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From: email@example.com (Bill Allison)
Subject: Re: Help - Power mosfets - difficult load
Date: 28 Oct 2002 13:37:39 -0800
References: <firstname.lastname@example.org> <email@example.com> <3DB81206.E28CA99F@rica.net> <firstname.lastname@example.org> <3DB963BE.939694C2@rica.net> <email@example.com> <3DBC2B63.146AA440@rica.net> <firstname.lastname@example.org> <3DBD434D.3BA1BB54@rica.net> <email@example.com>
NNTP-Posting-Date: 28 Oct 2002 21:37:39 GMT
I know that discontinuous low frequency operation will be inefficient,
1) Nobody has disagreed with my contention that those large low
frequency impulses will be *very* useful in overcoming stiction and
allowing the motor to be controlled smoothly at start-up and at very
low speed. I did produce some 20A speed controllers a while back
operated with 50Hz pulses and various motors could be made to creep
round at about 1 rev per second. That degree of control in this
application would be seriously useful.
2) *Provided* I can keep within fet ratings, I don't care too much
about efficiency - the unit is going to be powered up for only 10
seconds at most every 5 minutes at least so heating isn't a huge
problem and irrespective of frequency, at full throttle (i.e 100% duty
cycle) losses will be due only to RDSon.
If tests show this isn't do-able (fets-wise) at 50 Hz then circuitry
will need to be devised to produce a 500Hz PWM drive from the 50Hz
control signal but I want to avoid that complication if possible.
Tony Williams wrote in message news:<firstname.lastname@example.org>...
> In article <3DBD434D.3BA1BB54@rica.net>,
> John Popelish wrote:
> > I would call 40 mohm a confirmation of your previous 32 mohm value.
> > This pair also gives me some idea of the minimum frequency this motor
> > will run well on. I would estimate that the L/R time constant is a
> > good maximum pulse repetition period for your PWM circuit. That comes
> > out to just over 500 hz. Much slower than that will cause the current
> > to occur in big blasts with zero current between them.
> I believe this was mentioned in a similar thread here
> a few months ago. The maximum off-time should be small
> compared to the L/R, otherwise a discontinuous motor
> current results. This then produces a noisy+inefficient
> operation, with a weird torque/speed curve at low load.
> It is similar to the old L-C power supply smoothing at
> low load currents.
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