Braking of PWM motor - explanation

Discussion in 'The Projects Forum' started by sdukenneth, Jun 11, 2016.

  1. sdukenneth

    Thread Starter New Member

    Jun 11, 2016
    3
    0
    Hello everyone!

    We are working on a project with a scalextric car - where we have to make it automatic. It has to read the course and change its speed and brake in the turns.

    We have made a really good brake which can be controlled by a PWM signal from our ATmega32A. It works great - but we can't figure out how it theoretically works. Why does it brake faster?

    [​IMG]

    The motor is illustrated by the inductor.

    The IRLML0030TRPBF mosfet is controlled by a PWM aswell but that is for the gas. Our brake circuit is within the marked area.

    Can someone make a neat explanation on why this makes the motor brake faster?

    PWM Motor and PB3 = 5V in a duty cycle.
    The IRLML0030TRPBF and IRF9230 is not activated on the same time obviously.

    Thanks for the help!
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,809
    1,105
    The PFET short-circuits the motor. The motor's kinetic energy is converted to heat energy dissipated in the PFET.
    Welcome to AAC!
     
    sdukenneth likes this.
  3. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,132
    267
    When the motor is free-wheeling, it generates a back EMF proportional to the speed.

    The upper MOSFET effectively shorts the motor, causing this back EMF to force current through the motor coil windings, this current creates a torque opposing the rotation. This continues until the motor reaches zero speed, where the coil current also becomes = 0.

    Kinetic energy is dissipated as heat generated in the coil winding resistance and in the FET.
     
    sdukenneth likes this.
  4. sdukenneth

    Thread Starter New Member

    Jun 11, 2016
    3
    0
    Hey Guys !

    You guys are quick ! :D

    That is why we couldn't understand the theory behind it. We are only 2. semester students as Power Engineers, and EMF is something we learn about in 3. semester :)

    Thanks for the perfect answers !

    And thanks for the welcome :D
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
    224
    I'm surprised that a slot car needs brakes. I'd have thought that with a small mass and a fairly inefficient drive, the motor would come to a stop very quickly as soon as the power is turned off. Does the car really coast for any distance if you disconnect the power?
     
  6. sdukenneth

    Thread Starter New Member

    Jun 11, 2016
    3
    0
    One of the rules for the project is that we have to use strain gauges to detect the turns. So that combined with all the other circuits makes the car weight a lot more. Braking distance from full speed without brake is: 172 centimeters - and with the brake we could get it down to 58 centimeters :)
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
    224
    OK, you've made the observations, so I can't say the car behaves differently! I seem to have imagined something much smaller than it really is.
     
Loading...