On Sun, 19 Dec 2004 17:55:57 GMT, "Jerry Martes"
wrote:


wrote in message
.. .
On 18 Dec 2004 06:20:08 GMT, inch (Peter
H.) wrote:



Neither does he.


Fitch tried it ... didn't work.

If the motor was a wound rotor machine, it might have worked.

As an induction machine, as Fitch tried, no chance.


With the right machine it can work but starting is
difficult and it needs a vastly oversized efficient machine
because you are only applying power to one sixth of the
windings and this has to handle the whole of the output
power.

Both voltage regulation and efficiency suffer because
of the uneconomic utilisation of the windings.

Jim

Jim

I dont know how to analyze this 3 phase motor that is using only 1/2 its
windings (poles) to spin it. But, it surprizes me that an induction motor
can still run when the number of poles is reduced to 1/2.

I wonder if a 4 pole motor can be re-connected to be a 2 pole motor. But,
this gets too complex for me to analyze. I'd have guessed that the
re-connected motor would produce a torque that contained ripple.

I'd sure like to know more about how a 3 phase motor can be made to spin
when 1/2 itys poles are removed (not connected).

Jerry


Jerry

The right sort of machine needs to be one that is
wound with two independent sets of 220v three phase
windings. Either winding is a complete set and capable of
being used on its own from a 220v supply.

This means that each slot contains two bunches of
conductors - one for each set - series connected for 440v,
parallel connected for 220v. Because the bunches of
coductors are closely coupled, one leg can behave as an auto
transformer.

It's also possible to use the distribution that
you've assumed - a 4 pole distribution of the windings
which is arranged for 220v supply to one pair of poles and
220v to the remaining pair. The two pairs are series
connected for 440v or parallel for 220v operation.

With this arrangement the two sets of conductors
are widely separated and too poorly coupled to act as an
effective autotransformer.

An off the shelf 4 pole motor cannot be
reconnected to make it behave a 2 two pole motor because the
winding distribution is wrong - each phase is designed to
produce four field maxima 90 deg apart. Efficient 2 pole
operation needs two maxima 180 deg apart. The best that
reconnection of a standard motor an do is to produce a
distorted two pole field that has dip at the all important
180 deg point. Two speed motors have a different winding
distribution to overcome this problem.

Jim