The torque produced by a motor at full speed is given in its rating data, it is normal to give a motor’s locked rotor and breakdown torque as a ratio of its full load/speed torque.
The locked rotor or starting torque is the torque produced at the instant that power is supplied to the motor when it is at a standstill.
The actual ratio varies with the motor design, rating and nominal speed; the average for motors with a nominal speed of 1500rpm is 2.6x the full load torque.
Since starting methods other than direct on line and secondary resistance reduce the voltage across the motor windings during the starting period the starting torque is also reduced. The reduction in starting torque is a function of the voltage reduction. For a star delta starter where the voltage in each winding is reduced by √3 the torque is reduced to ⅓ of the direct on line locked rotor torque. For an auto transformer on a 50% tap the torque is reduced to 25% of direct on line locked rotor torque. For a rotor resistance starter the maximum starting torque is when the motor reactance equals the rotor resistance, the torque equals the direct on line locked rotor torque. For soft start and variable frequency starters the starting torque will be determined by the maximum current setting of the control unit, the torque/current curve for soft start and variable frequency units will need to be analysed to ensure adequate starting torque for a particular machine. Since the method of starting is normally selected to ensure compliance with the volt drop limitations of the electricity supply network soft start and variable frequency controllers which have a variable starting torque settings should have a current limiting primary control loop to ensure compliance with the volt drop limitations, which must take precedence over the starting torque control loop.