Re: Synchronous rotary gaps. - motors
Sent: Sunday, September 14, 1997 2:10 PM
Subject: Re: Synchronous rotary gaps. - motors
On Sun, 14 Sep 1997 10:02:31 -0500 Tesla List <tesla-at-pupman-dot-com> writes:
>From: William Noble[SMTP:William_B_Noble-at-classic.msn-dot-com]
>Sent: Sunday, September 14, 1997 12:44 AM
>To: Tesla List
>Subject: RE: Synchronous rotary gaps. - motors
>I had in mind the drive motors from the old removable pack hard disks
>- I find
>these things (like the old pertek units) at swap meets - I scrapped
>one a few
>years ago (the value of scrap aluminum was greater than I paid for it)
>unit spins a pair of 14 inch platters at so it ought to handle a gap,
>- maybe I'll try it out if I ever get home long enough.
>How can you tell FOR SURE that a motor is synchronous, versus a shaded
>motor???? is there some physical attribute that is evident, or
First off lets define synchronous a/c induction motors as in use for
rotary gaps with 60hz line frequency . B.T.W. Nickola Tesla discovered
the poly phase induction motor.
A sync. a/c induction motors speed is dependent on the input voltage"
frequency". If 60hz is the line freq. as in the U.S.A. then a two pole
motor will run at "exactly" 3600 R.P.M and a 4 pole motor will run 1800
R.P.M. This is for single phase a/c that is. A "non-sync." induction
motor operates at a "slip speed" usually rated around 3450 R.P.M for a
2-pole and 1725 for a 4-pole. There are other speeds depending on the
amount of poles. These speeds will vary according to motor load.
"Large" sync. a/c motors are not self starting and must be brought close
to sync. speed by another force, then power is supplied to the rotating
field winding and the motor operates at line freq. These are usually
3-phase motors and can have very high H.P!
In smaller sync. motors, several H.P. and below, a different system
locks the armature to an exact speed. Some use permanent magnets or iron
bars in the armature to lock it in speed with the rotating magnetic
field. These motors are self starting due to a ring of conductors which
are shorted on the ends (squirrel cage).
Anything that causes at least two dominant poles on the rotor
(armature) will follow the rotating mag. field developed in the stator
(outer) windings. Another thing that will bring it into sync. is
machining flats on an induction motors rotor. If two flats are made at
180 deg. apart, then it will have a north and south pole causing it to
follow the freq. dependent rotating mag. field (line freq.) or 3600 R.P.M
A sync. motor will state an R.P.M. 3600 or 1800 which are the most used
with single phase. Other R.P.M.s are possible, but scarce.
Checking a motor to see if it is indeed sync. is not difficult. All
you need is a hub, disk, or whatever to mount on the motors shaft, a dark
marker, and fluorescent lighting.
Make a mark on the item mounted on the motor shaft and power up the
motor at its rated voltage. With the 120Hz pulsating fluorescent lamp
illuminating the spinning item, the mark should appear stationary, that
is not rotating. If you see a moving shadow, then it is not in sync. with
the line freq. You can usually just look at the motor shaft in the
fluorescent light and see if it's locked in sync.
>Another idea - I have a pile (3 or 4) of 60 HZ synchros - I know that
>synchronously if you get them spinning. of course these have brushes
>to drive the armature. Also, it owuld seem that a stepper motor could
>good thing - it would be easy to step it in synch with 60HZ.
I am not aware of any "small" a/c induction sync. motors having brushes
in them. Repulsion start induction motors have shorting brushes in them
but are rare. Are you sure they are not a universal series motor?
Keeping a stepper motor in sync with the a/c line frequency would require
a solid state driver with an input from the 60 Hz line to lock it in
sync. The solid state circuit would be in danger form the high voltages
around the rotary motor disk.>
>Sent: Saturday, September 13, 1997 6:59 AM
>Subject: Re: Synchronous rotary gaps.
>On Fri, 12 Sep 1997 23:57:12 -0500 Tesla List <tesla-at-pupman-dot-com>
>>From: William Noble[SMTP:William_B_Noble-at-classic.msn-dot-com]
>>Sent: Friday, September 12, 1997 2:04 PM
>>To: Tesla List
>>Subject: RE: Synchronous rotary gaps.
>>aren't the motors used on the old disk drives (the ones driven by AC)
>>synchronous?? and also old phonograph motors (remember phonographs -
>>things that turned at 78RPM??) - I suppose I can test this with a
>>or something if I really want to.
>Yes there are small sync. motors out there. When I stated that
>fractional H.P motors were hard to find I meant small motors which had
>enough power to run a rotary disk with electrodes. The small motors
>found in phonographs (some cheapies use shaded pole non-sync motors)
>reel to reel tape decks are very low in power and have gear reduction
>increase torque. If they were used to direct drive a rotary disk at
>1800 or 3600 R.P.M. I feel there would not be enough torque available.
>As for computer disk drives I did not think about them being
>maybe one of the old 8" drive motors might work, however once you
>loading electrodes on a disk, the added weight and air resistance take
>toll on small motors.
>>Sent: Thursday, September 11, 1997 9:11 PM
>>Subject: Re: Synchronous rotary gaps.
>>On Wed, 10 Sep 1997 22:30:19 -0500 Tesla List <tesla-at-pupman-dot-com>
>>>From: Dale F. Pfaffle[SMTP:pfaffle-at-tele-net-dot-net]
>>>Sent: Wednesday, September 10, 1997 8:40 PM
>>>To: tesla list
>>>Subject: Synchronous rotary gaps.
>>>These synchronous rotary gaps, I am assuming (and I know what that
>>>are done with sync. motors. Is this something coilers are buying or
>>>that are being modified?
>>Sync. motors in the fractional H.P. range are very rare, even in the
>>integral H.P. range. Most sync. motors in use for rotary spark gaps
>>custom made, either by the coiler or a machine shop. You can modify
>>induction motor to run sync. by milling, filing or grinding flats on
>>armature. An example is given by John Freau on one of Richard Hull's
>>(TCBOR) video tapes that he sells somewhere in web land. I believe
>>I also sell modified sync. motors which are approx. 1/25 H.P. 3600
>>R.P.M. have ball bearings with a 5/16" x 1" shaft. I have been
>>them for $22.00 plus shipping.
>>This motor on my prototype will spin a 7" dia. phenolic disk 1/4"
>>with 2-electrodes running 120 breaks per second with power to spare.
>>If you are interested in one, e-mail me at:
>> teslaman-at-aol-dot-com or teslaman-at-juno-dot-com