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Re: [PSUBS-MAILIST] article on the OSS
I looked up some power transistors at Jameco.com, and found
this: 35A SCR $5.99 each, 27A FET $0.81 each. I have it from
the horse's mouth that SCRs are the prime choice, but that
FETs aren't that far behind. By the way, those current ratings
are at 100V! Two of those FETs would do the trick, at under $2.00.
Furthermore, you have a liquid-cooled steel hull to use as a great
heatsink. And yes, the cooler you keep the FETs (or SCRs), the
better!
Warren.
>
> I dont know about using PWM for something as large as a main drive for a Psub. Correct me if I am mistaken, but these size motors would use something on the order of 40 amps. This would require some considerably expensive FETs (with significant heat sinks). I would have to look at a catalog but I recall transistors of this size being sold for more than $80 each. And you would need 4 per thruster. I guess only the main thruster would require that kind of power though ; you could build smaller units for steering thrusters. Plus these things would waste quite a bit of energy as heat.
> Do most PSubs use relay-switched banks of batteries to control the speed of their main drive? This would have the advantage of nearly complete transmission of power to the motor, with the disadvantage of mechanical and control complexity. Could someone give me an estimate of the power requirements for a main drive motor for a ship this size and speed? I could come back with some hard data.
> thanks,
> --John
> Warrend Greenway <dub@linuxmail.org> wrote:Actually, a PWM motor controller is pretty dang simple. You could almost
> call it a no brainer. It does get more difficult as the current increases,
> because of the isolation, but it isn't rocket science. One of my first
> electronics projects was to make a PWM motor controller with a 555 timer.
> Reference TTL Cookbook. The MSP430 has a PWM built in, all you need to
> do is supply isolation and a couple FETs or SCRs. So, Dale wasn't that
> far off. This is also why I said that there were no if-and-or-buts in
> the motor controller method. PWM is easier, cheaper, more reliable, and
> vastly more efficient then manual switching or resistive methods.
>
> Warren.
>
> > Dale,
> >
> > At ease, you didn't offend anyone and it's possible I misread your article. I
> > was just making a comment that designing and building a PWM motor controller
> > is not an easy task.
> >
> > Sorry if you thought I was offended.
> >
> > Have fun,
> >
> > Ken Martindale
>
> --
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