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From: "Phil Allison"
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
Subject: Re: Help - Power mosfets - difficult load
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Wed, 23 Oct 2002 10:27:30 +1000
NNTP-Posting-Date: Wed, 23 Oct 2002 10:18:48 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Bill Allison" wrote in message
> "Phil Allison" wrote in message news:<3e8t9.58968$
> > ** Your assertion is wrong - 50 Hz PWM is inefficient as hell
> > any DC motor.
> I am encouraged by the fact that Futaba Corporation have made millions
> selling servos driven at 50Hz. I am further encouraged by their choice
> of a frequency as low as 120Hz for their newest servos in which (they
> are microcontroller-based) they could have chosen virtually any
** That is asinine. Servos are not drive motors and your example is
irrelevant to my point.
> We may to an extent be at cross purposes here. I am concerned with fet
> switching losses, because I know from the graphs that if I am not
> careful in this area I risk straying outside SOA. The less time the
> fet spends in the switching region the better from the point of view
> of heat generation in it. When fully on the power dissipated will be
> I^2 * R where R = 0.0033 ohm for the pair of FB180SA10 I will use i.e.
> 33 watt at 100 amp and of course when fully off the power dissipated
> in the fet will be zero. But when switching, the power dissipated,
> ignoring inductive lag, will be highest at the midpoint when 6 volts *
> 50 amps = 300w. So I want to be in that switching zone as infrequently
> as practicable. I am not concerned with motor or overall efficiency,
> because the motor has power aplenty for the job and battery capacity
> is not an issue either.
> I do accept that the *system* will operate more efficiently at higher
> frequencies (although the motor vibration from 50 Hz excitation is
> undoubtedly useful to overcome brush stiction and improve low speed
> starting). But as the control input available to me is a 50 Hz pulse
> train it is simplest and makes sense to stay with comparing that with
> a similar train derived from towline tension and expand the difference
> to (via a suitable drive IC) gate my fets.
> So I'm going with 50 Hz. It's the practical solution and will work.
** Tell me Bill, seeing as you already know all the answers why did you
bother to post here at all ?? You have wasted people's time and smugly
ignored facts that don't suit your misconceptions.
If you own a CRO and can use it to monitor the voltage across a low
value sense resistor you will see that 50 Hz drive produces huge current
pulses in the motor at low speeds and when the motor is stalled. When you
go to a higher frequencies the current wavefrom is basically DC with a small
ripple component. The magnitude of this current is many times less than
with 50 Hz drive.
It ought to be obvious that this takes the load off the switching
fets big time so they are not near their their current limits unless
something major goes wrong like a stalled motor or full drive is applied
Lots of luck with your project - I think you are going to need it.
. . ... . . ............... Phil
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