Wouter.
The speed will be load dependent as well as drive dependent. If you know
the charecteristics of the load & motor, then 50% drive may be all you
need. By putting the motor into reverse for 25% of the time you may get
better speed control but power consumption will increase dramatically.
Similarly, buy shorting out the drive to the motor on a 50% basis you will
be effectively breaking the motor - and again power consumption will
increase. The motor will work as a generator while spinning down and if an
"electrical" load is applied then  energy will be removed.
Is it a PM motor or shunt or series wound? If it is series the 25% - 75%
method "could"  just act like ~100% if the PWM was slow enough and the
inductance low. (In the extreame case)

My suggestion is to just drive it at 50% (on-off) and provide protection
against voltage spikes that will be created at the turn off points.  The
other methods will increase power output in the motor & drive considerably.

Richard P





I want to drive a modest (1A 24V) motor. The circuit must be potted, so
heat must be minimal, so I want to use FETs. For simplicity I will use
N/P fets.

Question: if I want to have the motor turn forward at half speed, should
I
- turn on left-high and right-low for 50%, and none for the other 50%,
or
- something like left-high/right-low for 75% and left-low/right-high for
25%?

In other words: to control the power in one direction, do you change the
on/off ratio, or the forward/backward ratio?

Wouter van Ooijen

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Van Ooijen Technische Informatica: www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: www.voti.nl/hvu




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