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.IEC 60050 (Publication date: 1990-10). Section 411-31: Rotation Machinery – General, IEV ref. 411-31-10: "Induction Machine – an asynchronous machine of which only one winding is energized". An induction motor therefore needs no electrical connections to the rotor.
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.
History
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 Electrical engineer, Volume 5. (February 1890)The Electrician, Volume 50. 1923Official gazette of the United States Patent Office: Volume 50. (1890)
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.Ion Boldea, Syed A. Nasar, The Induction Machine Handbook, CRC Press · 2010, page 2Thomas Parke Hughes, Networks of Power - Electrification in Western Society, 1880-1930, Johns Hopkins University Press -1993, page 117W. Bernard Carlson, Tesla - Inventor of the Electrical Age, Princeton University Press · 2015, page 110 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.
The General 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. Arthur E. Kennelly was the first to bring out the full significance of complex numbers (using j to represent the square root of minus one) to designate the 90º rotation operator in analysis of AC problems. 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 100-horsepower induction motor built in the 1970s had the same mounting dimensions as a 7.5-horsepower motor in 1897.
Principle
3-phase motor
In both induction and synchronous motors, the AC power supplied to the motor's stator creates a magnetic field that rotates in synchronism 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 somewhat slower speed than the stator field. The induction motor's stator magnetic field is therefore changing or rotating relative to the rotor. This induces an opposing current in the rotor, in effect the motor's secondary winding. 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.
For rotor currents to be induced, the speed of the physical rotor must be either slower or faster 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. 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. Another disadvantage of the induction motor generator is that it consumes a significant magnetizing current I0 = (20–35)%.
Synchronous speed An AC motor's synchronous speed, , is the rotation rate of the stator's magnetic field
where is the frequency of the power supply in hertz and is the number of magnetic poles. For synchronous speed in RPM, the formula becomes:
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.
Slip Slip, , is defined as the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm, or in percentage or ratio of synchronous speed. Thus
where is stator electrical speed, is rotor electrical speed.
For small amounts of slip, torque is approximately by:
Where, : Running Torque : Transformer ratio = Rotor turns/stator turns : Slip of Motor : Rotor Voltage : Rotor Resistance : Rotor Reactance
The equation is correct for small and moderate amounts of slip, but not for large amounts of slip, such as a stopped rotor. Double revolving field theory in the next section, provides a better approximation to actual torque. Slip, which varies from zero at synchronous speed and 1 when the rotor is stalled, influences the motor's torque.
Because the short-circuited rotor windings have small resistance, even a small slip induces a large current in the rotor and produces significant torque. At full rated load, slip varies from more than 5% for small or special purpose motors to less than 1% for large motors. These speed variations can cause load-sharing problems when differently sized motors are mechanically connected. Various methods are available to reduce slip, VFDs often offering the best solution.
Double revolving field The rotor of an induction motor is subject to two rotating magnet fields, which rotate in opposite directions. Torque is the result of the difference between these two rotating fields.
The illustration shows torque as a function of slip. The green line shows torque developed by a single-phase induction motor, without a starting winding. No torque is developed when the rotor is stopped. A slight push in either direction will cause the rotor to accelerate in the direction that it was pushed.
Torque is on the vertical axis shown as a green line, and slip is on the horizontal axis. Slip ranges from zero to zero, corresponding to the reverse and forward directions of the rotor, with 1.0 slip in the center as the point where the rotor is stopped and no torque is produced.
Torque is the net result of two counter-rotating magnetic fields, with one shown as a blue line, and the other shown as a red line. The red and blue lines are mirror images of the typical torque curve shown in the previous section.
Torque
Standard torque
The typical speed-torque relationship of a standard NEMA Design B polyphase induction motor is as shown in the curve at right. Suitable for most low performance loads such as centrifugal pumps and fans, Design B motors are constrained by the following typical torque ranges: Breakdown torque (peak torque), 175–300% of rated torque Locked-rotor torque (torque at 100% slip), 75–275% of rated torque Pull-up torque, 65–190% of rated torque.