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Homework Help: Current in DC motor

  1. Dec 17, 2009 #1
    Assume that a 12V permanent magnet DC motor is to be used for moving the robot legs.
    It has a stall current of 2 ampere and a no load speed of 3000 r.p.m.
    Assumer that the friction at no load spped was so little that it can be neglected .
    When the suply is at 11V, the motor is moving the legs at its rated speed of 2000 rom due to firction, what current would it draw?

    I try to answer:
    Vmax = 12V
    Imax = 12A
    ωmax = 3000 rpm <<<ω=omoega, i.e. rotational speed

    V1 = 11V
    I1 = unknown
    ω1 = 2000 rpm

    back e.m.f proportional to ω

    ω1/ ωmax = back e.m.f 1 / back e.m.f max
    and back e.m.f = V- Ir

    ω1/ ωmax = [ V1 - I1 ( r ) ] / [ Vmax - I max (r)]

    However, I find that I do not have the resistance, how can I calculate? thanks.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Dec 17, 2009 #2


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    Homework Helper

    You know that the stall current is 2 amperes, and when the motor is stalled there is no back emf. So the resistance is 12 V/2 A = 6 ohms.
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