Date of Award

2003

Document Type

Master Thesis

Degree Name

Master of Engineering (Research)

Department

Electronic Engineering

First Advisor

Dr. Richard Guinee

Abstract

Induction Motors (IM’s) are increasingly being considered for and used in high-performance motor drive (HPMD) systems. Their application has only gained acceptance since the 1970’s with the advent of vector control, microprocessors and improvements in power converter technology. D.C. motors were traditionally the preferred choice in HPMD systems. The main reasons for IM acceptance in HPMD systems are ruggedness, durability, low maintenance, cost, and small size by comparison with D.C. motors.

Alternating current asynchronous motors of the IM type are considered to be the universal workhorses of the manufacturing industry. It has been estimated that they are used in seventy to eighty per cent of all industrial drive applications, although the majority are in fixed speed applications such as pump or fan-drives. This thesis initially examines the relative advantages and disadvantages of the D.C. and IM motors. The main disadvantage of the asynchronous squirrel-cage IM is its control complexities in ASD applications and its non-linear operation, which can be overcome through low cost effective DSP solutions.

In general, the control of IM’s in HPMD systems can be classified into two distinct categories. The first of these is a traditional approach and is referred to as scalar control. Scalar control represents a means of obtaining speed control, and in the squirrel-cage IM this is achieved using both variable voltage/fixed frequency and variable frequency/fixed voltage supplies. For one such application of scalar control the constant volts per hertz (V/f) scheme is examined. Although a scalar controlled IM drive provides good speed control it does not provide a precise torque control capability with flux stabilisation. This was ascertained through simulation exercises of a scalar controlled IM drive in this thesis. In order to achieve the performance required by servo applications, IM’s have to be controlled using vector controllers.

The key features that differentiate between scalar and vector controllers are:

• Vector Control is designed to operate with a standard a.c., squirrel-cage asynchronous IM of known characteristics. If the characteristics of the IM are not known precisely then the vector control scheme can become totally inoperative. In many cases a Kalman observer is used for IM parameter estimation.

• A vector controller and its associated IM form an integrated drive; the drive controller and the motor have to be matched to achieve satisfactory performance.

• The vector controlled IM supplied currents are controlled both in magnitude and phase in real-time, in response to the demand and to external disturbances.

To examine the difficulties associated with the constant volts per hertz (V/f) scalar control technique a comprehensive study of the rotor flux-oriented vector control technique has been considered in this thesis.. Such a technique that relies heavily on the Parks transformation of the three-phase stator currents from the a-p stationary reference frame (s) to a special x-y reference frame for two-axis vector control is presented in this thesis. This transformation enables the asynchronous squirrel-cage IM to achieve a level of performance the same as that of a D.C. motor, in terms of precise torque control with flux stabilisation, but without the disadvantages associated with D.C. motor usage. Numerous IM drive simulations have been undertaken for a variety of load torque conditions to illustrate the essential differences between scalar and vector control.

With precise torque control, high-performance applications such as industrial machine tools, spindle drives, and cutters can be implemented by employing a two- axis controlled IM drive. To implement the rotor flux-oriented control technique particular attention is drawn to the development of a high-performance space vector pulse-width modulated (SV-PWM) voltage source inverter (VSI)- fed IM drive in the Matlab/Simulink software environment, that incorporates a mathematical model of a practical 2 kW IM and thereafter develop the rotor field-oriented control technique for such a drive. The SV-PWM technique is employed over the conventional sine-triangle PWM type since it is geared towards a digital implementation, and consequently allows the vector controlled IM drive to be used in a real-time application. It also provides a fifteen-percent increase in dc-link voltage utilisation compared with sine- triangle PWM technique.

Comments

Submitted to the Higher Education and Training Awards Council.

Access Level

info:eu-repo/semantics/openAccess

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