Re: Speed control

[Dave Everett <deverett@idx.com.au>]

At 11:43 14-05-99 -0500, Nathanael Henderson wrote:
>
>to provide 12, 24, 36, or 48 volts (giving four speeds.)   The psubs web
>site mentions 'pulse width modulation' but the term doesn't really mean
>anything to me, but I've taken a guess:   Switch the power to the motor on

You're guess was right!

>The problem is how on earth to
>do this?   A good relay seems like the first step. 

Ohh and you were going so well :) Relays are completely unsuitable for PWM,
the contacts would burnt out in no time and the coil takes time to pullin
and release, which would make the motor jerk.

The best choice for motors that draw the sort of current these do, is
IGBT's. I used IGBT's once in a plasma cutter and welder where the current
was about 100amps. Many people use FET's for this sort of thing, but FET's
that can handle this sort of current are very very expensive. If someone
can tell me what sort of current they are actually working with I can
advise better on the choices here, I would also need to know the stall
current.

>electricity to open and close the relay.    Generating the signal is a
>little more complicated, but I think a fairly cheap microprocessor and a
>variable resistor could do the job nicely.

You can get away with even less. A digital logic chip worth 50cents and
about 4 components is all you need to make a PWMer that could drive the
IGBT's. People still had to do things like this before the micro age (I
pity them).

>bad idea, or something else?  Is there an inherent flaw in the idea?

The only flaw is expense. IGBT's are more expensive than switches.

Dave Everett.