On 11 Jun 2005 10:26:54 -0700, wrote:

I find the rotor has 48 conductive bars. I could have this wrong.

By substituting the rotor, could I increase the efficiency of this
motor? That might be much easier than rewinding. I have removed and
replaced rotors from shafts before. It seems any rotor with the right
number of bars and less than or equal to the linear size, with the
right or smaller bore, would do to make *some* improvement. I'm
guessing I'd want a rotor with fewer bars, but I don't know the
physics.

Can any reader show me the math relating 60Hz, 36 poles, 400 rpm, 48
bars, and 225 rpm?

I don't get it. I take 7200 / 32 = 225. To get it, I need to go to
Falls Church's library, the Mary Riley Stiles library. There are not
one but two editions of Audel's there.

Doug

The precise number of bars in the rotor makes little difference to
motor efficiency and bears no particular relation to motor rated or
synchronous speed. The only thing that matters is the inductance to
resistance ratio of the effective shorted turn that surrounds the
rotor iron circuit i.e. the L/R time constant.

If there a lot of iron but not much copper/aluminium, the resistive
component is high and this is generally called a high resistance
rotor. This gives good starting torque but lousy efficiency because
of the extra power dissipated in the rotor. This results in full load
speed well below synchronous speed and it is this speed difference
that is responsible for the extra rotor dissipation.

Unless starting torque is a big problem, motors are
designed with the maximum possible amout of copper/aluminium in the
rotor. Only the minimum necesary amount iron is retained to enable it
to handle the stator induced flux density. This results in very low
slip frequency and high efficiency.

While fitting a low resistance rotor to your machine may
give some efficiency improvement it is unlikely to be enough to get it
to operate as useful induction generator. The main problem which will
still remain is the extremely high leakage inductance which is
inherent in the geometry of a small 36 pole stator.

Jim