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DC motor testing and repair

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DBB

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Can anyone help me troubleshoot a motor on a treadmill? I don't know a lot about motors so please pardon my basic questions.

The motor has brushes and the label says 90VDC. It has 2 wires going into it, one comes from the control board and the other comes from a transformer like looking thing (field coil?) that in turn comes from the control board. When I try to make it run, it shows -70VDC at both terminals on the control board. When I remove the wire at the control board that goes to the coil, the wire reads -70VDC and the terminal reads 0VDC. When I remove the wire going to the motor, the terminal reads -55VDC and the wire reads 0VDC. I am reading the voltage with the black wire of the DVM on the chassis and the red wire on the terminal. Since I'm getting -70V, should I suspect the control board? But shouldn't the motor run even with negative voltage? Are there other tests I can perform that would rule out the motor?

Thanks for your help.
 
The control board controls the speed of the motor by varying the voltage, 90 is provably the maximum speed. 70 volts will be pretty brisk. You can change the polarity on the motor, but that would make it go in the other direction, which is probably not what it should.

How are you measuring the motor voltage? It should be measured accross the two wires that go to the motor. If there are more than two (other than protective earth ground) then you'll need to research the motor details.
 
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