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Thread: Motor amp draw

  1. #14
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    Quote Originally Posted by hvacrmedic View Post
    The amperage in the two legs is actually different. This isn't due to measurement error, it's a result of phase shifting of the current and voltage in an inductive/capacitive circuit.
    The phase shift is the same on both legs of the motor. For a full explanation, look up Nortons Theorem and Kirchovs Law. Current out = Current in.
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  2. #15
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    Mr Bill and Twilli are like the village idiots they just walk by us and don't even notice....Twilli asks "some change for the homeless"
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  3. #16
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    Quote Originally Posted by twilli3967 View Post
    Mr Bill and Twilli are like the village idiots they just walk by us and don't even notice....Twilli asks "some change for the homeless"

    Thats them there "stoned" faced folks Mr. Bill tells Twilli about, they should have my
    signature line tattooed on their forehead.
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  4. #17
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    Quote Originally Posted by Kevin O'Neill View Post
    The phase shift is the same on both legs of the motor. For a full explanation, look up Nortons Theorem and Kirchovs Law. Current out = Current in.
    Yes. I misread. What I had in mind was that it's a common mistake to equate the run lead with line, in which case the half amp or so difference between common and run currents is not a measurement error. Thank you for catching my error.

  5. #18
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    color me stupid I guess, but in all my years of service I have never taken an amp draw on a neutral line to a 120v motor.
    I'm not tolerating Political Correctness anymore, from now on it's tell it like it is.

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  6. #19
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    If there is any thing in the loop that is 240v then that would affect the neutral readind since 240v does not use a neutral. Any amp draw test on a motor should be done with all covers in place to properly load the motor as in normal operation.

  7. #20
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    one will NOT get a magnetic field close to a capacitor -- at least not enuf to affect ammeters --

    having the clamp near another wire having high current flow will --

    mainly, not forcing the clamp closed was the problem with clamp-on meters when I took readings --

    remember, when the voltage fluctuates, the amperage will also --

    what is the accuracy of the meter?
    was the load on the motor constant?

    it is quite doubtful that the neutral & phase have different amperage -- just the measurement method is different
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  8. #21
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    Quote Originally Posted by cem-bsee View Post
    one will NOT get a magnetic field close to a capacitor -- at least not enuf to affect ammeters --
    I didn't used to think so either, but I have often seen it. The aluminum plates of the capacitor are wound inside the capacitor can. This creates a magnetic field just like any other coil. Some capacitors are more pronounced in this effect than others. The plastic cased capacitors seem to be the worst offenders.
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  9. #22
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    Just yesterday, I was setting up a hand-on lab for my students. They are learning about motors.
    I am setting up 4 of the 5 types of a/c induction motors for them to test.
    One of the motors is to be a shaded-pole motor.
    We have several gas furnaces with shaded-pole inducer fan motors, so I will use these to let the student check winding resistance, current flows, & voltages on the shaded-pole motors.

    I performed the tests myself ( to insure test results ) & waa laa, I find the current draw different between the 'hot' leg & the 'neutral' leg.
    the motor was rated for 1.6a...the hot leg pulled 1.72 amps while the neutral leg pulled 1.58. I then checked the other 3 units & they were all the same ( not exact measurement, but hot & neutral was different )

    Right now, I am trying to explain this, as I know my students will ask.....

    Richard

  10. #23
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    Quote Originally Posted by bornriding View Post
    Just yesterday, I was setting up a hand-on lab for my students. They are learning about motors.
    I am setting up 4 of the 5 types of a/c induction motors for them to test.
    One of the motors is to be a shaded-pole motor.
    We have several gas furnaces with shaded-pole inducer fan motors, so I will use these to let the student check winding resistance, current flows, & voltages on the shaded-pole motors.

    I performed the tests myself ( to insure test results ) & waa laa, I find the current draw different between the 'hot' leg & the 'neutral' leg.
    the motor was rated for 1.6a...the hot leg pulled 1.72 amps while the neutral leg pulled 1.58. I then checked the other 3 units & they were all the same ( not exact measurement, but hot & neutral was different )

    Right now, I am trying to explain this, as I know my students will ask.....

    Richard
    Did you check the motors with a megger too? Maybe there is a short to ground somewhere in the middle of the windings. I had that on a blower motor once. It still ran but drew high amps.

    If that is not it, then I am at a loss to explain it.
    Remember, Air Conditioning begins with AIR.

  11. #24
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    Quote Originally Posted by bornriding View Post
    Just yesterday, I was setting up a hand-on lab for my students. They are learning about motors.
    I am setting up 4 of the 5 types of a/c induction motors for them to test.
    One of the motors is to be a shaded-pole motor.
    We have several gas furnaces with shaded-pole inducer fan motors, so I will use these to let the student check winding resistance, current flows, & voltages on the shaded-pole motors.

    I performed the tests myself ( to insure test results ) & waa laa, I find the current draw different between the 'hot' leg & the 'neutral' leg.
    the motor was rated for 1.6a...the hot leg pulled 1.72 amps while the neutral leg pulled 1.58. I then checked the other 3 units & they were all the same ( not exact measurement, but hot & neutral was different )

    Right now, I am trying to explain this, as I know my students will ask.....

    Richard
    Richard,
    The difference is usually within the accuracy of the measuring instrument, since the principle is a transformer where the primary is the single "turn" of the wire and the secondary is a built-in multiturn coil(the clamp unit). The "error" is the variation in stray magnetic flux that surrounds the wiring. To verify this you should hook up a GFCI unit which is sensitive to differences of a few milliamps. So if you don't trip the GFCI you don't have any leakage ( to talk about).
    Last edited by sskzekeman; 02-19-2008 at 10:11 AM.

  12. #25
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    Quick check for current difference

    One quick way to check if there is a different current draw on the two wires feeding a motor is to put BOTH wires in the jaw of your current probe. If the current is the same in each wire, then the two magnetic fields will precisely cancel out and the meter should read zero current. If the currents are different then the meter will show the difference between them.

    Most unlikely that several motors have some kind of short to ground somewhere in their windings, what I suspect is that the winding itself has a capacitive path to ground and that explains the slightly higher current (which would only be in the "hot" wire).

    Any two wire device with only two wires connected to it (no ground on its frame or chassis) can only have identical current flow in each wire.

  13. #26
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    *

    Quote Originally Posted by sciencefreak614 View Post
    Whenever I am checking out a system and check amp draw on the motor(s), I am noticing that sometimes there is a difference in amperage on the hot wire and the neutral wire. Yes, I know this is not supposed to be and is more serious as the difference in the amp measurements increases. What are the guidelines on when it should be a concern and what exactly is happening to the motor? I know that in a series circuit, you have the same amount of current throughout. So, is this the current being divided somehow as in a parallel circuit?


    the guidelines on being concerned are, when motor is overamping past service factor!

    otherwise your gonna see a ton of different numbers when checking motor amperages

    especially when you start comparing their actual amperages with their R.L.A. and their F.L.A. specs

    and as someone already suggested depending where the wire is aligned in the amprobe makes a big difference!



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