Help stop a DC motor

Discussion in 'The Projects Forum' started by charlieb66, Feb 28, 2012.

  1. charlieb66

    Thread Starter New Member

    Feb 28, 2012
    2
    0
    New to the forum, and a novice at DC motors. I converted a DC treadmill motor to work on a piece of shop equipment. I was able to relocate the main circuit and controller along with the motor, all is working to my satisfaction. Problem is, the motor continues to turn momentarily when the pot is turned to off, I need the motor to stop imediately. If I turn off the safety switch it will stop. What I would like to do is wire the circuit so I can set the pot, and trigger a foot switch to make the motor turn as long I the pedal is down (on) and stop when it is released (off). Thanks.
     
  2. jimkeith

    Active Member

    Oct 26, 2011
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    99
    Is the drive a SCR type or PWM transistor type?
    Identifying is simple--the SCR drive has 5 power devices (2 SCR, 2 rectifier, 1 free-wheeling diode) and the PWM drive has an input bridge, large bus capacitor and one transistor power device.
     
  3. charlieb66

    Thread Starter New Member

    Feb 28, 2012
    2
    0
    It is an SCR. Thanks.
     
  4. Felo

    Member

    Feb 20, 2012
    91
    13
    Hi, to stop a DC motor I believe you have to somehow short the input the terminals of the motor while freewheeling, of course power supply must be first isolated "somehow" y say "somehow" because I do not know the particulars of your circuit.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Tread motors are BDC ones. and they free wheel.

    Either u come up with a clutch brake or shorting the motor terminals without blowing ur driver.
     
  6. jimkeith

    Active Member

    Oct 26, 2011
    539
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    The standard way to do this is to throw a dynamic braking (DB) resistor across the motor--this provides fast braking. A standard relay or contactor of the correct current rating is required, but it is necessary to interrupt the output of the control at the same time, so a 3pole relay is commonly used with the center pole NC contacts connected to the DB resistor and the two outside poles connected to either side of the armature.

    Next problem: To prevent relay contacts from arcing severely (relays do poorly at interrupting DC at this voltage level), an additional contact or means is employed to insure that the SCR bridge is phased back so that the relay does not do all the work of interrupting the arc. It does appear that your drive control does not phase back quickly and it is very important to connect some type of inhibit or disable contact to your drive. The most simple and crude means would be to connect a small relay that interrupts the SCR gate circuit, but it the relay must be located at the SCR bridge, because long wires connected to the SCR gates tend to act as antennae to EMI disturbances.

    Next problem: Drive may not be restarted until the motor is completely stopped, or else the DB contact must now interrupt DC, but now with the SCR bridge powering it at a much higher voltage. This is commonly done via an anti-plug relay (APR) that prevents the relay from picking up until the voltage across the motor is zero. There are different means of doing this, but a sensitive relay is generally used--one neat feature of DC relays is that they remain picked up until the coil voltage drops to 10 to 20% of coil voltage rating. Another approach would be to simply use a time delay relay to prevent restart until the braking period is completed--this is an application specific means.

    Next problem: Unwanted motion--actually, if an electro-mechanical relay is employed, there will be no possibility of motion when the foot pedal is not pressed, but if electro-mechanical means is not used, BEWARE because a noise disturbance could make your machine start unexpectedly and present serious danger to life and limb.

    Now you see what is behind the relatively high cost of industrial DC drive controls.

    If you would like to pursue this further, respond and I will rough up a sketch.
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,363
    To brake a DC motor in the shortest possible time you have to build what would be the equivalent of a negative resistor circuit. You need to apply current that opposes the back emf of the motor.
    I had to do this in a cassette data recorder.
     
  8. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    This is called "reverse plugging"
     
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