Hello Olin and company,

Hmmm... first, we are planning to use a PM DC motor.  Is this the same as a
brushless DC motor?

Given that it is, then we have not considered doing anything with the
windings during the off time.  What should we be doing?

From what we have read it appears to be a poor idea to let the motor "free
wheel" during the off time of the PWM pulse as then there is no path for the
inductive current to follow which results in a large voltage being produced.
Is this correct?  The way we understand it, is that if you have a current
going through an inductive load and you quickly turn off all the MOSFETs
then the voltage produced will be extremely large due to the rapid change in
current (v = L * di/dt).  However, if instead you turn on both the upper or
lower MOSFETs during the off time of the PWM signal then the current has a
path to follow and the di/dt term will be much lower (i.e. the current is
still being reduced as you are no longer supplying a voltage, but the
current cycles around for a finite amount of time).  That doesn't really
sound right now that I've written it out though.

So much to learn!

Regards,
Donovan Parks


> > The h-bridge is being driven at 20kHz and we are using a PM DC motor.  I
> am
> > indeed "shorting" the motor during the off time of the PWM signal.  The
> > rational for this is to reduce the power dissipated in the MOSFETs.  If
> the
> > MOSFET is not turned on then the power dissipated is greater due to the
> > relatively large voltage drop across the body diode.  As Spehro
mentioned,
> > the inductive nature of the motor allows this to work as long as the
> > frequency is high enough (and we have seen motor controllers that do
this
> > for the reason I mentioned).
>
> That's fine if your pulse frequency is indeed high enough so that you are
> always seeing flyback current between pulses.  However, I thought this was
a
> brushless DC motor, so each winding will be off during part of the cycle.
> That should be much longer than it takes the flyback current to die down.
> What are you doing with the winding during that time?

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