Austin Hughes

Inverter-fed Induction Motor Drives

The following areas of inverter-fed induction motor drives are discussed: pulse-width modulation (PWM) voltage source inverter (VSI), voltage and current waveforms; comparative performance of open-loop (without shaft sensor) and closed-loop (with sensor) drives; effects of inverter waveforms on induction motor, including acoustic noise, insulation, and derating; meaning of the term ‘inverter grade’ motor; effect of inverter-fed motor on the utility supply – harmonic currents and power-factor; inverter and motor protection; alternative converter topologies – active front end, multi-level inverter, and matric converter; and current source inverter (CSI) drives.

Machine/drive circuit interactions in small variable-reluctance stepping and brushless DC motor systems

The authors investigate the torque, losses, and efficiency of small brushless DC and stepping-motor systems, and explain how performance is influenced by excitation mode, drive circuit, and machine parameters. Quantitative deductions are made for a three-phase reluctance-type machine based on computed steady-state results. For a given machine and drive circuit, maximum torque is obtained with an excitation mode that allows each phase to be turned on for half of the complete excitation cycle. However, if maximum efficiency is the aim, phase excitation must occur for a shorter period, e.g. by exciting only one phase at a time. When making comparisons between drive circuits, the most important measure of drive-circuit capability is the circuit power available at low speeds. Drive circuits producing rapid current decay at phase turn-off benefit torque production in the two-phase-on excitation mode, but are detrimental with the one-phase-on mode. At high speed the pull-out torque depends on the unsaturated inductance parameters of the machine. For maximum torque these parameters must be correctly proportioned, their values being dependent on the excitation mode. >

Electric Motors – The Basics

Electromagnetic force production is presented using a simple straight wire as a primitive form of electrical machine. Magnitude of force is quantified in terms of the magnetic flux density and current. Methods of enhancing flux density are investigated. Concepts of magnetic circuit, magnetomotive force (m.m.f.), and reluctance are introduced. Linear model is extended to rotary. Fundamental expressions for torque as a function of rotor volume are developed. Motion at constant speed – the important idea of ‘motional e.m.f.’ in the conductor – is discussed. Basic equations governing the electromechanical energy conversion process are given. Transient (run-up) behavior is examined. Finally, efficiency and thermal limits are explored.

High-speed performance of variable-reluctance stepmotors

An analytical study is carried out with the aid of Blondel diagrams in order to identify the key parameters responsible for the performance limitations in a unipolar step-motor drive. It is shown that the fundamental component of the excitation voltage waveform and the total phase resistance govern the maximum output-power capability of the drive. Predicted and experimental results are presented for a multistack motor which demonstrated not only the validity of the theory but also highlights the effectiveness of using Blondel diagrams for stepping motor analysis. >

Variable Frequency Operation of Induction Motors

The following areas of variable frequency operation of induction motors are discussed: inverter-fed induction motor drives, and variation of torque–speed characteristics with frequency; operating regions, and limitations imposed by motor and inverter; four-quadrant capability; tutorial material required for understanding of field-oriented control, including transient and steady states, space phasor representation of m.m.f. waves, transformation of reference frames, and coupled circuit modeling; steady-state analysis of induction motor under current-fed conditions; importance of constant rotor flux linkage, and significance of flux and torque components of stator current; dynamic torque implemented by field-oriented control; the PWM controller and vector modulator; derivation of the rotor flux reference angle; and direct torque control – hysteresis control of stator flux and current.

Synchronous and Brushless Permanent Magnet Machines and Drives

The following areas of synchronous and brushless permanent magnet machines and drives are considered: clarification of terminology surrounding brushless machines; excited rotor and permanent magnet synchronous motors; equivalent circuits for synchronous motors; operation from utility supply, power-factor control; variable frequency operation of synchronous motors; field-oriented control, constant torque and constant power regions; field weakening; synchronous motor drive systems – permanent magnet motor and large multi-megawatt drives; performance of brushless motors – limitations; and reluctance and hysteresis motors.

Stepping and Switched-reluctance Motors

The following areas of stepping and switched reluctance motors are covered: stepping motors – open-loop position control; step and direction pulses and step response; high-speed running – ramping; principle of operation – variable-reluctance motor, hybrid motor; motor characteristics – static torque–displacement curves, step position error, holding torque; single- and half-stepping – step division, mini-stepping; steady-state behavior with constant-current drive; drive circuits and their torque–speed characteristics – chopper drives; resonances and high-frequency instability; optimum acceleration – closed-loop control; and switched-reluctance motor drives – principle of operation, torque prediction and control, power converter and drive characteristics.

Motor/Drive Selection

The following areas of motor/drive selection are discussed: power ratings and capabilities – conventional (brushed) d.c. motor drive, a.c. induction and synchronous motor drives, large synchronous motor drives, soft start, switched-reluctance and stepping motor drives; characteristics, typical applications, and pros and cons of drives of all types; specifying the load requirements – torque–speed curves; constant-torque and fan-type loads – inertia matching; regenerative operation and braking, duty cycle and rating, enclosures and cooling; dimensional standards; and supply interaction – harmonic currents.

Introduction to Power Electronic Converters for Motor Drives

Switching strategies to obtain efficient power conversion are explained. The transistor chopper for voltage control from fixed d.c. supply is explored, including overvoltage protection and freewheeling. Controlled rectification with single-pulse and 6-pulse circuits is introduced applied to the d.c. motor drive, in continuous current mode. The transistor inverter is described, with methods of output voltage control including pulse-width modulation (PWM). Cycloconverters are described. The principal switching devices are covered. Acoustic noise, heatsinks and thermal resistance are discussed.

5 – INDUCTION MOTORS – ROTATING FIELD, SLIP AND TORQUE

Abstract The fact that 4 of the 11 chapters of this book are devoted to the induction motor underlines its current dominance in electric drives, and we are therefore keen to start equipping the reader with a sound understanding of how it works. Attention is concentrated on how three stationary windings combine to produce a smoothly rotating magnetic field, and how the same windings produce torque on the rotor as it slips slowly past the travelling field. Several important relations governing the speed and torque are derived from a physical viewpoint and with minimum mathematics.