From Electric Power Systems: A Conceptual
Introduction by Alexandra von Meier, 2006
THE PAPER CLIP MOTOR
Building something with our own hands often provides a new quality of insight,
not to mention fun. With a few inexpensive materials, you can build your own
d.c. electric motor. The process of fiddling with your motor to get it to work
well illustrates the principles of physics as no textbook description can, and
4.1
THE SIMPLE GENERATOR
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watching it actually spin gives tremendous satisfaction. Putting your paper clip
motor together takes only a few minutes, and it is worth it!
Materials Needed
2 paper clips
1 small, strong magnet (from Radio Shack; most refrigerator magnets are too
weak)
1 C or D battery
1 yard of 20-gauge (AWG 20) coated copper wire
2 rubber bands or tape
1 small piece of sandpaper
Make a tidy wire coil by wrapping it 10 times or so around the battery. Leave a
few inches of wire at both ends. Tighten the ends around the coil on opposite
sides with an inch or more of wire sticking out straight from the coil to form
an axle on which the coil will spin (see Figure 4.4).
Attach the magnet to the side of the battery. It may stick by itself, or you may want
to secure it with a rubber band or tape. The magnet’s north or south pole should point
directly away from the battery (this is the way the magnet naturally wants to go).
Bend the two paper clips and attach them with a rubber band or tape to both
ends of the battery so as to form bearings on which the axle rests. The clips need
to be shaped so that they make good electrical contact with the battery terminals,
allow enough room for the coil to spin in front of the magnet, and keep the axle
in place with a minimum of friction.
After checking the fit of the axle on the bearings, use the sandpaper to remove
the red insulation coating on ONE end of the wire so that it can make electrical
contact with the paper clip. At the other end, remove the coating on only HALF
the wire by laying the wire coil flat down on a table and sanding only the top side.
This will interrupt the electrical contact during half the coil’s rotation, which is a
crude way to reproduce the effect of commutator brushes.
(Ideally, the direction of current flow through the coil should be reversed with
every rotation, which would then deliver a steady torque on the coil in one
direction; this is what commutator brushes do. If direct current were allowed
to flow continuously, the direction of the torque on the coil from the changing
magnetic flux would reverse with every half-turn of the coil. Simply interrupting
the current for half a turn interrupts the torque during just that period when it
would be pulling the wrong way. Once the spinning coil has enough momentum,
it will just coast through the half-turn without power until it meets the correct
torque again on the other side.)
When you place the coil on the bearings with the contact side down and current
flowing, you feel it being pulled in one direction by the interaction of the fields of
the permanent magnet and the coil (the “armature reaction”). Now give the coil a
little shove with your finger and watch it spin!
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GENERATORS
Figure 4.4
4.2
4.2.1
The paperclip motor. (From http://www.motors.ceresoft.org.)
THE SYNCHRONOUS GENERATOR
Basic Components and Functioning
Having characterized the essence of the generator’s functioning in the simplified
version just discussed, we need only make some “cosmetic” changes in order to
turn our description into that of a real generator. Let us begin by clarifying the
nomenclature. The rotating assembly in the center of the generator is called the
rotor, and the stationary part on the outside the stator. In the majority of designs,
and the only ones considered here, the rotor contains the magnet and the stator
the armature that is electrically connected to the load. One simple rationale for
this choice is that the armature typically carries much higher voltages, where
fixed and readily insulated connections are preferable to the sliding contacts required
for the rotating part of the machine. Furthermore, making the armature the stationary
outside part of the machine provides more space for those windings that carry the
most current.
Our first modification of the hypothetical simple generator concerns the rotor.
With the exception of some very small-scale applications, using a permanent bar
magnet to spin around is impractical because such magnets’ fields are relatively
weak compared to their size and weight. Instead, we mimic the permanent
magnet by creating a magnetic field through a coil of wire (a solenoid, as described
in Section 3.2) wound around an iron or steel core of high permeability (see
Section 2.4) that enhances the magnetic field (Figure 4.5). This conducting coil is
called the rotor winding, and its magnetic field the rotor field or excitation field.
As will become relevant later, such an electromagnet has additional advantages