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3.4
Exploration 12: Reverse Engineering of a DC Motor
In Exploration 12 and Extension 6 you saw that if a loop carrying current
was placed in a magnetic field, the resulting force could cause a rotation. In this
Exploration you will look at a device that uses this effect in a practical manner.
Throughout this Exploration take careful notes about your discoveries.
Equipment:
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Small 1.5-3 V DC motor
Compass
Battery pack (1.5-3 V)
Connecting wires
Small screwdriver
Magnifying glass
1.
Connect the motor to the battery pack and observe its motion. In particular,
note the direction of rotation as compared to the direction of current flow.
Disconnect the motor from the battery pack.
2.
Carefully dismantle the motor. Ask your instructor if you are unsure of how
to remove the case of the motor. Take care as you are working not to
damage any of the parts of the motor – you want to be able to reassemble
it.
3.
Locate the magnets in the motor and determine their polarity.
4.
Locate the windings (coils) and their electrical connections.
5.
You will also find a circular device with splits, or grooves in it. This is called
the commutator. If you are unsure of where this is located, ask your
instructor.
6.
Carefully draw a diagram showing the basic structure of the motor.
7.
Look carefully at the motor and try to determine the path of the current
through the motor. You will want to spend some time on this step, since the
circuit is probably more complicated than you think at first glance. Look at
the current flow for different positions of the motor windings. Is the current
flow always the same? What do you think is the role of the commutator in
the motor?
8.
Once you have figured out the current flow, check with your instructor to
see if you have it right. Then consider how the current interacts with the
magnetic fields of the magnets to make the motor turn. Remember to apply
the right-hand rule as necessary. When you think you have figured out the
interaction between the current and the magnetic fields, check with your
instructor to see if you are right.
9.
When you have completed your investigation and verified with your
instructor that you have correctly identified the function of each part of the
motor, carefully reassemble the motor.
10.
Using what you have learned, write a detailed summary of the structure and
operation of the electric motor.
3.5
Dialog 13: The DC Motor
In Exploration 13 you identified the parts of a DC motor. You may have
been surprised at the simplicity of its construction. It is a common misconception
that electric motors are very complicated devices, but in reality they have only a
few components. While dismantling your motor you should
have been able to identify the magnets, the coil (current loop),
and the commutator. How does each of these components
contribute to the operation of the motor?
The main purpose of the electric motor is to convert
electrical energy into useful mechanical work. The concept necessary to
create the mechanical work is to create a torque on a coil in an external
magnetic field. The torque occurs when forces are applied to the coil at a
distance away from the axis of rotation, or shaft. This causes the coil to rotate.
There are two ways to model this rotation. One model considers the magnetic
poles created by the current in the coil, which is an idea we discussed in section
2.17 of this module. The other model considers the forces on the currents in the
coil, as we discusses in section 2.8 of this module. They are both excellent
models, but they view what is occurring in a different manner. We will investigate
both models.
The Model Using Magnetic Poles
In this model the electric current going through the coil sets up a magnetic
field. The right hand rule, which we discussed in section 2.17 of this module, can
be used to determine which side of the coil is the north pole and which side of the
coil is south pole. The north and south poles will be attracted to or repelled by the
external permanent magnets.
To see this in more detail, let us start with the coil in the position of
maximum torque. A coil in the position of maximum torque between two
permanent magnets is shown in Position 1 and the graph below. Note the
direction of the current in the coil. It enters the coil in the wire that has an
identifying dot on it. The south pole of the coil is attracted to the north pole on
one of the permanent magnets, while the north end of the coil is attracted to the
south end of the other permanent magnet. This causes torque on the coil and
causes it to rotate.
The coil rotates approximately 45o and is at Position 2, shown below. The
torque is still in the same direction but is now smaller (see Position 2 on the
torque vs. time graph below). The torque is smaller because the perpendicular
distance between the forces is smaller. The current is still entering the coil on the
wire with the dot.
Another rotation of 45o brings the coil to Position 3. There is no torque at
this position (see position 3 on the torque vs. time graph). The current in the coil
is reversing direction at this position. The coil keeps turning because of inertia.
In Position 4 the current has already reversed its direction. The current
now enters the coil on the wire without a dot and leaves on the dotted wire. This
reversing of the current creates magnetic poles on the coil that keep the coil
rotating in the same direction. The new north pole of the coil is repelled by the
north end and attracted to the south end of the permanent magnets. The new
south pole of the coil is repelled by the south and attracted to the north ends of
the permanent magnets.
Position 5 is again at a position of maximum torque. The perpendicular
distance between the forces is at a maximum. When comparing the figures for
Position 1 and Position 5, they appear similar at first. However, there are subtle
differences between the two positions. The first difference is the direction of
current flow. In Position 1 the current flows into the coil on the dotted wire and
out of the coil on the wire without a dot. The current direction was reversed in
position 3 so now the current flows into the coil on the wire without a dot, and out
of the coil on the dotted wire.
