forward and reverse brake for dc motor control

Discussion in 'The Projects Forum' started by wewe, May 11, 2010.

  1. wewe

    Thread Starter Member

    Apr 9, 2010
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    hello , i have a project to control a 1500 watts dc motor
    i am controlling the motor with a h bridge of igbt to make the 4 quadrant dc chopper , i will use a microcontroller of course
    now i need to ask about forward brake and reverse brake , how to make them
    i don't need a code or anything just a very simple explanation about the basics of forward and reverse brake for a dc motor in order to write the appropriate code
    i know how to drive the motor in the 2 directions (2 quadrants) , now i need a very simple explanation about the other 2 quadrants , forward and reverse brake :)
    thx
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Simply shorting both terminals of the motor to either +V or -V will act as a brake. Basically, if the motor terminals are connected together, the reverse-EMF being generated; when shorted out will act to slow the motor fairly rapidly.
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    An example is an electric mower, 1kW universal, which on turn off, reversed field & shorted leads, bringing motor to stop in 3 seconds.Another one [ circ 1945] was a bang-bang motor using an electric brake momentarily at each reversal for fast, 2-3 sec, cycling of radio direction antenna. Magnetic particle clutches- brakes do a better job & not as noisey.
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    If you try to brake a motor that size by shorting it, you may destroy the bridge due to the high currents, or damage the motor brushgear.

    The correct way to implement speed control is by using a current control feedback loop.

    You measure the motor current and adjust the PWM to try and match the current setpoint.

    The current setpoint is in turn produced by comparing the actual speed to the command speed; the current command produced by the speed error.

    This setup is a double-loop speed controller or servo.

    It means when there is any sudden change in speed command, whether to a positive, negative or zero value, the motor speed will change smoothly, without drawing more current than the motor can safely handle (which you need to calibrate in whatever current sensor you use).

    The braking effect is inherent in the system.
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
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    A neck tie, while connected to your neck, it a horrible motor brake.

    However, a pulsed shorting, is a good way to stop a motor of decent size. You must size the bridge so it can handle the high current that the shorting creates but only during the pulses.

    You bridge will have a inrush or max current spike spec and how long it is designed to handle the spike. If you pulse the short and stay within these requirements, your bridge will be fine.

    The time required to stop the motor will be a determining factor in breaking technology used.

    Have you seen the Sawstop? It uses a spring and a fuse to force an aluminum block into the saw blade to stop the blade in 1/200th of a second. It keeps a constant voltage on the blade and monitors it. If anything conductive touches the blade (Like a finger, or a hotdog ;) ) the break stops the saw and retracts the blade below the table so quickly, the average injury is about a papercut in size.

    This is a cool video of it in action:
    http://www.sawstop.com/howitworks/videos.php
     
    Last edited: May 11, 2010
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    That's effectively what the current loop does, but with active control of the duty cycle.


    That saw stop is very impressive!

    It does also draw attention to the difference between a routine stop and an emergency stop.

    Under normal conditions you want to protect the equipment from overload, but under emergency stop conditions stopping is the only priority, the gear working again afterwards is a secondary consideration.
     
  7. wewe

    Thread Starter Member

    Apr 9, 2010
    31
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    thx for the help guys
    for the pulsed shorting , do u mean that instead of permanently closing the 2 low side transistors i should make them chop ???
    and i have another question , i now know how 3 quadrants work , 2 rotation directions and the brake , how about the last quadrant function ?
    as for the current feedback loop , what's the instrument that i can use for this ??? (i live in a third world country so try to help me here by not mentioning something extremely advanced , just any known instrument for this :)
     
  8. rjenkins

    AAC Fanatic!

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  9. retched

    AAC Fanatic!

    Dec 5, 2009
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    If by 'chop' you mean switch quickly on and off, then yes.

    The shorting frequency can be determined by how much current the bridge can take for how long.
     
  10. wewe

    Thread Starter Member

    Apr 9, 2010
    31
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    i will be driving a 1500 watts motor with a 280 volts for input
    1500/280 , that's more than 5 amperes
    someone suggested to use the irfp 460 because it can handle 500 volts and 20 amperes , but the datasheet says that it can only handle 500 volts for a current of a couple of milliamps , and 20 amperes only for a voltage of 10v
    is there any mosfet that i can use for this motor or i will have to go for the expensive IGBTs?
     
  11. John P

    AAC Fanatic!

    Oct 14, 2008
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    You haven't said how rapidly you want to stop your motor, nor how much momentum is being controlled (to suggest an extreme case, must this braking action also stop a vehicle that the motor is driving?)

    Beyond that, you haven't said whether the power source can absorb energy as well as deliver it. If it can't accept the energy that the load passes through the motor, then that energy must be dissipated as heat somewhere or other.

    Basically, the faster the deceleration and the more energy that's handled, the more carefully the hardware needs to be designed.
     
  12. wewe

    Thread Starter Member

    Apr 9, 2010
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    for stopping the motor i don't need it to stop instantaneously lol
    my project is to control the motor in all 4 quadrants , so nothing really specific for the type of the load or anything , i just should be able to brake the motor
    as for the power source , there's 2 ways :
    -either take a 220 alternative source from any outlet and use a diode bridge and a filter to get the dc voltage (in this case the dc voltage at the input will not reach 280 volts )
    -use the source that exists in the university in the lab , it's just some kind of a variable DC source (i don't know much about it)
    i think that in my case the source cant absorb energy if i will use an ac outlet and use the diode bridge , unless the ac outlets that exist in the labs and in our houses can absorb energy and i just don't know it lol
    anyway i wasn't talking about my deceleration , i was just asking if the irfp460 can drive this motor in normal conditions
     
  13. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    If you rectify 220V AC you will get somewhere around 310V DC (AC x 1.4)
     
  14. wewe

    Thread Starter Member

    Apr 9, 2010
    31
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    the formula is ( Ac x 1.4x 2 / pi ) (it's the formula on how to get Vmean using a diode bridge)
    so i need 310 ac to get 280 dc
    but the teacher allowed us to use a diode bridge on any 220 v ac source , which give us a voltage around 196 volts
    anyway , i read the datasheet of the irfp460 and what i understood is that on a voltage of 500 volts i can only have a current of a couple of micro amperes , while i can only have a big current (of 20 amperes) for a voltage of 10 volts
    did i misread the datasheet ???
    i want to know if i can use this mosfet or not :confused:
     
  15. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    I'm assuming you are smoothing the DC supply, otherwise you will get a serious ripple on your PWM current.
     
  16. wewe

    Thread Starter Member

    Apr 9, 2010
    31
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    i will use some capacitors as a filter , isn't that enough ?
     
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