Induction motors are widely used in various industries as prime workhorses to produce rotational motions with high power. Generally, variable-speed drives for induction motors require fast speed and torque response regardless of load variations. The field oriented control is the most successful in meeting the above requirements. Due to advances in power electronics and microprocessors, variable-speed drives for induction motors using the field oriented control have been widely used in many applications such as AC servo, electric vehicle drive system, and so on.
Using the field oriented control, a highly coupled, nonlinear, multivariable induction motor can be simply controlled through linear independent decoupled control of torque and flux, similar to separately excited DC motors. High-performance torque control requires fast current response enough for the current regulator to track the reference current. However, due to limitations of voltage and current ratings on the inverter dc link, input voltage and current of an induction motor are limited accordingly. Hence, developed torque in the motor should be limited for safe operation under these input constraints.
The objective of variable-speed control system for higher productivity is to track the reference speed as fast as possible. Therefore, under the constraints of input voltage and current, a control scheme which yields the maximum-torque over the entire speed range can be usefully applicable to high-speed control of induction motors.
In this paper, a new maximum-torque control scheme for induction motors is suggested under practical constraints on voltage and current. Also, a new high-speed high-efficiency control algorithm for induction motors is suggested to obtain good dynamic performance as well as high-efficiency. Maximum-torque control algorithms are derived in two ways. One is the steady-state maximum-torque control algorithm and the other is the maximum-torque control algorithm using an approximated model which can be applicable even during the transient-state. Also, two high-speed high-efficiency control algorithms for induction motors are suggested using respective torque control algorithm. In order to prove the advantage of the suggested torque and speed control algorithms, various simulations are performed. Simulation studies show that the suggested torque and speed controllers have superior response characteristics. Finally, hardware and software structure for the practical implementation of the proposed scheme are discussed in detail.
