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From: Tony Williams
Subject: Re: Help - Power mosfets - difficult load
Date: Mon, 28 Oct 2002 15:35:28 +0000 (GMT)
References: <firstname.lastname@example.org> <email@example.com> <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>
NNTP-Posting-Date: Mon, 28 Oct 2002 15:40:39 +0000 (UTC)
User-Agent: Pluto/1.14i (RISC-OS/3.60)
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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