Construction of an equivalent circuit for induction motors used with variable supply frequency, including inverter-fed applications.

Abstract

The following chapters give details of the construction of a new equivalent circuit for induction motors used in variable and modulated frequency supplies and one of its advantages for the determination of the additional losses in the machine due to harmonic frequencies. Chapter 1 includes the aim of the work and briefly explains the reason for obtaining such a model and its applications. In Chapter 2 the technique of constructing the new equivalent circuit is given. This is done as follows. With the aid of a digital computer and circle fit technique the frequency responses of the open and short circuit test results taken at various supply frequencies are used to find the values of the new equivalent circuit parameters. The new equivalent circuit takes care of the parameter changes arising from different supply frequencies and produces the correct input and output performance characteristics. The new equivalent circuit also produces the exact responses for mixed and modulated frequencies. In Chapter 3 the new equivalent circuit is analysed and performance equations are derived. This enables changes in the performance characteristics at different supply frequencies to be calculated directly from the performance equations. Using a digital computer the non-linear model of the induction motor is constructed from the test results for variable speed applications. The magnitude of the errors introduced in calculating the machine's performance characteristics are determined using the linear model in the non-linear region. Their magnitude becomes greater with the increasing value of the rotor speed or the supply frequency. An accurate torque speed plotting machine for extremely high speed induction machines is designed and constructed. Highly accurate results are achieved by sensing the rotor torque and speed without any physical contact with the rotor. In Chapter 4 the Fourier series analysis is used to analyse and derive the output harmonic contents of two popular types of inverter. The additional losses in the machine due to these harmonic frequencies of both inverter types are determined and compared. The new equivalent circuit is used to determine the magnitude of the errors induced in the calculation of these additional losses if the standard equivalent circuit is used.

Date of Award	1979
Original language	English
Awarding Institution	University of Bath