Re: 3-phase current

To: tesla-at-pupman-dot-com
Subject: Re: 3-phase current
From: "Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Sun, 06 Dec 1998 22:24:31 -0700
Approved: twf-at-verinet-dot-com

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> > Does anyone know the ballpark cost for piping in 3-phase
> > current, or what they usually charge for branching it in from a place
> > where it's already installed?

The cost is huge if there isn't 3 phase available near by. However, you
don't need three phase power to run your three phase supply. There are two
techniques (actually there are more, but 2 are relatively easy)

1) Get a rotary converter (essentially a three phase motor designed for
this application) which converts single phase to 3 phase. You seem them
surplus occasionally. I understand that you can actually use a run of the
mill three phase motor to do this once you get it spinning, etc. but I
haven't done it myself, and haven't calculated the loads involved. I could
see using a 15-20 HP motor to do this to get your 10 kVA of 3 phase power
without overloading the windings (since I think one winding will have to
carry the entire load power). A 20 HP low voltage (e.g. 240V) motor will be
a pretty big piece of gear (less than the 700 pounds for the power supply
though), and might turn up surplus. The scheme is to start the motor
unloaded (using a capacitor to get enough power in the third phase). Then,
once the motor is spinning, you put the load on, and it acts as a rotary
transformer.

2) Use capacitors to simulate the third phase. THis is done on welders all
the time. The trick is to calculate how much phase shift you need at the
rated current, and to size the caps accordingly. At lower than rated load,
the phase shift will be different, so the ripple on your power supply will
rise because the phases feeding the rectifier won't be nicely spaced at 120
degrees, and it will be less efficient, but, you are lower than rated load,
so who cares.


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