Some of the forum users probably remember the systems based on selsyn, currently the selsyn links have been practically completely replaced by modern solutions.
Imagine that you are given the task of designing a system that transmits information about the tilt of one of the machines to a remote control room. This can be, for example, the position of a wind direction indicator, ship's rudder, or the opening angle of a nuclear shelter door. ;)
Depending on the characteristics of the machine, an encoder (preferably absolute) or even a potentiometer can be used to determine the position of the element. The information from the sensor can be sent by wire as an analog signal or as a digital signal via a copper, fiber optic or radio link.
The indicator can be an LCD display, or even an analog indicator, generally anything that clearly communicates information to the operator.
You will probably give many more possible implementations of the layout.
Suppose that the project has been successfully implemented, the client's appetite grows and this time you are given the task of building a system that will allow you to move a part of the machine by a given angle depending on the settings on the control panel. This time we control a throttle that allows us to dose liquid to some machine.
The part that downloads data about settings can be an encoder, a potentiometer, as well as a numerical panel. Any transmission, as long as it is reliable. On the other side of the transmission channel, a servo motor can be used to position the axis in a predetermined position depending on the signal received. In some cases, you may decide to go with a driver and stepper motor or DC gear motor and axis tracking system. In less demanding applications, you will probably be tempted by a modeling servo.
The solutions can be multiplied for quite a long time.
Now suppose that, under strange circumstances, you go back 50 years and are given the task of doing the two above circuits. It turns out that there is a problem with buying modern automation systems After describing the problem, it turns out that the solution to both the task of transferring information about the angular position of the shaft, as well as remotely rotating the shaft (work performed) by a given angle is the selsyn.
What are selsins?
After reading the text so far, you know exactly what tasks can be performed by selsins, but how are they built?
Selsyn has a design similar to a motor with a three-phase stator winding and a single-phase rotor winding. Selsyn has three terminals connected to the star-connected stator winding and two terminals connected to the rotor winding. The current to the rotor is led through rings and brushes. This is how you can imagine the rotor:
Now we create a three-phase star-connected winding around the rotor:
The coils are shifted by an angle of 120 °.
We also lead out the terminals that supply the current to the rotor through the brushes to the housing:
We can schematically present our selsyn as follows:
And now the most interesting thing is how to combine selsins so that one of them reproduces the movements of the other? We need five wires. We connect the rotor windings in parallel (two wires). We connect each of the corresponding phases (three wires).
In order for the devices to work, we supply alternating current to the windings of the rotors:
The selsyn supply voltage was different, they could be 220V, 110V, 45V and many others, the frequency did not necessarily have to be 50Hz, the voltage with a frequency of 400Hz was often used.
As it is easy to imagine - more jointly connected units could work in the selsyn link, as a result it was possible to build a system with many indicators. In practical implementations there were also windings and systems (RC) damping vibrations. Selsyn could also have a single-phase stator and a three-phase rotor winding. There have been solutions with a three-phase stator and rotor. Most of you have probably noticed that selsyn can be used as a mechanical phase shifter. The Selsyn could be equipped with a gearbox, and could also be used to build a simple follow control with a separate drive motor (something similar to the current modeling servo).
On the YT website you can find many films showing the operation of selsyn, an example of the found operation presentation:
@TechEkspert The efficiency / performance of AC power sources at 400Hz is much higher (please do not correct me, I am not an energy specialist, I am giving the reason why most of the captured selsins are at the voltage of = 400Hz). On airplanes, every kilogram counts, and besides, at 400Hz we have a much greater accuracy of selsyn ... I can't tell you more, but if you know Russian you will learn everything you want (pages ended with .ru and .ua).
The principle of operation and the construction of the selsyn do not differ much from the principle of operation and construction of the resolver, at least from the setting selector side. Resolvers are still used in industrial robots today.
I dealt with them for 15 years at the Teltomat asphalt mixing plant. The transmitters are mounted on scales: flour and aggregate, the receiver is located in the control cabin where the reed switches are set, a mini magnet is attached to the pointer, which, when hovering over the reed switch, switches the fractions on the detector. Briefly..........
I remember Selsyny from my technical college classes, which I graduated 20 years ago. They had such a strange name, but I was intrigued by the fact that with the help of machines similar to motors, you can transmit the angular position from a "distance" and, as a result, smoothly control a device or indicator.