Download Introduction

Generators, Motors, and AC
Power
Discuss AC and DC.
Discuss Inductance, Generators and
Alternators.
Learn how transistors operate.
Drive a DC Motor using Pulsewidth
Modulation.
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Update: 7/24/2010
History of Electric Power
Generation
• DC power was made popular by Thomas
Edison in the late nineteenth century.
– Power could only be transmitted about one mile
- electrical losses were too great.
• In 1891, the London Electric Supply
Corporation became the first modern
power station, delivering AC power to
residents of London.
– George Westinghouse and Nikola Tesla
– AC power can be transmitted much greater
distances than DC power.
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Magnitude
AC - DC
Time
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Inductance
• If you move a magnetic field near a wire or
a coil of wire, you can create an electric
current. This principle is called inductance.
• Set your multimeter to 2VDC and attach it
to the terminals of your small DC motor.
Spin the rotor while holding the stator
(the body of the motor). You should see a
voltage measurement appear on the screen
as long as you turn the rotor.
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Generators and Alternators
• When a motor like this is used to create an
electric current it is called a generator
because it generates electricity.
• This is similar to the alternator in
automobiles.
• An alternator creates AC power, which is
later converted to DC power for use in
your car.
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Controlling a DC Motor with a
Microcontroller
• Microcontroller can’t (typically) supply
enough current to drive the motor
directly.
• (Power) Transistors are used to solve
this problems.
• A transistor can be switched “on” with a
small amount of current to allow a larger
amount of current to flow from a
source.
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Transistors
Schematic Diagram
for an NPN Transistor
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• Used to control motor speed.
• By sending pulses to the a
PWM circuit, we can control
the “Duty Cycle” and
therefore the power
delivered.
• Pulses are delivered to the
motor fast enough that it
maintains a constant speed
(at some % of full power).
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Magnitude
Pulsewidth Modulation (PWM)
Time
100% - Full Speed
0% - Stop
PWM Circuit
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PWM Command
Duty Cycle (0 – 255  0 – 100%)
Pin
PWM 9,
duty,
1
Duration (ms)
For this project, the duty variable is set to start at
132 because testing showed that for this setup,
duty cycles below 132 did not produce any
rotation of the motor.
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Motor Control Project
duty VAR Word
counter VAR word
time VAR Word
Main:
duty = 132
FOR counter = 0 TO 4 'Ramp to full power in 5 steps
duty = duty + 22
FOR time = 0 TO 500 'Pulse motor 500 times at
PWM 8, duty, 1 'current duty cycle.
next
next
GOTO Main
end
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Assignment
• Finish circuit and run motor.
• Change loop count as needed to clearly
observe changes in motor RPM
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