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Why should a series motor always start with a load?

  1. Jan 30, 2008 #1
    Why should a series motor always start with a load?
    Why should a shunt motor always start without a load?
  2. jcsd
  3. Jan 30, 2008 #2
    I'm pretty sure this is a homework question, so what do you think? By the way, there must be more to this question since it is not correct as written.
  4. Feb 9, 2008 #3
    In any DC motor the speed depends inversely on the flux. Consider when starting a series motor the current flowing will be high, but the flux that the motor produces will be moderate since the series winding has less number of turns. If u start the motor without load the speed will increase. as speed increases the back emf increases. this decreases the current through the series winding and hence flux decreases and the speed further increases. Theorerically speaking the speed becomes infinite. But practically the motor will accelerate to very high speed , which will damage the motor. But if you start the motor with the load, you are actually reducing the starting speed hence the motor runs safely. Since series motors has this property it is used in trains to pull high load.
  5. Feb 9, 2008 #4
    It is correct to say that a dc series motor can start at quite (even startlingly) high speeds, but don't you use any control? Are there no power electronics people on your railroad?

    BTW, the OP was about series motors not specifically dc series.
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