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Speed control by flux weakening

  1. Aug 21, 2013 #1
    In dc motors, speed can be increased by weakening or decreasing the field flux. The mathematical explanation is --- Speed is inversely proportional to flux. But why is it so? Should not it be that when flux decreases torque decreases which is driving the motor. Now decrease in Torque should decrease the speed.
    I need qualitative explanation of what happens inside the motor, when field flux is weakened& how speed increases.
  2. jcsd
  3. Aug 21, 2013 #2

    jim hardy

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    Gold Member

    Back to very basics,
    force F on a charge Q with velocity V in a magnetic field B : F = QV cross B (vector cross product) .
    That force F is responsible for counter-EMF.
    Counter EMF is what opposes inflow of armature current.
    Since counter EMF is proportional to product of velocity V (think RPM) and flux B (think field),,,

    IF you reduce either RPM or field
    THEN counter emf decreases so more current will flow into the armature.
    That increases electrical power input to the motor.
    So the motor will accelerate until counter emf once again is sufficient to balance applied voltage.

    Always keep in mind your two basic DC motor formulas:
    Counter EMF = (K X Flux) X RPM, where K is a constant for that particular motor
    and Torque = 7.04 X (K X Flux, both same as above) X Armature Amps.

    You can determine the value of (K X Flux) for a particular motor by placing it on a dynamometer and taking careful measurements of volts, amps and torque at various loads.

    any help ?

    old jim
  4. Aug 21, 2013 #3
    Agree with Jim, the speed increases but the torque decreases, if you reduce the field current to the motor you will find that it speeds up but you can stop it with you hand.
    If you continue decreasing the field current the time comes when the torque cannot overcome friction, then the motor will stop.
    With the motor stopped (stalled) the current still flowing through the armature will be very large (no back emf) and the armature could burn out.
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