SOURCE:

Faculty: Engineering
Department: Electrical Engineering

CONTRIBUTORS:

Obute, K.C;
Anih, L.U;

ABSTRACT:

This dissertation presents the analysis on improvement of performance indices of three-phase transfer field reluctance motor through induced rotor current. The inherent draw-backs of the three phase transfer-field reluctance motors are mainly the low output characteristics, which includes the low output torque, low output power, poor power factor etc. These characteristics of the motor are very much inferior and uncomparable to those of three phase induction motor with comparable ratings and sizes. This is the attribute of their low ratio of their direct axis to quadrature axis reactances. This is further worsened by the excessive leakage reactance, mainly contributed by the quadrature axis reactance. These are as a result of their rotor pole structures, which are projected (salient) in nature. To improve the output performance of the motor, the excessive leakage reactance must be minimized by optimsing the salient pole rotor design. This new configuration is conceived to reduce the overall leakage reactance of the existing motors with the corresponding increase in the maximum and starting torques at better power factor. This is achieved by the introduction of windings, known as rotor windings at the rotor pole structure. These windings (rotor windings) are connected in parallel with the auxillary windings. The reason for such parallel connection is based on the idea that when impedances (in the form of coils) are connected in parallel to the utility supply, their resultant impedance will be less than the least impedance and when impedance is reduced in a circuit, higher current flows in the circuit. This arrangement do not only give rise to an increase in the induced e.m.f, but also augument output power by effectively lowering the synchronous reactance of the output windings, thus leading to a higher output power, improved power factor and electromagnetic torque. The purpose of the work is to develop models of the machine necessary to explain its behaviour under steady-state and dynamic state conditions; derive mathematical equations necessary for the formation of the machine’s equivalent circuits, derive the machine’s output characteristics equations from the equivalent circuits; establish the machines circuit parameters necessary for the output characteristics plots through simulations for the machine. The three-phase transfer field reluctance motor with and without rotor windings was analyzed. The existing three-phase transfer field reluctance motor without rotor windings was modified and optimized. In addition, the output characteristics of the torque against slip, rotor current against slip, and power factor against slip were simulated. Thereafter, comparison of the existing three-phase transfer field reluctance motor with and without rotor windings was carried out. The results obtained showed that the three-phase transfer field reluctance motor without rotor windings has starting torque of 0.171N-m, while the three-phase transfer field reluctance motor with rotor windings has starting torque of 0.296N-m, thereby having a percentage improvement of 73.09%. Similarly, the power factor at start, for the three-phase transfer field reluctance motor with and without rotor windings were 0.180and 0.107 respectively, with a percentage improvement of 68%. Also the rotor/auxiliary currents at start for the three-phase transfer field reluctance motor with and without rotor windings were 7.475A and 5.409A respectively, with a percentage improvement of 38%. From the results obtained, it is important to state that the accelerated rotor current is a consequence of the rotor windings added to the rotor poles of the existing motor. Also, the increased induced rotor current is responsible for the improved output power, electromagnetic torque and power factor of the existing motor. Due to inherent half speed characteristics of the motor, it is envisaged that three phase transfer field reluctance motor with rotor windings for enhanced output performance will have future in a variety of special applications, such as low speed fixed frequency drives. It is common knowledge that low speed motors will find applications in domestic appliances requiring low speed drives such as grinding machines for perishables.