The Controlled Slip Static Inverter Drive

The ability to use the versatile characteristics of the semiconductor inverter to achieve optimized control of a squirrel cage induction motor is demonstrated. The technique has been made possible during the past few years due to the development of fast, powerful, and efficient inverters. These, in turn, are the direct result of advances in semiconductor technology, which have produced thyristors with improved qualities and the advent of integrated electronics. If the slip of an induction motor is constrained and controlled to values below breakdown, high efficiency, high power factors, and moderate currents result in performance comparable to that of a dc machine. General expressions defining torque and involving the quantities of slip and excitation are easily derived. Excitation can be expressed in terms of volts per cycle, current, or flux. Torque can be controlled by adjusting slip or excitation or both in combination. The ability to control slip and escitation precisely and accurately depends on the inverter which is used. The pulse width modulated (PWM) inverter is an extremely effective motor controller accomplishing voltage and frequency adjustment in a single circuit. It is a fast, linear device; its response is virtually instantaneous. As a power amplifier it is comparable to the duel converter of dc systems, and its speed of response makes it applicable to virtually any feedback loop. In considering the mating of the motor and inverter, several principal factors are involved in the optimization of the system.