ABSTRACT
This work presents the influence of rotor Caged winding on the performance of a Transfer Field (TF) machine. The transfer field (TF) machine comprises two identical machines that are integrally wound and mechanically coupled together but with their pole axes 90 electrical degrees out of phase. This project investigates the influence of rotor caged winding on the performance of a transfer field machine using the analysis of the caged and cage-less rotor winding configurations. From the analyses, a modeled direct and quadrature axes equations for both rotor configurations are presented for dynamic simulation. Basic parameters and machine performance such as rotor speed against time, electromagnetic torque against time, main winding current against time, and auxiliary winding current against time on load conditions were observed. The simulations were carried out using Embedded MATLAB function. It is observed from the results that when the load torque was applied to the cage and cage- less rotor winding of the TF machine, it settled at 5Nm and 4Nm respectively after a little transient during build up. It was equally observed that the main winding current for both cage and cage- less rotor on load application at 7secs were 6A and 4A respectively. The results obtained show that cage rotor winding of a Transfer field machine has a better performance characteristics than the cage- less rotor winding more especially on load.
CHAPTER ONE INTRODUCTION
1.1 BACKGROUND OF THE STUDY:
In alternators which have dc fed rotating fields, the magnitude of the current that can be passed through the rotating windings is limited by the practical consideration imposed by the slip-ring/brush combination. In order to keep the current low and still set up the required magnetic field a large number of turns of coils are buried in the coil and the rotor heating could therefore set the limit to the available output.
In wound rotor induction motors, the provision of slip rings and brush adds considerably to the cost of fabrication. For many applications using high speed motors, or in environments that are potentially explosive, sliding contacts cannot be tolerated.
Various electromagnetic machines have been developed which although completely stator excited, exhibit characteristics similar to that of conventional ac machines that have one of the two windings, excitation and output windings on the rotor. In these machines coupling between the output and excitation windings is either directly through the agency of rotor saliency or indirectly through their mutual coupling with a third passive winding on the rotor.
The earliest example of the saliency effect machine is the inductor alternator [1, 2]. Its characteristics are similar to those of the conventional alternator. It has found application mainly as a single phase high frequency
alternator; however poly phase types have been built. As a motor, it can only run synchronously, it is therefore not self starting. Some self starting motors have been described in literature [3,4,5]. The motors described in reference [3] and [4] like those of references [1] and [2] have basically two independent sets stator windings of different pole numbers p1 and p2 respectively and a multiple pole rotor. The p1 poles of mmf due to one set of windings will interact with the
2pr poles of the fundamental component of air gap permeance variation due to the pr poles of the rotor, to produce air gap flux distribution with 2pr ± p1 poles. If the second winding has 2pr + p1 or 2pr – p1 poles, energy conversion can be achieved at a suitable frequency. If the secondary current is obtained by induction from the first winding, the mode of operation will be asynchronous and the machine will be self starting. For normal synchronous operation, one of the windings will be excited with direct current.
The transfer field machine described in reference [5] and [6] differs from these other machines in that both sets of windings have the same number of poles which is the same number of poles as the rotor. Thus there is no restriction as to the choice of pole number of one winding in relation to the other winding pole number. The smallest number of pole pair that is unity can thus be used. Higher speeds can therefore be obtained with this machine than with any of the other reluctance effect types, and consequently a relatively high value of goodness factor would be expected. Furthermore, since the two sets of windings
have the same number of poles and are thus interchangeable, the possibility
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exists of operating both windings in series or parallel and hence double the output.
When the secondary/rotor winding of a conventional wound rotor induction machine is made to supply the primary (rotor) winding of an inverted induction machine to which it is rigidly coupled; and if the secondary (stator) winding of the second machine is short-circuited, the combination will operate at a synchronous speed different from those of the original machines. If the coupled machines have different pole numbers, one machine only may be used to house all four sets of windings; so that the stator has two sets of winding with different pole numbers and the rotor similarly has two different sets windings corresponding to the two stator windings. This form of brushless operation of a controllable motor is referred to as self-cascading [7]. The same objective can be achieved using a modified squirrel cage in the rotor; the result is analogous to the inductor alternator.
Another form of brushless operation involving two machines combines effects which are due to saliency and effects due to induced currents flowing in asymmetrical rotor circuits. In this case, two identical reluctance machines [9] are coupled together and their primaries connected in parallel in a manner so that apart from the current which drawn from the supply, currents of a different frequency are circulated in the closed path provided between the two machine windings. The combination will run as a motor with synchronous speed which
is half the normal synchronous speed of the normal reluctance machine. The rotor circuit effects are shown to compliment the effects due to saliency.
1.2 AIMS AND OBJECTIVES OF THE THESIS:
The major aim of this thesis is to determine, investigate and analysis the influence of rotor cage winding on the performance of a transfer field machine in a manner to the development of a simulation model for a rotor cage on the performance of an induction motor.
1.3 THESIS MOTIVATION:
The motivational factor in this thesis is that a cage winding in the rotor of TF machine could go a long way to improving its performance characteristics and a better ability to preserve the voltage wave shape following a sudden addition or removal of load than does a cage-less rotor machine. This implies that self- excited reluctance machines with shunt capacitors yields better performance characteristics if the rotor houses a cage.
1.4 THESIS OUTLINE:
The arrangement of the thesis is hereby elucidated as follows: The differences among Alternator, wound rotor induction motors and other various electromagnetic machines. It also explains the advantages of a Transfer-Field machine over induction machine. Chapter one of this work explains the. Chapter two discussed the detailed concept of the theory of transfer field machine, descriptions, derivations of some vital equations like voltage, current inductances and flux linkages. The chapter three deals with the derivation of transfer field machine model equations , chapter four deals with the simulation and modeling of the machines and finally, chapter five deals with the output results of the simulations which includes the voltage build-up , rotor and load currents ,inductance, speed and load characteristics all with respect to time .
1.5 Study limitations
Major limitation encountered in this thesis is that the TFM was not constructed because of some technical difficulties and consequently the study was limited to only simulation studies.
This material content is developed to serve as a GUIDE for students to conduct academic research
THE INFLUENCE OF ROTOR CAGE WINDING ON THE PERFORMANCE OF A TRANSFER FIELD MACHINE
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