The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: firstname.lastname@example.org (Bill Allison)
Subject: Re: Help - Power mosfets - difficult load
Date: 28 Oct 2002 13:37:39 -0800
References: <email@example.com> <firstname.lastname@example.org> <3DB81206.E28CA99F@rica.net> <email@example.com> <3DB963BE.939694C2@rica.net> <firstname.lastname@example.org> <3DBC2B63.146AA440@rica.net> <email@example.com> <3DBD434D.3BA1BB54@rica.net> <firstname.lastname@example.org>
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:<email@example.com>...
> 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.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup