Download The DC Motor

11/6/2015
SPH3U
UNIVERSITY PHYSICS
ELECTRICITY & MAGNETISM
L The DC Motor
(P.567-571)
Continuous Motion
As we saw with the galvanometer, a current-carrying coil pivoted in a
uniform magnetic field will begin to rotate. However, a closer examination
of the galvanometer design reveals that the coil will rotate only until it is at
right angles to the magnetic field and then it will stop.
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Continuous Motion
Thus, for the coil to continue to rotate, the direction of the force on it
would have to change every half rotation. This could happen only by
changing the direction of either the external magnetic field or the current
flowing through the coil.
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The DC Motor
One simple but ingenious idea was to
create a device called a split ring
commutator. The commutator is split so
that the circuit is incomplete when the
loop is aligned with the split. The split
ring commutator and the wire loop
(which are connected) are free to rotate
around an axis. The brushes are made
out of conducting bristles. They make
contact with the split ring commutator
but still allow rotation.
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The DC Motor
A DC motor uses an electric current in a
conductor which generates a magnetic
field that interacts with the external
magnetic field to cause rotation.
NOTE!
The rotor is the part of the motor that
rotates and the stator is the part of the
motor that remains stationary. In the
DC motor shown, the loop is the rotor
and the external magnets are the stator.
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The DC Motor
DC MOTOR
uses an electric current in a coil to
produce a magnetic field that
interacts with the external magnetic
field to cause rotation
consists of a rotor (part that rotates)
and a stator (part that is stationary)
PRACTICE
1. How does a DC motor work?
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The DC Motor – How Does It Work?
Conventional current is directed from the
positive terminal toward the brushes,
making contact with the purple part of
the split ring commutator. Charges flow
into the left-hand side of the loop and
exit from the right-hand side into the
pink part of the split ring commutator to
the brush and back to the negative
terminal. Using the right-hand rule for
the motor principle, we see that the
force is downward in the left-hand side
of the loop and upward in the right-hand
side. This will start a counter-clockwise
rotation.
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The DC Motor – How Does It Work?
The motor continues to rotate counterclockwise, and the situation is the same
as it was in the first slide. The current is
directed to the purple split ring of the
commutator, and charges flow into the
left of the loop and exit from the right of
the loop. The forces are still in the same
direction, as predicted by the right-hand
rule.
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The DC Motor – How Does It Work?
The wire loop continues to rotate to the
split. The circuit is now open, there is
no current, and no more magnetic fields
are being produced by the loop of wire.
However, the loop will continue to spin
due to inertia.
NOTE!
Law of Inertia – objects in motion tend
to stay in motion and objects at rest
tend to stay at rest.
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The DC Motor – How Does It Work?
The electric current is now directed into
the pink part of the split ring
commutator. This directs the current
into the left-hand side of the loop once
again and out of the purple part of the
split ring to the brush and to the
negative terminal. Using the right-hand
rule, you can see that the right-hand
side of the loop is being forced upward
while the left-hand side is being forced
downward. So the counter-clockwise
rotation continues.
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The DC Motor – How Does It Work?
The rotation of the loop continues
counter-clockwise until it again reaches
the split in the split ring commutator.
This will once again interrupt the electric
current until contact is made from the
positive terminal to the purple part of
the split ring commutator. At this point,
the process starts over.
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The DC Motor
PRACTICE
1. The conductors shown represent a
loop in a magnetic field. Will the
loop spin clockwise or counter
clockwise?
clockwise
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The DC Motor
PRACTICE
2. For the instant shown, will the loop
turn clockwise or counter clockwise?
clockwise (wrt to battery)
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The DC Motor
PRACTICE
3. Describe two possible ways of
forcing the loop shown to rotate
clockwise.
Î
Ï
switch the + and – leads
(changes the direction of the
electric current)
switch the N-pole and S-pole
(changes the direction of the
external magnetic field)
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The DC Motor – Improving The Design
This simple DC motor design is not very
powerful or efficient. To improve the
strength of the magnetic field in the loop
(and increase the speed), you can
increase the number of loops, increase
the current, or include a soft-iron core.
Increasing the current is not a desirable
choice because it will produce more
thermal energy as a side effect. So
designers increase the number of loops
and include a soft-iron core called an
armature. A strong electromagnet is
often used as the external magnet.
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The DC Motor – Improving The Design
DC MOTOR IMPROVEMENTS
to improve the strength of the
magnetic field in the loop you can:
U increase the number of loops
U include
a
soft-iron
core
(armature)
Y
increase the current (produces
too much heat)
NOTE!
A strong electromagnet is often used as
the field magnet.
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The Armature DC Motor – How Does It Work?
In diagram (a), electric current flows in through the bottom brush, into
split ring B, and through the coil, eventually entering split ring A and
leaving the motor through the top brush. Using the right-hand rule for a
coil, end A of the armature becomes a N-pole and is repelled by the N-pole
of the external magnet, causing it to move away and rotate clockwise.
