basic training industrial duty and commercial duty electric motors gear reducers pdf

Electric Motor History and Principles

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. 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 polyphase (three-phase) AC motors are produced by Marathon Motors and many other manufacturers. In polyphase 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. Singlephase motors do not produce a rotating field at a standstill, so a starter winding is added to give the effect of a polyphase 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.