SYnc motor mod's and Variacs
From: Gary Lau 29-Jan-1998 1532 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: Thursday, January 29, 1998 3:03 PM
Subject: SYnc motor mod's and Variacs
>> 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
>> Gary Lau
>If you are starting with only 1/8th HP motor and then grind the 4 flats you
>might not have enough power to run the RSG. If you are thinking of another
>motor in the future we have some nice already synchro 1800 RPM units for
>$30 apiece. You dont have to modify them and they run excellent with a 7
>inch rotor and 4 electrodes.
My modified 1/8 HP motor has far more torque than I need. With the rotor
attached, I can pinch the disk to increase the load and it still starts and
sync's right up, and the sync phase is solid and stable while I pinch it.
I wish I had known of your offer sooner, I'd spent way too long agonizing
over what to do for a motor!
>>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
>> Gary Lau
I should have added that this problem occurs regardless of whether or not
there is a disk load on the motor.
>> 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?
>I guess it would be close. I assume you kept your input voltage
>very low during the test? Without an installed and tuned secondary,
>the voltage can rise higher than usual and maybe harm the caps.
>Also in general, if you're using a resonant charging sized cap, it may
>see quite a high voltage even when the secondary is installed and
>tuned. In my 12kV, 30ma neon powered TC, I obtained about 32kV
>peak on my caps. So the caps need to be adequately rated voltage-
>Can you attach a long plastic pole to the gap, and use it to adjust
>the phase during coil operation, or rig up some sort of gear-motor
>to provide remote phase adjustment during operation?
In my eagerness to get my rig into operation, I was hoping to avoid
having to construct this sort of mechanical contrivance since it would
only be needed initially, but it would appear that there's no other way.
>From: Wysock, William C. [SMTP:Wysock-at-courier8.aero-dot-org]
>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.
The motor has 3 wires leaving it and an external 6 MFD cap but no
centrifical switch. I'm not sure what that makes it. Definately says
1/8 HP though. Changing the cap from 6 to 3 MFD increases the running
current slightly and changes the phase. Measured running current w/ 6 MFD
is 2.08 A -at-115VAC.
>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
The motor always sync's right up, no hunting. I've heard that sound on a
smaller unit I modified and probably removed too much rotor.
I managed to perform a test run of the complete rig last night. I turned
on the sync motor, then slowly cranked up the variac to my NST. The gaps
started firing, more and more, then less and less, then not at all, all
while increasing the variac! Flashbacks of the times I toasted my cap
and NST ran through my head. Damn the torpedoes, I still increased my
variac, and the gaps started firing, more and more. Huh, must have had a
bad spot on my variac? Now I read the rest of Bill Wysock's reply:
>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.
I think I see what's going on: Varying the NST's variac adds a variable
inductance in series with the NST primary and this changes the phase of
the charging cap. Thus, if I adjust the sync motor housing at 100%
variac, that setting is not valid at less than 100%. In fact, it
appeared that at one point on the variac, I was completely OUT of phase,
and resulted in near zero cap voltage at each gap presentation and no
firings, not a healthy situation for my cap or NST!
It would appear thet unless I can devise a PLL servo for my motor, sensing
the (now variable) phase of my cap, that the use of a variac could be
hazardous to my cap and NST. Do others using sync RSG's use variacs, and
if so, do you have to re-tune for phase at each variac setting?
Waltham, MA USA