All this misinformation about induction motors (synchronous or not) and
50/60Hz supplies. :-)
A basic lesson and some truths about running such motors designed for
60Hz 120v on 50Hz 100v supplies and vice versa, assuming the speed
differences are either of little to no importance or else taken care of
by a gearing adjustment.
If you're planning on using a 120v 60Hz induction motor on 50Hz supply,
you simply need to reduce the voltage in the same ratio. The torque will
remain the same although you will obviously lose 20% power capability
(and an increased risk of on load stalling if the motor rating was
marginal to begin with).
Going the other way, you'll need use a 120 to 288 volt 60Hz step up
transformer to power a 240v 50Hz motor if you wish to maintain the torque
output at the higher speed or else accept a lower maximum torque output
if merely doubling up the 120v to 240v - not always advisable if the
torque rating of the motor is marginal to begin with.
For record players and tape decks that rely upon the mains frequency to
calibrate the speed of their induction motors (asynchronous as well as
synchronous - either type can be used), as well as the obvious pulley
change, there should also be a matching voltage change included.
However, since the motors are well over-rated for this application in
order to minimise speed variations, in the case of asynchronous motors
particularly, as well as to minimise slip in belt drives, such a luxury
feature may be omitted in the interests of 'economy of manufacture',
acceptable where the motor is designed to cope with a 50Hz supply
frequency and 240 volt supply which will allow it to run faster on a 220
to 240v 60Hz supply but will obviously still require a 120 to 240 volt
step up transformer if designed for both 120v and 240v regions.