Committee Chair

Eltom, Ahmed Hassan


Dept. of Engineering


College of Engineering and Computer Science


University of Tennessee at Chattanooga

Place of Publication

Chattanooga (Tenn.)


This study investigates the behaviors and economics of standard and energy efficient polyphase induction motors under unbalanced phase voltage conditions ofup to 5 percent. Motors sizes of 25, 50, 100,150,200, 250 and 300 hp are used in this investigation. Computer simulations are used to study the efficiencies, power losses and economics of motor behavior when unbalanced phase voltage system conditions exist. Previous polyphase induction motor studies simulated unbalanced phase voltage conditions by assuming a high impedance fault at the motor terminals. In this study, a method is used to simulate unbalanced phase voltage conditions by varying the phase voltage at one of the motor terminals. The nonlinear variation of motor impedance with frequency is characterized using a model that is based on Maxwell's equations. This model has been verified in previous laboratory tests. In all cases considered, increasing phase voltage unbalance caused the efficiencies of the standard efficiency motors to drop more rapidly than those of the energy efficient motors. When unbalanced phase voltage conditions exist, the percentage increase in rotor losses are higher for the I 00, 150 and 200 hp standard efficiency motors and are higher for the 25, 50, 250 and 300 hp energy efficiency motors. Economic evaluations reveal that additional financial savings will occur if energy efficient motors are used in place of standard efficiency motors as unbalanced phase voltage conditions intensify. The payback time required to overcome the higher initial purchase costs of energy efficient motors are more quickly recovered when unbalanced phase voltage system conditions exist. It is noted that slight reductions in full load rotor speed and increases in current occur when the motors are subjected to unbalanced phase voltages while constantly maintaining rated output torque. Each unbalanced phase voltage level corresponds to a unique rotor speed that is close to the full load operating speed when phase voltages are balanced.


The author would like to express his thanks and appreciation to Dr. Eltom for his unlimited help and guidance throughout this work. Also, I would like to express thanks to the committee members for their review and suggestions of this thesis: Dr. Thomason, Dr. Parten and Dr. Keilany.


M. S.; A thesis submitted to the faculty of the University of Tennessee at Chattanooga in partial fulfillment of the requirements of the degree of Master of Science.




Electric motors, Induction


Electrical and Computer Engineering

Document Type

Masters theses




xv, 144 leaves



Call Number

LB2369.2 .B528 1998