[rescue] Onyx Rack

[James Lockwood]

On Fri, 7 Jun 2002, James Lockwood wrote:

> 3ph nowadays is usually supplied as 3ph Wye.  3 hots, 120V hot to neutral,
> 208V hot to hot.  Alternatives are 3ph Delta which is 240V hot to hot, and
> the deprecated high leg (red leg) Delta which runs a neutral from a center
> tap of one of the 240V Delta pairs.  This is bad because using the neutral
> for 120V circuits tends to unbalance the load.  3ph uses transformers as
> well even when voltage matching is not an issue to provide isolation and
> short-circuit protection.

Minor addendum: It's relatively easy to generate 3ph from 1ph using a
rotary converter.  This is basically a 3ph motor driven off of a single
phase, using one phase winding to produce rotary motion and the other two
windings to generate.  Balancing capacitors are necessary.  Since a 3ph
motor will not start on 1ph (but will run) it is necessary to either
hand-start or add in a starting circuit to spin it up.

This kind of setup is popular with people running large shop equipment at
home that come with big 3ph motors.  It is noisy and does consume some
power all on its own so is rarely used for "always-on" applications.

You'll get out on the motor the type of 3ph it is wired for.  A Y (Wye)
wound motor will give you 208/120 from hot to hot/neutral, and a Delta
motor will give you high leg Delta 3ph out (as the center of your 240V in
will be tapped for ground and neutral).

One really neat 3ph application: a "12-phase star" rectifier.  If you
combine the outputs from a Delta and a Wye connected full-wave rectifier
you get 12 pulses per 60Hz period, with tiny ripple (1%) and near constant
output voltage before any filtering is done.  If you need high power pure
DC, this is the way to go.  Bulk electrochemistry uses this trick.

-James