# Modification to induction motor for Synchronos operation

```Original poster: "Charles Hobson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <charles.a.hobson-at-btinternet-dot-com>

Hello all

I just acquired a 0.37kW (~1/2 HP) induction motor labelled: at 1370 RPM and
input voltage of 240V  at 50Hz. I have read numerous posts on modifying such
motors for synchronous operation and am about to grind flats on the rotor for
this purpose. However, some of the posts weren't clear as they referred to
other posts on the subject which I don't have.

1. The motor is evidently 4 pole as the rpm is 1370, just under the 1500 rpm
synchronous speed on 50Hz. Does that mean that 4 flats 90 degrees apart on the
rotor are required?

2. The motor has 2 sets of windings with one of them in series with a
capacitor. I recalled that motors in the old days had a centrifugal switches
which disconnected the winding (starting winding) with the capacitor in its
circuit. This one doesn't have such a switch. I don't know if that is of any
significance in my situation.

3. I have a copy of the text of a post containing a method for determining flat
width dimensions to be ground, but sadly I don't have the originator's details.
In essence, the information given was a formula for the flat width with rotor
radius as the variable. The information source was U. N. O. who ever that was.
There were two options: one with  a 38 degree arc for a motor with non dead
pole windings and one with a 40 degree arc for motors with dead poles. My width
calculations are 8.84mm and 9.78mm respectively. Are these figures reasonable?
Also, how does one recognise a non dead pole winding from a dead pole motor

4. I understand that European motors such as mine tend to have rotors with
smaller radii (mine has a radius of 28.6mm (1.125") than U. S. motors of
equivalent HP. I was wondering whether the U. N. O. formula will still be
applicable as I don't understand why flats on the rotors make induction motors
synchronous.

5. Are the flats ground along the entire length of the rotor (my rotor is
~125mm long)?

Any help in the above would be most helpful and appreciated.

Chuck/England

```