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The practical alternating current electro-magnetic motor which Tesla would attain There was an error working with the wiki: Code[1], Electric Motor (December 1889), is an induction motor.

Introduction

In 1882, There was an error working with the wiki: Code[69] principle, and pioneered the use of a rotary field of force to operate machines. Tesla had suggested that the There was an error working with the wiki: Code[70]s from a machine could be removed and the device could operate on a rotary field of force. Professor Poeschel, his teacher, stated that would be akin to building a There was an error working with the wiki: Code[78]. There was an error working with the wiki: Code[2]

{| width=44% border="0" border="1" style="float:right"

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! Stator energy || Rotor energy|| Total energy supplied || Power developed

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In the induction motor, the field and armature were ideally of equal field strengths and the field and armature cores were of equal sizes. The total energy supplied to operate the device equaled the sum of the energy expended in the armature and field coils. There was an error working with the wiki: Code[3] The power developed in operation of the device equaled the product of the energy expended in the armature and field coils. There was an error working with the wiki: Code[4] He exploited the principle to design a unique two-phase induction motor in 1883. In 1885, There was an error working with the wiki: Code[79] independently researched the concept. In 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

Introduction of Tesla's motor from 1888 onwards initiated what is known as the There was an error working with the wiki: Code[80], making possible the efficient generation and long distance distribution of electrical energy using the alternating current transmission system, also of Tesla's invention (1888) There was an error working with the wiki: Code[5] Before the invention of the rotating magnetic field, motors operated by continually passing a conductor through a stationary magnetic field (as in There was an error working with the wiki: Code[81]s). There was an error working with the wiki: Code[82] later invented a three-phase "cage-rotor" in 1890. A successful commercial There was an error working with the wiki: Code[83] of generation and long-distance transmission was designed by Almerian Decker at Mill Creek No. 1 in Redlands California, There was an error working with the wiki: Code[6] There was an error working with the wiki: Code[7]

Components and types

A typical AC motor consists of two parts:

# An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and

# An inside rotor attached to the output shaft that is given a torque by the rotating field.

There are two fundamental types of AC motor depending on the type of rotor used:

The synchronous motor, which rotates exactly at the supply frequency or a submultiple of the supply frequency, and

The induction motor, which turns slightly slower, and typically (though not necessarily always) takes the form of the There was an error working with the wiki: Code[71] motor.

Three-phase AC induction motors

Where a polyphase electrical supply is available, the There was an error working with the wiki: Code[72]) AC induction motor is commonly used, especially for higher-powered motors. The phase differences between the three phases of the polyphase electrical supply create a rotating electromagnetic field in the motor. Through There was an error working with the wiki: Code[84], the rotating magnetic field induces a current in the conductors in the rotor, which in turn sets up a counterbalancing magnetic field that causes the rotor to turn in the direction the field is rotating. The rotor must always rotate slower than the rotating magnetic field produced by the polyphase electrical supply otherwise, no counterbalancing field will be produced in the rotor. Induction motors are the workhorses of industry and motors up to about 500 kW (670 There was an error working with the wiki: Code[85]) in output are produced in highly standardized frame sizes, making them nearly completely interchangeable between manufacturers (although European and North American standard dimensions are different). Very large synchronous motors are capable of tens of thousands of kW in output, for pipeline compressors and wind-tunnel drives. There are two types of rotors used in induction motors.

Squirrel Cage rotors

Most common AC motors use the There was an error working with the wiki: Code[86], which will be found in virtually all domestic and light industrial alternating current motors. The squirrel cage takes its name from its shape - a ring at either end of the rotor, with bars connecting the rings running the length of the rotor. It is typically cast aluminum or copper poured between the iron laminates of the rotor, and usually only the end rings will be visible. The vast majority of the rotor currents will flow through the bars rather than the higher-resistance and usually varnished laminates. Very low voltages at very high currents are typical in the bars and end rings high efficiency motors will often use cast copper in order to reduce the resistance in the rotor.

