The question was directed to single phase motor which it is surmised to be of induction type. Depending on the load characteristics, your motor could stall or overheat (or simply not start) if the frequency is not the one it was designed for at a given load. Three phase motor is not exactly the same as a single phase motor in speed-torque characteristics. Single phase motors are meticulously designed to have start-up torque through shaded pole or capacitor, etc. They can handle certain loads like fans since the start-up load is low and increases with speed. Thus, one important change is the start-up torque if you apply 50 Hz or 60 Hz to a motor designed for 60 Hz or 50 Hz respectively. In the worst case, the motor may not even start running. Even if it runs, the slip will also increase meaning the efficiency will drop, losses go up and the motor overheats and may stall and burn. The importance of proportionately adjusting Voltage per Hz to avoid saturation but in reality this also ensures your maximum torque is maintained. My comments were based on assuming the motor was optimally designed for one frequency. It is possible to have a general purpose motor designed that could handle both frequency and a wider voltage range by compromising on efficiency and maximum torque. The size and cost would be higher than an optimal motor.