Original poster: "Scott Hanson" <huil888@xxxxxxxxxxxx> Phil -I don't believe that your Shinano Tokki motor is either a salient-pole or a hysteresis-synchronous motor; it is just a plain old synchronous AC motor.
There seems to be a lot of confusion as to what type of motor is needed to drive a SRSG (Synchronous Rotary Spark Gap) for an AC-charged Tesla coil. Many people assume that a "synchronous" motor is all that's required to obtain a "synchronous" rotary spark gap. This is absolutely NOT true. There are two completely different types of "synchronism" involved here.
First, an AC "synchronous" motor runs at a speed that is controlled by the frequency of the AC line current and the number of magnetic poles built into the motor's rotor/stator structure. In this context, "synchronous" means that when driven by a fixed frequency AC supply, the motor runs at a fixed speed. This is the primary characteristic of a synchronous AC motor. The motor can start and run with any of the rotor poles mechanically aligned with any of the stator poles.
The "synchronism" that is meant in the context of a Synchronous Rotary Spark Gap is a COMPLETELY DIFFERENT thing. This is an electro-mechanical synchronism that links the AC sine wave to a particular angular position of the motor's rotor relative to its stator. This allows you to set the "timing" of the rotating and stationary electrodes relative to some specific position on the AC sine wave of the HV supply that is charging the Tesla coil's tank capacitor.
The objective is to "time" the system so the SRSG electrodes align and fire at the instant of peak voltage on the tank capacitor. This requires some known, fixed, and repeatable relationship between the spark gap electrodes and the AC sine wave. Every time the motor is started, this electrode position/sine wave timing relationship must remain identical. Aside from using some type of servo motor with a rotary position encoder and feedback control circuit, the only was to easily obtain this electrode position/sine wave timing relationship is to use a salient-pole AC motor. The only true salient-pole motors I have ever seen, or heard about, were the motors used in '40's to '70's vintage Teletype machines. I'm not familiar enough with the operation of a Teletype machine to explain exactly how the electro-mechanical synchronism of the motor was actually used, but I can assure you that the motors operate as described above. I was fortunate enough to purchase a box of Teletype motors, and I've verified the electro-mechanical synchronism using my GenRad 1538-A Strobotach with the external trigger circuit linked to the AC line.
Fortunately, almost any single-phase synchronous capacitor-start AC motor can be modified to provide salient-pole operation by machining flats on the rotor. This process has been covered in great detail many times in the TCML archives. Just remember that after modification the motor will lose a significant percentage of its original starting and running torque, so begin with a motor having a larger HP rating than would normally be necessary.
Let me close this by stating that I have observed "pseudo-salient pole operation" with some standard AC synchronous motors under some conditions. I think this may be influenced by residual magnetism in the motor's rotor. If the motor's AC supply current is interrupted (motor shut off) while the current is at a peak, there may be more residual magnetism induced in the rotor vs if the motor is shut off at low current near the AC sine wave zero-crossing. The residual magnetism can be verified by removing the rotor and checking it with a Gaussmeter (or a compass). Repeat the power-on/power-off/measure residual magnetism cycle ten or twenty times and you will find several of the cycles leave the rotor with a stronger residual magnetism than the other cycles.
The bottom line is, if you are going to use your RSG with a NST-powered Tesla coil, you put your NST at risk unless your convert your motor to salient-pole operation and create a true SRSG. Otherwise, you'll end up with an ARSG (asynchronous RSG). See Richie Burnett's exceptional website ( www.richieburnett.co.uk ) for some great technical information on the differences between SRSG and ASRG operation, and the dangers of missed-presentations and voltage ring-up possible with an ASRG.
Regards, Scott Hanson ----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx> To: <tesla@xxxxxxxxxx> Sent: Wednesday, January 25, 2006 5:03 PM Subject: Re: Help with making SRSG phase control work?
Original poster: FIFTYGUY@xxxxxxxIn a message dated 1/25/06 2:24:51 PM Eastern Standard Time, tesla@xxxxxxxxxx writes:>Scott H. sent me a message inquiring if I really had a salient pole >Oriental Motor.Scott was kind enough to send a lengthy e-mail to me as well. I hope he'll understand if we continue the discussion in public, as I feel it's a perfectly relevant topic.>He said the company tells him that they have never >made any salient pole motors.Is "Oriental" a manufacturer, importer, distributor, or generic term for these brands?> The data sheet actually says "Reaction Synchronous Motor". What exactly is a "reaction" synchronous motor, anyhow? >So maybe it is a hysteresis motor after all? I >clearly have salient pole written in my notebook - maybe it was >advertised that way from C & H Sales. I can't absolutely testify >that it starts in the same place everytime.I'm not taking mine apart, as I've already got the whole SRSG built around it and aligned. I'm not too worried about how repeatable the start position is, so long as I can adjust it or get it right without too many restarts.But I'm sure my motor is synchronous, for the following reasons:1. It is completely identical to the one Terry Blake used. It looks to me like it works pretty consistently for Mr. Blake, who invented the prop gap I'm using.2. The label says "1500/1800 RPM", and the frequency is listed as "50/60 HZ". All the synchronous motors I've seen list an even 1500/1800 RPM.3. I did the salient-pole conversion to a 3/4hp motor. Using the stroboscope and a (voltage control) variac, I could see (and hear) the 3/4hp salient-pole motor drop in and out of synch with the steady 1800 rpm Oriental motor.I haven't had the time yet to try everybody's suggestions, but thanks to all for clarifying that the motor run cap is left in place, despite the addition of the phase control cap. I'll try with some smaller phase cap values, and see what happens.-Phil LaBudde