Here we list a small range of legacy self-synchronising signalling *transmitter" and low-torque indicating (receiving/resolver) devices comonally known as SELSYNS and otherwise known as AC-Motor-Transformers and Resolvers,

https://www.azosensors.com/article.aspx?ArticleID=319

Selsyns were comonally used during the 2nd WW as remote servo-indication (at a distance) between avionic and navigation systems and predicitive controlor indicator systems where no significant power was needed or consumed. In post WWW2 times they became poporor with radio Hams as indicators for beam-rotators.