There is a big problem with "high frequency" PWM like the 12 kHz you propose
: you can hear it ! unless it is more than #20 kHz (and at this frequency, a
poor 4 MHz PIC is a bit poor :-) ...
So, a railroad application isn't much convenient !!!

Hope this help

Jean-Yves
> ----------
> De :  Stefan Sczekalla-Waldschmidt[SMTP:ssw@OIKOSSW.DE]
> RŽpondre ˆ :      pic microcontroller discussion list
> Date :        jeudi 6 mai 1999 11:26
> A :   PICLIST@mitvma.mit.edu
> Objet :       Re: [OT] PWM and coreless motors
> 
> ^Hi,
> 
> Shahid Sheikh wrote:
> >
> > Yep. Dont tell me you've quit :)
> 
> How could I ....
> 
> Did you heard of the Danish Guys who made a Motorola-Protocol
> decoder for maerklin based on a PIC 16C84 ?
> 
> http://hjem.get2net.dk/formula.micro/indexeng.html
> 
> >
> > 19.somthing is the max I can generate with the Atmel chip I'm planning
> on
> > using so I think that will work.
> 
> AFAIK the higher the better but also 12 KHz may be ok, I think 20 KHz
> is just choosed because of being outside the audible range.
> 
> The high frequency pwm versus iron-core motors is necessary because
> the coreless motor has just a cooper-network glued tho the shaft.
> the forces acting on this network will be smaller if more short
> pulses of power are applied than only some long.
> 
> >
> > Anyone knows the benefits of using coreless over a iron core motor (or
> did I
> > already ask that)?
> 
> one main difference is the mass of the rotor (inertia). also coreless
> motors
> has virtually no "Rastmoment" - means if you just take both kind of
> motors,
> and try to turn them manually you will notice that there is some "force"
> necssary
> to turn the iron core motore whereas much less "force" is necessary to
> turn the
> coreless motor.
> 
> - oh, I forgot the real difference: Price. :-)
> 
> >
> > Also I was wondering how pronounced is the back emf of a coreless motor
> Vs.
> > a iron core motor. It should be better than an iron core motor right?
> 
> I'm sorry, but this need to be answered by someon else - never tested
> this.
> 
> Kind regards,
> 
>         Stefan
>