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. EM41 - M.Nelson NASA-Threads 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. EM41 - M.Nelson NASA-Threads

AC - DC Magnitude Time EM41 - M.Nelson NASA-Threads

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. EM41 - M.Nelson NASA-Threads

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. EM41 - M.Nelson NASA-Threads

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. EM41 - M.Nelson NASA-Threads

Transistors Schematic Diagram for an NPN Transistor EM41 - M.Nelson NASA-Threads

Pulsewidth Modulation (PWM) 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). Time Magnitude 100% - Full Speed 0% - Stop EM41 - M.Nelson NASA-Threads

PWM Circuit EM41 - M.Nelson NASA-Threads

PWM Command Pin Duty Cycle (0 – 255  0 – 100%) 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. EM41 - M.Nelson NASA-Threads

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 GOTO Main end EM41 - M.Nelson NASA-Threads

Assignment Finish circuit and run motor. Change loop count as needed to clearly observe changes in motor RPM EM41 - M.Nelson NASA-Threads