Re: "54Vac Three Phase" means what?

From: Scrim (none_at_none.com)
Date: 07/14/04 

Date: Wed, 14 Jul 2004 12:12:27 GMT

"Rich Grise" <null@example.net> wrote in message
news:2571560.nZIKurkq8H@entheos.thunderbird.ops.dsl-verizon.net...
> on Monday 12 July 2004 08:58 am, Scrim wrote:
>
> > "Rich Grise" <null@example.net> wrote in message
>
> >> It sounds like you're trying to design a power supply for the thing.
> >> Would that be an accurate guess?
> ...
> > My original plan was to use
> > the 3 phase variable frequency digital sine wave generator I've designed
> > (with help from this group - see thread: Digital Sine Wave Generation)
to
> > drive three audio ic power amps. Unfortunately I now realise that the
> > required 3 phase 54Vac means I'll need an 88Voutput swing, which is
beyond
> > the IC's I've been able to track down.
> ...
> > The alternative idea I've been considering is to use some logic to
> > produce 3 square wave signals which overlap in the required 120 degrees
(3
> > phase) fashion. I'd connect these to something like the HIP4086 3-Phase
> > Bridge Driver and 6 FETS.
> ...
>
> This sounds good to me - Plus, isn't the RMS of a square wave equal to the
> peak value?
>
> > The HIP4086 can't quite cope with the required 88V either, but I should
be
> > able to find something given the amount of mains powered switch mode
stuff
> > around these days. I'm concerned that this approximation to a sine wave
> > might be just too poor, especially given that it seems that the
limiting
> > factor with turbo pumps is power dissipation in the motor windings.
>
> I souldn't worry about this at all - You might need antiparallel diodes,
> but I'd think a motor's inductance would make a dandy low-pass filter. ;-)
>
> > However, I don't need the pump to run up to speed in just 60 seconds
(like
> > I understand it would with the recommended supply), and think I might
get
> > away with a fixed frequency 1250Hz 3 phase square wave supply - who
knows,
> > but I don't want to invest too much effort in this, given that the pump
> > may turn out to be beyond use anyway.
>
> I don't really get what you're saying here - do you mean that they used to
> ramp up the drive freq, or that it takes that long to spin up even with
> just the constant power supply? In either case, I'd try to duplicate the
> "real" operation as closely as I could.
>
> --
> Good Luck!
> Rich
>
 The rms/square wave thing is quite confusing, since, as you say, the motor
will filter it. It would certainly simplify things if I can just drive each
phase with a 0 to 54v square wave! I'm starting to feel motivated again :-)
Cheers - back to the HIP4086 3-Phase Bridge Driver and 6 FETS.

I'm not certain, but suspect the proper controller ramps frequency to get
maximum acceleration on the rotor all the way up to full speed. (I've read
that during the start-up phase it increases the "Volt/Hz" ratio, but that
doesn't reveal much). I also suspect it takes advantage of the windings
being cold at switch on, so it can dump more heat into them for a short time
without causing damage and hence get the motor from 0 to 75,000rpm in just
60 seconds. (The winding are fitted with a temperature sensing thermistor,
by the way). But I suspect the voltage might need to be reduced to some
extent at lower frequency to avoid excessive currents. This is too
complicated so I just intend to try using a fixed frequency 1250Hz supply
and accept it might take, say, 5 minutes to get to full speed. If it's
slower than that I'll try some frequency ramping.

Scrim

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