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From: firstname.lastname@example.org (Bill Allison)
Subject: Re: Help - Power mosfets - difficult load
Date: 28 Oct 2002 01:11:39 -0800
References: <email@example.com> <%hSu9.8156$Fj6.firstname.lastname@example.org>
NNTP-Posting-Date: 28 Oct 2002 09:11:39 GMT
"U231" wrote in message news:<%hSu9.8156$Fj6.email@example.com>...
> "Bill Allison" wrote in message
> > Hi
> > Be aware I'm not an electonics professional - advanced enthusiast is
> > about accurate.
> > Problem: I have to drive an automotive starter motor at variable
> > speed. The motor is used to drive a large model glider winch and
> > tow-line tension must be smoothly controlled. Tension is sensed by a
> > spring and potentiometer arrangemnent. Power is supplied by a 12v car
> > battery.
> > Solution? Use the pot to control pulse width (NE544 or similar) of
> > 50Hz pulse train fed via gate drive IC to bank of paralleled SMPS
> > mosfets (IRF3703 or similar).
> > Once you've stopped laughing, be aware that I have researched this
> > idea somewhat via postings here and all the IRF app notes etc and am
> > serious about having a go. I just wonder if driving this sort of nasty
> > load is feasible and whether anyone here has designed for a similar
> > situation. I am aware that transients due to the motor having a
> > brush-fed rotor and significant inductance is a major obstacle.
> > BTW I have a specific concern about the mosfets I've looked at so far
> > - the package TO220 seems wimpish to me for the package current rating
> > quoted of 75 amps - are there stud-mounted devices at reasonable cost?
> > Heatsink temperature rise is not so much of a problem as it might
> > otherwise be as the motor will be powered up at worst for 10-15
> > seconds every 5 minutes.
> > I do have a fall-back solution involving self-tapping screws and
> > multiple solenoid relays but it's horribly inelegant and has an
> > obvious drawback of unequal discharge of the cells in the battery!!
> > TIA for constructive replies
> > Bill Allison
> Could you use an automotive generator running as a constant torque motor
Don't know whether that's possible or of benefit. But it undoubtedly
would be more difficult. And I need controlled variable torque.
> Also have read of people using alternators as high powered stepper motors
> but I've never given it much thought.
Not relevant to this application I would have thought.
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