Sync motor modification

From:  Wysock, William C. [SMTP:Wysock-at-courier8.aero-dot-org]
Sent:  Wednesday, January 28, 1998 9:24 AM
To:  Tesla List
Cc:  ttr
Subject:  RE: Sync motor modification


Some answers and a question about your post.

Question:  Is the 1/8 h.p. motor you bought, capacitor start/induction
run?  If so, it shouldn't be stalling on you regardless of the rotor's
position.  A 3.5" dia. rotor sounds like higher then 1/8 h.p.!  If this
motor is an induction start/induction run, then I can understand the
problem you are seeing.  This latter type has much lower initial
starting torque then the former.  You are right on when you stay
conservative in the amount of material you mill off for each flat
on the rotor.  Too little material removed, no sync operation.  Too
much removed, no torque at sync speed and the field windings
will heat too much and the motor will draw much more then the
full load amp rating.  You can always remove more material, if
the motor (with the load of the rotary disc and electrodes) seems
to "hunt" (try to get up to sync speed, and then slip) You'll see
a surging in the line amps the motor draws when it "slips" and you'll
hear the motor "groaning".  When just enough material has been
removed, with the rotary disc/electrodes attached as a mechanical
load, the motor should produce a steady sound and a constant
line current (typically near the max. or slightly above) the full load
amps. spec. for your motor.  With a capacitor start/induction run
motor, the initial start-up current can be 4 or 5 times higher then the
full load amps rating, which is why you will benefit from using an
autotransformer to bring the motor up to speed gradually.  Some
coilers have built nichrome wire resistances with toggle switches
to "stair-step" ramp up the current to their motor.  This also works

As for the correct index "spot" on the rotation of the motor housing,
the point you have observed as "best" with just the Tesla primary
coil, may shift slightly, when the influence of the secondary L and C
are introduced.  An additional "tuning" element I use in my designs
(where non-saturable core H-V power transformers are used,) is
the addition of a variable series inductance (using a low voltage
high current autotransformer for this purpose,) connected in series
with the output of an autotransformer (used for voltage control,)
feeding the H-V trans.  With a sync gap that has a particular "dwell
index" setting, I might need say, 20% of the total series variac
inductance for best operation of the T.C.  If I were to rotate that
RSG index point say, 10 degrees, I can still find the same optimum
point of coil operation by adjusting the series variac to a different
point (typically by about  5% of the previous setting.)  Note that
when using this arrangement, and raising the voltage control
variac up to the point where the RSG just fires in a steady mode,
then at that point, adjusting the current control (series connected)
variac plus or minus of the initial point, will cause the gap to not
fire at all; and if you monitor the input amps to the transformer,
you'll see a sharp rise in current drawn.  This is an out-of-phase
current condition and represents reactive power that is not going
forward through the H-V transformer into the primary tank load.

Bill Wysock
Tesla Technology Research
From: Tesla List
To: Tesla List
Subject: Sync motor modification
Date: Wednesday, January 28, 1998 6:25AM

From:  Gary Lau  28-Jan-1998 0803 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent:  Wednesday, January 28, 1998 7:36 AM
To:  tesla-at-pupman-dot-com
Subject:  Sync motor modification

>> I have both acheived good outputs from neon systems using sync gaps,
>> without any tranny failures, but only time will tell how effectively the
>> gaps "protect" the trannies.  Perhaps if more folks start using sync gaps
>> on their neon systems, a long term consensus (on tranny longevity)
>> will emerge.

>Funny you should mention that.. I am in the process of seeking out
>the directions to modify a motor to sync operation for just that reason.
>[More power for less..] I figure if I smoke the tranny now, well, it
>gave me a good result, and I can't really learn too much more from it.

I'm just about ready to test out my sync RSG with a 15KV/60mA NST.  For a
motor, I bought a 1/8 HP 1725 RPM motor for $4.95 from H&R Surplus.  This
unit has a 3.5" dia rotor.  I milled four flats onto it, each being
1.00".  While the flats should have been 1.30" to each cover 1/8 of the
circumference, I thought it safer to remove less and see if it works.  It
sync's right up so I'll stick with the 1" flats.  Only problem is, there
are four angular positions, each 90 deg apart, from which the motor will
stall if started there.  Perhaps if the RSG works out well, I'll buy
another motor and remove more material and see if that resolves the

A question on adjusting the phase of a sync RSG.  It would be best if I
could do this without attempting the subjective test of when I have the
longest spark, turning off power each time.  I did a dry run last night
without my secondary installed, and as I rotated the motor case, saw a
clear point of maximum gap arcing.  Is this a valid indicator of the
correct rotor phase for when the TC is completely assembled?

Gary Lau
Waltham, MA USA