The reversal of the current also reverses which side of the coil is north and
which is south. In Position 5 the south pole of the coil is still on top. In Position 1
the same physical side of the coil was on the bottom and was the north pole.
Between Position 1 and Position 5 the coil has rotated 180 o or ½ rotation.
We have explained the first half of the rotation in detail. We will not explain
the second half of the rotation in as much detail because the corresponding
positions compared to those of Positions 1 through 4 would be very similar.
Therefore, the only position shown for the second half of the rotation is position
6. In position 6 the coil has rotated 270o or ¾ of the rotation. Here the torque is
zero, which is similar to Position 3. Position 6 is important because the current
reverses here. The reversal of the current creates the magnetic poles that will
carry the coil back to Position 1 and from there the whole process will repeat.
Observe the coil positions again to see if you understand the model. Can
you answer these questions?
1.
From the direction of the current in the coil, can you use the right hand
rule and determine the north and south poles of the coil?
2.
Once the poles of the coil are determined, can you determine which
way the coil will turn in the external field?
3.
Where are the positions of maximum and minimum torque? Do you
know why?
4.
At what positions does the current reverse?
5.
Using the six position diagrams, can you identify the two positions
where the current reversed?
6.
Can you explain why the current reversals are necessary keep the coil
rotating in the same direction?
If you can answer these six questions, you are ready to explore an
alternative model.
The Model Using Forces on Currents
This model considers the current that flows through the coil and its
interaction with the external magnetic field caused by two permanent magnets.
The sides of the coil will have forces on them given by F = ILB sin  (we
discussed this force in section 2.8 of this module). Since the forces applied to
the coil are at a distance away from the axis of rotation, or shaft, torque is applied
to the coil, causing it to rotate. This basic process is illustrated in the following
diagrams.
Our example will begin with Position 1, shown in the diagram and graph
below. The positions shown in this model are the same as shown in the previous
model. In Position 1 the torque is at a maximum. The wires at the opposite sides
of the coil are denoted by a lighter and darker shade. The wire on the right side
of the coil is darker and its current is coming toward us. Using the right hand rule
shows us that the direction of the force is up. The wire on the left side of the coil
is a lighter color and the current is going away from us. Using the right hand rule
shows us that the force is down.
In Position 2 the coil has rotated 45o, as shown in the diagram below. The
direction of the current and external magnetic field is the same. The only
difference is that the perpendicular distance between the two forces is closer
together so the torque is smaller (see the torque vs. time graph below).
In Position 3 the diagram shows that the coil has rotated 90o from Position
1. The direction of the current and external magnetic field is still the same. The
two forces are along the same line so will cause zero torque (see the torque vs.
time graph below). The coil will continue rotating by inertia. The current will
reverse itself at this point. We will see this current reversal in the next position.
In Position 4 the current has already reversed direction. Now the darker
wire, which used to have the current coming toward us, has reversed and is now
going away from us. This reversal of current is necessary to reverse the direction
of the forces so the torque on the coil will continue the rotation in the same
direction. The distance between the two forces is similar to the second position,
so the torque is smaller.
In Position 5 the coil has turned ½ rotation, or 180o from the Position 1, so
you will notice that the diagram below looks very similar to the diagram for
position 1. The only difference between these two diagrams is that the coil has
turned ½ rotation in Position 5, causing the dark and light wires to switch
positions. The two forces are at the maximum position apart so the torque is at a
maximum.
The second half of the rotation will be very similar to the first half.
Therefore, all the diagrams won’t be shown. The only diagram shown for the
second half of the rotation is when the coil has rotated ¾ or 270 o from the start.
At this position the forces are along a line so there is zero distance
between them. The torque is zero. The coil continues to turn by inertia. The
current will reverse a second time at this position. This reverses the forces and
keeps the coil turning back to its original position. After one complete rotation, the
process will continue to repeat itself.
Current Reversal
Current reversal is the role of the commutator. The simplest form of
commutator is made of two curved strips of metal attached to the coil of the
motor. These strips are in contact
with two conducting brushes,
which provide the electrical contact
for current to flow to the coil, as
shown in the diagram at right. The
brushes are mounted to the frame
so they stay stationary while the
commutator rotates between them.
The surface of the commutator
and the surfaces of the brushes
are in contact. As the coil turns,
each metal strip first comes into
contact with the positive terminal of the battery, then the negative. This
alternating contact reverses the flow of current through the coil every one-half
rotation, as required.
Note in the diagrams below that the brushes keep their same electrical sign.
The brush on the right is connected to the positive side of the battery and is
always positive. The brush on the left is connected to the negative side of the
battery and is always negative.
