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Starting a dc motor

  1. Jul 26, 2009 #1

    Question 1:

    can anyone tell me do we need a rheostat or face-plate starter for a 12V dc motor?

    I read in the book and it says the starting current for the motor is very high and it might burn out the armature. I was wondering if I could just directly connect the 12V supply to the 12V dc motor?

    Or do I need put a rheostat and gradually decrease the resistance as the motor picks up the speed.

    Question 2:

    I just bought some DC motors from the store and some of the motors have 4 pins. I can see the +ve and -ve sign on the motor but there are also 2 more pins labelled as " l ".

    Can anyone tell me what the 2 pins are for?

    Thank you for your kind help.
  2. jcsd
  3. Jul 26, 2009 #2
    All the DC motors, all small, I have used have had no starting resistors.

    The common electric, DC motor, drill has none but is run in AC of course.

    I think it more to reduce damage to the brushes.

    I would have thought if a soft start is required these days it would be done electronically, switch mode.

    Maybe there are 4 terminals to allow direction switching.
  4. Jul 26, 2009 #3
    How big a 12-volt motor are you talking about? Multi horsepower? Usually, 12-volt motors now are permanent magnet stators or rotors. Usually with DC armature/commutator and PM stator. 4 wires implies that the motor might have a wound stator, so that reversing polarity of one pair will reverse direction. This would be a shunt-wound dc motor. If you can reverse direction by reversing the polarity of the armature, the motor has a PM stator. Some specialized motors have a PM rotor, and wound stator that is switched by a Hall-Effect magnetic probe. We need more information,
  5. Jul 26, 2009 #4
    Thank you for replying.

    I would have put in more information if I have them.There was no spec available when i bought them. I only know it operates on 12V DC. There is a code or serial number.I googled it up but couldnt find the datasheet.

    I just want to know if I can just directly connect the supply to the pin or do I need to have rheostat to limit the starting current.

    If there is no need on rheostat,then i can just apply power to the 4 pins and test.
  6. Jul 26, 2009 #5
    I have a 12-volt automotive electric radiator fan motor. It is a 2-terminal motor with permanent magnet stator. I have connected 12 volts to it directly. The biggest problem is the tremendous reaction torque when the rotor accelerates. If it isn't bolted down, it will roll all over the floor when I plug it in. I suspect a soft start is better on the commutator.
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