Download Lab The Electric Motor

Name ______________________________________ Period ________
Lab: The Electric Motor
Objective: __________________________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
Concept Drawing
Parts of the St. Louis Motor
a. Field Magnet (or field coil). A device that sets up a stationary magnetic field.
b. Armature. An iron filled solenoid suspended on a axis that is free to rotate within the Field Magnet
stationary magnetic field.
c. Commutator. A part of the armature that allows reversal of the current to the solenoid of the
armature every 1/2 of a rotation.
d. Brushes. Metal tabs that slide against the commutator on the armature that allow current to be
placed into the solenoid of the armature.
Procedure
1. Remove the field magnets from the motor. Connect wire leads from the battery to the motor using the
connectors at the top of the motor or connecting the alligator clips to the metal posts below those
screw connectors. Use a compass to map out the magnetic field polarity of one end of the armature for
one complete revolution of the armature. Record your observations on the drawing below.
Connect battery here or here
Indicate the polarity of pole A at each location shown
A
Armature seen from above
Reverse the battery connections. Does the polarity of the armature reverse? Stay the same? Disappear
altogether? ________________________
2. Disconnect the battery. Replace the field magnets. Connect a voltmeter (set at 200 mV) to the same
connections that the batter was positioned. Rotate the armature rapidly and note the voltage measurement
that results. Rotate the armature in the opposite direction and note the voltage reading. Record your
maximum readings below:
a. Clockwise Rotation Voltage _____________ b. Counterclockwise Rotation Voltage _________
(This arrangement of this motor allows it to act as a generator. If you could provide a way to rotate the
armature continuously it would provide a steady source of current and voltage from the work that you
do rotating the armature.)
3. Disconnect the voltmeter and reconnect the battery. Make sure the brushes are touching the
commutator. Give the armature a gentle push and observe the result. Record the result here.
__________________________________________________________________________________________
4. What keeps the armature rotating when the polarity of the armature and the polarity of the field
magnets is not providing a repulsive force? ___________________
5. Temporarily disconnect the battery while you connect an Ammeter in series with the armature of
the motor. Reconnect the battery and again give the armature a gentle push. Record how/if the current
changes during this process. Supply measurements._____________________________________________
__________________________________________________________________________________________
6. Does reversing the polarity of the battery to the armature change the direction of rotation ? _________
7. Remove the permanent magnets. Replace them with the horseshoe shaped electromagnet (field coil).
Using the connecting wires provided connect the battery, armature, ammeter and this electromagnet in
series.
8. Permit the motor to run freely. What is the direction of rotation? ________________ Reverse the
battery connections. What effect does this change have on the direction of rotation? _________________
Reverse the connections to the field coil electromagnet only. What effect does this have on the direction of
rotation?_________________________________________________________________________________
9. What current does the series connected motor take to run? _____________ Stop the motor and
measure the current the motor uses. What was the change in current? _________________ What is
your explanation for this change in current? __________________________________________________
_________________________________________________________________________________________
10. Draw the schematic diagram for the series connected motor from step 7. Be sure and label all
components and use the correct symbols for each.
11. Rewire the motor so that the armature and the electromagnet are in parallel with each other. In
addition, connect two ammeters so that one registers the current only in the armature and the other
registers the current only in the electromagnet. Draw the schematic diagram for this circuit below.
Again, show all symbols and labels.
12. Measure the current flowing to the armature and to the electromagnet while the motor is stationary
and again while it is in motion. Record your observed measurements below.
Stationary
Armature Current (I)
Electromagnet (field coil)
________________
________________
In Motion
__________________
__________________
13. Measure and record the closed circuit voltage of the battery and the total current as it operates the
motor and calculate the following. Always begin with an equation and always show units.
Voltage Measurement of Battery ____________ Current While Motor Is Operating ________
a. What is the effective resistance of the motor?
b. What is the power consumption of the motor?
c. If this motor was used to raise a small mass of 0.0100 grams upward using a wheel and
pulley, how high would the mass be in 2.00 minutes assuming total conservation of energy?