Page 26 - Electric Machinery Fundamentals

2 ELECTRIC MACHJNERY FUNDAMENTALS

either a generator or a motor. Almost all practical motors and generators convert en-
ergy from one form to another through the action of a magnetic field, and only ma-
chines using magnetic fields to pelform such conversions are considered in this book.
The transformer is an electrical device that is closely related to electrical ma-
chines. It converts ac electrical energy at one voltage level to ac electrical energy at
another voltage level. Since transfonners operate on the same principles as genera-
tors and motors, depending on the action of a magnetic field to accomplish the
change in voltage level, they are usually studied together with generators and motors.
These three types of electric devices are ubiquitous in modern daily life.
Electric motors in the home run refrigerators, freezers, vacuum cleaners, blenders,
air conditioners, fans, and many similar appliances. In the workplace, motors pro-
vide the motive power for almost all tools. Of course, generators are necessaIY to
supply the power used by all these motors.
Why are electric motors and generators so common? The answer is very
simple: Electric power is a clean and efficient energy source that is easy to trans-
mit over long distances, and easy to control. An electric motor does not require
constant ventilation and fuel the way that an internal-combustion engine does, so
the motor is very well suited for use in environments where the pollutants associ-
ated with combustion are not desirable. Instead, heat or mechanical energy can be
converted to electrical form at a distant location, the energy can be transmitted
over long distances to the place where it is to be used, and it can be used cleanly
in any home, office, or factory. Transformers aid this process by reducing the en-
ergy loss between the point of electric power generation and the point of its use.

1.2 A NOTE ON UNITS AND NOTATION

The design and study of electric machines and power systems are among the old-
est areas of electrical engineering. Study began in the latter part of the nineteenth
century. At that time, electrical units were being standardized internationally, and
these units came to be universally used by engineers. Volts, amperes, ohms, watts,
and similar units, which are part of the metric system of units, have long been
used to describe electrical quantities in machines.
In English-speaking countries, though, mechanical quantities had long been
measured with the English system of units (inches, feet, pounds, etc.). This prac-
tice was followed in the study of machines. Therefore, for many years the electri-
cal and mechanical quantities of machines have been measured with different sys-
tems of units.
In 1954, a comprehensive system of units based on the metric system was
adopted as an international standard. This system of units became known as the
Systeme lnternational (SI) and has been adopted throughout most of the world.
The United States is practically the sale holdout-even Britain and Canada have
switched over to SJ.
The SI units will inevitably become standard in the United States as time
goes by, and professional societies such as the Institute of Electrical and Elec-
tronics Engineers (IEEE) have standardized on metric units for all work. How-
ever, many people have grown up using English units, and this system will remain