The diagrams below also show the coil in two positions. The first shows the
coil corresponding to Position 1 where the torque is at a maximum. The current
flows from positive to negative and is entering the coil on the wire with a dot. The
second shows the coil after it has rotated 180 o and is in Position 5. The current is
now entering the coil on the wire without the dot. The torque is again at a
maximum.
The corresponding position of the coil in Positions 3 and 6 are not shown.
These are the positions where the current reverses. These positions will occur
when the slits on the commutator are in contact with the brushes. At these
positions the commutator’s curved strips of metal are changing which brush they
are in contact with. This change in the electrical sign of the curved strips of metal
results in a current reversal in the coil.
There is still one problem that has to be addressed in a real DC motor. In
the simple motor we have been describing, there is no guarantee that the
brushes will be properly in contact with the commutator strips to start the motor
turning when it is turned on. It is possible that the motor coil is in a position such
that a brush was touching both strips of the commutator at once, or that the
brushes are at the gaps between the strips and not touching either one. In either
case, the motor won’t start. There is also no preferential direction of rotation: the
motor could spin either direction when it is turned on.
In a real motor these problems are prevented by having a number of coils
around the core, each coil with its own set of commutator strips. The
commutator for each coil causes the current direction in each coil to reverse
every one-half turn, as required for proper operation. The location of the coils
and their respective commutators is set so that each coil receives current only
when it is in the proper position to feel the maximum possible torque. This
arrangement assures that there is always a coil in position to receive current, and
the motor always spins when it is turned on. It also assures that the core always
spins in the same direction when turned on.
Attached to the core of the motor is the shaft. This shaft acts as the axis of
rotation for the core, but also serves as the means by which the motor
accomplishes its ultimate task—to do work. The rotating shaft can be connected,
via gears, to any number of devices, from toy cars to drills. The main purpose of
the electric motor is to convert electrical energy into useful work (mechanical
energy).
3.6
Optional - Exploration 13: Building a Simple DC Motor
You have seen how a DC motor is constructed and have learned about the
basic principles that make it work. The basic components are a magnetic field
and a loop of wire carrying current. In this Exploration you will apply those basic
principles as you try to build an operational DC motor – from scratch!
Equipment
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Battery in battery holder
One permanent ceramic magnet
2 Copper wire pivots
Magnet wire (thin, lacquer-coated
copper wire) – approx. 1 meter
Compass
Sandpaper for scraping
Connecting wires
Strobe light
White out
1.
Use the compass to test the polarity of your
magnets so that you know where the north and south poles are.
2.
Put the copper pivots (1” long copper with coiled ends) into the ends of the
battery holder. They should stand upright, and parallel to each other.
They will serve as electrical connections to the batteries, and the rings in
the ends of the pins will serve as supports for the coil of your motor.
3.
Wind a length of the magnet wire around the battery (take it out of the
holder ) many times to form a coil. You should leave about 4 centimeters
of wire loose at each end. Carefully remove the coil from the battery and
wrap the loose ends of the wire tightly around the coil a couple of times to
secure the loops. Leave 2-3 cm of wire sticking out on each side of the coil,
at right angles to the coil and directly across from each other as seen in the
diagram. These ends will serve as the shaft of the coil.
4.
Using sandpaper, carefully remove
the insulation from one side only of
both ends of the wire. You want
one side of the wire to make an
electrical connection with the pivot,
but not the other. Make sure you scrape the same side of both ends.
5.
Place the straight ends of the coil into the copper pivot. The coil should be
centered and balanced so that it rotates easily.
6.
Put the magnets on top of the battery below the coil so that either the north
or south pole faces the coil. What is the direction of the magnetic field in
the region of the coil? ________________________________________
7.
Wire the circuit shown below. The connection to the pivots can be made
with alligator clips.
8.
The moment of truth – does the motor work? Close the switch to test your
motor. You may have to give the coil a few gentle nudges to get it started.
What do you observe?
_______________________________________________ Don’t leave the
motor on very long or it will drain the batteries.
9.
When you get your motor working to the best of your ability, paint one
visible dot, or stripe on the coil with white out. The dot will be used to
determine the maximum rpm (revolutions per minute) of your motor.
○
(size of dot).
10.
Use the strobe light to determine the rpm of the motor. When you see two
dots (remember, you only painted one!) that means the strobe frequency is
twice that of your motor (go to the back of the room and dim the lights for
this). Motor rpm___________
11.
Summarize this Exploration by describing the operation of the motor and
drawing an appropriate sketch. The sketch should show the direction of the
current (I), the magnetic field (B), and the torque due to the magnetic force.
You should explain the operation using the right-hand rule to determine the
direction of force on the current-carrying loop and ultimately the direction of
rotation. Be sure to include in your explanation the reason for removing the
insulation from only one side of each wire. Attach this page to this lab.