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An electric motor uses electrical energy to produce
mechanical energy. The reverse process, that of
using mechanical energy to produce electrical
energy, is accomplished by a generator or dynamo.
Traction motors used on locomotives often perform
both tasks if the locomotive is equipped with
dynamic brakes. Electric motors are found in
household appliances such as fans, refrigerators,
washing machines, pool pumps, floor vacuums, and
fan-forced ovens.
The principle of conversion of electrical energy
into mechanical energy by electromagnetic means was
demonstrated by the British scientist Michael
Faraday in 1821 and consisted of a free-hanging wire
dipping into a pool of mercury. A permanent magnet
was placed in the middle of the pool of mercury.
When a current was passed through the wire, the wire
rotated around the magnet, showing that the current
gave rise to a circular magnetic field around the
wire. This motor is often demonstrated in school
physics classes, but brine (salt water) is sometimes
used in place of the toxic mercury. This is the
simplest form of a class of electric motors called
homopolar motors. A later refinement is the Barlow's
Wheel. These were demonstration devices, unsuited to
practical applications due to limited power.
The first electric motor using electromagnets for
both stationary and rotating parts was demonstrated
by Ányos Jedlik in 1828 Hungary, who later developed
a motor powerful enough to propel a vehicle. The
first commutator-type direct-current electric motor
capable of a practical application was invented by
the British scientist William Sturgeon in 1832.
Following Sturgeon's work, a commutator-type
direct-current electric motor made with the
intention of commercial use was built by the
American Thomas Davenport and patented in 1837.
Although several of these motors were built and used
to operate equipment such as a printing press, due
to the high cost of primary battery power, the
motors were commercially unsuccessful and Davenport
went bankrupt. Several inventors followed Sturgeon
in the development of DC motors but all encountered
the same cost issues with primary battery power. No
electricity distribution had been developed at the
time. Like Sturgeon's motor, there was no practical
commercial market for these motors.
The modern DC motor was invented by accident in
1873, when Zénobe Gramme connected the dynamo he had
invented to a second similar unit, driving it as a
motor. The Gramme machine was the first electric
motor that was successful in the industry.
In 1888 Nikola Tesla invented the first practicable
AC motor and with it the polyphase power
transmission system. Tesla continued his work on the
AC motor in the years to follow at the Westinghouse
company.
The classic division of electric motors has been
that of DC types vs AC types. This is more a de
facto convention, rather than a rigid distinction.
For example, many classic DC motors run happily on
AC power.
The ongoing trend toward electronic control further
muddles the distinction, as modern drivers have
moved the commutator out of the motor shell. For
this new breed of motor, driver circuits are relied
upon to generate sinusoidal AC drive currents, or
some approximation of. The two best examples are:
the brushless DC motor, and the stepping motor, both
being polyphase AC motors requiring external
electronic control.
There is a clearer distinction between a synchronous
motor and asynchronous types. In the synchronous
types, the rotor rotates in synchrony with the
oscillating field or current (eg. permanent magnet
motors). In contrast, an asynchronous motor is
designed to slip; the most ubiquitous example being
the common AC induction motor which must slip in
order to generate torque. |
