Testing 3 Phase Motor Windings
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    Default Testing 3 Phase Motor Windings

    I have a 5hp, 3 phase, dual wound motor. It has 9 leads. For 220, it is supposed to be wired

    L1, 2, 8 tied together
    L2, 1, 7 tied together
    L3, 3, 9 tied together
    4, 5, 6 tied together.

    I'm trying to make sure the windings are all good. In the interest of not completely taking all the wiring apart, I measured the resistance as follows:

    2, 8 to 1, 7 result .8 ohms
    2, 8 to 3, 9 result .8 ohms
    1, 7 to 3, 9 result .8 ohms

    My question is, if I get the results above is there any need to measure the resistance of each of the individual windings?

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    The odds of finding a problem the way you are approaching the motor is small. As far as windings are concerned, about the only ways I know of to test them is hipot to ground and to each other and surge test which can pickup shorted turns. Otherwise just wire it, put voltage to it and look for the magic smoke.

    Tom

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    Quote Originally Posted by TDegenhart View Post
    The odds of finding a problem the way you are approaching the motor is small. As far as windings are concerned, about the only ways I know of to test them is hipot to ground and to each other and surge test which can pickup shorted turns. Otherwise just wire it, put voltage to it and look for the magic smoke.

    Tom
    What Iíve done wouldnít reveal a broken winding?


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    You're not looking for a broken winding, per se. Megging the motor windings will reveal cracks and/or leaks (or moisture) in the insulation that would result in death on startup. It will of course spot any broken winding as well.

    Windings don't often fail from actually breaking, they usually vaporize when the insulation fails and high inrush current goes through that break to adjacent wires or ground.

    Stuart

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    Quote Originally Posted by TravisR100 View Post

    My question is, if I get the results above is there any need to measure the resistance of each of the individual windings?
    Your resistance is being measured with a low level dc current. Failures can occur at higher potentials and you won't know why with a dc resistance check.
    Connect power and use a clamp-on amp meter on each phase. That would be a good check.

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    Quote Originally Posted by rons View Post
    Your resistance is being measured with a low level dc current. Failures can occur at higher potentials and you won't know why with a dc resistance check.
    A clamp-on amp meter on each phase would be a good check.
    Thanks. Iíll try that.


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    Best approach: Check the resistance of each winding with a DVM that reads down to 0.01 ohm resolution. They should pretty well match.
    Then check between windings and ground with a DVM that reads up to 20 meg ohm or so. Should be infinite.

    If you have, or know somebody who has, a megger like one made by Biddle (crank type) then megger out the windings as well. Note the
    resistance.

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    As Jim says, it is hard to do anything meaningful with a typical DVM because the lead resistance gets in the act. I have one of the old Leeds & Northrup bridges that reads into milliohms that I got from a former Century Electric engineer, with Kelvin leads. If you can find something comparable, that will spot shorted turns. Otherwise, you can do about as much by visual and olfactory checks.

    Bill

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    Quote Originally Posted by 9100 View Post
    As Jim says, it is hard to do anything meaningful with a typical DVM because the lead resistance gets in the act. I have one of the old Leeds & Northrup bridges that reads into milliohms that I got from a former Century Electric engineer, with Kelvin leads. If you can find something comparable, that will spot shorted turns. Otherwise, you can do about as much by visual and olfactory checks.

    Bill
    When you get down to the milliohm range, measurement technique becomes very important. I ran into that when measuring OL relay heaters. I suspect a shorted turn would be found much more reliably by heating than measuring resistance.

    Tom

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    Many DVMs read down to 0.01 ohm resolution. You need to cancel out the lead resistance with that
    function on the meter (clamp one alligator clip to another when doing this) but it works pretty well.

    Tougher is finding a megger. General Radio makes a great high resistance ohmmeter that can be had
    relatively inexpensively:

    Used General Radio 1863 for sale by Test Equipment Connection Corp. | used-line.com

    Measures up to 2 e 13 ohms.

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    At $750 or so its cheaper just to take it to a motor shop and let them test it.

    Tom

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    Default Testing 3 Phase Motor Windings

    Quote Originally Posted by TDegenhart View Post
    At $750 or so its cheaper just to take it to a motor shop and let them test it.

