Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 1Spring Quarter Motor Dynamics Lab 2

Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 2Spring Quarter 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

Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 3Spring Quarter The Handyboard Designed for motors operating at: –9.6 V –1 A (maximum current)

Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 4Spring Quarter 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

Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 5Spring Quarter 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

Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 6Spring Quarter 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 9 Ω would mean a 1A draw (the maximum the Handyboard can deliver)

Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 7Spring Quarter 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

Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 8Spring Quarter 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

Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 9Spring Quarter Motor Performance Plots The maximum speed with no load attached is called the “no-load” speed (n r ) The load that stalls the motor is called the “stall torque” (also the starting torque)

Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 10Spring Quarter 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

Engineering H193 - Team Project Gateway Engineering Education Coalition Lab 2P. 11Spring Quarter Questions ?