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Thread: Motor amp draw

  1. #1
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    Motor amp draw

    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?
    Last edited by sciencefreak614; 02-17-2008 at 10:12 PM. Reason: grammar

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    Check your voltage from neutral to ground.
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  3. #3
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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?
    If your clamp amp meter is near the capacitor, it will sometimes read high due to the magnetic field around the capacitor. Otherwise there may be a partly grounded winding in the motor. Otherwise you need a new meter!
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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?
    Assuming you're using an "amp clamp", have you ever noticed how you can get the amp reading to vary just by moving the clamp closer to or further away from a wire while the clamp is clasped around the wire? Some amp clamps are marked with a line on both sides of the clamp, indicating where to align the wire with the clamp. Try this next time...read both sides of the motor leads, matching as closely as you can where the lead is positioned within the clamp.

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    .
    Last edited by sciencefreak614; 02-18-2008 at 11:49 AM. Reason: forgot quote

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    Quote Originally Posted by beenthere View Post
    Check your voltage from neutral to ground.
    Wouldn't I get zero volts then since all the voltage should drop across the load? Not sure what you want me to look for here.

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    You'll get a reading while the unit is running.
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    Quote Originally Posted by Kevin O'Neill View Post
    If your clamp amp meter is near the capacitor, it will sometimes read high due to the magnetic field around the capacitor. Otherwise there may be a partly grounded winding in the motor. Otherwise you need a new meter!
    Don't think I've ever been too close to a capacitor when measuring amp draw, also the lead and return wires are usually right next to each other so if this would happen to one wire I would think it would happen to the other.

    A partly grounded winding would explain the current being divided somehow but not sure I totally understand how it can be "partly" grounded. Wouldn't it be grounded or not grounded?

    LOL hope I don't need a new meter, it is pretty new. I don't see this on most motors, just some. So, I beleive my meter should be working fine.

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    Quote Originally Posted by shophound View Post
    Assuming you're using an "amp clamp", have you ever noticed how you can get the amp reading to vary just by moving the clamp closer to or further away from a wire while the clamp is clasped around the wire? Some amp clamps are marked with a line on both sides of the clamp, indicating where to align the wire with the clamp. Try this next time...read both sides of the motor leads, matching as closely as you can where the lead is positioned within the clamp.
    I have read where you get the best reading when the wire is in the center of the amp clamp ring (sorry, should have said I was using one). I am getting my measurements that way although I don't see much of a difference, maybe a tenth of an amp when I move the wire around.

  10. #10
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    Quote Originally Posted by beenthere View Post
    You'll get a reading while the unit is running.
    And this means the motor is bad? good? happy? sad?

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    Last edited by hvacrmedic; 02-18-2008 at 05:36 PM.

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    The greater the delta between the hot and ground, and the neutral and ground, the greater the amp delta. Thats all.
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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.

    Twilli has meter that will perform manual D on the motor before you take amp draw,
    it will equalize the phases so the amp draw will be the same, motor will be more efficient.
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