The breakdown torque of a synchronous motor is proportional to the square of the applied voltage and also varies with the load angle. This means if you double the voltage, the breakdown torque will approximately quadruple, and as the load increases, the angle between the rotor and stator poles (the load angle) increases, which is directly related to the developed torque.     

• Voltage (V): Breakdown torque is directly proportional to the square of the applied voltage. If the voltage is doubled, the torque will be about four times greater. 
• Load Angle (alpha): The torque developed by a synchronous motor is a function of the load angle, which is the phase difference between the stator and rotor magnetic fields. As the load increases, the rotor falls back, increasing the load angle and the torque until the breakdown torque is reached.