Self-Limiting Control of Induction Machines
Veysel (Tutku) Buyukdegirmenci with adviser P. T. KreinPast work has addressed the peak duty ratings of electric machines and control strategies to attain peak power until the maximum allowed temperature at the hot spot is reached. This research investigates control strategies to attain peak power after the maximum allowed temperature is reached. Conventional protection circuits turn off the drive or protect the motor after peak operation by reducing the current to rated. This protects the drive from overheating and avoids outages; however, the windings cool quickly because internal temperatures are still below rated, and then the entire machine slowly starts to heat and reach thermal steady state. These approaches imply that the machine thermal capacity is not utilized and overloading is permissible even after reaching maximum temperature.
A control law is derived that supplies the windings with the instantaneous thermal energy they lose and forces them into thermal steady state at the maximum allowed temperature. Experimental results presented in Fig. 4 for a limit temperature of 30°C show that the developed self-limiting controller keeps the winding temperatures at the limit and utilizes the machine’s full thermal capacity. The permissible currents for this operation are presented in Fig. 5 for various initial end-space temperatures. The results indicate that if the machine is internally cold as it reaches maximum allowed temperature at the windings, it is permissible to overload it until the internal temperatures reach their rated values while the hot spot temperature is kept at the maximum allowed temperature and the machine is not overheated. For the test case, the permissible overload duration of half an hour is identified as the time it takes the machine end space to reach its rated temperature.
This research is supported by the Grainger Center for Electric Machinery and Electromechanics and Rolls Royce, account number 2012-05196.