> We are working on a (PIC driven) application which uses a 90 Volt DC-PM
> motor. ...
> Does anyone know of a control technique (PWM maybe?) that
> allows lowering the speed while maintaining the torque?
> ...
> Am I trying to break the laws of physics here?

Yes.

For a DC electric motor, torque is proportional to current.  Speed is a
function of torque and load.  You can't dictate both the torque and speed of
the motor without controlling the load.  If the torque is contant the speed
will be constant if the load doesn't change.  Asking for the same torque at
a lower speed makes no sense.

So you need to decide, do you really care about the speed or the torque?  If
you could magically make the motor run at half the speed, why do you care
what torque is required to maintain that speed?  If you really care about
speed, then you probably need to do some speed feedback.  Not knowing the
situation, my generic answer is to control the motor via PWM, which is in
turn controlled by a PID control loop in the PIC that works with a speed
feedback signal.  I know this is doable because I've done it before.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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