INTRODUCTION

An inverter is used to generate an AC voltage from a DC source. Typically, a transformer is employed to step the voltage up significantly above the magnitude of the DC voltage. The DC source polarity is alternately switched across the transformer primary, which produces an alternating voltage at the transformer secondary, where the load is connected.

If the AC load is purely resistive, the actual waveform shape produced at the secondary is not critical. Examples of purely resistive AC loads are incandescent lights and heating elements. If, for example, a 120-VAC coffee pot is to be powered from an automobile starting battery, the inverter must generate the necessary RMS voltage that results in the average power required by the appliance. Since the desired effect is heat, the waveform shape across the load is not important; it need not even be AC.

For some loads, however, the waveform shape is essential. An AC induction motor, for example, is designed specifically to operate from a sinusoidal source. Motor torque variations and additional heat losses result if the phase current waveforms deviate from the sinusoidal shape.

The focus of this chapter is inverters that employ transformers to generate high-voltage AC from low-voltage DC. The presentation begins with a review of the fundamentals of the AC transformer and concludes with inverter simulation techniques in MATLAB.

AC TRANSFORMERS

The operation of the AC transformer is based on Michael Faraday's law of induction: a changing current in a coil induces a current in a...