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Suppose we are controlling a DC motor via PWM. The motor has a flyback diode across it.

Question: During the PWM off period, the motor current freewheels through the diode. Won't the motor experience a braking torque then? Is it desirable for efficiently controlling the motor?

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  • \$\begingroup\$ Unless you reverse the motor, the back EMF keeps the diode reverse biased. \$\endgroup\$
    – user16324
    Commented Apr 20, 2020 at 18:52
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    \$\begingroup\$ @BrianDrummond - no - when the transistor turns off the inductance results in current flowing forward through the diode. \$\endgroup\$
    – Kevin White
    Commented Apr 20, 2020 at 22:44
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    \$\begingroup\$ @KevinWhite Not quite. You are correct there will be a brief conduction spike but that's only from the motor's inductance. The braking in the question is from the back EMF which has the same polarity as the applied EMF keeping the diode reverse biased : therefore no braking as per your answer. Two separate phenomena, but often confused with each other. \$\endgroup\$
    – user16324
    Commented Apr 21, 2020 at 11:46
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    \$\begingroup\$ @BrianDrummond - In a normal PWM motor control system the frequency of the PWM is such that the diode conduction spike is the full length of the off-cycle so the back-emf never keeps the diode reverse biased as the next on-cycle will drive forward current into the motor and reverse bias the diode again before that happens. Although the effect you're describing would happen at a low-frequency PWM, that is not the normal scenario. \$\endgroup\$
    – Kevin White
    Commented Apr 21, 2020 at 14:25
  • \$\begingroup\$ @KevinWhite Fair point. There is no mention of PWM freq in the question so ... \$\endgroup\$
    – user16324
    Commented Apr 21, 2020 at 14:49

3 Answers 3

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During pwm off period, the motor current freewheels through diode. Won't the motor experience a braking torque then. Is it desirable for efficiently controlling the motor?

Firstly, Any PWM signal frequency should have a time period that is many times shorter than the physical response of the motor due to it's mechanical inertia.

And, as Brian Drummond reminded me, the diode will only conduct due to the initial back emf from the inductance of the motor. After this has settled-down, if the motor is continuing to free-wheel in the same direction, the diode won't remain forward biased. However, the motor free-wheeling will generate a voltage and so, it might be advisable to put a diode across the BJT - anode to ground - to prevent any excessive motor free-wheeling from reverse biasing the BJT to any great extent.

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  • \$\begingroup\$ The above question was derived from this question, could you take a look at it? electronics.stackexchange.com/questions/494442/… \$\endgroup\$
    – User
    Commented Apr 20, 2020 at 15:47
  • \$\begingroup\$ I don't use simulink or matlab. \$\endgroup\$
    – Andy aka
    Commented Apr 20, 2020 at 15:50
  • \$\begingroup\$ I could use your insight as to why any simulation model might behave like that \$\endgroup\$
    – User
    Commented Apr 20, 2020 at 15:52
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    \$\begingroup\$ @Andyaka I think this explanation is off here : the back EMF does not put the diode into conduction (though the inductive kick does, briefly). As a sanity check, consider that a short circuit would provide the hardest braking force : a series resistor would only reduce it (and take dissipation out of the motor) \$\endgroup\$
    – user16324
    Commented Apr 21, 2020 at 11:52
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    \$\begingroup\$ For PWM to be efficient the on-time must be much shorter than the L/R time constant or most of the power will be dissipated in the motor resistance. The off time will be the same as the on-time @50% duty cycle so it will also be << L/R. The inductive spike will be much longer than the off-time. Low-speed PWM is no different than using a resistor to drop the voltage. \$\endgroup\$
    – Kevin White
    Commented Apr 23, 2020 at 15:55
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The current through the motor is in the same direction during the PWM off period as during the on period - the voltage reverses but not the current.

So there is no braking torque.

It is a very efficient way of driving a motor - virtually all PWM motor driver use this arrangement or versions of it.

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Yes any diode will have a braking effect on the motor because the current through the diode experiences a voltage drop across the diode which causes heat and removes energy for the motor.

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