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The Armature DC Motor – How Does It Work?
In diagram (b), tracing the path of electric current through the motor the
right-hand rule for a coil verifies that end A remains a N-pole and is,
therefore, attracted toward the S-pole of the external magnet.
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The Armature DC Motor – How Does It Work?
In diagram (c), a significant change occurs. The top brush is now in
contact with split ring B. Electric current continues to flow in through the
bottom brush up through the coils, leaving by split ring B and the top
brush. End A of the armature now becomes a S-pole and is repelled by the
S-pole of the external magnet, causing the clockwise motion to continue.
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The Armature DC Motor – How Does It Work?
In diagram (d), tracing the flow of electric current through the motor again
the right-hand rule for a coil confirms that end A of the armature remains a
S-pole and is attracted toward the N-pole of the external magnet,
completing one full rotation of the motor.
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The DC Motor
DC MOTOR COMPONENTS (P.569)
coil of wire (wound on soft-iron core)
armature
(soft-iron
core
that
becomes an electromagnet when
current flows through the coil)
external magnets (provide external
magnetic field)
split ring commutator (changes the
direction of the current flow through
the coil of wire every 180E)
brushes (allows current to flow from
external circuit through the split ring
commutator to the coil)
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The DC Motor
PRACTICE
4. What should the external magnet
poles be in order for the motor to
spin counter-clockwise?
S
N
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The DC Motor
PRACTICE
5. What effect would each of the following changes have on a DC motor?
Consider each change separately.
(a) increasing the number of loops in the coil
(a) stronger/faster (F % N)
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The DC Motor
PRACTICE
5. What effect would each of the following changes have on a DC motor?
Consider each change separately.
(b) using a plastic core instead of a soft-iron core
(b) weaker/slower (F % :)
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The DC Motor
PRACTICE
5. What effect would each of the following changes have on a DC motor?
Consider each change separately.
(c) decreasing the current
(c) weaker/slower (F % I)
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The DC Motor
PRACTICE
5. What effect would each of the following changes have on a DC motor?
Consider each change separately.
(d) reversing the polarity of the external magnets
(d) reverses direction of rotation
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The DC Motor
PRACTICE
5. What effect would each of the following changes have on a DC motor?
Consider each change separately.
(e) reversing the polarity of the external magnets and reversing the
direction of the current
(a) nothing – one cancels the other
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The DC Motor – Further Improvements
The armature design greatly improves the power of an electric motor.
However, there are still some problems:
1. The magnetic force is strongest in the coil when it is lined up with the
magnetic fields of the external magnets. However, as the coil rotates
away from being lined up with the external magnetic field, the strength
of the magnetic force on the coil weakens and the motor slows down.
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The DC Motor – Further Improvements
The armature design greatly improves the power of an electric motor.
However, there are still some problems:
2.
If the motor is turned off just as it reaches the split in the split ring
commutator, then it will not be able to rotate when the motor is turned
on again because the circuit is incomplete. You would have to give the
motor a push.
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The DC Motor – Further Improvements
To overcome these issues, DC motor
designers put several coils into the motors
and use a split ring commutator with
several splits. This means that:
1. the speed of the motor does not
fluctuate as much and
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The DC Motor – Further Improvements
To overcome these issues, DC motor
designers put several coils into the motors
and use a split ring commutator with
several splits. This means that:
2. a segment of the multiple split ring
commutator is always in contact with
the external circuit (so the motor does
not need to be pushed by hand to
start it).
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The DC Motor – Further Improvements
FURTHER DC MOTOR IMPROVEMENTS
using several split rings and coils
prevents the need to manually start the
motor
by varying the current in the coils the
rate of rotation is easily controlled
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Applications of Motors
Electric motors are all around us and many mechanical movements you see
are caused by an electric motor. They can be found in household
appliances, cars, and trains. They are used to apply forces (i.e., power
tools), for cooling (i.e., in laptop computer fans), as starters (i.e., in cars),
or to move things (i.e., to spin a DVD).
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Applications of Motors
Some cars now rely on electric
motors for propulsion. Hybrid cars
use an electric motor alongside a
gasoline engine.
The electric
motor runs on battery power
which reduces pollution from the
gasoline engine. Once the battery
runs low, the electric motor can no
longer propel the vehicle. The car
then runs on its gasoline engine
and at the same time charges the
battery.
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Applications of Motors
While completely electric vehicles
have the potential to be
environmentally
friendly,
it
depends on how the electricity
used to charge the battery is
generated.
In addition, the
motors contain heavy metals,
which can be toxic to living things
once they are recycled.
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U Check Your Learning
TEXTBOOK
P.571 Q.1-3
WIKI (ELECTRICITY & MAGNETISM)
O.... 3U4 - PRJ1 (The DC Motor)
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3U4 – PRJ1 (DC Motor): hints for Q.3 & 5
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