Re: How to modify a motor for sync, was Design Problem

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 7/7/01 7:28:33 PM Eastern Daylight Time, tesla-at-pupman-dot-com 
writes:

> Original poster: "CJ Moore by way of Terry Fritz <twftesla-at-qwest-dot-net>" <
> wizard1234-at-home-dot-com>
>  
>  I already bought a motor - 1725 RPM 1/2 HP AC. I also bought 2 NST's, 9 kv 
> 60
>  ma each. I want to build a RSG for my coil and don't know how to make a
>  synchronous one. People have said that an asynchronous gap will destroy my
>  NST's with or without a protection circuit. What am I to do? Can I make a
>  synchronous gap with my motor? If I can is there any website with good
>  information on it, or anyone that can tell me how to do it without any
>  advanced/complicated tools. 
>   
>  CJ Moore
>  60871583

CJ,

You can easily modify your motor with a large simple hand file, to
make it synchronous.
For your motor, you need to file 4 flats equally spaced around
the squirrel cage rotor in the motor.  Open the motor, remove the
shaft with the rotor, and carefully measure the rotor diameter.
It's probably about 3.3" diameter.  Make each flat about 1/4 or
1/3 the total diameter.  For a 3.3" dia rotor, you could make
the flats 3/4" or 7/8" wide for instance.  Wider flats will make
the motor run hotter and draw more current, so there's no
point in making the flats wider than needed.   There is a dead
pole concept for modifying these motors, but many small 
motors don't have dead poles, so the concept should be ignored
in those cases.  I don't think it helps anyway.  Measure
carefully so the 4 flats are all teh same width and and equally
spaced around the rotor.  You can put the rotor into a vise to
hold it while you're measuring and filing it.  

After the motor is modified, you can test it by observing a line
drawn onto a sheet of cardboard and taped to the shaft.  Use
fluorescent light to observe the line.  The line will form an
unmoving pattern when the motor is locked.  Raise the voltage
gradually via a variac, and you'll first see the pattern spinning,
then slow, and lock as the voltage is increased.  The sound
will change too and become very steady.   Just before locking
the motor will make an unsteady hunting sound.

John Freau