Induction motor

Induction motor

Overview

An induction or asynchronous motor is a type of AC motor
AC motor
An AC motor is an electric motor driven by an alternating current.It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the...

 where power is supplied to the rotor by means of electromagnetic induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

. These motors are widely used in industrial drives, particularly polyphase
Polyphase system
A polyphase system is a means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful...

 induction motors, because they are robust and have no brushes. Their speed can be controlled with a variable frequency drive
Variable frequency drive
A variable-frequency drive is a system for controlling the rotational speed of an alternating current electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive...

.

The idea of a rotating magnetic field
Rotating magnetic field
A rotating magnetic field is a magnetic field which changes direction at a constant angular rate. This is a key principle in the operation of the alternating-current motor. Nikola Tesla claimed in his autobiography that he identified the concept of the rotating magnetic field in 1882. In 1885,...

 was developed by François Arago
François Arago
François Jean Dominique Arago , known simply as François Arago , was a French mathematician, physicist, astronomer and politician.-Early life and work:...

  in 1824, and first implemented by Walter Baily.
Discussion
Ask a question about 'Induction motor'
Start a new discussion about 'Induction motor'
Answer questions from other users
Full Discussion Forum
 
Unanswered Questions
Recent Discussions
Encyclopedia

An induction or asynchronous motor is a type of AC motor
AC motor
An AC motor is an electric motor driven by an alternating current.It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the...

 where power is supplied to the rotor by means of electromagnetic induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

. These motors are widely used in industrial drives, particularly polyphase
Polyphase system
A polyphase system is a means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful...

 induction motors, because they are robust and have no brushes. Their speed can be controlled with a variable frequency drive
Variable frequency drive
A variable-frequency drive is a system for controlling the rotational speed of an alternating current electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive...

.

History


The idea of a rotating magnetic field
Rotating magnetic field
A rotating magnetic field is a magnetic field which changes direction at a constant angular rate. This is a key principle in the operation of the alternating-current motor. Nikola Tesla claimed in his autobiography that he identified the concept of the rotating magnetic field in 1882. In 1885,...

 was developed by François Arago
François Arago
François Jean Dominique Arago , known simply as François Arago , was a French mathematician, physicist, astronomer and politician.-Early life and work:...

  in 1824, and first implemented by Walter Baily. Based on this, practical induction motors were independently invented by Nikola Tesla
Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

 in 1883 and Galileo Ferraris
Galileo Ferraris
Galileo Ferraris was an Italian physicist and electrical engineer, noted mostly for the studies and independent discovery of the rotating magnetic field, a basic working principle of the induction motor...

 in 1885. According to his 1915 autobiography Tesla conceived the rotating magnetic field in 1882 and used it to invent the first induction motor in 1883; Ferraris developed the idea in 1885. In 1888, Ferraris published his research to the Royal Academy of Sciences in Turin, where he detailed the foundations of motor operation; Tesla, in the same year, was granted for his motor. The induction motor with a cage was invented by Mikhail Dolivo-Dobrovolsky a year later.

Operation and comparison to synchronous motors



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
Polyphase system
A polyphase system is a means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful...

 they create a rotating magnetic field
Rotating magnetic field
A rotating magnetic field is a magnetic field which changes direction at a constant angular rate. This is a key principle in the operation of the alternating-current motor. Nikola Tesla claimed in his autobiography that he identified the concept of the rotating magnetic field in 1882. In 1885,...

 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 lower than that of the stator's rotating magnetic field (), or the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. If this happens while the motor is operating, the rotor slightly slows down, and consequently a current is induced again. The ratio between the speed of the magnetic field as seen by the rotor (slip speed) and the speed of the stator's rotating field is unitless and it is called slip. For this reason, induction motors are sometimes referred to as asynchronous motors. An induction motor can be used as induction generator
Induction generator
An induction generator or asynchronous generator is a type of AC electrical generator that uses the principles of induction motors to produce power. Induction generators operate by mechanically turning their rotor in generator mode, giving negative slip...

, or it can be unrolled to form the linear induction motor
Linear induction motor
A linear induction motor is an AC asynchronous linear motor that works by the same general principles as other induction motors but which has been designed to directly produce motion in a straight line....

 which can directly generate linear motion.

