LITMIR.BIZ

Fig. 1-1 Block diagrams of (a) electric machines and (b) electric drives (motoring mode shown).

However, in certain applications, it is required that for given quantities on the electrical or the mechanical side, the other quantities be controlled, as in a wind turbine. This is made possible in electric drives shown by their block diagram in Fig. 1-1b. It should be noted that in the literature and in trade publications, electric drives sometimes refer only to the power electronic converter and its control, excluding the motor. In this textbook, however, electric drives refer to the entire block, which is shown dotted in Fig. 1-1b, that includes power electronic converter (power processing unit – PPU) and its control, as well as the electric machine, whether it is in its motoring or the generating mode. We should also note that we will be looking at only the ac machines, hence the title of the book is ac drives.

1‐1 THE CLIMATE CRISIS AND THE ENERGY‐SAVING OPPORTUNITIES

The climate crisis, caused by the burning of fossil fuels, is the greatest and an existential threat facing humanity. To reduce the emission of carbon dioxide, a necessary solution is first to convert our energy use to electricity, as much as possible, and then to produce that electricity using renewables such as solar and wind. As we will see in the subsequent sections in this chapter, electric drives play a significant role in generating and efficiently consuming electricity and providing ample opportunity for energy savings.

      According to [1], “advances in integrated power electronics have the potential to develop a new generation of energy‐efficient, high‐power density, high‐speed motors and generators and, in turn, save significant energy.” In addition, a great deal of energy savings can be achieved by shifting from nearly constant speed motors to adjustable‐speed electric drives, as explained in this chapter.

      Prior to looking at the energy‐saving potentials, we should understand the meaning of primary energy. According to [2], the “Primary Energy is energy in the form that it is first accounted for in a statistical energy balance, before any transformation to secondary or tertiary forms of energy. For example, coal can be converted to synthetic gas, which can be converted to electricity; in this example, coal is primary energy, synthetic gas is secondary energy, and electricity is tertiary energy.” Often, the primary energy and the savings in the primary energy are expressed in quads, where a quad equals 10¹⁵ BTUs and 10 000 BTUs equal approximately 2.93 kWh.

1‐2 ENERGY SAVINGS IN GENERATION OF ELECTRICITY

Nearly 99% of electricity is produced through electric machines. This percentage was nearly the same, approximately 98.6%, in the United States in 2018. According to the US Energy Information Administration [3], about 4171 billion kWh (or 4.17 trillion kWh) of electricity was generated at utility‐scale electricity generation facilities in the United States in 2018. About 64% of this electricity generation was from fossil fuels (coal, natural gas, petroleum, and other gases). About 19% was from nuclear energy, and approximately 17% was from renewable energy sources. Out of the renewable energy sources, only 1.4% of the total electricity generated was by photovoltaic systems (PVs) that do not use electric machines, whereas all other sources of electricity generation use electric machines. Therefore, any improvement in increasing the efficiency of machines and electric drives will be very consequential.

      1‐2‐1 Energy‐Saving Potential in Harnessing of Wind Energy

One of the significant roles of electric drives is in harnessing wind energy. The block diagram for a wind‐electric system is shown in Fig. 1-2, where the variable‐frequency ac produced by the wind‐turbine‐driven generator is interfaced with the utility system through a power electronic converter (PPU). By letting the turbine speed vary with the wind speed, it is possible to recover a higher amount of energy in the wind compared to systems where the turbine essentially rotates at a constant speed due to the generator output being directly connected to the utility grid. The harnessing of wind energy involving ac drives is crucial for generating carbon‐free electricity [3], and this application is sure to grow rapidly.