In operation, the squirrel cage motor may be viewed as a Transformer with a rotating secondary - when the rotor is not rotating in sync with the magnetic field, large rotor currents are induced the large rotor currents magnetize the rotor and interact with the stator's magnetic fields to bring the rotor into synchronization with the stator's field. An unloaded squirrel cage motor at synchronous speed will only consume electrical power to maintain rotor speed against friction and resistance losses as the mechanical load increases, so will the electrical load - the electrical load is inherently related to the mechanical load. This is similar to a transformer, where the primary's electrical load is related to the secondary's electrical load.

This is why, as an example, a squirrel cage blower motor may cause the lights in a home to dim as it starts, but doesn't dim the lights when its fanbelt (and therefore mechanical load) is removed. Furthermore, a stalled squirrel cage motor (overloaded or with a jammed shaft) will consume current limited only by circuit resistance as it attempts to start. Unless something else limits the current (or cuts it off completely) overheating and destruction of the winding insulation is the likely outcome.

Virtually every There was an error working with the wiki: Code[73], There was an error working with the wiki: Code[87], etc. uses some variant of a squirrel cage motor.

Wound Rotor

An alternate design, called the wound rotor, is used when variable speed is required. In this case, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft. Carbon brushes connect the slip rings to an external controller such as a variable resistor that allows changing the motor's slip rate. In certain high-power variable speed wound-rotor drives, the slip-frequency energy is captured, rectified and returned to the power supply through an inverter.

Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, but they were the standard form for variable speed control before the advent of compact power electronic devices. Transistorized inverters with variable frequency drive can now be used for speed control and wound rotor motors are becoming less common. (Transistorized inverter drives also allow the more-efficient three-phase motors to be used when only single-phase mains current is available, but this is never used in house hold appliances, because it can cause electrical interference and because of high power requirements.)

Starting methods and speed

Several methods of starting a polyphase motor are used. Where the large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals. Where it is necessary to limit the starting inrush current (where the motor is large compared with the short-circuit capacity of the supply), reduced voltage starting using either series inductors, an Transformer, Thyristors, or other devices are used. A technique sometimes used is star-delta starting, where the motor coils are initially connected in wye for acceleration of the load, then switched to delta when the load is up to speed. This technique is more common in Europe than in North America. Transistorized drives can directly vary the applied voltage as required by the starting characteristics of the motor and load. This type of motor is becoming more common in traction applications such as locomotives, where it is known as the asynchronous There was an error working with the wiki: Code[88].

The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:

: N_{s} = 120F/p

where

:Ns = Synchronous speed, in revolutions per minute

:F = AC power frequency

:p = Number of poles per phase winding

Actual RPM for an induction motor will be less than this calculated synchronous speed by an amount known as slip that increases with the torque produced. With no load the speed will be very close to synchronous. When loaded, standard motors have between 2-3% slip, special motors may have up to 7% slip, and a class of motors known as torque motors are rated to operate at 100% slip (0 RPM/full stall).

The slip of the AC motor is calculated by:

:S = (N_{s} - N{r})/N_{s}

where

:Nr = Rotational speed, in revolutions per minute.

:S = Normalised Slip, 0 to 1.

As an example, a typical four-pole motor running on 60 Hz might have a nameplate rating of 1725 RPM at full load, while its calculated speed is 1800. The speed in this type of motor has traditionally been altered by having additional sets of coils or poles in the motor that can be switched on and off to change the speed of magnetic field rotation. However, developments in There was an error working with the wiki: Code[89] mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed.

Three-phase AC synchronous motors

If connections to the rotor coils of a three-phase motor are taken out on slip-rings and fed a separate field current to create a continuous magnetic field (or if the rotor consists of a permanent magnet), the result is called a synchronous motor because the rotor will rotate in synchronism with the rotating magnetic field produced by the polyphase electrical supply.

The synchronous motor can also be used as an There was an error working with the wiki: Code[90].

Nowadays, synchronous motors are frequently driven by transistorized There was an error working with the wiki: Code[74]. This greatly eases the problem of starting the massive rotor of a large synchronous motor. They may also be started as induction motors using a squirrel-cage winding that shares the common rotor: once the motor reaches synchronous speed, no current is induced in the squirrel-cage winding so it has little effect on the synchronous operation of the motor, aside from stabilizing the motor speed on load changes.

Synchronous motors are occasionally used as traction motors the There was an error working with the wiki: Code[91] may be the best-known example of such use.