    Tom
    Getting the motor out is the issue. On a lathe. Everything about it SEEMS ok except it bogs taking a .040 cut which seems light. It otherwise runs at the rated speed and makes no noise.

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    All the test equipment and measuring is academic. At what difference in resistance between phases will it be marked as a 'bad' motor? Unless there is a dead short or obvious issue.

    Run it. If it smokes then rewind it or scrap it. A scrap price will buy you a medium priced coffee.

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    Quote Originally Posted by TDegenhart View Post
    At $750 or so its cheaper just to take it to a motor shop and let them test it.
    Tom
    Don't believe everything you read on the internet. I think mine was 100 bucks via ebay. Granted
    a motor shop *would* have a megger on hand....

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    Default JST please read and comment.

    Is it possible that you have connected one of the phase windings backward so that the flux from it bucks the forward flux?

    For low voltage, one set of windings has the wye connection permanently connected. Connecting 4,5 and 6 together create the second wye point. The remaining leads are then wired together in parallel. Try this. Undo all the connections, then find the permanent wye set as these will all show shorted together. Connect just the those three leads to L1, L2 and L3. The motor should run fine but will have only half the power. Disconnect that set, connect the wye point of the second set and wire those to L1, L2 and L3. Should act the same as the first set and in the same direction. Now parallel the first and second sets together. Should work just fine as a 5 hp motor.

    This for JST. What would happen if say the 3-6 winding is connected backward? That is, the end that is supposed to be 3 is marked 6 and the other end 3?

    Another possibility is the 5, 6 and 9 get confused.

    Tom

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    Default Testing 3 Phase Motor Windings

    Quote Originally Posted by TDegenhart View Post
    Is it possible that you have connected one of the phase windings backward so that the flux from it bucks the forward flux?

    For low voltage, one set of windings has the wye connection permanently connected. Connecting 4,5 and 6 together create the second wye point. The remaining leads are then wired together in parallel. Try this. Undo all the connections, then find the permanent wye set as these will all show shorted together. Connect just the those three leads to L1, L2 and L3. The motor should run fine but will have only half the power. Disconnect that set, connect the wye point of the second set and wire those to L1, L2 and L3. Should act the same as the first set and in the same direction. Now parallel the first and second sets together. Should work just fine as a 5 hp motor.

    This for JST. What would happen if say the 3-6 winding is connected backward? That is, the end that is supposed to be 3 is marked 6 and the other end 3?

    Another possibility is the 5, 6 and 9 get confused.

    Tom
    Below is the diagram. Itís pretty clear. There are 9 wires coming out of the motor. My understanding is that 10, 11, and 12 are permanently tied together inside the motor. I get that tying 4, 5, and 6 create the second wye. You say ďfind the permanent wye set as these will all show shorted together.Ē That statement confuses me unless you simply mean 7, 8, and 9. If so then youíre saying connect 7, 8, and 9 to L1, L2, and L3. Basically run the motor on one set of windings. Then with 4, 5, and 6 tied together connect 1, 2, and 3 to L1, L2, and L3 and run on the other set of windings. Is that right?

    I can definitely try that and see what happens. In that case if there were a weak/bad set of windings what would I expect to see? And if a set of tags were backward then presumably it wonít run?

    Itís possible that I got tag confused but I donít think so. Iíll have to go back and check again. The tags on the wires are embossed metal tags crimped to the wires.




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    Just went over and looked at the tags. Theyíre pretty darn clear. Definitely no way to mistake the 5 for a 6 or a 9. The 6 and 9 tags however do not have an underscore below the number so i suppose itís possible those tags could have been put on backward. They look original to the motor but itís possible someone besides the original manufacturer put them on.


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    Just touching all bases. Running the two individual sets of winding is interesting.

    Tom

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    Quote Originally Posted by TDegenhart View Post
    Just touching all bases. Running the two individual sets of winding is interesting.

    Tom
    I agree. Canít hurt to pull the bundles apart and just verify that they are labeled correctly.

    Here are the tags and where the wires exit the motor.




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    I do find it odd that where the wires exit the motor there are 3 rows of 3 holes for the 9 wires. I would expect the top right hole to have wire number 3 coming from it and itís got the tag for 4 on it.


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