1 What are motors2 Principle of operation and comparison to synchronous motors3 Formulas4 Construction5 Speed control6 Starting of induction motor7 Types of starters

An electric motor is an electromechanical device that converts electrical energy to mechanical energy. The mechanical energy can be used to perform work such as rotating a pump impeller, fan, blower, driving a compressor, lifting materials etc.

Advantages: Simple designInexpensive High power to weight ratio Easy to maintain Direct connection to AC power source

An induction motor works on transforming action. The stator works as the primary while the rotor works as the secondary. It is also called asynchronous motor. It consists of two parts: 1. Stator - It is the stationary part of the motor. 2. Rotor - It is the rotating part of the motor.

Stator has three main parts: Outer Frame It is the outer body of the of the motor. It protects the inner part of the machine. Stator Core Built up of high grade silicon steel. Carries the alternating magnetic field. Stator winding Has a three phase winding.

The basic difference between an induction motor and a synchronous AC motor is that in the latter a currentis supplied onto the rotor. This then creates amagnetic field which, through magnetic interaction,links to the rotating magnetic field in the statorwhich in turn causes the rotor to turn. It is calledsynchronous because at steady state the speed of therotor is the same as the speed of the rotatingmagnetic field in the stator.

A 3 phase induction motor derives its name from the fact that the rotor current is induced by the magnetic field, instead of electrical connections.The operating principle of a 3 phase induction motor is based on the production of r.m.f.

According to Faradays law an emf induced in any circuit is due to the rate of change of magnetic flux linkage through the circuit. As the rotor winding in an induction motor are either closed through an external resistance or directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar and due to this emf a current flows through the rotor conductor.

Here the relative velocity between the rotating flux and static rotor conductor is the cause of current generation; hence as per Lenzs law the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity.

In a three phase induction motor, the induced emf in the rotorcircuit depends on the slip of the induction motor and themagnitude of the rotor current depends upon this induced emf(electromotive force). When the motor is started, the slip is equalto 1 as the rotor speed is zero, so the induced emf in the rotor islarge. As a result, a very high current flows through the rotor. Thisis similar to a transformer with the secondary coil short circuited,which causes the primary coil to draw a high current from themains. Similarly, when an induction motor starts, a very highcurrent is drawn by the stator, on the order of 5 to 9 times the fullload current. This high current can damage the motor windingsand because it causes heavy line voltage drop, other appliancesconnected to the same line may be affected by the voltagefluctuation. To avoid such effects, the starting current should belimited

it may observed that the rotor speed should not reach the synchronous speed produced by the stator. If the speeds equals, there would be no such relative velocity, so no emf induction in the rotor, & no current would be flowing, and therefore no torque would be generated. Consequently the rotor can not reach at the synchronous speed. The difference between the stator (synchronous speed) and rotor speeds is called the slip. The rotation of the magnetic field in an induction motor has the advantage that no electrical connections need to be made to the rotor.

The induction motor does not have any directsupply onto the rotor; instead, a secondarycurrent is induced in the rotor. To achievethis, stator windings are arranged around therotor so that when energised with a polyphasesupply they create a rotating magnetic fieldpattern which sweeps past the rotor At the moment illustrated, the current in the statorcoil is in the direction shown and increasing. Theinduced voltage in the coil shown drives current andresults in a clockwise torque. Note that this simplified motor will turn once it isstarted in motion, but has no starting torque.Various techniques are used to produce someasymmetry in the fields to give the motor a startingtorque.

In a DC machine, the stator winding is excited by DCcurrent and hence the field produced by thiswinding is time invariant in nature. In this machine theconversion of energy from electrical to mechanical form orvice versa is possible by one of the following ways:1.rotating the rotor in the field produced by the stator2.feeding external dc current through carbon brushes tothe rotor

Electric motors convert electrical power to mechanicalpower in its rotor (rotating part).There are several ways to supply power to the rotor.AC motor this power is induced in the rotating device. An induction motor can be called a rotatingtransformer because the stator(stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side.Induction motors are widely used, especially polyphaseinduction motors, which are frequently used in industrialdrives.

The relationship between the supply frequency, f, thenumber of pole pairs, p, and the synchronous speed, n,is given by f = p*n.From this relationship:Speed of rotating field (n) = f/P (revs.s-1)Speed of rotor = n(1-S) (rev.s-1)where S is the slip.Slip is calculated using:% slip = (n - r) / n * 100where r is the rotor speed. In contrast, a synchronous motor always runs at either a constant speed N=(120f)/P or zero.