A Simple Shaded Pole AC Motor
few tens of miles from
the generators due to
losses in the transmission lines. The advantage that AC (Alternating
Current) power could be
transmitted over long
distances had already
been recognized, but its
advantage could not be
utilized until the AC
motor was developed.
DC power design
had been rather simple
and developed by trial
and error. AC power
development, however,
required a fundamental
understanding of AC theory. In 1888, Galileo Ferraris —
an Italian professor — published his observations that two
out-of-phase light waves would produce a rotating beam
of light. From this simple idea, he was led to describe a
rotating magnetic field produced by two out-of-phase
magnetic fields. In this paper, he described how a single
AC current could be split into two out-of-phase components that would produce a traveling magnetic field, but
— unfortunately — he erroneously concluded that such a
motor was impractical and was only another laboratory
curiosity.
A year earlier, Nikola Tesla (1856-1943) applied for
a patent on an AC induction motor that employed a
traveling magnetic field that rotated. Tesla was born in
Yugoslavia and moved to the US in 1884 to work with
Thomas Edison. Unfortunately, the two had a personality
conflict and Tesla left Edison a year later to begin an arc
lamp business. The conflict between these two inventors
continued with Edison pushing for DC systems and Tesla
supporting AC systems. During the period of 1888 to
1896, Tesla obtained extensive patent coverage over
most of the features of AC motors, including multiphase
systems.
By 1893, both Westinghouse and General Electric had
introduced AC induction motors. At this time, Tesla
demonstrated his system of lighting at the Chicago World
Columbian Exposition in the futuristic “White City.” The
commercial success of AC power was assured in 1896 by
the construction of the Niagara Falls hydroelectric plant
that provided the staggering power of 11 megawatts. Later,
several more generators were added to raise this level to
37 megawatts.
Eventually, this power would enable the Pittsburgh
Reduction Company (later to become the Aluminum
Company of America) to produce the aluminum that
would nurture the aircraft industry. Thus, by the 1900s, all
the major features of both DC and AC electric power
systems were firmly in place.
JANUARY 2005
Figure 2. A typical shaded pole motor.
Figure 3. A shaded pole AC relay.
In some AC motor designs, two out-of-phase magnetic
fields can be generated by the reactive impedance of a
capacitor, as illustrated in Figure 1. Such motors are called
capacitive motors. One design employs the extra phased
winding only for starting torque and these motors are
called capacitive start motors. Still another version — the
shaded pole motor — produces two out-of-phase magnetic
Circle #68 on the Reader Service Card.
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