This fifth edition was prompted in part by the great increase of computers in industrial controls and automation, which has produced computer programs that can simulate relays and relay contacts. These on/off discrete controls have eliminated the wiring and installation of hardware components in favor of virtual relays and contacts that can be programmed on a keyboard. The devices that perform these operations are called Programmable Logic Controllers (PLCs), or simply programmable controllers. These devices were initially stand-alone computers that controlled a specific robot or manufacturing operation. However, with the advent of the Internet, they have now been integrated with the overall manufacturing process, leading seamlessly to integration with management, sales, procurement, and consumer satisfaction. The relay control of machines covered in Chapter 20 is now supplemented by coverage of PLC controls in Chapter 31. Chapter 31 covers the basic principles of PLCs and shows, by way of example, how they are used in running the activities of a large service enterprise. This new chapter illustrates how these trend-setting computer-based activities involving controls and automation are being integrated with other business activities, including e-commerce.

As I mentioned in the last edition, similar upheavals have occurred in power technology. It is simply amazing to witness the entrance of power electronics into every facet of industrial drives. Thus, it is no longer pertinent to discuss do and ac machines in isolation because wherever they are being installed, an electronic control forms part of the package. Consequently, the termdrivenow involves not the motor alone but the entire unit that directs the torque and speed of the machine. This is having a direct influence on the way electrical machinery courses are being taught. How has this dramatic change come about? It is mainly due to the high-power solid state switching devices, such as insulated gate bipolar transistors (IGBTs), which can operate at frequencies of up to 20 kHz. The change has also been driven by thyristors and gate turn-off thyristors (GTOs) that can handle currents of several thousand amperes at voltages of up to 5 kV Another key element is the computing power of microprocessors that can process signal data in real time with incredible speed. The high switching frequencies of IGBTs permit the use of pulse-width-modulation techniques in power converters.

This, in turn, enables torque and speed control of induction motors down to zero speed. This was not feasible in rectangular-wave converters that were employed only a few years ago. Most industrial drives are in the fractional horsepower to the 500 hp range. That is precisely the range now available for control by IGBTs. The result has been an explosion in the retrofitting of existing drives. Lower maintenance costs, higher efficiency, and greater productivity have made such changeovers economically attractive. Thus, do drives are being replaced by induction motor drives, which require less maintenance while offering equal and often superior dynamic performance. Every sector of industrial and commercial activity is therefore being affected by this revolutionary converter technology.

Electric elevators, electric locomotives, electric transit vehicles, servomechanisms, heating, ventilating and air conditioning systems, fans, compressors, and innumerable industrial production lines are being modified to utilize this new technology. The change is also affecting the transmission and distribution of electric power--an industry that has been relatively stable for over 50 years. Here, we are seeing large rotating machines, such as synchronous condensers and frequency changers, being replaced by solid-state converters that have no moving parts at all. Important development work, carried out by the Electric Power Research Institute (EPRI) in Palo Alt.