An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction, rather than

by slip ringsand commutators as in slip-ring AC motors. These motors are widely used in industrial drives, particularly polyphase induction motors,[citation needed] because they are robust, have no friction caused by brushes, and their speed can be easily controlled.
Contents
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1 History 2 Operation and comparison to synchronous motors o 2.1 Synchronous speed o 2.2 Slip 3 Construction 4 Speed control 5 Equivalent circuit 6 Starting 7 Sources 8 References 9 External links

[edit]History The idea of a rotating magnetic field was developed by Franois Arago in 1824,[1] and first implemented by Walter Baily.[2] Based on this, practical induction motors were independently invented by Nikola Tesla in 1883 and Galileo Ferraris in 1885.[3] Tesla conceived the rotating magnetic field in 1882 and used it to invent the first induction motor in 1883;[4] Ferraris developed the idea in 1885.[5] In 1888, Ferraris published his research to the Royal Academy of Sciences in Turin, where he detailed the foundations of motor operation;[6] Tesla, in the same year, was granted U.S. Patent 381,968 for his motor. The induction motor with a cage was invented by Mikhail DolivoDobrovolsky a year later.

[edit]Operation

and comparison to synchronous

motors

A 3-phase power supply provides a rotating magnetic field in an induction motor.

In a synchronous AC motor, the rotating magnetic field of the stator imposes a torque on the magnetic field of the rotor, causing it to rotate steadily. It is called synchronous because at steady state, the speed of the rotor matches the speed of the rotating magnetic field in the stator. By contrast, an induction motor has a current induced in the rotor; to do this, stator windings are arranged so that when energised with a polyphase supply they create a rotating magnetic field that induces current in the rotor conductors. These currents interact with the rotating magnetic field, causing rotational motion of the rotor. For these currents to be induced, the speed of the physical rotor must be less than that of the stator's rotating magnetic field (ns), or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If this happens while the motor is operating, the rotor slows slightly until a current is re-induced, and it continues as before. The ratio between the speed of the magnetic field as seen by the rotor (slip speed) to the speed of the rotating stator field is unitless and is called the slip; due to this, induction motors are sometimes referred to as asynchronous machines.[7] As well as generating rotary motion, induction motors may be used as induction generators or unrolled to form the linear induction motor which can directly generate linear motion. [edit]Synchronous

speed

To understand the behaviour of induction motors, it is helpful to understand their distinction from a synchronous motor. A synchronous motor always runs at a shaft rotation frequency that is an integer fraction

of the supply frequency; the synchronous speed of an induction motor is the same. It can be shown that ns in rpm is determined by

where f is the frequency of the AC supply in Hz and p is the number of magnetic poles per phase.[8] Some texts refer to the number of polepairs per phase; a 6 pole motor would have 3 pole pairs. In this case, P, the number of pole pairs, takes the place of p in the equation. [edit]Slip

Typical torque curve as a function of slip.

The slip s is a ratio relative to the synchronous speed and is defined as

where nr is the rotor rotation speed in rpm.[9] [edit]Construction

Typical winding pattern for a 3 phase, 4 pole motor (phases here are labelled U, V, W). Note the interleaving of the pole windings and the resulting quadrupole field.

Squirrel cages were invented by Mikhail Dolivo-Dobrovolsky.

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, the 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 orthree-phase power, but twophase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings and a capacitor can be viewed as two-phase motors, since the capacitor generates a second power phase 90 degrees from the single-phase supply and feeds it to a separate motor winding. Single-phase power is more widely available in residential buildings, but cannot produce a rotating field in the motor, so they must incorporate some kind of starting mechanism to produce a rotating field. There are three types of rotor: squirrel cage rotors made up of skewed (to reduce noise) bars of copper or aluminum that span the length of the rotor, slip ringrotors with windings connected to slip rings replacing the bars of the squirrel cage, and solid core rotors made from mild steel. [edit]Speed

control

Typical torque curves for different line frequencies. By varying the line frequency with an inverter, induction motors can be kept on the stable part of the torque curve above the peak over a wide range of rotation speeds. However, the inverters can be expensive, and fixed line frequencies and other start up schemes are often employed instead.

