Motor system energy efficiency in the nylon industry: A comparison of PWM and square wave inverters

Escalating energy prices and increasing environmental concerns have renewed interest in improving motor system efficiency during normal operating conditions and when the motor is fed from a polluted power system. In this study a laboratory study and a mathematical motor model are used to evaluate and compare energy consumption and performance of a 3 phase induction motor fed by a square wave and a sinusoidal width modulated (PWM) inverter. Motor harmonic voltages and currents, input and output power, torque, voltages, currents and motor efficiency were measured in the laboratory and calculated using the mathematical model. The model accounts for the skin effect in the rotor bars to predict the dependency of motor impedance on frequency. Motor losses and efficiency at 60, 40, and 30 Hz frequencies were evaluated and compared at different motor's loads for each inverter and potential energy savings are discussed. It is noted that inverter harmonics negatively impacted motor losses and hence motor efficiency. It is also noted that motor efficiency decreased with decreasing inverter frequency and motor load. The decrease in motor efficiency is more significant when using a square wave inverter compared to a PWM inverter. The laboratory and the calculated results using the electrical model show close agreement.