[Synchrodyne]

Some indirect evidence, perhaps: Hi Fi Year Book 1973 lists both the EMT 930st and 928, the latter appearing to be a newer model, as it was not listed in HFYB 1972, whereas the 930st was. Anyway, the 928 is described as having a three-phase synchronous motor fed by an electronic three-phase generator. The same description does not apply to the 930st, from which one might (circumstantially) infer that it did not have a synchronous motor.

Anyway, as Kalee20 has already said, 1450 rev/min and the speed brake pretty much outrule a synchronous motor. The use of induction motors with speed brakes (usually of the eddy current type) seems to have been reasonably common for “transcription” turntables in the 1960s; the Garrard 301 & 401, and Thorens TD124 were of this type. Thorens moved to an oscillator/amplifier-driven low-speed synchronous motor (16-pole) with the TD125 circa 1969.

Three-phase motors have higher starting torque than capacitor split-phase motors, which may have been a reason for EMT’s choice. In TV studios at least three phase supplies might be available as I think that some telecine units, for example, have or had three-phase drives, and Selsyn evidently works better with three-phase. Thus a three-phase motor could be used as such, or in a capacitor split-phase mode where only a single phase supply was available. Normally capacitor split-phase motors seem to have two-phase windings, but as the phase shift is not that exact, I think that the technique could be applied to pretty much any polyphase winding.

Cheers,