Transcript ENG H193 - Prelab for Lab 2

Engineering H193 - Team Project
Motor Dynamics
Lab 2
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 1
Engineering H193 - Team Project
Pulse Width Modulation
• Pulse Width Modulation (or “PWM”) is the manner by which
most of today’s DC motors are controlled
– Rather than varying the voltage supplied to the motor to
control the speed (or power level), the voltage to the
motor is simply switched on and off at a very high rate
– This rate is expressed as the “Duty Cycle” or the ratio of
“on-time” versus “off-time”
• Example: a motor with a 50% Duty Cycle is only on
half of the time, and therefore only runs with 50% of
the power
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 2
Engineering H193 - Team Project
The Handyboard
• Designed for motors operating at:
– 9.6 V
– 1 A (maximum current)
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 3
Engineering H193 - Team Project
When does Max Current Occur ?
• A motor will typically draw the maximum amount
of current when it is stalled (i.e. not turning or
stopped)
– This current is known as the “stall current”
– A large spike in current (approaching the level
of the stall current) will also occur whenever a
motor is started
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 4
Engineering H193 - Team Project
How to Estimate Max Current
• Measure the motor’s internal resistance using a
multi-meter
– Set the multi-meter to its most sensitive scale
(typically 0 to 200 Ω)
– Apply the probes to the motor terminals and
gently rotate the motor shaft by hand until the
smallest possible reading is obtained
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 5
Engineering H193 - Team Project
How to Estimate Max Current
• Apply Ohm’s Law ( V=IR ) to find the current draw
based on a voltage of 9.6V
– A reading of 9Ω would mean a 1A draw (the
maximum the Handyboard can deliver)
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 6
Engineering H193 - Team Project
Incompatible Motors
• Typically the Handyboard will not run properly
with:
– 3V-4.5V toy car motors
• They are too electrically noisy and may
draw several amps of current
– High end radio controlled car motors
• They draw large amounts of power and
require 25, 50 or more starting amps
• Drawing too much current from the Handyboard
will lead to a board reset or possibly a lot worse
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 7
Engineering H193 - Team Project
Motor Characteristics
• The output properties of primary interest for a DC
motor are shaft torque and speed
– Typically these quantities are represented as a
plot of Torque vs. Speed
• The type of DC motors commonly used in
this lab will exhibit an operating
characteristic known as “negative speed
regulation” – which just means that the
motor’s speed decreases as the load
connected to its shaft increases
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 8
Engineering H193 - Team Project
Motor Performance Plots
Torque vs Speed
• The maximum speed with
no load attached is called
the “no-load” speed (nr)
Stall Torque
20
15
Torque
• The load that stalls the
motor is called the “stall
torque” (also the starting
torque)
25
10
No Load Speed
5
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Speed
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 9
5
Engineering H193 - Team Project
Lab Guidelines
• BE SURE TO PRINT OFF EXTRA COPIES (one per team) OF
YOUR TABLE AND TORQUE vs. SPEED GRAPH
• Lab Report Guidelines:
– Due one week after lab
– Individual lab reports
– Five pages maximum (including cover page, sketches,
attachments)
• See Lab Write-Up for more specific instructions
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 10
Engineering H193 - Team Project
Questions ?
Spring Quarter
Gateway Engineering Education Coalition
Lab 2
P. 11