Two-phase AC servo motors

A typical two-phase AC servo motor has a squirrel-cage rotor and a field consisting of two windings: 1) a constant-voltage (AC) main winding, and 2) a control-voltage (AC) winding in quadrature with the main winding so as to produce a rotating magnetic field. The electrical resistance of the rotor is made high intentionally so that the speed-torque curve is fairly linear. Two-phase servo motors are inherently high-speed, low-torque devices, heavily geared down to drive the load.

Single-phase AC induction motors

Three-phase motors inherently produce a rotating magnetic field. However, when only single-phase power is available, the rotating magnetic field must be produced using other means. Several methods are commonly used.

A common single-phase motor is the There was an error working with the wiki: Code[92], which is used in devices requiring low Torque, such as There was an error working with the wiki: Code[93]s or other small household appliances. In this motor, small single-turn copper "shading coils" create the moving magnetic field. Part of each pole is encircled by a copper coil or strap the induced current in the strap opposes the change of flux through the coil (There was an error working with the wiki: Code[94]), so that the maximum field intensity moves across the pole face on each cycle, thus producing the required rotating magnetic field.

Another common single-phase AC motor is the split-phase induction motor, commonly used in There was an error working with the wiki: Code[95]s such as There was an error working with the wiki: Code[96]s and There was an error working with the wiki: Code[97]s. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special There was an error working with the wiki: Code[98] in conjunction with a There was an error working with the wiki: Code[99].

In the split-phase motor, the startup winding is designed with a higher Electrical resistance than the running winding. This creates an There was an error working with the wiki: Code[75] which slightly shifts the phase of the current in the startup winding. When the motor is starting, the startup winding is connected to the power source via a set of spring-loaded contacts pressed upon by the not-yet-rotating centrifugal switch. The starting winding is wound with fewer turns of smaller wire than the main winding, so it has a lower inductance (L) and higher resistance (R). The lower L/R ratio creates a small phase shift, not more than about 30 degrees, between the flux due to the main winding and the flux of the starting winding. The starting direction of rotation may be reversed simply by exchanging the connections of the startup winding relative to the running winding.

The phase of the magnetic field in this startup winding is shifted from the phase of the mains power, allowing the creation of a moving magnetic field which starts the motor. Once the motor reaches near design operating speed, the centrifugal switch activates, opening the contacts and disconnecting the startup winding from the power source. The motor then operates solely on the running winding. The starting winding must be disconnected since it would increase the losses in the motor.

In a capacitor start motor, a starting Capacitor is inserted in series with the startup winding, creating an There was an error working with the wiki: Code[100] which is capable of a much greater phase shift (and so, a much greater starting torque). The capacitor naturally adds expense to such motors.

Another variation is the Permanent Split-Capacitor (PSC) motor (also known as a capacitor start and run motor). This motor operates similarly to the capacitor-start motor described above, but there is no centrifugal starting switch and the second winding is permanently connected to the power source. PSC motors are frequently used in air handlers, fans, and blowers and other cases where a variable speed is desired. By changing taps on the running winding but keeping the load constant, the motor can be made to run at different speeds. Also provided all 6 winding connections are available separately, a 3 phase motor can be converted to a capacitor start and run motor by commoning two of the windings and connecting the third via a capacitor to act as a start winding.

Repulsion motors are wound-rotor single-phase AC motors that are similar to universal motors. In a repulsion motor, the armature brushes are shorted together rather than connected in series with the field. Several types of repulsion motors have been manufactured, but the repulsion-start induction-run (RS-IR) motor has been used most frequently. The RS-IR motor has a centrifugal switch that shorts all segments of the commutator so that the motor operates as an induction motor once it has been accelerated to full speed. RS-IR motors have been used to provide high starting torque per ampere under conditions of cold operating temperatures and poor source voltage regulation. Few repulsion motors of any type are sold as of 2006.

Single-phase AC synchronous motors

Small single-phase AC motors can also be designed with magnetized rotors (or several variations on that idea). The rotors in these motors do not require any induced current so they do not slip backward against the mains frequency. Instead, they rotate synchronously with the mains frequency. Because of their highly accurate speed, such motors are usually used to power mechanical clocks, audio There was an error working with the wiki: Code[76]s, and There was an error working with the wiki: Code[101]s formerly they were also much used in accurate timing instruments such as strip-chart recorders or telescope drive mechanisms. The There was an error working with the wiki: Code[102] is one version.

