360 Chapter 12 Electric Current and Related Parameters
Thefarad(F)is the basic unit used to designate the size of a capacitor. One farad is equal
to 1 coulomb per volt. Because the farad is a relatively large unit, many capacitor sizes are
expressed in microfarad ( F 10
6
F) or picofarad (pF 10
12
F).12.5 Electric Motors
As engineering students, most of you will take at least one class in basic electrical circuits wherein
you will learn about circuit theory and various elements, such as resistors, capacitors, inductors,
and transformers. You will also be introduced to different types of motors and their operations.
Even if you are studying to be a civil engineer, you should pay special attention to this class in
electrical circuits, especially the part about motors, because motors drive many devices and
equipment that make our lives comfortable and less laborious. To understand the significant role
motors play in our everyday lives, look around you. You will find motors running all types of
equipment in homes, commercial buildings, hospitals, recreational equipment, automobiles,
computers, printers, copiers, and so on. For example, in a typical home you can identify a large
number of motors operating quietly all around you that you normally don’t think about. Here,
we have identified a few household appliances with motors:- Refrigerator: compressor motor, fan motor
- Garbage disposer
- Microwave with a turning tray
- Stove hood with a fan
- Exhaust fan in the bathroom
- Room ceiling fan
- Tape player in a VCR
- Hand-held power screwdriver or hand-held drill
- Heating, ventilating, or cooling system fan
- Vacuum cleaner
- Hair dryer
- Electric shaver
- Computer: cooling fan, hard drive
Some of the factors that engineers consider when selecting a motor for an application
are: (a) motor type, (b) motor speed (rpm), (c) motor performance in terms of torque output,
(d) efficiency, (e) duty cycles, (f ) cost, (g) life expectancy, (h) noise level, and (i) maintenance
and service requirements. Most of these are self-explanatory. We discussed speed and torque in
previous chapters; next we will discuss motor types and duty cycles.Motor Type
Selecting a motor for an application depends on a number of factors, including the speed of the
motor (in rpm), the power requirement, and the type of load. Some applications deal with
difficult starting loads, such as conveyors, while others deal with easy starting loads, such as a
fan. For this reason, there are many different types of motors. Here we will not discuss the prin-
ciples of how various motors work. Instead, we will provide examples of various motors and
their applications, as given in Table 12.3. Most of you will take some classes later that will be
devoted to the theory and operation of motors.Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