Transient negative torques in induction motors due to rapid reconnection of the supply

The first part of the paper deals with a combined experimental and analytical study on a 5 hp laboratory set. It establishes the way in which the transient negative-torque peak ↑R− varies with the residual voltage on the machine terminals at the time of reconnecting the supply, and the phase angle that could exist between this voltage and the supply voltage which is being reconnected. An approximate expression for the peak negative torque is developed, which helps to explain why the characteristics showing how ↑R− varies diverge significantly from what would be expected by simple reasoning. A 100hp motor is then considered, and the variation of ↑R− with the time delay td in reconnecting is established. The effects of the load at which the motor is operating at the instant of supply disconnection and of external inertia are examined. Finally, for a range of machines from 100 to 2700hp, operating at various loads and with various external inertias, patterns of ↑R−/td are computed and reduced to a single normalised curve, from which the largest possible value of ↑R− for any machine, under any operating conditions, can quickly be assessed for any selected delay in reclosing. The application of this normalised curve is illustrated for three motors not already used, and the results are compared with values obtained from complete-transient solutions.