EFFICIENCY ANALYSIS OF INDIRECT VECTOR CONTROLLED THREE PHASE INDUCTION MOTOR DRIVE

ABSTRACT: This paper analyses the overall system level efficiency of an indirect vector controlled induction motor drive. Both the induction motor losses and inverter losses are considered. A 5.4-hp induction motor drive is simulated to analyse the efficiency under different operating conditions of the drive. The variation of efficiency under load change and speed change is observed. Simulation results show that efficiency increases as the operation of induction motor drive shifts from unrated to rated conditions. At light loads, the iron losses in the induction motor are significant. The efficiency of the motor can be increased by keeping the flux level below the rated value. Also the influence of hysteresis band on the switching losses of the inverter is analysed. The results suggest that, for efficient operation of the drive, an optimum combination of the various control variables are required. Keywords: Vector control, Hysteresis band, Indirect field oriented control, Switching losses, Simulink I. INTRODUCTION It is estimated that electric machines consume more than 50% of the world electric energy generated. Economic saving and reduction of environmental pollution are the two factors that highlights the importance of analyzing the efficiency in electric drives. The induction motors are widely used in the electrical drives and are responsible for most of the energy consumed by electric motors. In recent years the control of high-performance induction motor drives for general industry applications and production automation has received widespread research interests. Many schemes have been proposed for the control of induction motor drives, among which the field oriented control, or vector control, has been accepted as one of the most effective methods. Vector control offers a number of benefits including speed control over a wide range, precise speed regulation, fast dynamic response, and operation above base speed. In the field oriented control of an induction motor drive system, current control technology plays the most important role in current-controlled pulse width modulation (PWM) inverters, which are widely applied in high performance dynamic drives system. Among the various current control techniques, considering easy implementation, the conventional hysteresis PWM current control method is a popular one. The advantages of this scheme are its simplicity, good accuracy, good response, and high robustness. The major drawback of this scheme is that high switching frequency can happen at lower hysteresis band so the switching loss of the inverter will be increased. In addition, the current error is not strictly limited. The current ripple can reach twice the hysteresis band. This paper analyses the overall system level efficiency of a vector controlled induction motor drive which employs a hysteresis current controller for PWM generation. The losses in the induction motor and the switching losses in the inverter circuit of the drive are considered. Induction motors have a high efficiency at rated speed and torque. However, the operation of the machine with rated flux at light loads, iron losses increase dramatically, reducing considerably the efficiency. The hysteresis band of the current controller is the main factor deciding the switching losses in the inverter circuit. It also affects the current ripple and torque ripple of the induction motor drive