Induction Motor

Introduction

Electrical equipments such as fridge, washing machine, fan, clothes drier, the fish tank pump that circulate water in the tank, dish washer and even more are equipments that we are familiar with and most of us uses them at home, you may want to know what is inside these electrical equipments that enable them carry out their operation, these electrical equipments operate on induction motor. The induction motor has useful application in domestic and industrial use. The induction motor was said to have been invented in the year 1882 in France by NIKOLA TESLA but his invention at that year was with a wrapped rotor and the initial patent was in the year 1888. The cage type induction motor was later invented by MIkHAIL in the year 1889, although so many invention on the induction motor was done by different inventor. The usefulness of an induction motor is as a result of the reliability,ruggedness and economical nature of it.

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The induction motor is an electromechanical device that covert power in an electrical form to a mechanical form in its rotation, due to its lesser speed of rotation compared to that of the synchronous machine, the induction motor is then referred to an asynchronous motor. The induction motor is an A.C (alternating current)electric motor that have two major part which are The Stator and The Rotor .The direct current (DC) motor has its power delivered directly to the armature winding at the rotor from a direct current source, but in the induction motor the alternating current is supplied to the motor through the stator, the rotor and stator are connected in a non-electrical connection , so the rotor then receive his own electric current that needed to produce torque for rotation by electromagnetic induction from the stator magnetic field winding, the rotor of an induction motor is of two type and can either be slip-ring rotor or squirrel cage rotor.

The induction motor is also referred to as a rotating transformer, having two side which are the primary side and the secondary side, the stationary part( Stator ) of an induction motor is referred as the primary side while that of the rotating part( Rotor )is referred to as the secondary side. Like the direct current motor that has brushes, the induction motor is designed without brushes which aid the control of the motor speed, its preferred nature is as a result of the rugged construction, reliability and economical nature of the induction motor. The induction motor can be of two type which are the single-phase induction motor and the three-phase induction motor, most of our household electrical equipments are of the single-phase type such as fan,pump,compressors, electric shaver. The three-phase induction motor has great industrial applications as a result of its good power factor and high efficiency, the three-phase induction motor doesn't get started by any form of starting device, so for this reason they are referred to as a self-starting induction motor, although the three phase induction motors have some disadvantages, in that when the speed of the motor tend to vary, the efficiency will be affected and also the speed of the machine is inversely proportional to the applied load, that is when the applied load increases, the speed of the machine decreases.

Parts Of An Induction Motor

1• Stator

The stator is the primary side of an induction motor, a stationary part of the motor that produces a rotating magnetic field, the field produced in the rotor is as a result of these rotating magnetic field produced in the stator. The stator is made up of electrical insulated winding, the insulation is made up of varnish and these winding are of copper wires. In other to avoid the flow of eddy current in the winding, the electrical winding Is then laminated with high graded alloy steel but in some cases, stack iron slices can be used in other to reduce the loss in the electrical system,the alternating current source is connected through the stator winding which are slotted on the stampings, the winding in the stator correspond with the poles number as a result of the required speed, the magnetic field speed equals the frequency that the alternating current operate and it's referred to as a synchronous speed. The slots in the stator of an induction motor is of same similarity of the synchronous machine, the slots carrying the winding either in a three-phase or single phase winding.

2• Rotor

The Rotor is another major part of an induction motor which can be said to be the secondary side of the motor, the rotor act as the rotating part of the induction motor and it's centrally located within the stator . Same as the stator, the rotor is made up of laminated slices in other to have a reduction in the system electrical losses. the rotor is connected to a shaft to enhance a better rotation. In a general form, the rotor are made up of aluminium or copper strips which is attached to each of the fixture end. The magnetic field produced by the stator cuts through the air gap between the stator and rotor and then induce current on the rotor on which then produces the rotor magnetic field and the interaction between both magnetic field of the stator and rotor causes a mechanical torque to be form and this torque is the rotating force that result in the rotation of the rotor. The rotor are of two common different types which are

i• Squirrel-cage rotor

ii• Slip-ring rotor

Squirrel-Cage Rotor

The squirrel cage rotor have parallel slots that carries laminated cores which are connected in a cylindrical form, this core are made up of heavy bar of aluminium or that of copper instead of wires, these bars are arranged in each of the slots to form a cylindrical space, end rings or metal rings are used to electrically weld these bar at each end of the rotor, forming a short circuit. The arrangement of these bar are in a squirrel cage form, resulting to the name.

