Hello Estelle, thanks for asking Galco.
A Sync Transformer is something we are used to being used in a DC drive or AC REGEN drive. Basically any circuit that has to connect directly to the incoming power lines and control power into or out of a controller will require these type of transformers. They provide isolated timing signals that allow control circuits to precisely time pulses to the power devices being controlled. So the Control circuits are synchronized to the line power that the power devices are directly connected too.