The long history and its three main kinds of Electric motor
2008-08-07

                                                                                         The long history and its three main kinds of Electric motor

       An electric is a motor that uses electrical energy to produce mechanical energy. The reverse process£¬which of using mechanical energy to produce electrical energy, is accomplished by a generator or dynamo. Electric motors are generally found in household appliances such as fans, refrigerators, washing machines, pool pumps, floor vacuums, and fan-forced ovens.        Electric motor has a history of nearly two hundred years.        British scientist Michael Faraday invented the electric motor, which is used to convert electrical energy to mechanical energy, in 1821.        Michael Faraday, FRS (September 22, 1791 – August 25, 1867) was an English chemist and physicist (or natural philosopher, in the terminology of that time) who contributed to the fields of electromagnetism and electrochemistry.        Faraday studied the magnetic field around a conductor carrying a DC electric current, and established the basis for the magnetic field concept in physics. He discovered electromagnetic induction, diamagnetism, and laws of electrolysis. He established that magnetism could affect rays of light and that there was an underlying relationship between the two phenomena.His inventions of electromagnetic rotary devices formed the foundation of electric motor technology, and it was largely due to his efforts that electricity became viable for use in technology.        Electric current supplied from the power lines can only be used directly in heating, lighting and other applications. To use this power to run devices like pumps, automobiles, domestic appliances and machine tools, the electrical energy must be converted to mechanical energy, which rotates shafts and gear trains.        The first electric motor using electromagnets for both stationary and rotating parts was demonstrated by Ányos Jedlik in 1828 Hungary, who later developed a motor powerful enough to propel a vehicle. The first commutator-type direct-current electric motor capable of a practical application was invented by the British scientist William Sturgeon in 1832. Following Sturgeon's work, a commutator-type direct-current electric motor made with the intention of commercial use was built by the American Thomas Davenport and patented in 1837. Although several of these motors were built and used to operate equipment such as a printing press, due to the high cost of primary battery power, the motors were commercially unsuccessful and Davenport went bankrupt. Several inventors followed Sturgeon in the development of DC motors but all encountered the same cost issues with primary battery power. No electricity distribution had been developed at the time. Like Sturgeon's motor, there was no practical commercial market for these motors.         The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected the dynamo he had invented to a second similar unit, driving it as a motor. The Gramme machine was the first electric motor that was successful in the industry.         Zénobe Gramme (April 4, 1826 - January 20, 1901) was a Belgian electrical engineer. In spite of the fact that he was semi-literate and had no advanced knowledge of mathematics, in 1869, he invented the Gramme machine, a type of direct current dynamo capable of generating smoother (less AC) and much higher voltages than the dynamos known to that point. In 1873 he accidentally discovered that the device was reversible and would spin when connected to any DC power supply. The Gramme machine was the first usefully powerful electrical motor successful in industry.         In 1888 Nikola Tesla invented the first practicable AC motor and with it the polyphone power transmission system. Tesla continued his work on the AC motor in the years to follow at the Westinghouse Company.         Michail Osipovich Dolivo-Dobrovolsky later invented a three-phase "cage-rotor" in 1890 which is now used for the vast majority of commercial applications.         Electric motors can be classified into three major types, which are DC motors (direct current), AC motors (alternating current) and Universal motors that can operate on either AC or DC current. Universal Motors         These motors can use both DC and AC current and are commonly used in vacuum cleaners, food mixers, blenders, small power tools and hair dryers and other appliances that operate at high speed but are not used continuously. They are a variant of the wound DC motor and special care is taken to cover the impedance and reluctance of AC motors. Thyristors or stepped speed control circuits are used for continuous speed control. DC Motors         DC motors provide momentary power bursts of up to five times the rated torque. The speed can be brought down to zero smoothly and immediately raised in the opposite direction without any power interruption.         DC motors have an electromagnet with two poles, which serve as a rotating armature. A commutator or rotary switch is used to reverse the current direction twice in each cycle. This causes the poles of the electromagnet to push and pull against the external permanent magnets. When the poles of the armature pass through the poles of the permanent magnet, the commutator reverses the polarity of the armature. The inertia maintains the current direction at the instance when polarity is switched. AC Motors        The AC motor allows long-range distribution of alternating current. This motor played a very important role in the rapid growth of industrialization. An AC motor has two main parts, a fixed external stator and an internal rotor. The stator has coils through which AC current flows and it produces a rotating magnetic field. The rotor is attached to the output shaft and gets a torque by the rotating magnetic field.