Индукциялық электр қозғалтқыштары: Түрлері мен тарихы

Type of AC electric motor

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor therefore needs no electrical connections to the rotor. An induction motor's rotor can be either wound type or squirrel cage type. Three phase squirrel cage induction motors are widely used as industrial drives because they are self starting, reliable, and economical. Single phase induction motors are used extensively for smaller loads, such as garbage disposals and stationary power tools. Although traditionally used for constant speed service, single and three phase induction motors are increasingly being installed in variable speed applications using variable frequency drives (VFD). VFD offers energy savings opportunities for induction motors in applications like fans, pumps, and compressors that have a variable load.

In 1824, the French physicist François Arago formulated the existence of rotating magnetic fields, termed Arago's rotations. By manually turning switches on and off, Walter Baily demonstrated this in 1879, effectively the first primitive induction motor. The first commutator free single phase AC induction motor was invented by Hungarian engineer Ottó Bláthy; he used the single phase motor to propel his invention, the electricity meter. The first AC commutator free polyphase induction motors were 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 US 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 having 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 a separately excited DC supply to the 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 invented the cage rotor induction motor in 1889 and the three limb transformer in 1890. Furthermore, 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. GE's Charles Proteus Steinmetz improved the application of AC complex quantities and developed an analytical model called the induction motor Steinmetz equivalent circuit. Induction motor improvements flowing from these inventions and innovations were such that a modern 100 horsepower induction motor has the same mounting dimensions as a 7.5 horsepower motor in 1897. The rotating magnetic flux induces currents in the rotor windings, in a manner similar to currents induced in a transformer's secondary winding(s). The induced currents in the rotor windings in turn create magnetic fields in the rotor that react against the stator field. The direction of the rotor magnetic field opposes the change in current through the rotor windings, following Lenz's Law. 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 turns in the direction of the stator magnetic field. The rotor accelerates until the magnitude of induced rotor current and torque balances the load on the rotor. Since rotation at synchronous speed does not induce rotor current, an induction motor always operates slightly 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 motor's essential character is that torque is created solely by induction instead of the rotor being separately excited as in synchronous or DC machines or being self magnetized as in permanent magnet 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. The generating mode for induction motors is complicated by the need to excite the rotor, which begins with only residual magnetization. In some cases, that residual magnetization is enough to self excite the motor under load. Therefore, it is necessary to either snap the motor and connect it momentarily to a live grid or to add capacitors charged initially by residual magnetism and providing the required reactive power during operation. Similar is the operation of the induction motor in parallel with a synchronous motor serving as a power factor compensator. A feature in the generator mode in parallel to the grid is that the rotor speed is higher than in the driving mode. Then active energy is being given to the grid. For example, for a four pole, three phase motor, = 4 and = 1,500 RPM (for = 50 Hz) and 1,800 RPM (for = 60 Hz) synchronous speed. The number of magnetic poles, , is the number of north and south poles per phase. For example; a single phase motor with 3 north and 3 south poles, having 6 poles per phase, is a 6 pole motor. A three phase motor with 18 north and 18 south poles, having 6 poles per phase, is also a 6 pole motor. This industry standard method of counting poles results in the same synchronous speed for a given frequency regardless of polarity.

There are three basic types of small induction motors: split phase single phase, shaded pole single phase, and polyphase. In two pole single phase motors, the torque goes to zero at 100% slip (zero speed), so these require alterations to the stator such as shaded poles to provide starting torque. 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 copper wire turn around part of a pole; such a pole is referred to as a shaded 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 are split phase motors and 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. A resistance start design uses a starter inserted in series with the startup winding, creating reactance. Self starting polyphase induction motors produce torque even at standstill. Available squirrel 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, variable frequency drives (VFDs). Large slip ring motor drives, termed slip energy recovery systems, some still in use, recover energy from the rotor circuit, rectify it, and return it to the power system using a VFD.

The speed of a pair of slip ring motors can be controlled by a cascade connection, or concatenation. The rotor of one motor is connected to the stator of the other. If the two motors are also mechanically connected, they will run at half speed. This system was once widely used in three phase AC railway locomotives, such as FS Class E.333. By the turn of this century, however, such cascade based electromechanical systems became much more efficiently and economically solved using power semiconductor elements solutions.

In many industrial variable speed applications, DC and WRIM drives are being displaced by VFD fed cage induction motors. The most common efficient way to control asynchronous motor speed of many loads is with VFDs. Barriers to adoption of VFDs due to cost and reliability considerations have been reduced considerably over the past three decades such that it is estimated that drive technology is adopted in as many as 30–40% of all newly installed motors. Variable frequency drives implement the scalar or vector control of an induction motor. With scalar control, only the magnitude and frequency of the supply voltage are controlled without phase control (absent feedback by rotor position). Scalar control is suitable for application where the load is constant. Vector control allows independent control of the speed and torque of the motor, making it possible to maintain a constant rotation speed at varying load torque. But vector control is more expensive because of the cost of the sensor (not always) and the requirement for a more powerful controller.

The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the magnetic field, windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles. Induction motors are most commonly run on single phase or three phase power, but two phase motors exist; in theory, induction motors can have any number of phases. Many single phase motors having two windings can be viewed as two phase motors, since a capacitor is used to generate a second power phase 90° from the single phase supply and feeds it to the second motor winding. Single phase motors require some mechanism to produce a rotating field on startup. Induction motors using a squirrel cage rotor winding may have the rotor bars skewed slightly to smooth out torque in each revolution. Standardized NEMA & IEC motor frame sizes throughout the industry result in interchangeable dimensions for shaft, foot mounting, general aspects as well as certain motor flange aspect. Since an open, drip proof (ODP) motor design allows a free air exchange from outside to the inner stator windings, this style of motor tends to be slightly more efficient because the windings are cooler. At a given power rating, lower speed requires a larger frame.

The method of changing the direction of rotation of an induction motor depends on whether it is a three phase or single phase machine. A three phase motor can be reversed by swapping any two of its phase connections. Motors required to change direction regularly (such as hoists) will have extra switching contacts in their controller to reverse rotation as needed. A variable frequency drive nearly always permits reversal by electronically changing the phase sequence of voltage applied to the motor. In a single phase split phase motor, reversal is achieved by reversing the connections of the starting winding. Some motors bring out the start winding connections to allow selection of rotation direction at installation. If the start winding is permanently connected within the motor, it is impractical to reverse the sense of rotation. Single phase shaded pole motors have a fixed rotation unless a second set of shading windings is provided.