"Small multi-phase synchronous motors may be started by ramping the drive frequency from zero to the final running frequency. The multi-phase drive signals are generated by electronic circuits, and will be square waves in all but the most demanding applications. Such motors are known as brushless DC motors. True synchronous motors are driven by sine waveforms"
So how do i know if a certain motor can be driven buy square wave or not?
The square wave has a large amount of harmonics which will produce currents and power loss. The power supply may not like the load either, it will be far from resistive.