• The applied voltage (starting torque is proportional to the square of the applied voltage, T proportional to V^2).
• The rotor resistance (increasing rotor resistance increases the starting torque up to a maximum, and shifts the maximum-torque point towards standstill).
• The rotor and stator reactances (leakage reactance) - higher leakage reactance reduces the starting torque.
• The number of poles and the synchronous speed (torque depends on the air-gap power and synchronous speed).
• The rotor and stator winding resistances and the turns ratio.
• The supply frequency.
• The air-gap flux.
• Comparison with rated torque: the starting torque of a standard squirrel-cage motor is typically about 1.5 to 2 times the full-load (rated) torque. It is generally greater than the rated torque so that the motor can start the load, but not excessively high. For a slip-ring motor the starting torque can be increased up to the maximum torque by adding rotor resistance.

• (i) Synchronous speed Ns = 120 f / P = 120 x 50 / 4 = 1500 rev/min.
• (ii) Speed at 4% slip: N = Ns (1 - s) = 1500 x (1 - 0.04) = 1500 x 0.96 = 1440 rev/min.
• (iii) Rotor frequency when rotor speed is 600 rev/min:
• Slip s = (Ns - N)/Ns = (1500 - 600)/1500 = 900/1500 = 0.6.
• Rotor frequency fr = s x f = 0.6 x 50 = 30 Hz.

So synchronous speed = 1500 rev/min, rotor speed at 4% slip = 1440 rev/min, rotor frequency at 600 rev/min = 30 Hz.