I've converted about 8 or more induction motors to make them synchronous. All worked well. In each case I just used a file. In a couple of cases I drilled a bunch of holes to the correct depth at the area of the required flats just so less filing would be needed. I used a drill press for this, and set the depth screw so I could not drill too deep. The needed depth is quite shallow. For a 1750 rpm motor I make the flats 1/4 the diameter of the armature. If the armature (which really is a squirrel cage rotor), is 3" in diameter, I made the flats 3/4" wide. To locate the four flats in the right positions, I measured carefully using a steel flexible tape measure. Then I scribed lines showing the desired width of the flats. Then I filed between the lines until the depth reached the lines. When the job seemed to be done, I again measured the width of each flat, and also the spaces between flats to make sure everything was accurate. For 1800 rpm motors I didn't detect any vibration. For 3600 rpm motors there was sometimes a little vibration but nothing objectionable. For 3600 rpm motor I made the flat widths about 1/2 the rotor diameter. I think for a 1-7/8" dia armature (rotor), I made the flats 1" in width, but it can be a little less than that. The widths of the flats is not super critical as long as they are all the same and 90 (or 180) degrees apart depending on if it's an 1800 or 3600 target speed. Some people like to use a dead pole method to judge the width of the flats, but I never bothered with that. The largest motor I converted was 1/4 HP. The more metal you remove, the hotter the motor will run. Since Tesla coils are usually not run that long, I did not have any problems with overheating. Of course the load on the motor will affect the heating too. Motors will of course lose quite some torque when they are modified. Usually after modification, I've have to raise the input voltage to somewhere between 55 and 75 volts before the motor would lock into sync mode for a 120 volt motor with no load.
John
>
> ----- Original Message -----
> From: "Tedd Dillard" <tedd.dillard@xxxxxxxxx>
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Sent: Tuesday, February 16, 2021 8:43:03 AM
> Subject: Re: [TCML] sync motor
>
> Are there any comments on machining flats on the rotor of an induction
> motor. I understand that has been done with some success. Most of the
> issues I have seen are about the machining.
> Teddy
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