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Homework Help: Circuit Breakers in practice

  1. Feb 8, 2010 #1
    1. The problem statement, all variables and given/known data

    In a certain machine shop, if everyone starts their machines at the same time circuit breakers will blow, even if all the machines can normally operate simultaneously. Why?

    2. Relevant equations


    3. The attempt at a solution

    I know that typically direct current is used to supply electricity to buildings while alternating current is used to transport electricity long distances. I know that circuit breakers blow when the current passing through them is higher than their rating. So, I can't think of anyway using a DC circuit that this could occur.

    Using AC power the RMS current is lower than the max rating on the circuit breaker, so the circuit breaker never blows if the machines are started one at a time. However, if all the machines are started simultaneously the instantaneous current is too high for the circuit breaker and it blows....
  2. jcsd
  3. Feb 8, 2010 #2
    I'm not positive about my answer, but maybe it's because if you simultaneously start all the machines, they are all definitely in phase whereas the machines are normally not in phase so that when one inductive load is at Imax, another may be at Imin.
  4. Feb 8, 2010 #3


    User Avatar

    Staff: Mentor

    Nope. Search on torque versus RPM for electric motor.
  5. Feb 10, 2010 #4
    Re: Circuit Breakers in practice <solved>

    When a motor is running, back EMF is produced which lowers the applied current. By Lenz's Law we know that the induced current in in a direction so that the flux it causes opposes the changing magnetic flux (caused by current being introduced in the the motor). Initially, right when turned on, there is no back EMF, so the current draw by the motor is higher than that the circuit can handle, so when all motors are started simultaneously the circuit blows. They reason they can all run simultaneously when not started together is that the back EMF lowers the total current draw enough to prevent the total current from exceeding the circuit breaker's limit.
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