Induction motor

Type of AC electric motor

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"Tesla motor" redirects here. For the electric car company, see Tesla, Inc.

Three-phase totally-enclosed fan-cooled (TEFC) induction motor with end cover on the left, and without end cover to show cooling fan on the right. In TEFC motors, interior heat losses are dissipated indirectly through enclosure fins, mostly by forced air convection. Cutaway view through stator of TEFC induction motor, showing rotor with internal air circulation vanes. Many such motors have a symmetric armature, and the frame may be reversed to place the electrical connection box (not shown) on the opposite side.

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.[1] An induction motor therefore needs no electrical connections to the rotor.[a] 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.

History

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A model of Nikola Tesla's first induction motor at the Tesla Museum in Belgrade, Serbia Squirrel-cage rotor construction, showing only the center three laminations

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.[2][3][4][5][6][7][8]

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.[9][10]

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.[5][11] 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)[12][13][14][15] [16] 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.[17] 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.[12][18][19][20] 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.[21][22] 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.[23] 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.[12]

The General Electric Company (GE) began developing three-phase induction motors in 1891.[12] By 1896, General Electric and Westinghouse signed a cross-licensing agreement for the bar-winding-rotor design, later called the squirrel-cage rotor.[12] 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.[24] GE's Charles Proteus Steinmetz improved the application of AC complex quantities and developed an analytical model called the induction motor Steinmetz equivalent circuit.[12][25][26][27]

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.[12]

Principle

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3-phase motor

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A three-phase power supply provides a rotating magnetic field in an induction motor. Inherent slip – unequal rotation frequency of stator field and the rotor

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 stator's 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.[28] The rotating magnetic flux induces currents in the rotor windings,[29] 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.[30] 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.[28]

For rotor currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field ( n s {\displaystyle n_{s}} ); 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".[31]

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.[2] Another disadvantage of the induction motor generator is that it consumes a significant magnetizing current I0 = (20–35)%.

Synchronous speed

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An AC motor's synchronous speed, f s {\displaystyle f_{s}} , is the rotation rate of the stator's magnetic field,

f s = 2 f p {\displaystyle f_{s}={2f \over p}}

where f {\displaystyle f} is the frequency of the power supply, p {\displaystyle p} is the number of magnetic poles, and f s {\displaystyle f_{s}} is the synchronous speed of the machine. For f {\displaystyle f} in hertz and n s {\displaystyle n_{s}} synchronous speed in RPM, the formula becomes:

n s = 2 f p ⋅ ( 60   s e c o n d s m i n u t e ) = 120 f p ⋅ ( s e c o n d s m i n u t e ) {\displaystyle n_{s}={2f \over p}\cdot \left({\frac {60\ \mathrm {seconds} }{\mathrm {minute} }}\right)={120f \over {p}}\cdot \left({\frac {\mathrm {seconds} }{\mathrm {minute} }}\right)}

[32][33]

For example, for a four-pole, three-phase motor, p {\displaystyle p} = 4 and n s = 120 f 4 {\displaystyle n_{s}={120f \over 4}} = 1,500 RPM (for f {\displaystyle f} = 50 Hz) and 1,800 RPM (for f {\displaystyle f} = 60 Hz) synchronous speed.

The number of magnetic poles, p {\displaystyle p} , 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

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Typical torque curve as a function of slip, represented as "g" here

Slip, s {\displaystyle s} , 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

s = n s − n r n s {\displaystyle s={\frac {n_{s}-n_{r}}{n_{s}}}\,}

where n s {\displaystyle n_{s}} is stator electrical speed, n r {\displaystyle n_{r}} is rotor mechanical speed.[34][35] Slip, which varies from zero at synchronous speed and 1 when the rotor is stalled, determines the motor's torque. Since the short-circuited rotor windings have small resistance, even a small slip induces a large current in the rotor and produces significant torque.[36] At full rated load, slip varies from more than 5% for small or special purpose motors to less than 1% for large motors.[37] These speed variations can cause load-sharing problems when differently sized motors are mechanically connected.[37] Various methods are available to reduce slip, VFDs often offering the best solution.[37]

Torque

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Standard torque

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Speed-torque curves for four induction motor types: A) Single-phase, B) Polyphase cage, C) Polyphase cage deep bar, D) Polyphase double cage Typical speed-torque curve for NEMA Design B Motor

Transient solution for an AC induction motor from a complete stop to its operating point under a varying load

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:[30][b]

• 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.

Over a motor's normal load range, the torque's slope is approximately linear or proportional to slip because the value of rotor resistance divided by slip, R r ′ / s {\displaystyle R_{r}'/s} , dominates torque in a linear manner.[38] As load increases above rated load, stator and rotor leakage reactance factors gradually become more significant in relation to R r ′ / s {\displaystyle R_{r}'/s} such that torque gradually curves towards breakdown torque. As the load torque increases beyond breakdown torque the motor stalls.

Starting

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