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Current measurement of a PM DC generator

  1. Aug 20, 2014 #1

    I'm trying to use a small PM DC motor as a generator to convert mechanical energy to electrical energy.

    I couple that small generator to another DC motor with spur gears and I measure the current of that small generator while it is driven by the DC motor at fixed speed. The circuit of the DC generator is closed by means of connecting an external resistor in series to the generator terminals.

    The initial current reading is, say 0.1A, which is approximately same with theoretical prediction. However, the current reading drops gradually as time progresses and eventually settles down at, say 0.02 A. The speed of the motor remains unchanged.

    Why does the current reading of the generator drop while the rotation speed remains unchanged? Does that imply something is wrong with the generator?

    Thank you.

  2. jcsd
  3. Aug 22, 2014 #2


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    Science Advisor

    Welcome to PF.
    Is the generator getting hotter as the current falls?
    It may have a shorted turn or a commutator fault.
    Does your resistor stay the same value? It might be a PTC thermistor!
    Is your meter working correctly?
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