From obxwindsurf@yahoo.com Thu Dec 25 18:30:30 2003
Subject:Re: Speed Control Pedal

Respectfully I must agree with Claire on everything below. A common
garden variety volume pedal cannot pass mains voltages directly.
Rather the pot must be used as some kind of control voltage to a
hybrid control circuit designed to control the speed of induction
motors. This may be as simple as a PWM circuit which gates the
control input of a solid state relay. During the "on" time, full
voltage and a specified number of cycles are sent to the motor
allowing it to speed up to a certain speed. During the "off" time,
the motor slows down due to friction in the bearings. By varying
the on and off time very quickly (1 or 2 Hz) you vary the average
CURRENT through the motor while maintaining full voltage at all
times. The effect is no different than if you manually switched the
voltage to the motor through a momentary switch, although it is more
precise and achieves the desired result.

Light dimmers won't work because they work on the principle of phase
angle controlled turn on of applied ac voltage for partial cycles.
The waveform sent to the motor would vary from a spike to a half
sine-wave hardly the type of voltage that an induction motor likes
to see.

Hope this helps to confirm a safe method of speed control for single
speed motors. If you have experience in high-voltage/current design
this would be easy to build, but the commercial solutions take care
of the engineering for you.

Best regards,
Kevin

> Claire
>
> Australia >>
>
> more light at the end of the tunnel....