On Sat, 13 Mar 2004 18:38:45 -0600, "Bob Swinney"
wrote:
Gary sez:

Because the motor won't actually try to spin up when fed only 1 ph, the
starting surge isn't all that much over normal running current, impedance
limited. So the breaker probably won't trip. But the converter motor will
just sit there humming, no cooling air flow, and will rapidly overheat.
The chance of electrical fire is high. That's why dropout protection is
a must."

Since the rotor can't turn, I imagine the 1 ph current is more like locked
rotor current, considerably higher than normal inrush.

Yes, but the Line breakers have to be sized big enough to hold
against the Locked Rotor currents long enough to get the idler motor
up to speed, and then 15 seconds later they have to hold the idler
motor's current and the Locked Rotor starting current of the load
connected equipment's largest motor...

According to the inviolable principles of Murphy's Law, the Line
side breakers could hold against the Locked Rotor current of the idler
motor /just/ long enough for something to burn out, burn up or blow
up. All three things that aren't too safe, especially if the
components that torch aren't properly enclosed to prevent a fire from
getting out into the shop.

Large electrolytic capacitors can go off like dynamite, with a big
bang, tons of foil chaff everywhere, and sometimes a good gout of
flame. Not that I've ever watched as an induhvidual who lived down
the block blew some electrolytics up with straight 120V across the
terminals in his backyard for cheap laughs, mind you...

A TEFC motor and all the control circuitry and capacitors properly
enclosed in a NEMA-1 or NEMA-3R rated enclosure would not be a
problem.

But if someone's using a normal open ventilated motor - or worse, a
good old 1940's style fully open frame motor as their idler, and with
their correction capacitors out in the open screwed to a board with
plumber's tape, and an inch of sawdust from the table saw on top of
everything...

This is the kind of stuff that has to be thought through.

-- Bruce --