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IT 318 - Lab #7
Motors
Objective
Observe the operational characteristics of the three major types of motors relative to
speed, torque, current, and motion.
Equipment Required
DC power supply, variac, DMM, integrated brushless DC motor, DC motor on stand, AC
shaded-pole induction wall-plugged motor.
Procedures
These 3 groups of procedures may be done in any order; it does not matter which motor
you begin with.
DC Motor
1. With the motor disconnected, turn the shaft at various speeds with your fingers and
feel the way it rotates. Record your observations.
2. Connect the motor to +6 Volts with no load and measure the armature current.
3. Briefly stall the motor with your fingers and measure the armature current. Do not
leave the motor stalled for more than a few seconds. Record this value.
4. Vary the load on the armature; observe the resulting armature current. Do this by
applying a 10g weight to a cotton string draped over the flywheel. Then increase to 20g, 30g,
40g and so on up to 100g. Plot the resulting relationship.
5. Continue incrementing the load on the motor by increasing the weights in 10g
increments (or more if necessary); observe the weight required to stall your motor. Remember to
not leave the motor stalled for more than a few seconds.
6. Create a speed/voltage plot by measuring the RPM of the motor at +2 Volts, then again
at +3 Volts, +4 Volts, +5 Volts, and +6 Volts. The RPM may be measured by measuring the
frequency of the tachometer output wires, then dividing by 3 (the number of armature windings).
7. Reverse the power supply leads to the motor, so that you are essentially applying -6
Volts to the motor. Record your observation of what happens.
Brushless DC Motor
It is important to remember that all brushless DC motors require external commutation.
This particular model has the commutation electronics built into the motor. Be sure to observe
the proper connections for the motor, or it will not function as you would expect. Use the 24 Volt
output of the power supply for wire #1 (red) and wire #2. Make sure you do not reverse these
two wires. Use another variable output of the power supply for wire #4 (speed command), but
make sure you do not exceed the 5V maximum for this input. Note: for this motor, it is
essential that you never reverse the polarity of the applied voltage, or you will burn out the
commutation electronics.
1. With the motor disconnected, turn the shaft at various speeds with your fingers and
feel the way it rotates. Record your observations.
2. With the motor wires properly connected, measure the no-load current with a speed
voltage of 1.0 Volts. Repeat for speed voltages of 3.0 Volts, 4.0 Volts, and 5.0 Volts.
3. Apply a variable load to the motor by placing a metal rod on and perpendicular to the
shaft. Observe the current response as you change the frictional load using the metal rod.
4. How would you reverse the direction of this type of motor? (study the spec sheet).
Once you’ve figured it out, try it and record your observations.
Shaded-Pole AC Induction Motor
AC induction motors use the commutation effect of the AC voltage; no brushes,
commutator, nor external commutation is required. These are some of the simplest motors made,
but the shaded-pole variety of AC induction motor suffers from extremely low starting torque.
1. With the motor disconnected, turn the shaft at various speeds with your fingers and
feel the way it rotates. Record your observations. How freely does the shaft rotate?
2. Connect the motor to the output of the variac. Start with the variac set to 0. Record the
armature current and observe the rpm as you change the output of the variac to 20, 40, 60, 80,
then 100.
3. Turn the variac back down to 20. Invert the plug from the motor into the variac; record
your observations when you plug it back in with the leads thus reversed. What do you think it
would take to reverse the direction of rotation of a shaded-pole AC induction motor?
4. Turn the variac to 50. Hold the motor and rotor firmly and then plug it in; feel the
starting torque. Would you expect it to be more?
Conclusion
Summarize the results of this lab in relation to the objective. Build a table of comparison
for these three motor types. What did you learn? What more would you like to know? Were any
problems encountered? If so how were they resolved.
Write this as though you are teaching the concepts from this lab to a student just getting
into electronics and computers.