RE: Synchronous motors for rotary gaps (fwd)
[ This may be a re-post, but I found it in a directory that it shouldn't be.
-- Chip ]
---------- Forwarded message ----------
Date: Fri, 03 May 96 09:49:00 PDT
From: Richard Hull <RICHARDH-at-whitlock-dot-com>
To: bin <bin-at-poodle.pupman-dot-com>
Subject: RE: Synchronous motors for rotary gaps
I haven't heard this mentioned in this conversation so I'll throw it out.
Bill Wysock in California has long championed the Synchronous rotary
system. He has used "salient pole motors" This are synchronous motors
which also lock to the same point each time they are started. That is the
same point on the shaft will align with a given point on the case every
For a long time, this was a deep dark secret. This motors are made up from
common, garden variety squirrel cage induction motors (el cheapos).
Two/four flats are milled or plained off of the armature rotor in a fashion
which does not disturb the copper conducting bars within the armature. In
this manner, anyone can make a nice salient pole synchronous motor.
Recently, TCBOR good buddy and long time correspondent, John Freau of the
Tesla coil Builders of New Jersey has done a huge amount of research into
these motors and the synchronous system in general. He has fashioned a
number of motors with crude hand tools and a drill press. (no milling
machine). All of them work well. All of his finished gaps work well, too.
Again, what one hand gives the other takes away. It seems that if you take
a 1/4 hp induction motor and turn it into a salient pole motor, you will be
lucky to retain 1/2 to 1/3rd its original horsepower rating!!! Thus your
old 1/4hp might now be a 1/10 hp unit, but synchronous. This is not bad if
you start with a 1/2 hp motor, but don't experiment with milling flats on a
large unit like this!! If you screw up, you now have a worhless motor! If
you decide to piddle with salinet pole motors, get a fist full of surplus
1/8-1/10 hp units and dink around until you have a feel for the work.
John has shown no real advantage exists in the synchronous system over say a
normal fully controlled variable speed rotary. This is based on actual
tests with working coil systems, where a like non-synchronous gap is swapped
out after the synchronous gap is run. The synchronous system has a very
distinct spark pattern and can be recognized instantly at sight by the
I am not a booster for synchronous systems, but it sounds like a few of you
might be. I hope this info is of some value. John Freau like myself, tends
to use the non-synchronous gap most often in his medium sized magnifier
Richard Hull, TCBOR
Subject: Synchronous motors for rotary gaps
Date: Thursday, May 02, 1996 8:42PM
>From sgreiner-at-wwnet-dot-comThu May 2 20:37:17 1996
Date: Thu, 02 May 1996 19:41:43 -0700
From: Skip Greiner <sgreiner-at-wwnet-dot-com>
To: tesla list <tesla-at-pupman-dot-com>
Subject: Synchronous motors for rotary gaps
Jim Fosse and the group
All synchronous motors run at synchronous speed which is some multiple of
the line frequency. They come in various flavors with the best and most
expensive being hysteresis synchronous. In 1800 and 3600 rpm versions
their motor shafts will always run in exactly the same position with
respect to the motor stator or frame. Therefore once the rotor is firmly
attached to the motor shaft, you know that when the motor runs, the
moving contacts on the rotor will always be in the same position. If the
motor is mounted in such a way that you can rotate the motor in the
mount, you can rotate the position of the rotating contacts with respect
to the stationary contacts while the motor is running. I have built a
varible phase strobe light to set contacts easily. The strobe runs at 60
hz and is triggered from the line frequency. The trigger is varible from
0 to 180 degrees of the line frequency. The unit has been calibrated with
a scope so I can set the firing point to any specific point in the line
sine wave. Before I had the strobe I had to play with the firing point
until the discharge max'd out. Now I set it in advance. With this device
it is a simple matter to calibrate a motor in its mount by marking a
couple of labels which are stuck on the motor and mounting clamp.
Hysteresis synchronous motors are very expensive. Reliance built a 1/2 hp
unit for me for $180.00.
There are other flavors out there. I watch all the magazine ads and
fliers for motors listed with rpms of 1800 or 3600. Then I call to make
sure that they really are as advertised.
THe last motors I got were from Surplus Center, 1015 O Street, Po box
82209, Lincoln, NE 68501, TEl: 402-474-4055. The motors are part number
10-1052. They appear to be shaded pole motors of some sort but they lock
in at 1800 rpm and the rotor always takes exactly the same position. The
motors cost 12 or 13 dollars each. THese motors are about 1/15 hp and
they will drive an 7" rotor which is what I use. I saw an ad a couple of
months ago for some 1800 rpm motors which were advertised as hysteresis
synchronous for about 50 dollars. If you or anyone is interested I will
try to find the flier.