Basics of energy saving
The history of the effects of the first induction motor. Record status induction improvements and the development of three-phase asynchronous motors. The use of electric motors in modern technology. Features of the nature of induction machines.
|Рубрика||Физика и энергетика|
|Размер файла||19,1 K|
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Министерство образования и науки Российской Федерации
федеральное государственное бюджетное образовательное учреждение высшего профессионального образования
УЛЬЯНОВСКИЙ ГОСУДАРСТВЕННЫЙ ТЕХНИЧЕСКИЙ УНИВЕРСИТЕТ
Кафедра «Иностранные языки»
Тема: Энергосберегающие технологии
студент(ка) гр. Эбд-11
Principle of operation
Induction machine is an electromechanical transducer in which the occurrence of torque on the rotor shaft is only possible at different speeds and the magnetic field of the rotor.
Asynchronous machines the most widely used as motors. This is the main engine used in industry, agriculture and households. Only asynchronous motors unified series ranging from 0.6 to 400 kW in our country annually produces about 10 million Induction micromotors power from 0.6 kW produced tens of millions per year. electric motor induction machines
Electrical industry produces asynchronous motors in a wide range of capacities. Limiting power induction motors - a few tens of megawatts. In indicator systems use asynchronous motors from fraction of a watt to hundreds of watts. Rotational speed general-purpose engines - from 3000 to 500 rev / min.
In generator mode asynchronous machines are rarely used. To create a field in the gap of an induction machine requires reactive power, which is taken from the network or from other sources of reactive power. Induction motors cannot work with costs = 1. This is a significant drawback of asynchronous machines, limiting their use in regenerative mode.
The presence of losses in the rotor is proportional to the slip-dependent, - one of the features of asynchronous machines that determine how they differ from other types of electrical machines.
If the rotor windings are closed loops, then the slip s = 1 all the power flowing to the rotor is converted into heat. When slip s = 0, the power is not supplied to the rotor. With slides, other than 0 or 1 , the electromagnetic power is converted into motor mode, mechanical power and heat, and in generator mode - into electricity and heat.
In the embodiment asynchronous motors - the most simple, they are most widely.
Three-phase totally enclosed fan-cooled (TEFC) induction motor, with and, at right, without end cover to show cooling fan. In TEFC motors, interior losses are dissipated indirectly through enclosure fins mostly by forced air convection.
An induction or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is induced by electromagnetic induction from the magnetic field of the stator winding. An induction motor therefore does not require mechanical commutation, separate-excitation or self-excitation for all or part of the energy transferred from stator to rotor, as in universal, DC and large synchronous motors. An induction motor's rotor can be either wound type or squirrel-cage type.
Three-phase squirrel-cage induction motors are widely used in industrial drives because they are rugged, reliable and economical. Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans. Although traditionally used in fixed-speed service, induction motors are increasingly being used with variable-frequency drives (VFDs) in variable-speed service. VFDs offer especially important energy savings opportunities for existing and prospective induction motors in variable-torque centrifugal fan, pump and compressor load applications. Squirrel cage induction motors are very widely used in both fixed-speed and VFD applications.
In 1824, the French physicist Franзois Arago formulated the existence of rotating magnetic fields, termed Arago's rotations, which, by manually turning switches on and off, Walter Baily demonstrated in 1879 as in effect the first primitive induction motor. Practical alternating current induction motors seem to have been independently invented by Galileo Ferraris and Nikola Tesla, a working motor model having been demonstrated by the former in 1885 and by the latter in 1887. Tesla applied for U.S. patents in October and November 1887 and was granted some of these patents in May 1888. In April 1888, the Royal Academy of Science of Turin published Ferraris's research on his AC polyphase motor detailing the foundations of motor operation. In May 1888 Tesla presented the technical paper A New System for Alternating Current Motors and Transformers to the American Institute of Electrical Engineers (AIEE) describing three four-stator-pole motor types: one with a four-pole rotor forming a non-self-starting reluctance motor, another with a wound rotor forming a self-starting induction motor, and the third a true synchronous motor with separately excited DC supply to rotor winding. George Westinghouse, who was developing an alternating current power system at that time, licensed Tesla's patents in 1888 and purchased a US patent option on Ferraris' induction motor concept. Tesla was also employed for one year as a consultant. Westinghouse employee C. F. Scott was assigned to assist Tesla and later took over development of the induction motor at Westinghouse. Steadfast in his promotion of three-phase development, Mikhail Dolivo-Dobrovolsky's invented the cage-rotor induction motor in 1889 and the three-limb transformer in 1890. However, he claimed that Tesla's motor was not practical because of two-phase pulsations, which prompted him to persist in his three-phase work. Although Westinghouse achieved its first practical induction motor in 1892 and developed a line of polyphase 60 hertz induction motors in 1893, these early Westinghouse motors were two-phase motors with wound rotors until B.G. Lamme developed a rotating bar winding rotor. TheGeneral Electric Company (GE) began developing three-phase induction motors in 1891. By 1896, General Electric and Westinghouse signed a cross-licensing agreement for the bar-winding-rotor design, later called the squirrel-cage rotor. GE's Charles Proteus Steinmetz was the first to make use of the letter "j" (the square root of minus one) to designate the 90-degree rotation operator in electrical mathematical expressions and thereby be able to describe the induction motor in terms now commonly known as the Steinmetz equivalent circuit. Induction motor improvements flowing from these inventions and innovations were such that a 100 horsepower induction motor currently has the same mounting dimensions as a 7.5 horsepower motor in 1897.
