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Author
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Topic: Selsyn motor question
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Bill Purdy
Expert Film Handler
Posts: 139
From: Seattle, WA
Registered: Oct 1999
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posted 10-20-2000 11:11 AM
What fun, I love interlock systems and Selsyn ones particularly. I would like to expand on Gordon's correct input, but first, a bit of tutoring. "Selsyn" is (or was) a trademarked name of GE's, but like other such names has become generic. Strictly speaking, a Selsyn is not a motor. It is not intended to run by itself and will not do so except under strange circumstances. It really is a funky transformer. Envision three transformer windings mechanically arranged in a triangle just as you would draw a delta connected 3 phase transformer. Now envision another single winding mounted such that it can be rotated within the delta windings. Apply an AC Voltage to the single winding and start measuring the induced voltages in the three outside windings and they will all be different and what's more, as you rotate the single one, the voltages in the delta will change as the magnetic coupling changes. We will now call the single winding connections R1 and R2 as they are the "rotor", and the delta windings are S1, S2, and S3 as they are the stator windings. Next we tie all of the like marked leads of one Selsyn to one or more other Selsyns and as all of the rotors are connected to the same excitation voltage they will create the same poles within body of the unit. The varying magnetic forces induced in the stator windings will now cause the rotors to align themselves so that they are all in equilibrium and this is what makes the whole thing work. Turn one of them and the others will do their best to follow. There is a lot more to know about these great gadgets, but now you have the basics. Clamp a pair down to your work bench and play with them. If you reverse the rotor connections the armature will flip 180 degrees. Reverse any pair of the stator leads and one will rotate in the opposite direction. What you want to avoid is operating them in the region where they might become a natural synchronous motor. In the case of the GEs this would be around 3600 rpm as the rotors are two pole devices. In your case, you are probably only going to interlock two like projectors and all you care about is matching two normal drive motors which are probably 1764 rpm or thereabouts. One further note, until you become more versed in their care and feeding, don't interconnect dissimilar units as they do not all have the same secondary voltages and smoke might result. ------------------
Bill Purdy
Component Engineering
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Bill Purdy
Expert Film Handler
Posts: 139
From: Seattle, WA
Registered: Oct 1999
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posted 10-23-2000 10:34 AM
Barry,
What you need to do is to arrange the ratios between the motor and the Selsyn such that the Selsyn makes one revolution per frame (1440 rpm) and then once you get your gear belts on in sync, the Selsyns will always lock up in the right position.That ratio can be a tough one depending upon the actual speed (and subsequent gearing) of your projector's drive motor. If you are lucky enough to be using the 1800 rpm synchronous motors so common today, it is a ratio of 1.25 to 1. Set up your pulleys such that the Selsyn rotates one revolution for each 1.25 on the motor. (Another way to figure it is that the Selsyn should make 4 revolutions for every five of the motor.) If, on the other hand, you are using older motors such as found on SH-100's and RCA's, it turns out that the bizarre ratio is 49 to 40. One does not just run out and buy 49 tooth pulleys. The old National Theatre Supply used to have kits available during the heyday of 3D. Perhaps you can find some in somebody's collection of old stuff. If you do, you will need the matching belts as they were not one of the standard pitches. Failing that, for whatever pulleys you use, set it up such that one of them does not have flanges and then you can slide the belt off and on. Then, you thread up, align your shutters, energize the Selsyns and slide the belt on. ------------------
Bill Purdy
Component Engineering
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