Motor and Drive Notes

For the motor I have:

• pulley diameter on motor = 9/32 inch
• pulley diameter on disk drive = 2 31/64 inch
• ratio motor/drive = 6/53
• a 360K floppy disk rotates at 300 RPM (source: The Indispensable PC Hardware Book, Hans-Peter Messmer)
• therefore, at about 5 V (determined in the lab using a photo tach) (previously assumed at 12 V, an incorrect assumption!) the motor rotates at approximately 2,650 RPM (laboratory test confirms this)
• at 12 V, unloaded, the motor rotates at approximately 7,450 RPM (determined in the laboratory using a photo tach)
• evidence from the laboratory indicates that the built in tachometer has one period for every 7.6 shaft rotations (go figure!)
• the armature resistance is 280 W (<--also incorrect! it is actually about 130 W????)

For the wheels:

• wheel diameter is approximately 1 3/4 inch
• wheel circumference is approximately 5.4978 inches

If we want the vehicle to move at 10 ft/sec (a brisk walk is about 3 MPH, or 4.4 ft/sec and 10 MPH, for 6 min. per mile, a fast mile (for a human running), is 14.67 ft/sec), then:

distance traveled per revolution is (5.4978/12)ft = 0.45815 ft

this means that the wheel must make 10/0.45815 = 21.827 revolutions per second,

that is 1,309.6 RPM

This results in a requirement of a 5.69:1 gear ratio (that's (driven gear):(drive gear)) to achieve 10 ft/sec (6.82 MPH) at 12 V

The equation (assuming 12 V):

= (driven gear diameter or # of teeth)/(drive gear)

d = wheel diameter in feet

v = desired maximum speed in ft/sec

Determining the Torque-Speed Curve

Prediction: It should be linear.

Tools required:

• photo tachometer
• 2 voltmeters
• 1 ammeter
• oscilloscope
• variable voltage power supply (0 to 12 volts)
• variable power resistor

Test Setup Procedure:

• Attach two identical motors to a plate
• Couple them with a belt
• For the drive motor, connect the power supply, voltmeter, and ammeter to the motor input and one channel of the oscilloscope to the built-in tachometer
• For the driven motor, or generator, connect the variable power resistor, and voltmeter to the generator output and the other channel of the oscilloscope to the built-in tachometer
• As a test start the motor off at 12 volts with the load resistor on the generator at some high value. This should result in a rotational speed of approximately 7,450 RPM. Measure the motor shaft speed using the photo tachometer. Now set the motor voltage to 5 volts and measure the shaft speed. It should now be about 2,650 RPM; confirm using the photo tachometer.

Procedure to Determine Efficiency:

• With the motor voltage still at 5 volts, adjust the power resistor on the generator until the voltage across it is 2.5 volts.
• Measure the voltage (ensure it is close to 5 volts) and current being supplied to the motor and the current and resistance on the generator. The power input to the motor is IV and the power output from the generator is I²R. Assuming the efficiencies of both motors are the same, as generator and motor, the efficiency may be determined by taking half the difference between the power input and the power output, adding it to the power output, and dividing the power input by that number. The equation:

power input / (power output + (power input - power output)/2)

Torque-Speed Curve Procedure:

• Now, with the efficiency you can determine the shaft horsepower of the motor using the power input to the motor.
• Beginning at 1 volt, repeat the following procedure up to 12 volts
• Handed to Group 2 at this bullet.
• Repeat in the opposite direction of rotation. (There will be a difference, unfortunately.)

What parameter are we looking for? We want to know how much torque the motor can deliver at a given speed at the various applied voltages.

From electric machines, the power equation for a motor is known to be

hp = (torque x RPM)/560

and 1 horsepower is equal to 745.7 Watts.