Re: Servo disk motors for rotary spark gaps

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Scott Hanson by way of Terry Fritz <teslalist-at-qwest-dot-net>" <huil888-at-surfside-dot-net>

Ken -

You may be missing my point. These motors look interesting because they are
very well made and relatively inexpensive.

Right now, many of these DC servo motors (some brand new in sealed
packaging) are available on the surplus market. While they originally cost
$600 or more, I have picked up several for less than $20. With large
diameter shafts and bearings, and zero shaft endplay, they can support large
disks and allow good control of electrode spacing.

They should make excellent motors as-is for variable-speed rotary gaps,
which are probably best suited for DC coils.

No Tesla coil rotary gap need to start and stop nearly instantaneously. This
motor characteristic was noted as a warning because these motors will
accelerate so quickly that they will destroy a simple set-screw connection
to a rotary disk hub if the motor is just "hit" with full voltage to turn it
on, instead of being ramped up to speed with a variac.

For the motors that have integral optical encoders, I agree that even a
simple control circuit ought to be able to lock the motor into a speed
synchronous with the AC sine wave, give phase control, and also allow synch
operation at multiple speeds (1800, 3600, and 7200 RPM).

Regards,
Scott Hanson


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, June 03, 2003 6:12 AM
Subject: Re: Servo disk motors for rotary spark gaps


 > Original poster: "Crow Leader by way of Terry Fritz <teslalist-at-qwest-dot-net>"
<tesla-at-lists.symmetric-dot-net>
 >
 > So basically they are overgrown basket wound motors, like Maxon makes. I
 > still don't see a reason to pick one of these for a spark gap over any
other
 > sort of motor, unless you have these laying around. A spark gap is not a
 > legacy tape drive or check processing equipment and does not need instant
 > stop/start abilities.
 >
 > I've tried for fun to mimic an 1800 rpm synchronous AC motor with a DC
 > motor, PWM drive, radio shack potentiometer and regulated power supply.
The
 > AC motor was replaced with the DC one and spark gap left alone otherwise.
 > Surprisingly, with no feedback other than my eye, it was almost boringly
 > easy to maintain 1800 RPM dead on like the AC motor. Things would drift
 > after about 10 seconds or so, clearly even the junkiest servo system with
 > even a one if not a few counts per revolution sensor would be sufficient
to
 > control a DC motor in fake sync mode.
 >
 > KEN
 >
 > ----- Original Message -----
 > From: "Tesla list" <tesla-at-pupman-dot-com>
 > To: <tesla-at-pupman-dot-com>
 > Sent: Monday, June 02, 2003 7:34 AM
 > Subject: Servo disk motors for rotary spark gaps
 >
 >
 >  > Original poster: "Scott Hanson by way of Terry Fritz
 > <teslalist-at-qwest-dot-net>" <huil888-at-surfside-dot-net>
 >  >
 >  > Ken -
 >  >
 >  > How long does the first revolution take, from a dead stop? Literally
 >  > milliseconds, depending on the size of the motor. Because the armature
 >  > consists only of a very thin disk of copper conductors, with absolutely
no
 >  > iron core or laminations of any sort, the armature mass is a tiny
fraction
 >  > of the mass of any conventional AC or DC motor. In fact, the motor
shaft
 >  > itself can have more mass than the rest of the armature.
 >  >
 >  > Also, because of the lack of discrete iron poles in the armature, these
 >  > motors have almost no "cogging" at low speed. The lack of iron means no
 >  > inductive spikes during commutation, and no arcing or burning of the
brush
 >  > and commutator surfaces. These motors require only very small brushes
and
 >  > commutator surfaces. A 1/2 HP motor of this type has tiny brushes, and
 > will
 >  > run with hydraulic smoothness at only a few RPM with only 2 volts
applied.
 >  > During full acceleration they can draw 50A or more.
 >  >
 >  > Lots of these motors are now available on the surplus market, and many
of
 >  > them are equipped with high resolution optical encoders for precise
speed
 >  > control.
 >  >
 >  > These encoder-equipped motors could be used with an external feedback
 >  > control circuit as an "electronically controlled" synchronous rotary
spark
 >  > gap motor, as opposed to a modified (salient pole) induction motor that
 >  > achieves its synchronism internally via "electromagnetic control".
 >  >
 >  > Regards,
 >  > Scott Hanson
 >  >
 >  > ----- Original Message -----
 >  > From: "Tesla list" <tesla-at-pupman-dot-com>
 >  > To: <tesla-at-pupman-dot-com>
 >  > Sent: Sunday, June 01, 2003 3:43 PM
 >  > Subject: Re: Need a variable-speed 2hp or greater AC motor for serious
gap
 >  > drive
 >  >
 >  >
 >  >  > Original poster: "Crow Leader by way of Terry Fritz
 > <teslalist-at-qwest-dot-net>"
 >  > <tesla-at-lists.symmetric-dot-net>
 >  >  >
 >  >  >  > Original poster: "Scott Hanson by way of Terry Fritz
 >  >  > <teslalist-at-qwest-dot-net>" <huil888-at-surfside-dot-net>
 >  >  >
 >  >  > [cut]
 >  >  >
 >  >  >  > Also, there are large number of high-performance, ironless-core
DC
 >  > servo
 >  >  >  > motors (Servo Disk type) on the surplus market now (ex-computer
tape
 >  > drive
 >  >  >  > or large disk drive) that can accelerate from a dead stop to over
 > 5,000
 >  >  > RPM
 >  >  >  > in less than 1 revolution. This type of acceleration can destroy
 >  >  > couplings,
 >  >  >
 >  >  > How long does this first revolution take?
 >  >  >
 >  >  > KEN
 >  >  >
 >  >  >
 >  >
 >  >