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LCR meter

Discussion in 'Electronic Basics' started by Bart, Nov 23, 2005.

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  1. Bart

    Bart Guest

    <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
     
  2. DaveB

    DaveB Guest

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

    Best

    Daveb
     
  3. Cliff

    Cliff Guest

    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.
     
  4. Cliff

    Cliff Guest

    "Perfect" how? Hysteresis losses?
     
  5. 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
     
  6. DaveB

    DaveB Guest

    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
     
  7. Don Young

    Don Young Guest

    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
     
  8. Cliff

    Cliff Guest

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

    John Fields Guest

     
  10. DaveB

    DaveB Guest

    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
     
  11. John Fields

    John Fields Guest

    ---
    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.
     
  12. DaveB

    DaveB Guest

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

    Gary H. Lucas
     
  14. Jeff Lowe

    Jeff Lowe Guest

    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.
     
  15. Cliff

    Cliff Guest

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

    Cliff Guest

    Eddy-current losses are an issue in the coils?
     
  17. Cliff

    Cliff Guest

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