Page 144 - Electromechanical Devices and Components Illustrated Sourcebook

106 Electromechanical Devices & Components Illustrated Sourcebook

One of the most important divisions in electromechanics is
rotating equipment. The two most noteworthy pieces of rotat-
ing equipment are the motor and the generator. We encoun- M
tered electric motors virtually everywhere we go. Our
kitchens have a variety of motors inside the appliances that
dot our countertops and cabinets. Our automobiles use elec- Schematic Symbol
tric motors to drive the windshield wipers, heater fan, power
windows, automatic doors, and fuel pumps. Every time we
ride an elevator we experience the work generated by an elec-
tric motor. The air-conditioning and heat we enjoy would not
be possible without the motors that drive the air handlers and
compressors within the systems. Permanet
The electric motors that make our lives so comfortable must Magnet
have a ready source of power; this is where generators come Rotor Pole
into play. Although most of us do not encounter generators in Rotor Core
our day-to-day lives, they are closer than you might think. All Armature
Rotor Coil
of the electric power that you rely on at home and work is pro-
duced by generators. Most people take their access to reliable Brush
electric power for granted and never really consider where it Axle
comes from or how it’s produced. Even the common terms that N S
are associated with power generation like coal, hydroelectric,
and nuclear have no real meaning to the average person. Brush Spring Magnet Pole
Probably the closest generator to most people is the alterna- Terminal
tor in their automobile. Without this critical piece of equipment,
the electrical system of your car would quickly fail. Many of us Figure 6-1 Permanent Magnet DC Motor
have experienced a failed alternator while driving. When the
alternator stops producing power, the car’s electrical system
starts to operate on the battery alone. It doesn’t take long before
the load depletes the battery and the car shuts down.
Permanent
Magnet
Permanent Magnet DC Motors

The basic permanent magnet DC motor, as shown in Figure 6-1, Axle
is fairly easy to understand. It generates rotation by manipu-
lating the interaction of the fields between a permanent mag- Rotor Core
net and an electromagnet. When the rotor poles are vertical,
N S
power is connected to the rotor coil, which, in turn, generates
a magnetic field in the core of the rotor. The two magnetic
fields attract one another and the rotor aligns to the permanent Rotor Coil
magnet (dotted lines). When the rotor spins into the horizon-
tal position, the rotating armature disconnects the power to Terminals
the coils and the rotor freewheels toward the vertical. As the
Armature
rotor approaches vertical, the armature reconnects the coil and Brushes
a reverse field is generated within the core. In this manner the
Figure 6-2 Permanent Magnet DC Motor Schematic
magnetic field in the rotor is reversed every half revolution
and a spinning motion is created. Figure 6-2 shows a schematic
representation of a two-pole permanent magnet electric
motor. Figure 6-3 shows a typical permanent magnet DC
motor. Note that most of these motors have facilities to easily a permanent magnet motor, except that an electromagnet
replace the brush set, the part of a DC motor that most often replaces the permanent magnet. Shunt wound motors are
requires maintenance. generally used for higher horsepower motors because the
electromagnet can supply a much higher field strength than a
permanent magnet can. Figure 6-4 shows a two-pole shunt
Shunt Wound DC Motors wound motor. Notice that the only difference between this
unit and a permanent magnet unit is that this design uses an
A common variation of the permanent magnet DC motor is electromagnet. Otherwise the two motors operate in the same
the shunt wound motor. A shunt wound motor is the same as fashion. Figure 6-5 shows a shunt wound motor schematic.