Is this motor synchronous? (fwd)
From: Tesla List Owner[SMTP:listown-at-pupman-dot-com]
Sent: Wednesday, December 24, 1997 5:08 PM
Subject: RE: Is this motor synchronous? (fwd)
---------- Forwarded message ----------
Date: Wed, 24 Dec 1997 07:35:27 -0700
From: "Wysock, William C." <Wysock-at-courier8.aero-dot-org>
To: tesla-request <tesla-request-at-pupman-dot-com>
Cc: ttr <ttr-at-ocws-dot-com>
Subject: RE: Is this motor synchronous?
Your problem with the motor is that while it is synchronous, it is not
salient pole synchronous. The latter type always locks in only
where the "dominant poles" are located. I introduced a technique
of modifying any a.c. motor to become salient pole synchronous
back in 1975, for rotary spark gaps used in Tesla Coils, although this
technique is not really new in motor technology. In a 1,725 r.p.m.
(typical) non-synchronous motor, there are four separate field or
stator windings. The rotating a.c. magnetic field couples to the
shunt constructed armature links (seen as an eddie current winding)
and the motor spins at a rate which is determined by the number of
stator segments, the number of field windings, and the number of
armature links. In the case of a 1725 r.p.m. motor, if you grind (the
hard way) or fly-cut on a vertical end mill with an index chuck (the
easy way,) four flats across the cross-section of the armature, then
you have an armature with a certain amount of core material that
is still in close magnetic coupling to the field windings, and four
sections (where the material was removed,) that do not couple to
the stator's rotating magnetic field. In this manner, you now have a
motor that runs synchronous as a multiple of the a.c. frequency (60 Hz,)
and the number of stator magnetic fields times the four (now dominant)
poles of the armature, thus 1,800 r.p.m. "Salient pole" is synonymous
for "dominant pole." In the case of a 3,450 r.p.m. motor, there are just
two sets of field windings, and the armature will need to have two
180 degree opposite "flats" machined across the length of the armature.
In this case, you "bump" up the speed to 3,600 r.p.m. For the 1,800 r.p.m.
armature, you need four "flats" on 90 degree centers. In either case,
the amount of material removed from the armature is critical to a
certain degree. Remove too little material, and the motor will still be
non-synchronous, and will not be dominant pole. Remove too much
material, and you will reduce the amount of available torque to the
point that virtually any load on the armature shaft, will cause the motor
to "hunt." This phenomena is characterized by the field windings drawing
up to as much as four times (or more) line current. The motor heats
rapidly and will be prone to burning the field windings. By removing the
correct amount of material, the motor will only draw its normal full load
current, but the amount of torque will be reduced from the nameplate
rating. For an RSG with up to 12 tungsten electrodes on a 10 inch
diameter disc, a 1/3 h.p. motor rated at 1,725 r.p.m. may be successfully
run up to 1,800 r.p.m. For the same rotary disc with a 3,450 r.p.m. motor,
it is best to use at least a 1/2 (or larger) h.p. motor for conservative
at 3,600 r.p.m.
Bodine as well as several other manufacturers make synchronous motors
that are not salient pole. You have one of these now. You did not say
what h.p. rating your motor is; but at a rating of only 0.85 amp, it sounds
like somewhere in the 1/16 h.p. range; not good for your RSG. As soon
as you have some amount of rotating mass (rotary electrodes) the
torque required to spin your disc at 1,800 r.p.m. will exceed that which is
available from your motor. If you had a 1/4 h.p. sync motor, you could
make successful use of it for your RSG. You would simply have to have
a sufficient number of rotary electrodes to match the number of possible
phase lock points the motor is capable of. A much better solution is to
acquire the correct h.p. size non-synchronous motor (in your case 1,725
r.p.m.) and modify it to become salient pole 1,800 r.p.m. synchronous.
Be sure that you specify a capacitor start/induction run type of motor, and
make sure it is a ball bearing type. Other types such as induction start/
induction run or capacitor start/capacitor run are not suitable for this
modification/application. It is best to have the modification to the
done by a machinist in a machine shop; you need to make sure that
the resultant modified armature is still dynamically balanced and that the
angles of the flats (either two at 180 degrees or four at 90 degrees,) is
Tesla Technology Research
To: 'Tesla List'
Subject: Is this motor synchronous?
Date: Tuesday, December 23, 1997 8:06PM
From: Gary Lau 23-Dec-1997 1018[SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: Tuesday, December 23, 1997 8:29 AM
Subject: Is this motor synchronous?
I'm attempting to build a synchronous RSG. I ordered a motor from the H&R
Surplus catalog for $7.50, a Fugi motor, 115VAC, .85A, 1800 RPM. To
verify that it was a synchronous motor, I attached a 9.5" load disk with
a small bar magnet in the center, and positioned a pickup coil as shown,
connected to a scope sync'ed to LINE.
It spun right up to speed, the period of the waveform on the scope was
33.3 msec and was rock-solid in phase. If I pinch the disk to increase
the torque, the phase shifts slightly, but bounces right back to where it
was when released. However, if I pinch to apply above some critical
torque, the phase shifts and stays shifted. Same thing if I briefly
switch off power. At first I thought that the phase was shifting in some
even sub-multiple of one revolution, but find that I can shift any
arbitrary amount from 0-360 degrees. I have verified that the
disk/magnet cannot shift relative to the shaft, and cannot believe that
the shaft is shifting relative to the motor rotor.
Is it possible for a motor to be synchronous but at an arbitrary and
variable phase? Is this motor unsuitable for a sync RSG, or is my
analysis technique flawed?
Waltham, MA USA