Transcript Electric Motors - Mr. Hoover's Science Classes CASS: 2011

ELECTRIC MOTORS
Lesson 11
The motor principle

Michael Faraday was the first person to create a
device that used electromagnets with a permanent
magnet to apply a directed force. This device was
the first electric motor and it used the motor
principle.

Two magnetic fields interact with each other to produce a
force. If the conductor that carries a current cuts through
an existing magnetic field, it experiences a force
perpendicular to both the conductor’s charge flow and the
external magnetic field. The strength of this force depends
on the strength of the external magnetic field and the
current through the conductor.

The direction of the force of the force can be
predicted by using a left hand rule
Left Hand Rule #3 (LHR#3)

Open your left hand so that your fingers point in the
direction of the magnetic field (from north to south).
Rotate the middle finger so that it points in the
direction of electron (-) current flow. The orientation
of the thumb the direction of the force produced.
Electric Motors

The electric motor is a device that directs electric
force full circle, without stopping part way. A DC
current motor is safe, efficient, and cost effective. In
a DC motor, the electromagnet changes its polarity
once every rotation. This polarity change is made
because the split ring communicator reverses the
current flow in the armature.
How the armature works


Stage 1:
The current enters the black half-ring and leaves the
gray half-ring, causing the split ring and the
attached loop to rotate in a clockwise direction.
How the armature works


Stage 2:
The black and gray conducting halves of the ring lose
contact with the black and blue electrodes. At this point,
the insulating portion of each ring is in direct contact with
the electrodes. No current is present in the attached loop.
The loop’s momentum carries it forward until the black
half-ring is now in contact with the negative electrode.
How the armature works


Stage 3:
With the black half-ring in contact with the negative
electrode, the current in the attached loop is now
reversed. Use the left hand rule to verify that this
allows the rotation to continue in the original
direction.
How the armature works


Stage 4:
The motion continues in the same direction. The
rotational motion will be continuous with the split
ring communicator attached. The rotating loop in
the magnetic field may also be used as the basis
for the galvanometer.
The Simple Induction AC Motor


There are several types of AC motors that exist. We
will look at the most basic type.
As previously stated, AC current alternates back
and forth; the current does not flow in one constant
direction as it does in DC. In North America, this
cycles 60 times per second.

The AC motor works on the same principle as the
DC motor, like charges repel causing the motor to
spin. If the motor seen to the right was used it would
spin at around 30 RPS because the current is cycling
at 60 Hz.

There are many other types of AC motors, these are
three examples of 3 phase motors
Building a basic DC motor

Starting about 3 inches from the end of the wire,
wrap it 7 about times around a toilet paper tube or
a large marker. Remove the tube (you don't need it
any more). Cut the wire, leaving a 3 inch tail
opposite the original starting point.


Wrap the two tails around the coil so that the coil is
held together and the two tails extend
perpendicular to the coil. See illustration left:
Note: Be sure to center the two tails on either side
of the coil. Balance is important. You might need to
put a drop of glue where the tail meets the coil to
prevent slipping

On one tail, use fine sandpaper to completely
remove the insulation from the wire. Leave about
1/4" of insulation on the end and where the wire
meets to coil. On the other tail, lay the coil down
flat and lightly sand off the insulation from the top
half of the wire only. Again, leave 1/4" of full
insulation on the end and where the wire meets the
coil.


Bend the two paper clips into the following
shape:
Use the rubber band to hold the loop ends to the
terminals of the "D" Cell battery:

Place the coil in the cradle formed by the right ends of the
paper clips. You may have to give it a gentle push to get it
started, but it should begin to spin rapidly. If it doesn't
spin, check to make sure that all of the insulation has been
remove d from the wire ends. If it spins erratically, make
sure that the tails on the coil are centered on the sides of
the coil. Note that the motor is "in phase" only when it is
held horizontally (as shown in the drawing).