Path: utzoo!attcan!uunet!lll-winken!lll-ncis!helios.ee.lbl.gov!pasteur!ucbvax!hplabs!hpda!hpcupt1!hpirs!ld
From: ld@hpirs.HP.COM (Larry Dwyer)
Newsgroups: sci.electronics
Subject: Re: Variable speed motor control ... need help.
Message-ID: <3660001@hpirs.HP.COM>
Date: 11 Jan 89 00:16:33 GMT
References: <288@heurikon.UUCP>
Organization: Hewlett Packard, Cupertino
Lines: 67

The Pulse Width Modulator (PWM) also has the advantage that it overcomes
Sliding Friction.  An example of Sliding Friction is obvious in the
following graph (which I lifted from my Physics notes):

       ^
    friction
         Fs __|                                                      
              | \                                                    
              |   \                                                  
              |     \                                                
         Fk __|       \________________________________              
              |                                                      
              |                                                      
              |                                                      
              |                                                      
              |                                                      
              |                                                      
              |________________________________________                                                      
                           velocity >                                
                                                                     
 	Fs = static friction
	Fk = kinetic friction

This friction is noticeable when one is trying to start a "classic" DC
motor at low RPM.  The motor will not start as the voltage is applied,
then it suddenly springs to a higher RPM than the operator intended.
After the static friction is overcome, the motor operates linearly (I
know, part of this friction is caused by the position of the rotor and
the residual internal magnetic fields that tend to keep the shaft in
place plus a number of other factors I probably don't know about).

Since a PWM kicks the motor with the full voltage, if only for a very
short duration during low RPM operation, the kick overcomes the static
friction in the motor (and the rest of the transmission system).

Another advantage is that the transistor which is switching the
voltage on and off need not be of as high a power rating as one
which is acting as a variable voltage divider.  It is either on
(low voltage, high current) or off (high voltage, no current).
This is why switching regulators are more efficient than linear
series regulators.

A disadvantage that has been reported to me is that a PWM is rough on
some kinds of DC motors.  At the very least, it requires limiting diodes
to shunt the back EMF when the current is removed during the off cycle.

PWM's are available for model railroad use (generally 12-18 volt systems).

A PWM can be built with a dual LM555 (or equiv.), a transistor for
driving the motor and one to isolate the modulation input to the LM555,
some resistors, some capacitors, a potentiometer and diodes for
kick-back.  The configuration for the LM555 can be ascertained in any
good product guide for that IC.  The rest of the components depend upon
the specifics of the motor to be driven (voltage, current, etc.).
All of these can be purchased at Radio Shack.  If the requester of this
response needs information about how to design such a circuit, I suggest
he contact the responders directly (the schematic is too difficult to
enter here but simple enough to be build by a hobbiest).

Don't forget a current limit circuit if you are going to use this on a
model railroad.  Someone may inadvertently place a small screwdriver on the
tracks and bask in the glow of molten metal.

		Larry Dwyer

Disclaimer: The statements made here are those of an individual and
do not represent any recommendations Hewlett Packard Company may make.