Because There was an error working with the wiki: Code[103] makes it difficult to instantly accelerate the rotor from stopped to synchronous speed, these motors normally require some sort of special feature to get started. Various designs use a small induction motor (which may share the same field coils and rotor as the synchronous motor) or a very light rotor with a one-way mechanism (to ensure that the rotor starts in the "forward" direction).

Patents

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There was an error working with the wiki: Code[17] 310/66 310/166 33/320 74/5R 74/5.5

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There was an error working with the wiki: Code[31] 318/759 188/159 310/166 322/47

There was an error working with the wiki: Code[32] 318/824 200/80R 310/166

There was an error working with the wiki: Code[33] 310/82 310/163 310/166 310/46 310/70R

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There was an error working with the wiki: Code[48] 318/824 200/80R 310/166

There was an error working with the wiki: Code[49] 318/824 200/80R 310/166

There was an error working with the wiki: Code[50] 363/155 307/24 310/166

There was an error working with the wiki: Code[51] 318/817 307/109 310/166 310/172

There was an error working with the wiki: Code[52] 318/767 310/166 310/211

There was an error working with the wiki: Code[53] 310/163 310/166 388/808

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There was an error working with the wiki: Code[55] 310/166 310/172 310/195 318/781

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There was an error working with the wiki: Code[59] 18/714 307/87 310/166 318/462 318/705 318/719

There was an error working with the wiki: Code[60] 310/166 12/142C 310/211 310/46 388/836

There was an error working with the wiki: Code[61] 310/176 310/166 335/223 335/224 335/243

There was an error working with the wiki: Code[62] 310/166 292/275 310/40R

Notes

There was an error working with the wiki: Code[63] U.S. Patent 0416194, "Electric Motor", December 1889.

There was an error working with the wiki: Code[64] "Tesla's Early Years". There was an error working with the wiki: Code[104].

There was an error working with the wiki: Code[65] U.S. Patent 0416194, "Electric Motor", December 1889.

There was an error working with the wiki: Code[66] http://www.tfcbooks.com/tesla/system.htm

There was an error working with the wiki: Code[67] http://www.electrichistory.com

There was an error working with the wiki: Code[68] http://www.redlandsweb.com

References ands external articles

General references

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition, McGraw-Hill, New York, 1978, ISBN 0-07-020974-X.

Edwin J. Houston and Arthur Kennelly, Recent Types of Dynamo-Electric Machinery, copyright American Technical Book Company 1897, published by P.F. Collier and Sons New York, 1902

Kuphaldt, Tony R., "Lessons In Electric Circuits", Volume II, Chapter 13 'AC MOTORS'. http://www.ibiblio.org/obp/electricCircuits/AC/AC_13.html

A. O. Smith: The AC's and DC's of Electric Motors http://www.aosmithmotors.com/pdf/brochures/bulletin3100/ACDC.PDF

Further reading

Shanefield D. J., Industrial Electronics for Engineers, Chemists, and Technicians, William Andrew Publishing, Norwich, NY, 2001. A self-teaching textbook that briefly covers electric motors, transformers, speed controllers, wiring codes and grounding, There was an error working with the wiki: Code[105]s, digital, etc. Easy to read and understand, up to an elementary level on each subject, not a suitable reference book for technologists already working in any of those fields.

Fitzgerald/Kingsley/Kusko (Fitzgerald/Kingsley/Umans in later years), Electric Machinery, classic text for junior and senior electrical engineering students. Originally published in 1952, 6th edition published in 2002. Authors still listed as Fitzgerald/Kingsley/Umans although Fitzgerald and Kingsley are now deceased.

Bedford, B. D., R. G. Hoft, et al, "Principles of Inverter Circuits". John Wiley & Sons, Inc., New York1, 964 ISBN 0471061344 (Inverter circuits are used for There was an error working with the wiki: Code[77])

B. R. Pelly, "Thyristor Phase-Controlled Converters and Cycloconverters: Operation, Control, and Performance" (New York: John Wiley, 1971).

See Also

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