Download Reading: Motors

Electric Motors
Prentice Hall
The wire in the magnetic field of a galvanometer cannot rotate more than half a turn. Suppose you could make a loop of
wire rotate continuously. Instead of moving a pointer, the wire could turn a rod, or axle. The axle could then turn
something else, such as the blades of a fan or blender. Such a device would be an electric motor. An electric motor is a
device that uses an electric current to turn an axle.
An electric motor converts electrical energy into mechanical energy. An electric motor is different from a
galvanometer because in a motor, a loop of current-carrying wire spins continuously.
How a Motor Works: How can you make a loop of wire continue to spin? The direction of the force on the wire
depends on the current and the magnetic field surrounding the coil. In a motor, current is reversed just as the loop gets to
the vertical position. This reverses the force on each side of the loop. The side of the loop that was pushed up on the left
is now pushed down on the right. The side of the loop that was pushed down on the right is now pushed up on the left.
The current reverses after each half turn so that the loop spins continuously in the same direction.
Electric Motor A loop of wire in a motor spins continuously. A. The magnetic field of the loop makes it rotate to a vertical position. B.
As the loop of wire passes the vertical position, each half of the commutator makes contact with the opposite brush. The direction of
current flow changes, and so does the direction of the magnetic force on the loop. The loop continues to spin in the same direction.
Parts of a Motor: A commutator is a device that reverses the flow of current through an electric motor. You can see
in the diagram above that a commutator consists of two parts of a ring. Each half of the commutator is attached to one
end of the loop of wire. When the loop of wire rotates, the commutator rotates as well. As it moves, the commutator
slides past two contact points called brushes. Each half of the commutator is connected to the current source by one of
the brushes.
As the loop of wire gets to the vertical position, each half of the commutator makes contact with the other brush. Since
the current runs through the brushes, changing brushes reverses the direction of the current in the loop. Changing the
direction of the current causes the loop of wire to spin continuously.
Instead of a single loop of wire, practical electric motors have dozens or hundreds of loops of wire wrapped around an
iron core. This arrangement of wires and iron core is called an armature. Using many loops increases the strength of
the motor and allows it to rotate more smoothly. Large electric motors also use electromagnets in place of permanent
magnets.
Armature This armature contains hundreds of coils of wire.