22 Jan
2009
22 Jan
'09
9:54 a.m.
On Wednesday 21 January 2009 12:08:02 pm Paul Koning wrote:
I suspect it's a lot higher than that.
>>>> "Guy" == Guy Sotomayor <ggs at shiresoft.com> writes:
>
> Note that you can generate 3 phase power from a 1 phase supply.
> There are electronic devices to do this ("variable frequency
> converters", devices intended to drive motors) up to 2 kW or so.
> Those can also produce odd frequencies, so they might be a
> suitable substitute for a CDC motor generator (for 400 Hz power).
> For higher power you can get a "rotary phase converter" which is
> just a 3 phase motor with some trick wiring. Those are more
> expensive though they are easy to build from a suitable size
> surplus motor plus some capacitors.
Guy> One of the big problems in dealing with this is the in-rush
Guy> current which is *really* impressive. Go look at the KL10
Guy> installation manual on the specs. It's a bit scary to have an
Guy> in-rush current that high (for up to 1-2 seconds). I don't know
Guy> if those converters are set up for those kinds of in-rush
Guy> current. My understanding was they were set up more for
Guy> inductive loads (ie motors) but never having dealt with them,
Guy> I'm working from ignorance.
Motors are the original "high starting current" devices. That's why
protection devices for motors are different from conventional circuit
breakers. A "locked rotor" motor draws a LOT more current than a
spinning one -- some vague memory says it might be 10x or so. And the initial surge as the motor is accelerating
looks like the locked
rotor case, briefly. How briefly depends on the machinery that the motor is
driving; if it's a high load it could take some time before the motor
is spun up to speed.
paul
I remember one small window A/C unit we had, years ago, that was nominally
rated at "7.5A" but the starting current on that thing was a whopping 40A!
--
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