If you want a constant RPM, then the torque output of the motor must match
the resistance of the load. The tach feedback PWM technique mentioned before
is a way of varying the torque to match the load.

If the motor is driving a wheel on a vehicle, when the vehicle starts from
rest, it requires a lot of torque to accelerate the mass of the vehicle and
overcome the static friction forces, but then when it gets up to speed, the
torque requirement drops to just enough to match the resistance to the
motion (air resistance, wheel/drive friction).  Likewise, going up a hill
requires more torque and going down a hill requires less torque.

If you drove this motor (in the vehicle) at half voltage, the torque may not
be sufficient to overcome the static friction/inertial forces, so the
vehicle just sits there until you give it a push to get started. If on the
other hand you drove the motor with a 50% duty cycle, full voltage signal,
the torque during the 'on' part of the signal would be the full rated
torque - enough to overcome the static friction/inertial forces, so the
vehicle starts up better than when half voltage is applied (but not as well
or as fast as if full voltage is applied)

It would accelerate slower than if full voltage were constantly applied
because the power output of the fixed duty cycle system is about constant so
there is less than full power accelerating the vehicle. Also because of the
relatively constant power, it would slow down when going up a hill.

If you have a tach based feedback system and you gave it the 'half speed'
command, it would drive the motor at full voltage until it accelerated up to
speed then it would drop the duty cycle down to what ever it needed to match
the resistance of motion.  When the vehicle goes up a hill, the feedback
system would raise the duty cycle to match the power output to the increased
power requirement of going up the hill and the speed would stay constant.

So, you do not need feedback to get the higher torque at less than full
power advantage of a PWM drive, but with feedback you can get full power
automatically applied when you need it.

--BobG


-----Original Message-----
From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On
Behalf Of Russell McMahon
Sent: Monday, August 06, 2001 7:27 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE}: DC-PM motor, speed vs torque?

> We are working on a (PIC driven) application which uses a 90 Volt DC-PM
> motor. In some circumstances we want to lower the speed to about half of
its
> full-scale value, but we still need the torque we get out of the motor at
> its full speed. Of course we can lower the speed by lowering the DC drive
> voltage, but it seems that the torque also falls off proportionately to
the
> lowered voltage.  Does anyone know of a control technique (PWM maybe?)
that
> allows lowering the speed while maintaining the torque?  I am obviously
not
> a motor expert.  Am I trying to break the laws of physics here?  Any
> suggestions welcome. Thanks very much,
>

Presumably you do mean torque and not power. Torque should be able to be
maintained with decreasing speed as noted by others but power, being the
product pf speed and torque, will decrease as you slow unless you increase
the torque.

What is your current doing as you reduce speed?




            Russell McMahon
            apptech@clear.net.nz

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