Braking a DC Motor

Discussion in 'General Electronics Chat' started by scorca, Apr 10, 2009.

  1. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    Hi all,

    I am doing a project that make use of a dc motor. There is a 24 VDC motor with nominal current of 1.17 A.

    I have a problem with emergency stop. The requirement is that when E-stop button is pushed or safety hood is open, the motor has to stop on a dime.

    My question is: how to stop the motor electrically in short period of time without damaging it?


    scorca
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    You can brake the motor with a power resistor (to limit the current) and a switch contact
    (that breakes the power and switches the resistor across the motor).

    Greetings,
    Bertus
     
  3. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    Do you know how to calculate the resistor value and the its power rating?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    If you are using an H-bridge to drive your motor, you can brake the motor by driving both ends of the motor to the same potential. That is to say you can drive both ends with 0V or conversely you can drive both ends with +24V.

    In effect you are shorting the two motor terminals together. This will bring the motor to a very rapid halt. The technique is referred to as self-braking.

    hgmjr
     
    Last edited: Apr 10, 2009
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    A resistor will get very hot if it doesn't have the proper power rating. It is better to use a MOS and PWM to control the braking current of the motor.

    If you use an H-bridge rectifier you can control the on-off time of the two lower MOS (the upper MOS will be off) as to control the braking current. Note that the MOS has to have a built in protection diode otherwise you need to put external ones.
     
  6. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    I'm using a built in motor controller and don't know what kind of mos they put in there so any changes on the H bridge wouldn't be possible.

    My only way (so far I know) is to use a power resistor but I'm still looking for the way to calculate the power rating and how long it takes untill it stops.
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    You could try to persuade the motor controller into the braking regime.
    Because this is an emergency stop and not the main brake, you don´t have to worry too much about the resistor, just try it without.
    But you need to know what energy is stored in the system at full RPM to be able to tell if the motor and the driver will hold the shorted current. Knowing the internal resistance of the motor would help too.
     
  8. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    I have calculated and got like 250 Joule and the internal motor resistance is 2.19 ohm.
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    So it takes about 9 seconds for the motor to reach the full speed when starting? Is that right, or is it longer?
    I know the voltage and current on the motor when stopping and starting will be exponential, so if I assume linear behavior for calculation, the actual results for stopping should be longer by the above time ratio.

    If you shorted the motor, the inital power dissipated is cca 260W, so it should stop in about a second, with the current peaking at 11A. Is the driver capable of taking that current, and what is it´s internal resistance when the fets are on?
     
  10. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    How did you come to 9 sec and 260 W, 11A?

    Yes, the voltage and current will be exponential.
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    E=W.t so making 250J with 28W takes 250/28=8.9s
    But this is only true when the motor does 24W all the time from zero to full speed.

    I=V/R so 24V/2.19, when the motor is shorted at full speed, makes 11A.
     
  12. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    ok, now I have 250 J brake energy, my regenerative power is 260 W and my internal resistance is 2.19 ohm. I want the motor stops after 2 sec, The dynamic brake resistor would be:

    Pbrake= 250 J/2 sec = 125 W

    Rbrake= 24^2/125 = 4.6 ohm

    Rextra = Rbrake - Rinternal = 4.6 - 2.19 = 2.4 ohm

    is that right?
     
  13. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    It seems right, but all my assumptions were based on the motor spinning up in 9 seconds, which you must confirm first.
    Do you have access to the motor, so that you could try it?
     
  14. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    Nope, I dont have. We are still in designing phase. What I got from my coleague is that the acceleration of the motor is 360 rad/s^2. The stall torque is 0.234 Nm and the mass inertia is 0.00065 kgm^2.

    Based on that, I think it wont take 9sec to get the full speed but it would be shorter.
     
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