Motors, motor control and drives
Introduction
Electric motors can be found in applications from computer disk drives, domestic appliances, automobiles to industrial process lines, liquid pumping, conveyors, weaving machines and many more. Modern building services are heavily reliant upon motors and drives, which can be found at the heart of air handling and elevator systems. Even in the world of theatre and film, the electric motor is at the centre of the action, moving scenery and allowing actors to perform death-defying feats in complete safety. If it moves, it is reasonable to expect to find that an electric motor is somehow responsible.
The flexibility of power transmission that was introduced by the electric motor has been harnessed and controlled by the application of torque, speed and position controllers, which all fall under the generic term of drives or Variable Speed Drives (VSD). This precision of control has been central to all aspects of industrial automa- tion and has opened up new and demanding applications such as automatic production and sectional process lines, machine tool axis control, glass engraving, embroidery machines and precision polishing machines. It has also facilitated considerable reduc- tions in energy consumption by regulation of flow through speed control in fan and pump type loads, where power consumption is proportional to the cube of the speed. This ability to reduce energy consumption continues to be a major stimulus to growth in the variable speed drives market.
Many ac motors operate at fixed speed, being connected directly to the fixed-frequency supply system, but of the 70 per cent of all electrical power which flows through electric motors, a significant proportion passes through semiconductor conversion in the form of drives. The importance of this technology is self-evident, though the selection of appro- priate equipment is often less clear. It would be a difficult, if not impossible, task to detail every motor type and associated power conversion circuit available, and the focus here will be on those of greatest practical importance in the broad base of industries.
All electric motors have a stationary component, the stator, and a moving component, the rotor. In conventional motors, the rotor turns within the stator, but special motors are available for applications such as material handling conveyors where the rotor rotates outside the stator. In linear motors, the rotor moves along the path of the stator. In all cases, the rotor and stator are separated by an air-gap. The stator and rotor usually have a laminated steel core to reduce the losses arising from time-varying magnetic fields (as explained in section 3.2).
Almost all electric motors adhere to common fundamental principles of operation. This so-called unified theory of electric machines is important in the design and analysis of motor performance, but is not always helpful in understanding the principles of operation, and is therefore not included here. Only the characteristics relevant to operation and control of the practically important motors are presented.