Quote:
Originally Posted by Ether
Black Jags alternately connect the motor to power and short it.
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So what happens to the current in a spinning motor when it is shorted?
The simple answer is, the motor's inductance forces the current to keep flowing for a few microseconds, before the motor's back emf overpowers the inductance and reverses the current.
This momentary forward current continues applying torque during the few microseconds that it is flowing.
With a Jag, operating at 15000Hz, the pulse width is shorter than the decay time, so it makes sense to short the motor during the OFF cycle, to let the good (forward) current keep flowing.
With a Victor, operating at 150Hz, the pulse width is far longer than the decay time, so it the motor were shorted, the back emf would have time to overpower the inductance, and you'd get reverse current (and reverse torque) during the OFF portion of the PWM.
See attached PDF (analytical solution) or PNG (numerical solution) for more technical detail.