PWM Control DC Motors

gyansorova@gmail.com wrote:

This PWM H bridge does the direction reverse, as well as the
speed control.  No reversing switch is needed.Note the red
and green arrows representing forward and reverse current
that produces forward and reverse torque.

gyansorova@gmail.com wrote:

He's describing a situation where they go out of their way to avoid
switching the high-side FETs.  Once you get over that hurdle, it is
easier to design an H-bridge PWM amplifier than a bipolar PWM amplifier.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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gyansorova@gmail.com wrote in news:1181596391.678516.16790
@g37g2000prf.googlegroups.com:


The control signals on the gates of the H-bridge determine the direction


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Scott
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wrote:

I know that but when do you switch over? Is it smooth? With old-
fashioned DC control and a bipolar supply it is smooth through zero
and into negative voltage.

gyansorova@gmail.com wrote:


If you pulse width modulate the average current to zero as
you switch directions, the torque reversal is smooth.

You can think of the motor as a giant low pass filter.  What you do on any
given pulse on the PWM would have very little impact on the motion profile.

Bottom line, just change the direction, and don't worry too much about
when.  During direction changes, chances are the pulses will be small
anyway.

As a practical matter, motors have a pulse width at each frequency, below
which they won't move at all.  Be sure to add this dead width to all your
widths.

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Scott
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