This is not about “Selsun” Blue Shampoo, it is about “Selsyns.” The electric kind. So if you are looking for hair care and styling advice I am NOT your guy. “Selsyn” is one of the names bestowed upon a group of motors that also include such terms as “Synchro” …”servo” …or “rotary transformer” Selsyn is a fusing of the words “self” and “synchronizing.”
Prior to digital encoders and microprocessor controls, remote indication and positioning tasks were accomplished by mechanical coupling…via pulleys and shafts, teleflex cables, gears, rods, etc…..until the era of the Panama Canal with its need of control and indication over a great expanse of area. By this time, the Selsyn had been perfected. A selsyn is essentially a star or delta wound 3 phase motor with an additional separate winding in the rotor used to apply an alternating current to excite the system. Instead of applying 3 phase power to make it spin, a single phase AC current is applied to this other winding. The effect is 3 separate voltages induced in each of the 3 phase windings. These are 120 degrees out of phase with each other and of magnitudes that are dependent upon their angular position with respect to the rotor’s winding as this angle affects the coupling between them and thereby the voltage induced within the winding.
If two such devices are connected together as shown in the diagram, they will remain stationary. If you rotate the shaft of one, however, the voltage and phase angles induced at each winding will be altered. The other unit, in response, will attempt to equalize and reach a corresponding balance…..in so doing, it too, will rotate a corresponding amount as the first. This enables remotely monitoring the position of a valve, antenna, or other item……even the big guns on a battleship! In addition, variations of this can be used to actually effect the change remotely rather than simply indicate the position. This technology was initially applied to water level sensors, lock, and valve position indicators of the Panama Canal and then quickly spread into other uses such as repeating compass indicators on board large vessels (allowing a master gyro compass to provide indications at several stations on board) antenna positioning for radio and radar antenna arrays, aircraft navigation aids such as the ADF units, and a variety of industrial indication and control functions. It is much easier to string some multiple conductor wire over a long and circuitous route than to try to accomplish the same by a hard mechanical connection.
The selsyn units shown in the video are old WWII vintage USN units of heavy and impressive bronze construction. They are also somewhat powerful and can apply pretty good torque. I am uncertain of their original application….possibly part of a ship’s fire control system –the mechanical fire control computers on the battleships needed to have a lot of data about the ship’s speed and direction, those of the target (fed from radar or optical sights) and the position of the turrets….such data could easily be tapped from the various sensor points on the ship and fed below decks to the computer’s input dials with the use of selsyns like these.
Why the “electric dominatrix?” ….well…..one unit is considered the master…..and the other is the slave. One other point of note, obvious in the video, is that with all such relationships the roles are reversible.