Rob Paisley wrote:
Bill wrote in message ...

wrote:


In sci.electronics.basics Rob Paisley wrote:

| For larger horsepower motors, Three Phases means that they will
| start with no external mechanical assistance.

And, as I understand it, they can then "stand on one leg" although they
will be a bit "shaky".


You can also reverse the rotation by swapping two of the phases. If one
phase is blown you can start it by giving it a spin in the right
direction, 'without' getting your hand caught

Bill Baka


I am not sure if a 3 phase motor would even rotate unloaded on 2
phases but it certainly would not develop a usable amount of
horsepower. I would suspect that running on 1 phase would be out of
the question.

I do know that if a loaded 3 phase motor blows a fuse it will almost
always blow a fuse in one of the other lines due to an overload. Due
to the way the control power for the motor is connected the starter
will then open the circuit if it did not already do so when the first
fuse failed.

Also, there is a number of 1.57 being bandied about in this thread
for the relative horse power of a 3 phase versus single motor for a
given current. This number should be 1.73 (The square root of 3.

Rob.
The motor I replaced was on an air conditioning blower and would rotate
if given a boost before turning on the power, but as you said not with
enough power. It just would not start without the third phase. The fix
was to replace a 3/4 horse motor with a 1 horse motor in the same NEMA
frame size. It was a 480 volt unit so we did not play much with it and
just tossed it. Seems us engineer types prefer 5 volt stuff.
Bill Baka