The theoretical unloaded speed (with slip approaching zero) of the induction motor is controlled by the number of pole pairs and the frequency of the supply voltage. When driven from a fixed line frequency, loading the motor reduces the rotation speed. When used in this way, induction motors are usually run so that in operation the shaft rotation speed is kept above the peak torque point; then the motor will tend to run at reasonably constant speed. Below this point, the speed tends to be unstable and the motor may stall or run at reduced shaft speed, depending on the nature of the mechanical load. Before the development of semiconductor power electronics, it was difficult to vary the frequency, and induction motors were mainly used in fixed speed applications. However, many older DC motors have now been replaced with induction motors and accompanying inverters in industrial applications. [edit]Equivalent

circuit

The equivalent circuit of an induction motor.

The equivalent circuit of an induction motor has the equivalent resistance of the stator on the left, consisting of the copper and core resistance in series, as Rs. During operation, the stator induces reactance, represented by the inductor Xs. Xr represents the effect of the rotor passing through the stator's magnetic field. The effective resistance of the rotor, Rr, is composed of the equivalent value of the machine's power and the ohmic resistance of the stator windings and squirrel cage. The induction motor equivalent circuit when idle is approximately Rs + Xs, which is why this machine takes up mostly reactive power. The idle current draw is often near the rated current, due to the copper and core losses existing without load. In these conditions, this is usually more than half the power loss at the rated load. If the torque against the motor spindle is increased, the active current in the rotor increases by Rr. Due to the construction of the induction motor, the two resistances induce magnetic flux, in contrast to synchronous machines where it is induced only by the reactive current in the stator windings. The current produces a voltage drop in the cage factor of Rr and a slightly higher one in the stator windings. Hence, the losses increase faster in the rotor than in the stator. Rs and the copper factor 2 of Rr both cause I R losses, meaning the efficiency improves with increasing load and reduces with temperature.

Xs gets smaller with smaller frequency and must be reduced by the

delivered drive voltage. Thus, increases engine power losses. In continuous operation, this is an approximation because a nominal torque generated by the cooling of the rotor and stator is not included in the calculation. Above the rated speed or frequency, induction motors are more effective at higher voltages. Today, Rs and Rr are measured automatically and thus can be used on a motor to automatically configure itself and thus protect it from overload. Holding torques and speeds close to zero can be achieved with vector controls. There can be problems with cooling here, since the fan is usually mounted on the rotor.[citation needed] [edit]Starting

Main article: Motor controller

Torque curves for 4 types of asynchronous induction motors: A) Single-phase motor B) Polyphase squirrel cage motor C) Polyphase squirrel cage bar deep motor D) Polyphase double squirrel cage motor

It is necessary to provide a starting circuit to start up an induction rotor; alternatively, rotation may be commenced by manually giving a slight turn to the rotor. A single phase induction motor rotates either way; only the starting circuit determines rotational direction. The four methods of starting an induction motor are direct on-line, reactor, auto-transformer and star-delta. Although these methods can also be used to start a slip ring induction motor, a rotor resistance starter is usually used instead[citation needed]; it cannot be used in squirrel cage induction motors (SCIMs) because external resistance cannot be introduced in the cage rotor. For small motors of a few watts, starting is done by a turn of copper wire around one corner of the pole. The current induced in this turn is out of phase with the supply current, creating an out-of-phase magnetic field, imparting to it sufficient rotational character to start the motor. These are called shaded poles; such shaded-pole motors are typically used in applications such as desk fans and record players, as the starting torque is very low and efficiency is not a problem. Larger 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 have different values of inductance and resistance from the main winding. In some 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. Other designs keep the second winding on when running, improving torque.