1. mahmoud shendy

    Thread Starter Member

    Dec 23, 2007
    Dear All, I still in my design problem:
    I have a DC motor ( Field and Armature ), both Field and Armature Resistances are 0.8 Ohm.
    The motor has a label written on it:

    8Amp (20Amp)

    Now, I need to control the speed of this motor and brake it after power off
    where it takes about 15 minutes to stop rotation.

    What is the best way to such control?
    Shunt or Series wound recommended?
    How to overcome the Extremely Large starting currents?

    By the way, I know much about power electronics and switching devices such as MOSFETs
    and made many times PWM control for DC motors but NOT in such ratings.

    I need your advice, many thanx..... SHENDY
  2. alwayslearning


    Feb 27, 2008
    You say you have designed these systems using less power-then just scale it up; design again but with higher power components (MOSFET`s, SCR`S, Diodes, ...)

    There are basically three ways to control the motor speed:
    1)Field Control (shunt field)
    -Rf proportional to speed
    -limited at low speeds (basic speed) due to winding resistance which is
    -high speed drawback, unstable due to armature reaction
    -provides continuous adjustment of speed and easy to use

    2)Armature resistance control (shunt field)
    -easy to implement no need for a special starter
    -allows speed control below the basic speed
    -max speed at minimum Ra
    -under load armature currents high will need large power rating rheostat
    for Rs
    -delay in time between Rs change and speed stabilization, therefore
    few adjustments to attain deisred speed
    -sensitivity to load causes poor speed regulation

    3)Armature voltage (Va) control (field excitation, seperate DC source)
    -since want to change armature voltage w/o changing the flux must
    be seperately excited
    -Armature voltage proportional to speed
    -if polarity of Va reversed then direction of rotation reversed
  3. Bernard


    Aug 7, 2008
    Since it takes some time to stop you must have seen it in operation, can you tell us how it was wired series or shunt , if shunt was there resistor in field ckt ?