: Hi, I've been talking to a colleague who is trying to modify a squirrel cage
: single phase electric motor to syncronise, at 3000 rpm, with the 50Hz
: mains frequency of the supply by machining 2 flats on the rotor. I just
: can't see how this, based on motor theory, would work and even if it did I
: think the motor power and torque would be seriously compromised to the
: point that the motor would drive its self but that's about it. Can any
: one give me some technical details / theory about how the concept works?
: Many Thanks, Rustic.
This technique has been successfully used by many Tesla Coilers to convert non-synchronous squirrel-cage induction motors to synchronous operation. Two or four flats are milled or ground on the rotor to make it operate at 3000 or 1500 RPM (50 Hz line) or 3600 or 1800 RPM (60 Hz). Since induction motors are commonly available at little or no cost, the modification can be a cheap way to make a "poor man's" synchronous motor. The modified motors are typically then used to drive a synchronous rotary spark gap in a Tesla Coil. This permits the gap to consistently fire at one or more optimal phase angles versus incoming AC power. Some Tesla Coilers also add a variac and motor run capacitor to permit precise adjustment of motor shaft angle versus incoming line phasing "on the fly" - see the second article below.
The modification increases the effective air gap between rotor and stator, and it does indeed reduce the available torque. If too much material is removed, the motor loses too much torque to be useful. A modified motor runs hotter because of the reduced back EMF seen by the stator windings and the reduced stator self inductance.
Following is some more detailed information about the technique as well as some theory:
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