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Thread: Measuring Impedance of a Coil

  1. #1
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    Measuring Impedance of a Coil

    I am trying to measure the resistance of a coil with a digital meter. I know that with an AC coil there will be a resistive componenet and a reactive component, and together these make up the total impedance of the coil. It is this total impedance that determines current.

    I know that you cannot measure reactance with a meter, and when you measure a coil with a meter, you are only reading the resistive portion of the coil with may be very low and only a fraction of the total impedance of the coil.

    But what if the wattage rating is given for the coil? Since the wattage rating is a result of the resistive component can you simply use the P=V^2/R to determine what the resistance of the coil should be? Should you then be able to see this resistance component with a meter? (All of this ignoring the fact that resistance changes with temperature). Total coil kVA will be a result of including the reactive component with total impedance being Z = R+xj. This is why you cannot use P=IV then V=IR to determine resistance based off of rated voltage and current?

    If this were a DC coil then would this be a direct measurment since there is no reactance?

  2. #2
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    From what I remember, you can place a variable resistor in series with the coil, and adjust it until you read one half of the supply voltage across the coil.

    Then, disconnect the circuit and measure the resistance of the variable resistor without disturbing its setting. Its resistance equals the coil's impedance.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  3. #3
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    Quote Originally Posted by Pitt123 View Post
    But what if the wattage rating is given for the coil? Since the wattage rating is a result of the resistive component can you simply use the P=V^2/R to determine what the resistance of the coil should be? Should you then be able to see this resistance component with a meter? (All of this ignoring the fact that resistance changes with temperature). Total coil kVA will be a result of including the reactive component with total impedance being Z = R+xj. This is why you cannot use P=IV then V=IR to determine resistance based off of rated voltage and current?
    The resistive portion of the impedance of the coil cannot be figured from the wattage specification of that coil. Different coil constructions are available, some of which have higher temperature specs than others. I understand what you are trying to do but the only remedy that I can think of is to measure an identical coil that is known to be a good one, unless you have access to the manufacturer's specs.

    If this were a DC coil then would this be a direct measurment since there is no reactance?
    Correct. Reactance doesn't show up on a dc coil unless that dc current is fluctuating.

    Marc

  4. #4
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    Quote Originally Posted by Marc L View Post
    The resistive portion of the impedance of the coil cannot be figured from the wattage specification of that coil. Different coil constructions are available, some of which have higher temperature specs than others. I understand what you are trying to do but the only remedy that I can think of is to measure an identical coil that is known to be a good one, unless you have access to the manufacturer's specs.
    Marc
    I understand what you are saying here about temperature specs and resistance. But ignoring temperature is there any other reason why resistance cant be determined from the wattage rating?

  5. #5
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    Consider an air core coil. Then for low frequencies the DC resistance measurement of the coil is approximately the same as the equivalent series AC resistance of the coil. In this case all power loss is in the series resistance of the coil.

    For a given applied voltage you will also need to know the reactance and thus impedance to determine the current flow and therefore the power dissipated in the coil.

    An iron core coil has additional losses beyond the wire resistance loss and therefore the series equivalent resistance is different than the DC resistance, still assuming low frequencies and no skin effect.

    You can use an LRC bridge to measure the inductive and resistive components of the inductor.

    If the inductor is a solenoid, then its inductance is very sensitive to the position of the core. This is why an AC solenoid coil will burn out if the core does not move to its maximum inductance position. Whereas a DC solenoid can be stuck in any position and not burn out. However, the AC solenoid has a greater pull-in force.

    .

  6. #6
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    You can estimate the inductance with a transient measurement.

    In a series LR circuit the current will rise to about 63% of its final value in one time constant. This you should have covered in a class on transient analysis.

    .

  7. #7
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    You asked about measuring the impedance using a digital meter that is capable of measuring resistance.
    Can this meter measure ac current?
    I so then wouldn't the impedance be Z = E/I ?
    You know the voltage and you measure the current flow at rated frequency.
    Of course this is not telling you the inductance or reactance value but you asked about impedance.
    Last edited by ELA; 01-14-10 at 09:52 AM.
    - Resistance is Futile ..... (if less than < 1 ohm) -

  8. #8
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    Quote Originally Posted by ELA View Post
    ...Of course this is not telling you the inductance or reactance value but you asked about impedance.
    Measure the resistance with an ohmeter. Using the impdeance value and the resistance, the reactive component is

    X^2 = Z^2 - R^2

    cf

  9. #9
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    Quote Originally Posted by gar View Post
    100114-0833 EST



    For a given applied voltage you will also need to know the reactance and thus impedance to determine the current flow and therefore the power dissipated in the coil.

    This I understand. The reactance and impedance will determine the current and thust the total kVA of the coil. But with just measuring the resistance can we find the kW rating of this coil using V^2/R?

  10. #10
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    Pitt123:

    This I understand. The reactance and impedance will determine the current and thus the total kVA of the coil. But with just measuring the resistance can we find the kW rating of this coil using V^2/R?
    Are we talking about a coil or a transformer?

    What is the meaning of the KVA of a coil?

    Suppose we have a coil and its magnetic field couples to nothing else. Then as a first approximation we have a resistance in series with a pure inductor, but the resistance is distributed uniformly thru the coil winding. The current thru the resistance will dissipate heat. This will heat the coil. From empirical information there will be a maximum temperature allowed for the wire. There is also an ambient temperature around the coil. There is thermal resistance from the coil to the ambient environment. All of this will determine the maximum coil current.

    I suppose you can define the KVA rating of this coil as the maximum current times whatever voltage is required to force that current thru the coil.

    What do you mean by the KW rating of the coil? Having determined the max current above, then I suppose that you might define the KW rating of the coil as I^2*R. It certainly is not V^2/R unless you somehow determined the equivalent voltage across the equivalent resistance.

    .

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