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[DPRG] PWM and motor response

Subject: [DPRG] PWM and motor response
From: larry barello lbarello at barello.net
Date: Tue Feb 20 12:13:11 CST 2007

I gave a presentation that covers (implicitly) your situation.
www.barello.net/Papers/H-Bridge.pdf  I could demonstrate each hand drawing
on my oscilloscope using a small H-Bridge controller and a motor.  Voltage
was measured across the motors, current across a small resister in-line with
the bridge.

I am guessing you are applying the PWM to the ENABLE line.  Per my
investigations (and resulting presentation) the problem is that most of the
energy you put into the coil windings are wasted during the off period
recirculating through the battery and recirculation diodes.

A much better way to handle the H-Bridge is by Synchronous-Rectification or
Locked-anti-phase.  The latter requires much higher PWM frequencies to be
efficient.  It all depends upon the inductance of your motor circuit (you
can add series inductance...).

I recommend sign-magnitude, synchronous rectification.  Then you will get
motor movement with sub 1% pwm (depending upon the motor).  Once I had a bug
in a motor drive where at 0% pwm there was a microsecond glitch which
represented .25% drive.  It was a 12 Hp ETEK motor and at zero drive would
very slowly rotate the shaft... (like 2 rpm vs 4krpm when 100% drive...)

To cut to the chase: Apply your PWM to one side of the H-Bridge and use the
other side for direction control.  You will need to "invert" the PWM when
going in reverse.  If the PIC has four PWM outputs, you can just apply a
high level signal to one side and apply PWM to the other side, swapping for

-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On Behalf
Of Vimal Shankar
Sent: Tuesday, February 20, 2007 8:04 AM
To: dprglist at dprg.org
Subject: [DPRG] PWM and motor response

I am facing a peculiar problem.

I am using PIC16F877A @ 20MHZ PWM to control two DC
Geared motors 12v at 450ma via l298.

The motors respond fine to PWM at 1250hz (that's about
the lowest freq the PIC can go) but when I change it
to 5khz ... the motors don't move until I hit 80% duty
cycle !!
Is this solely due to the motor's response ?

If so, how do I find out the freq at which I get good
response ? What tools do I have to use ?
Visual judgement and intution in core electrical are
things which I don't have much because of my being a
Comp Sc Major.
I am more at ease with (simple) math eqns and DSOs...

Any pointers ?

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