Hello Marvin and MMD, Years ago, in a former life, I worked in the
field of professional audio and I serviced Ampex tape recorders at
local radio stations. One station had an Ampex 300 whose hysteresis
synchronous drive motor was run from AC power created by a variable
frequency audio oscillator and a large high power vacuum tube amplifier.
This allowed the normally 7.5 and 15 inch per second tape transport
to operate at 15 and 30 inches per second. To accomplish this, the
oscillator was set to output a 120 Hz sine wave to the amplifier. The
amplifier had an output transformer that would provide 120 volt AC at
whatever frequency it was fed, and the synchronous motor ran at a
Today, this can be done easily as high powered solid state amplifiers
capable of driving a small AC induction motor are readily and
inexpensively available. Likewise, a variable frequency sine wave
oscillator can be easily come by. This approach is safe to use on
non-"inverter rated" motors because its output is pure sine wave and
will not cause insulation piercing voltage spikes that chopper
inverters can produce.
Wow, I can't believe I remembered that! Hope it helps.