH .. and that it applies to your question.

 

"Perfect" how? Hysteresis losses?

 

Bart,
You're in an area I don't know a lot about, even though I've done lots of
motor work. My suggestion would be to test known good motors of various
types and sizes and make yourself a chart. Then if you can find a known bad
one you could test it to see where it falls in your chart. I did this with
ohm meter readings and AC motors many years ago. My basic chart gave me a
good idea of where the phase resistance falls for say a 5 hp motor. One
that has open or shorted coils was then quite easy to pick out. Use a
megohm meter also to test insulation resistance and you could almost
guarantee a motor was good or bad.

Gary H. Lucas

 

Happy Thanksgiving Cliff,

In reality there is no perfect inductor (he is talking motors) so
hystersis is always considered.

All inductors we deal with on a day to day basis have a resistive
component, in addition to many other variables

In some permanet magnet motors, hystersis losses are almost non
existent ( in advertising only) but you know what I mean.

This is a topic that there is no end .

Regards


..
Daveb

 

Basically, "Q" is the ratio of inductive reactance to resistance, so an
inductor generating 1000 ohms of inductive reactance and having a resistance
of 100 ohms has a Q of ten. Since inductive reactance is different at
different frequencies and resistance is basically the same, Q generally
increases with frequency up to a point. There are other complications
because not only resistance but all energy losses including core hysteresis
reduce Q. Q is actually defined as the ratio of energy stored in the
inductor to energy lost in the inductor. Since energy storage and inductive
reactance are generally desireable properties of an inductor and energy loss
is not, high Q means you are getting more of what you want and less of what
you do not want. Most consideration of Q is in design of tuned radio
frequency circuits where high Q gives sharp or narrow band tuning and low Q
gives broad or wideband tuning.
Don Young

 

Might not the lead/lag of voltage & current have an impact? And
that might depend on what else is on the powerline ..... ?

 

In any inductor being a single coil or dual cores (transformer) there
are always hystersis losses, one of the causes of heat in a coil.

Hystersis losses are frequency related, as the higher the frequency
the larger the losses.

Also eddy-current losses change the resistive element of an inductor.

This could go on and on ,but the basis fact is when you have a magnet
these conditions are part of the deal.

But as stated before by Don Young:

Basically, "Q" is the ratio of inductive reactance to resistance, so
an
inductor generating 1000 ohms of inductive reactance and having a
resistance
of 100 ohms has a Q of ten.

But, lol I still dont have a clue how the OP is to use his meter.

Regards


Daveb

 

---
I don't believe that's true with air-wound coils, at any frequency,
since it's impossible to saturate air.

There is skin effect, though, and the propensity for charge to flow
closer and closer to the outside of a conductor as frequency
increases is what causes losses to increase as frequency increases.

 

Your correct, I thought we were talking iron-core

Again, iron core type inducters start to heat up as frequency
increases, (ferrite cores are probably the worse)
Eddy current can develop in the cross section of the wire itself at
high frequency. Thats why they make Litz wire to help this condition.

I think the term "Effective Resistence" is used when dealing with the
combination of all the fore mentioned losses
bad term-should have said mf.
Daveb

 

Nope,
All measurements were made using the DC current put out by the meter. I was
only measuring static conditions.

Gary H. Lucas

 

Motors usually fail with a short or open, or by de-magnitizing. Typical
inductances will be from tens of microHenrys to ten or so milliHenrys. DC
motors will be lower than induction motors. Resistances are usually low
enough to be hard to read with a DVM. Use your LCR meter at 120 Hz since
this approximates line frequency. The best bet is to test a few typical
motors you use and get an idea what to expect.

 

Hystersis losses cause the coils to heat up?
Resistive losses do but .... any hystersis losses
in the coils are rather minor indeed I suspect.

 

Eddy-current losses are an issue in the coils?

 

Ferrites would be better as they have almost no conductive
paths to produce eddy-current losses.