Re: Speed control

[Dick Morrisson <rmorrisson@unidial.com>]

And an "IGBT" is a what?
Integrated Great Big Transister
Irritated Goose Bite Tester
Illogical Giant Banana Transformer
Ok, I give up, please enlighten me (an ME) oh great one (assumed EE)!

Dave Everett wrote:

> 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.