Re: good results and sync gapsl

From: 	FutureT-at-aol-dot-com[SMTP:FutureT-at-aol-dot-com]
Sent: 	Saturday, August 09, 1997 2:34 AM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: good results and sync gapsl

In a message dated 97-08-08 13:24:54 EDT, you write:

<< snip>
> I have also noted that the sparkgap quenching needs greater airflow at
> higher freqs. I am presently using two squirel cage fans (out of a
> microwave oven) one blowing and one sucking air through the gap and this
> works well at 15kV 60Ma but I intend to try a rotary sync gap as soon as
> I find a decent sync motor.  

> I have a DC sync motor from a large hard disk drive. Has anyone on the
> list successfully used one of these in a rotary gap?

> Cheers,
> Peter E.


In order to use a DC sync motor, you would have to build a speed
and phase servo circuit using speed and phase feedback to control
the motor.  I was thinking of trying this...the advantage is you could
control the phase of the sync-gap electronically rather than needing
a mechanical phase adjustment device...but it's still probably easier
to just use an AC sync-motor.  (I don't know if these hard drives have 
a a built-in servo?)

BTW, you can modify a regular induction motor to create a sync-motor
by grinding two flats onto the armature of a 3450 rpm motor, or 4 flats
onto a 1740 rpm motor.  (if you have 50Hz AC, these rpms will
be different...2950, and 1450 rpm, I suppose).  The flats can be ground
using a milling machine, or by drilling numerous holes into the area to
be removed, and then filing (use a large file) the excess away.  It's a
lot of work if you do it by hand...but it can be done.  The two flats 
should be parallel, and of equal size to keep the motor balanced.
You'll have to use a larger motor than normal because the modification
causes the motor to lose some of its power.  I am able to use a 1/10HP
motor because I'm using a small, light rotor, but if you're using a heavier
rotor, you may need a 1HP motor.   The flats should be 1/2 as wide as
the armature dia for two flats, 1/3rd the dia of armature for 4 flats, if I 
remember correctly (I have this written down somewhere from when
I did my experiments).  Be sure not to cut 4 flats onto a 3600 rpm 
motor...you'll lose too much power...and the depth of cut will be very

Coiling for today and tomorrow,

John Freau