Breaking DC motor

Discussion in 'General Electronics Chat' started by mahmoud shendy, Oct 15, 2008.

  1. mahmoud shendy

    Thread Starter Member

    Dec 23, 2007
    Dear All, my problem is that I have a large DC motor ( permanent magnet DC motor ), it operates at about 180VDC / 8 Amp.
    When it rotates and reaches its speed ( 7200 rpm ) and I turned it off it still rotates for about 6 minutes..

    I need to break it even this take HALF or ONE minute..

    What is the best and safe way to make that..

    By the way, when the motor is turned off it becomes a generator ( magnet rotates in a coil ), can I connect a load such a 200W
    lamp to break it or connecting such load will not affect?!!

    is the principle of connecting load will break the motor TRUE or NOT?

    thanx in advance.. SHENDY
  2. mik3

    Senior Member

    Feb 4, 2008
    Yes you can apply a load on its terminals to break it because as correctly you said it acts like a generator. Another method which will slow it down more quickly is to apply a voltage on its terminals but with opposite polarity of the working voltage. When it stops remove this reverse voltage as not to start rotating backwards. Also, because the EMF of the rotor and the reverse voltage you apply are additive a large amount of current will flow and may destroy your motor. Thus you have to put a resistor in series with this reverse voltage as to limit the current to a safe value. A good and safe value is twice the operating current.
  3. John P

    AAC Fanatic!

    Oct 14, 2008
    There is a lot of difference between "braking a motor" and "breaking a motor"!

    Note that in addition to acting as a generator, the motor will function as an inductor. If you simply open a contact to disconnect power, there'll be a spark, and after some number of operations, the contacts will be destroyed. You should connect a snubber across the contacts, usually a resistor and capacitor in series.

    Figuring out the effect of a light bulb to brake a motor sounds like an interesting exercise. After a first instant when the bulb is cold and almost a short circuit (rapid deceleration, briefly) the resistance will be high, but then as the motor slows down, the voltage drops, so the bulb will get dimmer and the resistance drops, but the current rises, at least proportionally to the voltage. It seems as if the power consumption would be closer to constant with a bulb than with an ordinary resistor. Apparently the change in resistance as a bulb goes from cold to normal operating temperature is about a factor of 10.
  4. KMoffett

    AAC Fanatic!

    Dec 19, 2007
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    For that large a motor, a 400 volt 35 amp bridge and a 100 ohm 100 watt resistor might work. A lamp is not a good braking load.

    At least you know the bearings are good.
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
    Why not switch a FET connected through a resistor? You can PWM it to low down the motor.