1
POWER ELECTRONICS: AN ENABLING TECHNOLOGY

Power electronic systems are essential for energy sustainability, which can be defined as meeting our present needs without compromising the ability of future generations to meet their needs. Using renewable energy for generating electricity and increasing the efficiency of transmitting and consuming it are the twin pillars of sustainability. Some of the applications of power electronics in doing so are as mentioned below:

• Harnessing renewable energy such as wind energy and solar energy using photovoltaics .
• Storage of electricity in batteries and flywheels to offset the variability in the electricity generated by renewables.
• Increasing the efficiency of transmitting electricity.
• Increasing efficiency in consuming the electricity in motor-driven systems and lighting, for example.

This introductory chapter highlights all the points mentioned above, which are discussed in further detail in the context of describing the fundamentals of power electronics in the subsequent chapters.

1.1 INTRODUCTION TO POWER ELECTRONICS

Power electronics is an enabling technology, providing the needed interface between an electrical source and an electrical load, as depicted in Figure 1.1 [1]. The electrical source and the electrical load can, and often do, differ in frequency, voltage amplitudes, and the number of phases. The power electronics interface facilitates the transfer of power from the source to the load by converting voltages and currents from one form to another, in which it is possible for the source and load to reverse roles. The controller shown in Figure 1.1 allows management of the power transfer process in which the conversion of voltages and currents should be achieved with as high energy efficiency and high power density as possible. Adjustable-speed electric drives, for example in wind turbines, represent an important application of power electronics.

FIGURE 1.1 Power electronics interface between the source and load.

1.2 APPLICATIONS AND THE ROLE OF POWER ELECTRONICS

Power electronics and drives encompass a wide array of applications. A few important applications and their role are described below.

1.2.1 Powering the Information Technology

Most of the consumer electronics equipment such as personal computers (PCs) and entertainment systems supplied from the utility need very low DC voltages internally. They, therefore, require power electronics in the form of switch-mode DC power supplies for converting the input line voltage into a regulated low DC voltage, as shown in Figure 1.2a. Figure 1.2b shows the distributed architecture typically used in computers in which the incoming AC voltage from the utility is converted into DC voltage, for example, at 24 V. This semi-regulated voltage is distributed within the computer where onboard power supplies in logic-level printed circuit boards convert this 24 V DC input voltage to a lower voltage, for example, 5 V DC, which is very tightly regulated. Very large-scale integration and higher logic circuitry speed require operating voltages much lower than 5 V; hence 3.3 V, 1 V, and eventually, 0.5 V levels would be needed.

FIGURE 1.2 Regulated low-voltage DC power supplies.

Many devices such as cell phones operate from low battery voltages with one or two battery cells as inputs. However, the electronic circuitry within them requires higher voltages, thus necessitating a circuit to boost input DC to a higher DC voltage as shown in the block diagram of Figure 1.3.

FIGURE 1.3 Boost DC-DC converter needed in cell-operated equipment.

1.2.2 Robotics and Flexible Production

Robotics and flexible production are now essential to industrial competitiveness in a global economy. These applications require adjustable-speed drives for precise speed and position control. Figure 1.4 shows the block diagram of adjustable-speed drives in which the AC input from a 1-phase or a 3-phase utility source is at the line frequency of 50 or 60 Hz . The role of the power electronics interface, as a power-processing unit, is to provide the required voltage to the motor. In the case of a DC motor, DC voltage is supplied with an adjustable magnitude that controls the motor speed. In the case of an AC motor, the power electronics interface provides sinusoidal AC voltages with adjustable amplitude and frequency to control the motor speed. In certain cases, the power electronics interface may be required to allow bidirectional power flow through it, between the utility and the motor load.

FIGURE 1.4 Block diagram of adjustable-speed drives.

Induction heating and electric welding, shown in Figures 1.5 and 1.6, respectively, by their block diagrams, are other important industrial applications of power electronics for flexible production.

FIGURE 1.5 Power electronics interface required for induction heating.

FIGURE 1.6 Power electronics interface required for electric welding.

1.3 ENERGY AND THE ENVIRONMENT: ROLE OF POWER ELECTRONICS IN PROVIDING SUSTAINABLE ELECTRIC ENERGY

As mentioned in the preface of this textbook, power electronics is an enabling technology in providing sustainable electric energy. Most scientists now believe that carbon-based fuels for energy production contribute to climate change, which is threatening human civilization. In the United States, the Department of Energy reports that approximately 40% of all the energy consumed is first converted into electricity. Potentially, the use of electric and plug-in hybrid cars, high-speed rails, and so on, may increase this to even 60%. Therefore, it is essential that we generate electricity from renewable sources such as wind and solar, which, at present, represent only slightly over 4%, build the next-generation smarter grid to utilize renewable resources often in remote locations, and use electricity in more energy-efficient ways. Undoubtedly, using electricity efficiently and generating it from renewable sources are the twin pillars of sustainability, and power electronic systems discussed in this textbook are a key to them both!

1.3.1 Energy Conservation

It’s an old adage: a penny saved is a penny earned. Not only does energy conservation lead to financial savings, but it also helps the environment. The pie chart in Figure 1.7 shows the percentages of electricity usage in the United States for various applications. The potential for energy conservation in these applications are discussed below.

FIGURE 1.7 Percentage use of electricity in various sectors in the US.

1.3.1.1 Electric-Motor Driven Systems

Figure 1.7 shows that electric motors, including their applications in heating, ventilating, and air conditioning (HVAC), are responsible for consuming one-half to two-thirds of all the electricity generated. Traditionally, motor-driven systems run at a nearly constant speed, and their output, for example, the flow rate in a pump, is controlled by wasting a portion of the input energy across a throttling valve. This waste is eliminated by an adjustable-speed electric drive, as shown in Figure 1.8, by efficiently controlling the motor speed, hence the pump speed, by means of power electronics [2].

FIGURE 1.8 Role of adjustable-speed drives in pump-driven systems.

One out of three new homes in the United States now uses an electric heat pump, in which an adjustable-speed drive can reduce energy consumption by as much as 30% [3] by eliminating on-off cycling of the compressor and running the heat pump at a speed that matches the thermal load of the building. The same is true for air conditioners.

A Department of Energy report [4] estimates that operating all these motor-driven systems more efficiently in the United States could annually save electricity equivalent to the annual electricity usage by the entire state of New York!

1.3.1.2 Lighting Using LEDs

As shown in the pie chart in Figure 1.7, approximately one-fifth of the electricity produced is used for lighting. LEDs (light-emitting diodes) can improve this efficiency by more than a factor of six. They offer a longer lifetime and have become equally affordable as incandescent lamps. They require a power-electronic interface, as shown in Figure 1.9, to convert the line-frequency to supply DC current to the LEDs.

FIGURE 1.9 Power electronics interface required for LED.

1.3.1.3 Transportation

Electric drives offer huge potential for energy conservation in transportation. While efforts to introduce commercially viable electric vehicles (EVs) continue with progress in battery [5] and fuel cell technologies [6] being reported, hybrid electric vehicles (HEVs) are sure to make a huge impact [7]. According to the US Environmental Protection Agency, the estimated gas mileage of the hybrid-electrical vehicle shown in Figure 1.10 in combined city and highway driving is 48 miles per gallon [8]. This is in comparison to the gas mileage of 22.1 miles per gallon for an average passenger car in the United States [9]. Since automobiles are estimated to account...