Although the slots of this type of rotor and the shaft are not always parallel to each other, the bar are then twisted in an angle which is called the Skew angle and this is done to have a reduction in the rotor locking tendency, to cause an increment in the transformation ratio of the stator and the rotor and also increase the resistance of the rotor due to the length of the bars. High percentage of the used induction motor are made of the squirrel cage rotor, very simple and robust in construction, although external resistance can't be added to the armature circuit of the rotor.

Slip-ring Rotor

The construction of this type of rotor is made in a way that the end of the star connected Rotor winding are lay out and have it connected to three insulated slip ring which is mounted on the shaft that have brushes on it (the brushes is made lifted above the slip rings and this is done to ensure less frictional losses in the system ) this is made so in other to have connection for starting. The winding on the slip ring rotor are of three-phase distribution and of double layer,which is similar to that of the winding on the stator, having same number of poles and are connected in a star connection.

Like that of the squirrel cage rotor that doesn't have an external resistance connected to it, the slip ring rotor have an arrangement in other to have room for additional external resistance in the rotor circuit, this is done in other to have a control on the characteristics of the speed and torque and also to aid the starting of the rotor. When the induction motor is working on its rated speed, the metal collar causes the slip ring to be short circuited automatically. The slip ring rotor is usually applied for enormous induction motor of three phase, although slip ring rotor are more expensive when compared to the squirrel cage rotor and take a lot to maintain the brushes and slip ring and as a result of this, most industry find the squirrel cage rotor more applicable.

3• Shaft and Bearings

The shaft of an induction machine is placed inside the rotor,and when the rotor rotates, resulting to a corresponding rotation between the rotor and shaft, the connection of the shaft is brought outside the induction motor frame, having one end of it connected a fan which ensure adequate circulation of cooling air to the system during operation, while the other end is connected to allow connection to external system to have its rotating power transmitted. When getting a shaft for the motor construction, one should consider the torque that the rotor produces, doing this will avoid the shaft for breaking. The shaft of the rotor is held with bearings at both end, to help reduce friction during rotation and by that increasing the motor efficiency.

4• Casing

The casing house all components of the motor, there by providing a mechanical protection and also support for all form of winding in it, with the design of the casing, it also allows ventilation to the system, reducing heat in the system.

Working Principle Of A Three-phase Induction Motor

The winding of the stator are of two type, which are the main winding and the auxiliary winding. Let's assume that the stator is of one winding which is the main winding, knowing that in an induction motor, the alternating current is supplied through the stator and since the current are alternating and current flowing through a conductor produces a magnetic field around the conductor but in this case, an alternating magnetic field is produced which depends on the direction of the current and when a rotor is introduced into this field, fluxes from the rotating magnetic field of the stator will cut through the air gap between the stator and the rotor, there by inducing current into the rotor winding due to electromagnetic induction, these current then produce his own magnetic field resulting to production of torque on the rotor and this alternating field is equal to two magnetic field that are opposite, this concept is know called Double Revolving Field Theory.

The torque produced by this two opposite magnetic field are equal but opposite, resulting to a net effect of Zero torque on the motor rotor and instead of rotating the rotor, it will only buzz but when an initial external rotation is made on the rotor, one of the torque produced will be greater than the other then no zero torque but the problem to this was how to give the rotor an initial external rotation. When an auxiliary winding is added to the stator winding, the auxiliary winding will then produce an alternating magnetic field of two equal but opposite magnetic field, in this case one of them will cancel one from that main winding while the other will add up and this will bring about one rotating magnetic field, resulting to a starting torque on the rotor. The auxiliary winding makes the induction motor to be self-starting.

Equation Of The Torque Produced

The Torque produced is directly proportional to the flux that resulted to the electromotive force , the current that flow through the rotor and the rotor circuit power factor.

T ~♧Icos◇

T= the torque produced.

♧= the flux that result in the induced E.M.F.

I= the current that flow in the rotor.

cos◇= the circuit power factor.

The flux in the stator is directly proportional to the E.M.F on the stator ( E1)

♧~E1

I =(s*E2)/Z2

where

s= motor slip.

E2= rotor E.M.F.

Z2=total impedance on rotor.