Download Lecture 5 - Auburn Engineering

Announcements

Bring two motors to lab this week.
Motors can be purchased at the Scientific
Supply Store - 211 Sciences Center
Laboratory (2nd Floor).
 Prepare the leads as shown on the class
website. Can do this in lab if desired.

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Password Sync
 …to
login to Engineering labs, sync
your password at
www.auburn.edu/password.
 Still having trouble? Visit Engineering
Network Services in 270 Ross Hall
during normal business hours for
assistance.
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How a Motor Works
As current goes
through armature it
creates a magnetic
field.
 Opposites attract and
likes repel causing
rotational motion.

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Motor Parts

The brushes make electrical contact to the
moving armature. Wires would get twisted.
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Motor Parts Continued

Current through the armature causes a
magnetic field. The more current and more
coils, the faster it turns.
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Motor Parts Continued

PTC (Positive
Temperature
Coefficient)
Resistor » As temperature
increases, so does
the resistanceprevents burning up
the motor.
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Characteristics of Lego Motor
Constant voltage:
Higher torque
requires more
current → lower
speed. (Lego Car)
 Constant torque:
Voltage, speed, and
current increase
together. (Lab)

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Motors/Generators

Motor
» Appling a voltage, turns the motor

Generator
» Turn the rotor (gas engine, water power, etc.)
generate a voltage
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Motor Equivalent Circuit
kf


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The faster the motor turns, the more the
internal voltage cancels the driving voltage.
k is a constant that depends on the motor.
f is the rotational speed in revolutions/min
(RPM).
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Motor Control Signals
Pulse Width Modulation (PWM)
 Pulse Position Modulation (PPM)

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Pulse Width Modulation (PWM)
Waveform in which
the width of a pulse
is varied
 Can be used as a
communication
scheme
 Can be used to vary
the speed of a motor
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Important Terms




Period (T) – the time to complete one cycle of the
waveform (Units: seconds)
Frequency (f=1/T) – the number of cycles per
second (Units: hertz, Hz=1 cycle/s)
Pulse width – amount of time the wave is high
during one period
Duty Cycle – Pulse width / Period
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Controlling Motors with PWM
Think of the pulses
as someone pulsing
the switch below
 The longer they hold
it, the larger the
pulse width
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PWM used to vary speed
As the duty cycle
decreases, so does
the average voltage
delivered to the
motors.
 Hence your speed
will decrease
 Max speed = 100%
duty cycle

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PWM for SERVOS
Servos are motors
that you can tell to
turn to a position.
 Based on the duty
cycle you send them,
they move to a
corresponding
position.

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Pulse Position Modulation

Communication
scheme in which the
position of pulse
varies in order to
communicate a
message.
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