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AC vs. DC
The production of and use of electricity is an expensive
business.Therefore the most efficient use of the
resource is required.
The most efficient way of generating electricity is
through the use of an alternator. Additionally, alternating
current is the most efficient to transport over long
distances.
Direct current transmission is limited to the distance it
can travel.
DC Motor
Field winding
The production of and use of
electricity in a motor is done
through the use of an armature,
field winding, brushes, slip ring
or commutator for a DC motor.
Brushes
Brush
holders
Commutator
Armature
Sine Waves
To generate or use electricity,
the rotor of the generator
passes through the magnetic
field produced by the field
winding. As the rotor travels
through its 360 degree
rotation the field is charged
and discharged twice in
different directions. This in
turn, develops the pictured
sine wave.
Single Phase Sine Wave
The pictured sine wave is
a single phase.
In a single phase
generator the rotor is
rotated 3,600 times per
minute, or 60 revolutions
per second.
This rotation produces
the 60hz line voltages
used today.
3 Phase Generator
A three phase generator
has three generator
windings equally
spaced around the
circle.
The windings will
produce output voltages
120 degrees out of
phase with each other.
3 Phase AC Voltage
The advantages of three
phase AC voltage is more
efficient distribution of
power.
AC induction motors are
self-starting, and the AC
ripple is easier to filter out
when rectifying to DC.
Motor Control Centers
Motor control centers combine individual control units
into standard modular structure joined together for a
convenient control center in one location.
The motor control circuit is usually supplied from a
panel board or switchboard.
Basically the function of the control center is to take the
incoming power and deliver it to the motor loads that
are being controlled.
DC Motor Control
Direct current (DC) motors are used in industrial
applications that require variable speed control, high
torque, or both.
Speed of most DC motors can be controlled smoothly
and easily form zero to full speed. They are used in
many acceleration and deceleration applications.
DC motors can be controlled by the use of a simple
rheostat (variable resistor).
Silicon Control Rectifiers(SCR)
Silicon control rectifiers (SCR) control motors by
controlling conduction time with a pulsed gate. A
rheostat controls motor speed with resistance
SCRs are smaller in size for the same rating, energyefficient by not wasting power within itself, and less
expensive.
For these reasons most DC speed controls use SCRs
instead of rheostats.
DC Advantages
In addition to having excellent speed control, the
DC motor is ideal in applications that call for
momentarily higher torque outputs.
This is because the DC motor can deliver three
to five times its rated torque for short periods of
time.
AC Motors
Alternating current motors are basically a constant
speed motor. This is because the synchronous speed of
an induction motor are designed for 60 hertz use.
Unlike the speed of a DC motor, the speed of an AC
motor should not be changed by varying the applied
voltage.
In fact, if the supply voltage is varied to an AC motor
more than 10% above or below the rated voltage,
damage may be done.
Changing AC Motor Speed
The frequency or number of poles must be changed to
change the speed of an AC motor, two methods of
speed control are available.
 Changing the frequency applied to the motor
 Reconnecting the windings to create a different
number of poles.