Construction of an Electric Motor©98
Experiment 11
Objective: To construct and understand the principles underlying the operation of an
electric motor, and to determine the magnetic field created by a self-made coil.
DISCUSSION:
See your text to review the nature of magnetic field produced by a current moving
through a coil. Consider the implications of having two magnetic fields that are not in
the same direction and what must result from this conflict. This is the basic principle
behind the electric motor. We omit other details in order to proceed with the construction
of the motor.
EXCERSISES:
There are no partners for this experiment. Each person should construct their own
motor. Of course feel free to discuss design principles and share innovations you may
think of with your TA and fellow students.
1. Using the long segment of copper wire, make a one inch diameter coil of wire by
wrapping it around the small dowel rod provided.
The smaller the diameter creates more turns, but that
does not necessarily mean that the motor will go
faster. Comment on why this is true in your report.
2. Straighten the ends of the wire as illustrated
in Figure 1. To make the windings stay together, you
can twist an additional piece of wire around the loops.
Figure 1: The motor coil.
3. Make two paper clip brackets to support
the coil. They should be fastened to the wood
block with thumbtacks. The battery will be
connected to the paperclips, which will feed the
current to the coil.
4. Bend the paperclips so they freely
support the motor coil. Refer to Figure 2 for
reference.
Figure 2: Paperclip supports for the motor coil.
5. Remove the shellac insulation from one side of
one end of the motor coil, as shown in Figure 3, using
a razor blade and finishing with sandpaper. This
creates a commutator, giving the coil a pulse of current
on every half rotation, without which the motor will not
Figure 3: Half stripped wire.
continue to turn. It is up to you to think through and
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explain in your write-up which half, in relation to the plane of the coil, should be
removed for the best performance of your motor.
6. Remove all the insulation from the
other end of the coil.
7. Secure the magnets, using the doublesided tape, on edge on either side of the coil
as shown in Figure 4. Place them as close as
possible to the coil, yet not so close that the
coil cannot rotate freely. Should opposite or
like poles face each other? Secure them with
small pieces of mounting tape.
Figure 4: Overall picture of complete motor.
8. Connect the paperclips to the battery with the light gauge wire, one to each end to
provide electricity for the motor. Don't forget to scrape the insulation off the wire with a
razor blade or sandpaper. The battery should be secured to the board with a strip of
double-sided tape. The ends of the wire can be secured to the battery also with doublesided tape.
9. Measure the speed of your motor with a stroboscope (your laboratory instructor
will help you). Think about how this is possible and explain in your report what is being
done. A prize possibly will be given to the people with the fastest rotating motors, and
the cleverest designs.
10. In your report, discuss the speed of your motor. Your grade for the experiment
could be partially based on the speed of your motor. The fastest may receive a 100%
grade, and each slower motor the percentage of the fastest speed… In any event, include
diagrams showing the directions of the current, magnetic fields, and forces involved
when the motor is running. Explain the operation of the motor. Also, include an
explanation of exactly how the stroboscope is used to measure rotational speed.
11. If all fails and your motor is not made operable even with the TA’s assistance, be
prepared in your report to discuss exactly what the problems were, and what could be
done to improve it or repair it.
12. If your motor is not being kept, be sure to do the following: Remove all tape and
tacks from the wood block base. Discard bent paperclips and tacks. Put the block, good
tacks, unbent paperclips, magnets, and battery and bag in the appropriate in-house recycle
in boxes at back.
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