In article , dnichols@d-
and-d.com says...
In article ,
Nate Weber wrote:
"Ned Simmons" wrote in message
...
In article ,

[ ... ]

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.

Well ... the heaters sense the current to the motor, and the
third can be no worse than the sum of the two being monitored, so it
should be possible to detect any kind of fault circuit (the most common
being that of power applied to only two phases, which would be feeding a
stalled motor, and thus be sufficiently over-current to trip the circuit
fairly quickly anyway.)

A fault to ground in a leg with no heater would not trip
the overload if the feed was Y connected.

This is intended to protect the motor, not
anything else, as there should be a breaker or a fuse in each hot anyway
to protect everything else.

That's the primary purpose of the overload, but since the
overcurrent protection in a motor branch circuit can be
much higher than the ampacity of the conductors, the
overloads appear to provide some protection for the
conductors as well.


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.---

This means that my earlier circuit to reverse a 220V
single-phase motor with a single three-pole double-throw center off
switch would be legal, as long as there was a disconnect provided
(which in my case would have been the twist-lock power connector near
the lathe).

However, I agree with others that the idea of keeping one side
of the motor live does not feel right anyway.


And most industrial practice would agree. I've seen
hundreds, if not thousands, of installed motor starters and
can't recall any where all the hots were not switched, even
on old installations with two heaters on three phase
starters.

Ned Simmons