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Homework Help: How does current affect the speed of a DC motor

  1. May 8, 2014 #1
    Say I was run a homemade motor using a 9v battery, and then ran it again using two 9v batteries... would it go faster with the 18v??
  2. jcsd
  3. May 8, 2014 #2


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    In general if you double the voltage the current also doubles. The power will go up by 4 times (depending on the efficiency off the motor) so your motor will go faster (up until you break it).
  4. May 8, 2014 #3
    The speed of a motor is determined by the voltage and the torque by the current. If a motor is running at a certain speed with a constant torque and the load increases, the current will increase and so also the torque to maintain the same speed.

    To be a little more precise there is a small voltage loss in the motor that does not contribute to the speed. This is called the IR drop. The current through the motor times the resistance of the motor creates a small voltage that must be subtracted from the applied voltage. As the load and current increase, this IR drop also increases causing the motor to lose a little speed.
  5. May 8, 2014 #4


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    For a DC permanent magnet motor (as found in many toys)...

    The voltage normally determines the no load speed. Double the voltage and you will roughly double the no load speed (within limits!).

    The actual speed and current is normally determined by the LOAD on the motor. It's not always safe to generalise and say the current will do this or that when the voltage is changed. It depends how the load responds to the increased rpm. For example the wheels on a toy car might loose traction, the propeller of a toy plane or the fan blades or a cooling fan might become more or less efficient, stall or unstall etc. Get used to thinking that the current depends on the load.
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