Nuts & Volts - January 2005

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