AbstractAn inverter-fed three phase squirrel cage induction motor drive system with improved mechanical characteristics is presented. A detailed analytical review of the ideal constant flux control method and the basic f v control strategy were undertaken before an improved f v control method, which utilises a low frequency boost-voltage, was developed. This method, unlike the basic f v control method, provides a boost-voltage at low frequencies thereby compensating for the stator impedance drop, offering constant flux operation with maximum motoring torque from zero to rated speed. Index TermsInduction Motor, Constant Volts/Hertz f v , Constant Flux, Motor Torque and Speed) I. INTRODUCTION Induction machine is the most used in industry because of its high robustness, reliability, low cost, high efficiency and good self-starting capability [1,2,3,4]. The induction motor, particularly with a squirrel cage rotor, is the most widely used source of mechanical power fed from an AC power system. Its low sensitivity to disturbances during operation make the squirrel cage motor the first choice when selecting a motor for a particular application [5]. In spite of this popularity, the induction motor has two inherent limitations: (1) the standard motor is not a true constant-speed machine, its full-load slip varies from less than 1% (in high-horse power motors) to more than 5% (in fractional-horsepower motors) and (2) It is not, inherently, capable of providing variable speed operations[6,7]. These limitations can be solved through the use of adjustable speed controllers [8,9]. The basic control action involved in adjustable speed control of induction motors is to apply a variable frequency variable magnitude AC voltage to Manuscript received January 14, 2010. This work was supported by the Grant the Principal Author received from the African Network of Scientific and Technological Institutions (ANSTI) Grant Number No. ANSTI.269.10 C. U. Ogbuka (Member IAENG) is a Lecturer in The Department of Electrical Engineering University of Nigeria, Nsukka, Enugu State, Nigeria. He is also a Ph.D Research Student under Prof. M.U. Agu, the co-author. His research interests are in Electric Machine Drives and Power Electronics. Phone: +2348032616466. E-mail: ucogbuka@yahoo.com. M. U. Agu, is a Professor of Power Electronics and Head of Department of Electrical Engineering, University of Nigeria, Nsukka, Nigeria. E-mail: Phone: +2348076361747 E-mail: drmarcelagu@yahoo.co.uk the motor to achieve the aims of variable speed operation [10]. The most common AC drives today are based on sinusoidal pulse-width modulation SPWM. However, voltage source inverters with constant volts/hertz f v are more popular, especially for applications without position control requirements, or where the need for high accuracy of speed control is not crucial [11]. However, since the introduction of field-oriented control theory, almost all research has been concentrated in this area and little has been published about constant f v operation. Its application at low frequencies is still challenging due to the influence of the stator resistance and the necessary rotor slip to produce torque [12]. In addition, the nonlinear behaviour of pulse-width modulated voltage inverter in the low voltage range makes it difficult to use constant f v drives at frequencies below 3Hz [13]. II. CONSTANT FLUX CONTROL: PRINCIPLES AND MOTOR PERFORMANCE The ideal of the variable frequency, variable voltage control methods is the constant flux control where the magnetizing current is kept constant [10, 14]. As the frequency varies, all the reactances vary accordingly. Taking the operating frequency as s Z and b Z or sr Z as the base (rated) frequency at which the reactances ls x , ’ lr x , and m x are determined, then the familiar equivalent circuit for the steady state analysis of squirrel cage induction motor modifies as shown in figure 1 below. Figure 1: Steady State Equivalent Circuit of a Squirrel Cage Induction Motor for Constant Flux Control (a  1) A Modified Approach to Induction Motor Stator Voltage and Frequency Control Cosmas .U. Ogbuka, Member, IAENG and Marcel. U. Agu, MIEEE s V s r r ’ m jax ar aE ’ lr jax ls jax s r s I ’ r I m I Proceedings of the World Congress on Engineering 2011 Vol II WCE 2011, July 6 - 8, 2011, London, U.K. ISBN: 978-988-19251-4-5 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) WCE 2011