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 in rpm is determined by



where f is the frequency of the AC supply
Utility frequency
The utility frequency, line frequency or mains frequency is the frequency at which alternating current is transmitted from a power plant to the end-user. In most parts of the world this is 50 Hz, although in the Americas it is typically 60 Hz...

 in Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 and p is the number of magnetic pole pairs per phase. Some texts refer to the number of pole pairs per phase; a 6 pole motor would have 3 pole pairs. In this case, P, the number of poles, takes the place of p in the equation. For example, a two-pole motor (one pair of poles) would have a synchronous speed of 3600 revolutions per minute at a power frequency of 60 Hz, but at 25 Hz the speed would be only 1500 r/m.

Slip



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


where is the rotor rotation speed in rpm.

Construction




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
Single-phase electric power
In electrical engineering, single-phase electric power refers to the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors...

 or three-phase
Three-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...

 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 degrees from the single-phase supply and feeds it to the second 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 rotor
Squirrel cage rotor
A squirrel cage rotor is the rotating part used in the most common form of AC induction motor. An electric motor with a squirrel cage rotor is termed a squirrel cage motor.- Structure :In overall shape, it is a cylinder mounted on a shaft...

s made up of skewed (to reduce noise) bars of copper or aluminum that span the length of the rotor, slip ring
Slip ring
A slip ring is a method of making an electrical connection through a rotating assembly. Slip rings, also called rotary electrical interfaces, rotating electrical connectors, collectors, swivels, or electrical rotary joints, are commonly found in electric motors, electrical generators for AC...

 rotors with windings connected to slip rings replacing the bars of the squirrel cage, and solid core rotors made from mild steel.

Speed control



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.

Equivalent circuit



The equivalent circuit
Equivalent circuit
In electrical engineering and science, an equivalent circuit refers to a theoretical circuit that retains all of the electrical characteristics of a given circuit. Often, an equivalent circuit is sought that is the simplest form of a more complex circuit in order to aid analysis. In its most common...

 of an induction motor has the equivalent resistance of the stator on the left, consisting of the copper and core resistance in series, as . During operation, the stator induces reactance, represented by the inductor . represents the effect of the rotor passing through the stator's magnetic field. The effective resistance of the rotor, , 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 , which is mostly reactive. Induction motors generally have a poor power factor
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...

, which can be improved by a compensation network.

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 . 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 and a slightly higher one in the stator windings. Hence, the losses increase faster in the rotor than in the stator. and the copper factor of both cause losses, meaning the efficiency improves with increasing load and reduces with temperature.

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, and 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.

Starting


A single phase induction motor is not self starting; thus, it is necessary to provide a starting circuit and associated start windings to give the initial rotation in a single phase induction motor. The normal running windings within such a motor motor can cause the rotor to turn in either direction, so the starting circuit determines the operating direction.

The four methods of starting an induction motor are direct on-line, reactor, auto-transformer and star-delta. Unlike a wound-rotor motor, the rotor circuit is inaccessible and it is not feasible to introduce extra resistance for starting or speed control.

For small single-phase shaded-pole motor
Shaded-pole motor
A shaded-pole motor is a type of AC single-phase induction motor. It is basically a small squirrel cage motor in which the auxiliary winding is composed of a copper ring surrounding a portion of each pole. This auxiliary winding is called a shading coil. Currents in this coil delay the phase of ...

 of a few watts, starting is done by a shaded pole, with a turn of copper wire around part of the 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 character to start the motor. These 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 single phase 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
Thermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...

 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.

Polyphase motors have rotor bars shaped to give different speed/torque characteristics. The current distribution within the rotor bars varies depending on the frequency of the inducted current. At standstill, the rotor current is the same frequency as the stator current, and tends to travel at the outermost parts of the squirrel-cage rotor bars. As the rotor accelerates, the slip frequency declines, and current tends to travel deeper within the squirrel cage bars. Polyphase motors can generate torque from standstill, so no extra mechanism is required to initiate rotation. The different bar shapes can give usefully different speed/torque characteristics as well as some control over the inrush current at startup.

Linear induction motor



A linear induction motor (LIM) is an AC asynchronous linear motor
Linear motor
A linear motor is an electric motor that has had its stator and rotor "unrolled" so that instead of producing a torque it produces a linear force along its length...

 that works by the same general principles as other induction motors but which has been designed to directly produce motion in a straight line.

Linear motors frequently run on a 3 phase power supply.

Their uses include magnetic levitation
Magnetic levitation
Magnetic levitation, maglev, or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields...

, linear propulsion, and linear actuators. They have also been used for pumping liquid metals.

External links