12.5 The Motor Principle When English physicist Michael Faraday
saw that an electric current in a wire caused a compass needle to
move, he was curious if the reverse could be true. Could a magnetic
field cause a current-carrying conductor to move? Supporting a bar
magnet in a pool of conducting liquid mercury, Faraday suspended a
copper wire, allowing it to make contact with the pool. The wire
and the mercury were connected to a battery to complete the
circuit. Once connected, the wire rotated around the bar magnet:
The first electric motor.
Slide 2
12.5 The Motor Principle The copper wire in Faradays experiment
was able to rotate around the permanent bar magnet because the
magnetic field in the copper wire interacted with the magnetic
field of the bar magnet. Where two interacting magnetic fields are
pointing in the same direction, there is a repulsion force; when
the interacting fields are pointing in the opposite direction,
there is an attraction force. Here the fields interacting cause a
downward force.
Slide 3
12.5 Right Hand Rule for the Motor Principle Motor Principle A
current-carrying conductor that cuts across an external magnetic
field experiences a magnetic force perpendicular to both the
external magnetic field and the direction of electric current.
Right Hand Rule for a the Motor Principle (RHR #3): If the fingers
of your open right hand point in the direction of the external
magnetic field, and your right thumb points in the direction of the
conventional current, then your palm faces in the direction of the
magnetic force on the conductor.
Slide 4
12.5 Homework Questions # 1-4 p.566
Slide 5
12.6 The Direct Current Motor A simple DC motor uses an
electric current in a looped conductor, which generates a magnetic
field that interacts with an external magnetic field to cause
rotation. How do you make this motion continuous? Scientists wanted
to find a way to temporarily interrupt the current flow, to change
its direction, thus changing the direction of the magnetic force
produced One simple, but ingenious idea was to use a split-ring
commutator. Brushes, made out of conducting bristles, make contact
with the commutator, but still allow rotation.
Slide 6
12.6 DC Motor Step 1 A conventional current is directed from
the +ve terminal through the left brush, into the purple end of the
split- ring commutator. The charges flow into the left end of the
loop and exit from the right side into the pink end of the
commutator, through the right brush, back to the -ve terminal.
Using RHR #3 for the motor principle, the magnetic force produced
acts downward on the left side of the loop, and upwards on the
right side of the loop. This starts the counter-clockwise
rotation.
Slide 7
12.6 DC Motor Step 2 The motor continues to rotate counter-
clockwise as in Step 1. The current is directed into the purple end
of the split-ring commutator. The charges continue to flow into the
left end of the loop and exit from the right side into the pink end
of the commutator, through the right brush. The forces continue to
act in the same direction, as per RHR#3.
Slide 8
12.6 DC Motor Step 3 The wire loop has now rotated to the
split. At this point, the circuit is now open; there is no current
flow, and no more magnetic fields being produced by the loop of
wire. The loop however, continues to rotate forward due to its
inertia.
Slide 9
12.6 DC Motor Step 4 The current is now directed through the
left brush, however into the pink end of the split-ring commutator.
The charges continue to flow into the left end of the loop and exit
from the right side into the purple end of the commutator, through
the right brush, back to the -ve terminal. Using RHR #3, the
magnetic force produced continues to act downward on the left side
of the loop, and upwards on the right side of the loop. The
counter-clockwise rotation continues. Every half turn the loop
makes, the current will be interrupted, continuously changing which
end of the commutator it enters, keeping the loop turning.
Slide 10
12.6 Improving the Design A motor with one loop will not be
very strong. In order to improve the strength of the magnetic field
in the loop, you can; Increase the number of loops, increase the
current, and include a soft-iron core Increasing the current
however is not a desirable choice because it produces more thermal
energy as a side effect. Designers typically focus on more loops
and include a soft iron core called an armature. In order to
maintain a high constant speed of rotation, several coils are put
into the motor and several splits are included in the
commutator.
Slide 11
12.6 Armature DC Motor Step 1 A current is directed from the
+ve terminal through the lower brush, into split-ring B. The
charges flow around the coil, upwards on the front side, and exit
from split-ring A, through the upper brush and back to the -ve
terminal. Instead of using RHR #3 for the motor principle, we use
RHR #2 for coiled conductors. The north pole produced on the left
side of the armature repels the north pole of the external magnet.
This starts the clockwise rotation.
Slide 12
12.6 Armature DC Motor Step 2 The current is still directed
into split-ring B and the clockwise rotation continues. At this
point of the rotation, the north pole of the armature is now
attracted to the south pole of the external magnet on the right
side; the south pole of the armature is attracted to the north pole
of the external magnet on the left side. The armature continues to
rotate until it reaches the split. The circuit is interrupted and
there is no current. The armature continues to spin due to
inertia.
Slide 13
12.6 Armature DC Motor Step 3 The current is now directed into
split-ring A. The charges continue to flow around the coil, upwards
on the front side, and exit now from split-ring B back to the -ve
terminal. Using RHR #2, the left side of the armature is a north
pole again. It repels the north pole of the external magnet,
continuing the clockwise motion. Due to the split, the magnetic
poles of the armature continue to reverse every half rotation,
allowing the coil to continue to rotate.
Slide 14
12.6 Applications of Electric Motors Electric motors are used
in many electric devices. Many mechanical movements are caused by
electric motors. They are found in cars, trains, and household
appliances. Power tools use them to apply forces; laptop fans for
cooling; DVD players for spinning. Some hybrid cars rely on
electric motors for propulsion. Electric motors run on battery
power, reducing pollution from gasoline engines. Once the battery
runs low, the gasoline engine kicks in, while at the same time
charging the battery.