Synchronous motors run at a speed governed by the supply frequency. Therefore to change the speed, you must change the supply frequency.
How far do you want to change the speed, and are you stuck with the synchrounous motor? It is possible to change the speed, but is there an easier solution such as change the motor type?
For such a small load, you should be able to do run the motor from a a small amplifier and a step-up transformer, the amplifier being driven from a variable frequency oscillator circuit.
The oscillator with variable frequency output may be the solution. However, many oscillators are designed for higher frequency ranges than 0Hz to 60Hz. There may be a manufacturer who already produces these low watt synchronous motor drives.
well my idea is to run the synchronous motor at two speeds.we have both 12v/120V 60 Hz ,2/3 W syn motors.
We are thinking of rotating this motor at 6rpm during the first on and after one more on should rotate at 2 rpm synchronosly with the other units connected.The motor is to be controlled using PIC.
Shiju