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Motor Dynamics

Motor Dynamics. Lab 2. Pulse Width Modulation. Pulse Width Modulation (or “PWM”) is the manner by which most of today’s DC motors are controlled

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Motor Dynamics

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  1. Motor Dynamics Lab 2 Spring Quarter

  2. 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

  3. The Handyboard • Designed for motors operating at: • 9.6 V • 1 A (maximum current) Spring Quarter

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. Motor Performance Plots • The maximum speed with no load attached is called the “no-load” speed (nr) • The load that stalls the motor is called the “stall torque” (also the starting torque) Spring Quarter

  10. 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

  11. Questions ? Spring Quarter

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