See http://www.astromag.co.uk/inverter.html  This is a home-built
12-volt to AC adapter whose frequency and output voltage can be altered
to suit a motor.  It doesn't look like it would be too tough to build,
and the circuit could be hacked such that the 12v supply isn't needed.

It's for a telescope drive motor, so I'm not sure that the output
transistors (a pair of TIP41's) would be up to the job, but these could
be replaced with more robust parts.

Mark Kinsler