The electric motor in its simplest terms is a converter of electrical energy to useful mechanical energy. The electric motor has played a leading role in the high productivity of modern industry, and it is therefore directly responsible for the high standard of living being enjoyed throughout the industrialized world.
The beginnings of the electric motor are shrouded in mystery, but this much seems clear: The basic principles of electromagnetic induction were discovered in the early 1800’s by Oersted, Gauss and Faraday, and this combination of Scandinavian, German and English thought gave us the fundamentals for the electric motor. In the late 1800’s the actual invention of the alternating current motor was made by Nikola Tesla, a Serb who had migrated to the United States. Nikola Tesla was a Serbian-American inventor, electrical engineer, mechanical engineer, and futurist who is best known for his contributions to the design of the modern alternating current (AC) electricity supply system. One measure of Tesla’s genius is that he was granted more than 900 patents in the electrical field. Before Tesla’s time, direct current motors had been produced in small quantities, but it was his development of the versatile and rugged alternating current motor that opened a new age of automation and industrial productivity.
An electric motor’s principle of operation is based on the fact that a current-carrying conductor, when placed in a magnetic field, will have a force exerted on the conductor proportional to the current flowing in the conductor and to the strength of the magnetic field. In alternating current motors, the windings placed in the laminated stator core produce the magnetic field. The aluminum bars in the laminated rotor core are the current-carrying conductors upon which the force acts. The resultant action is the rotary motion of the rotor and shaft, which can then be coupled to various devices to be driven and produce the output.
Many types of motors are produced today. Undoubtedly, the most common are alternating current induction motors. The term “induction” derives from the transference of power from the stator to the rotor through electromagnetic induction. No slip rings or brushes are required since the load currents in the rotor conductors are induced by transformer action. The induction motor is, in effect, a transformer – with the stator winding being the primary winding and the rotor bars and end rings being the movable secondary members.
Both single-phase and poly-phase (three-phase) AC motors are produced by
LEESON and many other manufacturers. In poly-phase motors, the placement of the phase winding groups in conjunction with the phase sequence of the power supply line produces a rotating field around the rotor surface. The rotor tends to follow this rotating field with a rotational speed that varies inversely with the number of poles wound into the stator. Single-phase motors do not produce a rotating field at a standstill, so a starter winding is added to give the effect of a poly-phase rotating field. Once the motor is running, the start winding can be cut out of the circuit, and the motor will continue to run on a rotating field that now exists due to the motion of the rotor interacting with the single-phase stator magnetic field.
The development of power semiconductors and microprocessors has brought efficient adjustable speed control to AC motors through the use of inverter drives. Through this technology, the most recent designs of so-called pulse width modulated AC drives are capable of speed and torque regulation that equals or closely approximates direct current systems.
LEESON Electric also produces permanent-magnet direct current motors. The DC motor is the oldest member of the electric motor family. Technological breakthroughs in magnetic materials, as well as solid state electronic controls and high-power-density rechargeable batteries, have all revitalized the versatile DC motor.
DC motors have extremely high torque capabilities and can be used in conjunction with relatively simple solid state control devices to give programmed acceleration and deceleration over a wide range of selected speeds. Because the speed of a DC motor is not dependent on the number of poles, there is great versatility for any constant or variable speed requirement.
In most common DC motors, the magnetic field is produced by high strength permanent magnets, which have replaced traditional field coil windings. The magnets require no current from the power supply. This improves motor efficiency and reduces internal heating. In addition, the reduced current draw enhances the life of batteries used as power supplies in mobile or remote applications.
Both AC and DC motors must be manufactured with a great deal of precision in order to operate properly. LEESON and other major manufacturers use laminated stator, rotor and armature cores to reduce energy losses and heat in the motor. Rotors for AC motors are heat treated to separate the aluminum bars from the rotor’s magnetic lamination. Shaft and bearing tolerances must be held to ten thousandths of an inch.
The whole structure of the motor must be rigid to reduce vibration and noise. The stator insulation and coil winding must be done in a precise manner to avoid damaging the wire insulation or ground insulation. And mountings musts meet exacting dimensions. This is especially true for motors with (National Electrical Manufacturers Association) NEMA C face mountings, which are used for direct coupling to speed reducers, pumps and other devices.
The electric motor is, of course, the very heart of any machine it drives. If the motor does not run, the machine or device will not function. The importance and scope of the electric motor in modern life is attested to by the fact that electric motors, numbering countless millions in total, convert more energy than do all our passenger automobiles. Electric motors are much more efficient in energy conversion than automobiles, but they are such a large factor in the total energy picture that renewed interest is being shown in motor performance. Today’s industrial motors have energy
conversion efficiency exceeding 96% in larger horsepower.
This efficiency, combined with unsurpassed durability and reliability, will continue to make electric motors the “prime movers” of choice for decades to come.