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Main Menu Session: # 3233 (Effective Energy Laboratory Ideas)

A FREQUENCY AND VOLTAGE CONTROLLABLE POWER SOURCE FOR LABORATORY USE

K. A. Nigim G. T. Heydt Senior Member Fellow University of Waterloo Arizona State University Electrical and computer Engineering Dept. Tempe, AZ, USA knigim@ece.uwaterloo.ca heydt@asu.edu

Abstract A technique is presented to generate a low voltage, frequency controllable power source for educational laboratory use. The proposed power supply has the merits of generating a sinusoidal AC low voltage waveform with controllable frequency and voltage from a wound rotor induction machine driven by a DC motor. The generator does not require a variable external excitation power source or any automatic voltage regulator. The presented configuration has the inherent capability of protecting the load under test from short circuit. The variable frequency generator is proposed as an educational laboratory power source for experiments in energy conversion, control, and electric power quality.

Keywords: Power engineering education; power supply; induction generator; frequency changer; pulse width modulation; power electronics.

1. INTRODUCTION

Since induction machines are not equipped with field windings, the AC excitation current for an induction generator must be supplied externally. The process, known as self- excitation, entails excitation current that is supplied from a source that is continually replenished by the machine itself. This current magnetizes the reactive stator windings of the machine, providing a lagging current in these windings. This needs to be balanced by a source of leading reactive current. The leading reactive var current must be adjustable in order to be able to supply the demand at varying levels of load. Figure (1) lists different possible configurations of self-excited induction machine generating systems both for squirrel cage and wound rotor machines. The external source commonly employed for self-excitation of induction generators is a capacitor bank across the machine terminals with large enough capacitance to provide the necessary leading current. The value of the effective capacitance must be controlled so that it will increase or decrease according to the level and type of load applied to the generator. Many researchers have focused on the induction machine as a generator; the self-excitation phenomenon giving terminal voltage as function of the rotor speed and load is presented in [1- 4]. For a wound rotor machine, results are presented in references [5,6]. It is well known that the generated terminal voltage and frequency decrease with increasing load. The main control problem that has been addressed for the induction generator terminal voltage relates to the control concepts that guarantee smoothness of the voltage magnitude as the external vars are controlled. These methods ensure smooth voltage variation by incorporating a continuously controlled leading var source. Advantages and disadvantages of four alternative designs depicted in Figure (1) are listed in Table (1). A typical system (see Figure (1B)) involves the connection of a fully controlled thyristor bridge converter loaded with a large inductance. The converter is

“Proceedings of the 2002 American Society for Engineering Education Annual Conference & Exposition Copyright 2002, American Society for Engineering Education” Main Menu

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Heydt, G., & Nigim, K. (2002, June), A Frequency And Voltage Controllable Power Source For Laboratory Use Paper presented at 2002 Annual Conference, Montreal, Canada. 10.18260/1-2--11081