On Sun, 13 Jun 2004 20:46:18 -0500, "Nate Weber"
wrote:

"Ned Simmons" wrote in message
...
In article ,

I think it may have been legal at one time to switch only
two leads of a three 3 phase motor. My 60s vintage IR
compressor has a Square D pressure switch nameplate rated
5HP @ 240V, 3 phase, but is only a 2-pole switch.

I know that it used to be acceptable to put overload
heaters on only two poles of a motor starter, and this is
no longer the case.

Ned Simmons

From the 2002 NEC

Article 430.84
The controller shall not be required to open all conductors
to the motor.
Exception: Where the controller serves also as a disconnection
means, it shall open all ungrounded conductors to the motor
as provided in 430.111
---A controller that does not also serve as a disconnecting
means must open only as many motor circuit conductors as
may be necessary to stop the motor, that is, one conductor
for a DC or single-phase motor circuit, two conductors for
a 3-phase motor circuit, and three conductors for a 2-phase
motor circuit.---


Nate


Hey Nate,

And as just a follow-up to that, we used MG sets that were operated
Star-Delta. These were effectively 6 lead motors, at 575 to 600 volts
AC producing 0 to 240 volts DC, or sometimes PWM DC. The AC end of
the MG's had three "hot" leads at all times, and the motor is started
and run by switching the other three leads at the controller. That
way, a whole contactor, valued at from $50 to $250 (depending on size)
was "saved". Only problem is, these MG's sit for at least 50% of the
time in a shut-down state, but the 600 volts is still a potential on
the windings, and these MG's would blow the AC ends at a much higher
rate than the same models in a more conventional "all motor leads
open" type.

Take care.

Brian Lawson,
Bothwell, Ontario.