Ask A Scientist© Engineering Archive

Index Key:   ENG012
Author:      andrew m childs
Subject:     Variable speed motor
Text:        This is an electrical engineering question.  I am setting up a 
device that needs to run a motor at variable speeds, controlled by a knob or 
slider.  How should I do this?  I have tried a large number of combinations of 
motors, different voltage batteries, and various potentiometers connected in 
series or parallel with the motor, but nothing gives me a wide range of 
control.  What should I do?

Response #:  1 of 1
Author:      dipper
Text:        It really depends upon how closely you want to control the speed 
of the motor.  The most precise way is by "closing the loop", or creating a 
servo system.  If you can come by a tachometer (really a very precise DC 
generator), you can monitor the speed of the motor by coupling the two 
together.  By adding a reference voltage (say +/- 10 v), and summing that with 
the tachometer voltage, you can build an amplifier that will give you quite 
precise speed control.  For DC motors SCR's can work, although good PWM 
amplifiers are better (and considerably more expensive). Several books from 
your local library can give you many common circuits for closed loop speed 
control, depending upon the size and type of motor you are planning on using.  
If you want to use a 3 phase motor, you need to build a phase invertor that 
literally changes the frequency of the 3 phase current.  A DC stepper motor 
would require a phase sequencing circuit with drivers.  There are simpler ways 
to get rough, less precise speed control from a variety of motors.