dc motor break

Discussion in 'General Electronics Chat' started by mdv1982, Jul 22, 2009.

  1. mdv1982

    Thread Starter New Member

    Jun 22, 2009
    7
    0
    Guys good day. im having trouble with a DC motor in my current project. im upgrading a german made slicer from microcontroller control to PLC control. this DC motor is controlled by a relay which is connected to its positive line.when the coil is energized, the contact will close and the motor will run. The thing is that when I deenergize the coil, it does not actually stop the motor.it just cuts the supply.since the motor is free running,it would still run because of the residual force,which makes it inaccurate. so basically i need an electronic break for this motor. but I have to meet this conditions
    1. only 1 PLC output will trigger the on and break of the motor
    2. i cannot change the motor to any different type
    thanks, please i really need help with this, thanks
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    A resistor, and a DPDT relay controlled by the PLC. The motor leads go to the two C (common) contacts. The power supply goes to the two NO (normally open) contacts. A low ohms resistor goes between the NC contacts. I don't know resistance/wattage it will take, but the lower the resistance the faster the breaking. Maybe someone were with more direct experience can suggest the values. What's the voltage and current rating on the motor?

    ken
     
  3. mdv1982

    Thread Starter New Member

    Jun 22, 2009
    7
    0
    24V, 3.5A, you said that this low value resistor would be connected to the NC of the relay, where would I connect the other end of the resistor?thanks sir for the reply
     
  4. mdv1982

    Thread Starter New Member

    Jun 22, 2009
    7
    0
    ok I got it sir, i thought a resistor is needed for each NC output, thus making it 2, sorry for misunderstanding your reply, so in short,if I turn off the relay, the contact would be shorted with the NC contact thus shorting the two poles which in between is a low valued resistor,can I ask the theory about this,thanks sir
     
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Yes, you are shorting the resistor across the motor leads to break it.

    Ken
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    If there is a heavy load on the motor it can spin for a significant amount of time. Thus, take care of the value of the resistor as not to cause a current, which is greater than the rated current, for long enough. This will burn the motor. Also, note that the motor will not stop instantly. If you need precise movements you have to use a stepper or a servo motor.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If the brush and commutator wear is too high with the moderated short, a mechanical brake on the shaft is an alternative.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    If you use a constant current source (load?) instead of the resistor, you can set the maximum brake current, and it may even brake quicker than the resistor as it will brake with a higher current than the resistor in the latter part of the braking cycle.
     
  9. mdv1982

    Thread Starter New Member

    Jun 22, 2009
    7
    0
    the motor is a brushless type sir, I cant afford to put a mechanical break because Im short of space.I'm opting for the resistor break for now,also I need a relay which can switch fast enough,less than 100ms,thus Im opting for 2 electronic relays,one for the on switching of the motor and one for break. my motor specs are 24V and 3.5A,thus ill be using a resistor valued to 8 ohms 60W resistor. ill also put a fuse for safety. sir can you enlighten me about the break current and what are the risks involved when I use it, thanks for your replies
     
  10. mdv1982

    Thread Starter New Member

    Jun 22, 2009
    7
    0
    in the main process of the machine, the motor is activated almost every second.I can control the on state of the motor by the encoder function, which I encoded on my PLC program,if my setpoint pulse is achieved, an auxillary memory bit would trigger the off state of the motor. thus summing it all up, the whole process will be done less than 1.5s. at first I thought that the weight of the gears connected on the motor and the mechanical chain would be sufficient enough to stop the residual force of the motor. It helped but not that enough, the motor still have a small (but not negligible) residual force that makes the inaccurate drop of the sliced meat (its raining meat:D).
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That kind of precision may call for a mechanical modification. Disengaging the motor with an electromagnetic clutch and halting the slicer with an electromagnetic brake may be the solution. What kind of rotating mass are you dealing with?
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Do you want to break the motor or do you want to brake the motor??
    A huge difference (you need to learn spelling).
     
  13. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Where's the spell checker when you need it?....Oh!...It was did it's job! Where's the brain checker when you need it? :)

    Ken
     
  14. mdv1982

    Thread Starter New Member

    Jun 22, 2009
    7
    0
    sorry for my spelling,its BRAKE obviously, hehehe, i was more focused using the word 'break' on breaking the run of the motor, not the motor itself,sorry again, the mass it carrying is about half a kilo
     
Loading...