Principle of operation
In both induction and synchronous motors, the AC power supplied to the motor's stator creates a magnetic field that rotates in time with the AC oscillations. Whereas a synchronous motor's rotor turns at the same rate as the stator field, an induction motor's rotor rotates at a slower speed than the stator field. The induction motor stator's magnetic field is therefore changing or rotating relative to the rotor. This induces an opposing current in the induction motor's rotor, in effect the motor's secondary winding, when the latter is short-circuited or closed through an external impedance. The rotating magnetic flux induces currents in the windings of the rotor; in a manner similar to currents induced in a transformer's secondary winding(s). The currents in the rotor windings in turn create magnetic fields in the rotor that react against the stator field. Due to Lenz's Law, the direction of the magnetic field created will be such as to oppose the change in current through the rotor windings. The cause of induced current in the rotor windings is the rotating stator magnetic field, so to oppose the change in rotor-winding currents the rotor will start to rotate in the direction of the rotating stator magnetic field. The rotor accelerates until the magnitude of induced rotor current and torque balances the applied load. Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates slower than synchronous speed. The difference, or "slip," between actual and synchronous speed varies from about 0.5 to 5.0% for standard Design B torque curve induction motors. The induction machine's essential character is that it is created solely by induction instead of being separately excited as in synchronous or DC machines or being self-magnetized as in permanent magnet motors.
For rotor currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field (); otherwise the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The ratio between the rotation rate of the magnetic field induced in the rotor and the rotation rate of the stator's rotating field is called slip. Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as asynchronous motors. An induction motor can be used as an induction generator, or it can be unrolled to form a linear induction motor which can directly generate linear motion.
There are five basic types of competing small induction motor: single-phase capacitor-start, capacitor-run, split-phase and shaded-pole types, and small polyphase induction motors.
A single-phase induction motor requires separate starting circuitry to provide a rotating field to the motor. The normal running windings within such a single-phase motor can cause the rotor to turn in either direction, so the starting circuit determines the operating direction.
In certain smaller single-phase motors, starting is done by means of a shaded pole with a copper wire turn around part of the pole. The current induced in this turn lags behind the supply current, creating a delayed magnetic field around the shaded part of the pole face. This imparts sufficient rotational field energy to start the motor. These motors are typically used in applications such as desk fans and record players, as the required starting torque is low, and the low efficiency is tolerable relative to the reduced cost of the motor and starting method compared to other AC motor designs.
Larger single phase motors have a second stator winding fed with out-of-phase current; such currents may be created by feeding the winding through a capacitor or having it receive different values of inductance and resistance from the main winding. In capacitor-start designs, the second winding is disconnected once the motor is up to speed, usually either by a centrifugal switch acting on weights on the motor shaft or a thermistor which heats up and increases its resistance, reducing the current through the second winding to an insignificant level. The capacitor-run designs keep the second winding on when running, improving torque.
Self-starting polyphase induction motors produce torque even at standstill. Available cage induction motor starting methods include direct-on-line starting, reduced-voltage reactor or auto-transformer starting, star-delta starting or, increasingly, new solid-state soft assemblies and, of course, VFDs.
Polyphase motors have rotor bars shaped to give different speed-torque characteristics. The current distribution within the rotor bars varies depending on the frequency of the induced current. At standstill, the rotor current is the same frequency as the stator current, and tends to travel at the outermost parts of the cage rotor bars (by skin effect). The different bar shapes can give usefully different speed-torque characteristics as well as some control over the inrush current at startup.
In wound rotor motors, rotor circuit connection through slip rings to external resistances allows change of speed-torque characteristics for acceleration control and speed control purposes.
Operating characteristics of the induction motor corresponds to its stable operation zone: Three-phase asynchronous motor
This feature allows you to find all the basic values that define the operation of the engine at different loads. They can be obtained either by calculation on the equivalent circuit, or experimentally.
Maximum engine torque is called tipping point. When the engine load torque to values less than the maximum moment, but close to it, accidental motor overload causes it to stop and to rule as to its failure.
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