Electric Motor

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Introduction Electric motors, both ac and dc types, come in many shapes and sizes. Some are standardized versions for general-purpose applications. Others are intended for specific tasks. In any case, motors should be selected to satisfy the dynamic requirements of the machines on which they are applied without exceeding rated motor temperature. Thus, the first and most important step in motor selection is determining load characteristics -- torque and speed versus time. Selection is also based
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  Introduction Electric motors, both ac and dc types, come in many shapes and sizes. Some are standardizedversions for general-purpose applications. Others are intended for specific tasks. In any case,motors should be selected to satisfy the dynamic requirements of the machines on which theyare applied without exceeding rated motor temperature. Thus, the first and most importantstep in motor selection is determining load characteristics -- torque and speed versus time.Selection is also based on mission goals, power available, and cost.    Ele ctric motor A n electric motor uses electrical energy to produce mechanical energy, usually through theinteraction of magnetic fields and current-carrying conductors. The reverse process, producingelectrical energy from mechanical energy, is accomplished by a generator or dynamo. Tractionmotors used on vehicles often perform both tasks. Electric motors can be run as generators andvice versa, although this is not always practical. Electric motors are ubiquitous, being found inapplications as diverse as industrial fans, blowers and pumps, machine tools, householdappliances, power tools, and disk drives. They may be powered by direct current (for example abattery powered portable device or motor vehicle), or by alternating current from a centralelectrical distribution grid. The smallest motors may be found in electric wristwatches.Medium-size motors of highly standardized dimensions and characteristics provide convenientmechanical power for industrial uses. The very largest electric motors are used for propulsionof large ships, and for such purposes as pipeline compressors, with ratings in the thousands of kilowatts. Electric motors may be classified by the source of electric power, by their internalconstruction, and by their application.The physical principle of production of mechanical force by the interactions of an electriccurrent and a magnetic field was known as early as 1821. Electric motors of increasingefficiency were constructed throughout the 19th century, but commercial exploitation of electric motors on a large scale required efficient electrical generators and electricaldistribution networks.By convention, electric engine refers to a railroad electric locomotive, rather than an electricmotor.  Th e princip le   The conversion of electrical energy into mechanical energy by electromagnetic means wasdemonstrated by the British scientist Michael Faraday in 1821. A free-hanging wire was dippedinto a pool of mercury, on which a permanent magnet was placed. When a current was passedthrough the wire, the wire rotated around the magnet, showing that the current gave rise to acircular magnetic field around the wire. This motor is often demonstrated in school physicsclasses, but brine (salt water) is sometimes used in place of the toxic mercury. This is thesimplest form of a class of devices called homopolar motors. A later refinement is the Barlow'sWheel. These were demonstration devices only, unsuited to practical applications due to theirprimitive construction.Electromagnetic experiment of Faraday, ca. 1821.In 1827, Hungarian Ányos Jedlik started experimenting with electromagnetic rotating deviceshe called lightning-magnetic self-rotors . He used them for instructive purposes in universities,and in 1828 demonstrated the first device which contained the three main components of practical direct current motors: the stator, rotor and commutator. Both the stationary and therevolving parts were electromagnetic, employing no permanent magnets. A gain, the deviceshad no practical application.Jedlik's lightning-magnetic self-rotor , 1827. (Museum of  A pplied A rts, Budapest.)    Th e first ele ctric motors   The first commutator-type direct current electric motor capable of turning machinery wasinvented by the British scientist William Sturgeon in 1832. Following Sturgeon's work, acommutator-type direct-current electric motor made with the intention of commercial use wasbuilt by the A merican Thomas Davenport and patented in 1837. His motors ran at up to 600revolutions per minute and powered machine tools and a printing press. Due to the high cost of the zinc electrodes required by primary battery power, the motors were commerciallyunsuccessful and Davenport went bankrupt. Several inventors followed Sturgeon in thedevelopment of DC motors but all encountered the same cost issues with primary batterypower. No electricity distribution had been developed at the time. Like Sturgeon's motor, therewas no practical commercial market for these motors.In 1855 Jedlik built a device using similar principles to those used in his electromagnetic self-rotors that was capable of useful work. He built a model electric motor-propelled vehicle thatsame year. There is no evidence that this experimentation was communicated to the widerscientific world at that time, or that it influenced the development of electric motors in thefollowing decades.The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected thedynamo he had invented to a second similar unit, driving it as a motor. The Gramme machinewas the first electric motor that was successful in the industry.In 1888 Nikola Tesla invented the first practicable A C motor and with it the polyphase powertransmission system. Tesla continued his work on the A C motor in the years to follow at theWestinghouse company.The development of electric motors of acceptable efficiency was delayed for several decades byfailure to recognize the extreme importance of a relatively-small air gap between rotor andstator. Early motors, for some rotor positions, had comparatively huge air gaps whichconstituted a very-high-reluctance magnetic circuit. They produced far-lower torque than anequivalent amount of power would produce with efficient designs. The cause of the lack of understanding seems to be that early designs were based on familiarity of distant attractionbetween a magnet and a piece of ferromagnetic material, or between two electromagnets.Efficient designs, as this article describes, are based on a rotor with a comparatively small airgap, and flux patterns that create torque.Note that the armature bars are at some distance (unknown) from the field pole pieces whenpower is fed to one of the field magnets; the air gap is likely to be considerable. The text tells of the inefficiency of the design. (Electricity was created, as a practical matter, by consuming zincin wet primary cells!) In his workshops Froment had an electromotive engine of one-horse power. But, though an interestingapplication of the transformation of energy, these machines will never be practically applied on the
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