An electric motor is a device used to convert electrical energy to
mechanical energy. Electric motors are extremely important in modern-day
life. They are used in vacuum cleaners, dishwashers, computer printers,
fax machines, video cassette recorders, machine tools, printing presses,
automobiles, subway systems, sewage treatment plants, and water pumping
stations, to mention only a few applications.
Principle of operation
The basic principle on which motors operate is Ampere's law. This
law states that a wire carrying an electric current produces a magnetic
field around itself. Imagine that current is flowing through
the wire loop shown in the figure below. The presence of that current
creates a magnetic field around the wire. Since the loop itself has become
a magnet, one side of it will be attracted to the north (N) pole of the
surrounding magnet and the other side will be attracted to the south (S)
pole of the magnet. The loop will begin to rotate, as shown by the arrow
marked F.
AC motors.
What happens next depends on the kind of electric current used to run the
motor, direct (DC) or alternating (AC) current. With AC current, the
direction in which the current flows changes back and forth rapidly and at
a regular rate. In the United States, the rate of change is 60 times per
second, or 60 hertz (the unit of frequency).
In an AC motor, then, the current flows first in one direction through the
wire loop and then reverses itself about 1/60 second later. This change of
direction means that the magnetic field produced around the loop also
changes once every 1/60 second. At one instant, one part of the loop is
attracted by the north pole of the magnet, and at the next instant, it is
attracted by the south pole of the magnet.
But this shifting of the magnetic field is necessary to keep the motor
operating. When the current is flowing in one direction, the right hand
side of the coil might become the south pole of the loop magnet. It would
be repelled by the south pole of the outside magnet and attracted by the
north pole of the outside magnet. The wire loop would be twisted around
until the right side of the loop had completed half a revolution and was
next to the north pole of the outside magnet.
If nothing further happened, the loop would come to a stop, since two
opposite magnetic poles—one from the outside magnet and one from
the wire loop—would be adjacent to (located next to) each other.
And unlike magnetic poles attract each other. But something further does
happen. The current changes direction, and so does the magnetic field
around the wire loop. The side of the loop that was previously attracted
to the north pole is now attracted to the south pole, and vice versa.
Therefore, the loop receives another "kick," twisting it
around on its axis in response to the new forces of magnetic attraction
and repulsion.
Thus, as long as the current continues to change direction, the wire loop
is forced to spin around on its axis. This spinning motion can be used to
operate any one of the electrical appliances mentioned above.
DC motors.
When electric motors were first invented, AC current had not yet been
discovered. So the earliest motors all operated on DC current, such as the
current provided by a battery.
Capacitor
A capacitor is a device for storing electrical energy. Capacitors are
used in a wide variety of applications today. Engineers use large banks
of capacitors, for example, to test the performance of an electrical
circuit when struck by a bolt of lighting. The energy released by these
large capacitors is similar to the lightning bolt. On another scale, a
camera flash works by storing energy in a capacitor and then releasing
it to cause a quick bright flash of light. On the smallest scale,
capacitors are used in computer systems. A charged capacitor represents
the number 1 and an uncharged capacitor a 0 in the binary number system
used by computers.
How a capacitor stores energy
A capacitor consists of two electrical conductors that are not in
contact. The conductors are usually separated by a layer of insulating
material known as a dielectric. Since air is a dielectric, an additional
insulating material may not have to be added to the capacitor.
Think of a capacitor as consisting of two copper plates separated by 1
centimeter of air. Then imagine that electrical charge (that is,
electrons) are pumped into one of the plates. That plate becomes
negatively charged because of the excess number of electrons it
contains. The negative charge on the first copper plate then induces
(creates) a positive charge on the second plate.
As electrons are added to the first plate, one might expect a current to
flow from that plate to the second plate. But the presence of the
dielectric prevents any flow of electrical current. Instead, as more
electrons are added to the first plate, it accumulates more and more
energy. Adding electrons increases energy because each electron added to
the plate has to overcome repulsion from other electrons already there.
The tenth electron added has to bring with it more energy to add to the
plate than did the fifth electron. And the one-hundredth electron will
have to bring with it even more energy. As a result, as long as current
flows into the first plate, it stores up more and more electrical
energy.
Capacitors release the energy stored within them when the two plates are
connected with each other. For example, just closing an electric switch
between the two plates releases the energy stored in the first plate.
That energy rushes through the circuit, providing a burst of energy.
The primary difference between a DC motor and an AC motor is finding a way
to change the direction of current flow. In direct current, electric
current always moves in the same direction. That means that the wire loop
in the motor will stop turning after the first half revolution. Because
the current is always flowing in the same direction, the resulting
magnetic field always points in the same direction.
To solve this problem, the wire coming from the DC power source is
attached to a metal ring cut in half, as shown in the figure. The ring is
called a split-ring commutator. At the first moment the motor is turned
on, current flows out of the battery, through the wire, and into one side
of the commutator. The current then flows into the wire loop, producing a
magnetic field.
Once the loop begins to rotate, however, it carries the commutator with
it. After a half turn, the ring reaches the empty space in the two halves
and then moves on to the second half of the commutator. At that point,
then, current begins to flow into the opposite side of the loop, producing
the same effect achieved with AC current. Current flows backward through
the loop, the magnetic field is reversed, and the loop continues to
rotate.
In your house, almost every mechanical movement that you see around you is caused by an AC or DC electric motor.
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