Electric Motor
The modern electric motor dates back to 1831, when American physicist Joseph Henry published a paper describing an electric motor which, he showed, was basically the reverse of the electric generator. Instead of converting mechanical movement into an electric current, like the generator, his motor used electric current to produce mechanical movement. Henry's motor was the first to be constructed, although inefficiency limited its potential. In 1834 American blacksmith Thomas Davenport improved the motor's operating principles, using four magnets, two fixed and two revolving. Davenport used his motor to operate his own drills and wood-turning lathes. He went on to incorporate his motor in the electric railway, electric trolley, electric piano, and electric printing press.
However, across the Atlantic Ocean, English physicist Michael Faraday had been making advances of his own. Some of Faraday's discoveries were anticipated by Henry, but Faraday received credit by publishing the results of his experiments first. Faraday, having learned of Hans Christian Oersted's (1777-1851) 1821 discovery that an electric current created a magnetic field which could deflect a compass needle, set out to reverse the results and create an electric current from a magnetic field. Faraday built a device that consisted of rods, wires, and magnets, some of which were fixed in position, others mounted on pivots. When he sent an electric current through his device, the moveable wire pivoted around the fixed magnet, and the movable magnet pivoted around the fixed wire. Although it did not perform work tasks, it did convert electrical and magnetic force into mechanical movement, the essence of a electric motor.
Ten years later, Faraday constructed the first electric generator, making a continuous supply of electricity available for use. Hippolyte Pixii built a hand-driven generator that produced alternating current (AC), but, on the suggestion of André Marie Ampère (1775-1836), he added a commutator to convert the power into direct current (DC). The scientific world had concentrated on DC since 1800, when Alessandro Volta invented his battery. Although AC had a number of advantages over DC, there was little interest in it, and early electric motors operated on the principle of direct current.
Alternating current took a step forward in 1867 when Belgian-French inventor Zénobe Gramme built an improved dynamo for producing AC; two years later he improved the DC dynamo. Using the principles of Henry's and Faraday's discoveries, Gramme, with his associate Hippolyte Fontaine (1833-1917), opened a factory that manufactured electrical devices, setting the standards for the industry. In 1884, Nikola Tesla went to work for Thomas Edison, and tried to convince him of the advantages of AC power. Edison, whose electric company was already established with DC, could not be convinced. A year later, Tesla took his AC technology and induction motor to industrialist George Westinghouse, initiating a conflict between Edison and Westinghouse that was only resolved in 1893, when AC generators were successfully used to provide power for the World Columbia Exposition in Chicago. Westinghouse made a fortune building AC motors, and Tesla, ironically, was awarded the Edison medal in 1917 for his work in electricity.
All motors have two basic parts: a rotor and a stator. The stator is usually stationary, and the rotor revolves around it. The stator is a magnet that shapes a magnetic field. The rotor is a conductor connected to the electric circuit that interacts with this magnetic field, and in turn produces a magnetic field that acts on the stator.
Modern motors can be classified into two groups: electromagnetic motors and magnetic motors. Electromagnetic motors improve in performance as they are enlarged; magnetic machines improve as they are scaled-down. Electromagnetic motors include the induction, AC polyphase commutator, AC single-phase commutator, DC, synchronous, and repulsion motors. The magnetic motors include the solenoid, relay, reluctance, and hysteresis motors.
An induction motor uses AC and a ring of fixed electromagnets (the stator) to produce a rotating magnetic field. The moving electromagnetic field causes the rotor to spin, producing mechanical energy. More than 90% of the world's motors are of this type. A synchronous motor uses either permanent magnets or DC-fed electromagnets to produce a magnetic field. Unlike the induction motor, whose rotor "chases" after the rotating magnetic field, the synchronous motor has a magnetized rotor. The rotor's magnetic field matches the rotating magnetic field, resulting in a synchronized mechanical motion that has very little slippage.
There are two main disadvantages to induction and synchronous motors: without a variable power supply, they cannot provide efficient speed variation over a wide range. They are also limited to speeds under 3000 revolutions per minute (rpm) when they draw power directly from power mains. When these factors are a problem, commutator motors are used.
The commutator motor has insulated coils whose ends are connected to a pair of conducting segments. Carbon blocks or copper brushes make direct contact with the pairs of commutator segments from which the current flows. The chief problem with this type of motor is the sparking and arcing that occurs between the brushes and the commutator segments. The reluctance motor, used in clocks, is a synchronous motor whose magnetized rotor has been replaced by a piece of metal that is shaped so that it fits into a number of preferred positions, where the resistance is minimized. The hysteresis motor is similar to the reluctance motor, except the shaped piece of metal is replaced by a smooth cylinder. The magnetic field passes over the cylinder, leaving it permanently magnetized. The motor operates more and more like a synchronous motor as it speeds up, until it locks on the rotating magnetic field. The linear induction motor is a hybrid of its rotary cousin. In this type of motor, the rotor, which does not rotate, and the stator move past each other, separated by a small gap of air, resulting in a magnetic field that moves in a straight line instead of in a circle. A liner induction motor is superbly suited for use in high-speed magnetic levitation trains, whose upper speed is limited only by wind resistance and safety considerations. There are more than 200 different types of linear induction motors in use today.
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