Free-wheeling in PWM motor control

Discussion in 'General Electronics Chat' started by Robin66, Sep 7, 2016.

  1. Robin66

    Thread Starter Member

    Jan 5, 2016
    PWM drive of a motor involves a "free wheeling" period due to the winding's inductance. Eg. in the pic below, if S1 & S2 are switched during an ON period, the OFF period will result in free-wheeling thru the body diodes of S4 & S3. My understanding is that this will push charge back into the supply (DC below). However, if only S1 is pulsed (S2 remains ON even during OFF periods) then free-wheeling will occur thru S4 & S2, thus not pushing charge back into the supply. Some considerations:
    1. This would reduce losses in the reservoir Cap (parallel with DC).
    2. This would considerably extend the free-wheeling period since dI/dt is proportional to V across L.
    3. It's important that there isn't a gap between the end of the free-wheeling period and the next ON phase otherwise the back-emf in the windings will abruptly oppose rotation.
    4. This could help sensing of the zero-crossing point in sensorless BLDC motor controller due to the reduction of transients and the opening winding's crossing V being referenced to ground.
    Is there anything else I've missed or could someone point me to some literature on design considerations?

    Last edited: Sep 7, 2016