Three Phase Induction Motors

“A machine which converts three-phase electrical energy into mechanical energy based on principle of electro-magnetic induction is known as three-phase induction motor”. These motors are quite different from D.C. motors,because the three-phase electric supply is given to its stator only. There is no electrical connection to the rotor from the main supply and also no other source of supply is required to run the induction motors. Whereas in case of D.C. motor, both the stators as well rotors (Armature) are connected across the supply to run themotor. These motors are mostly used in factories for different applications such as for lathe, drill, blower, printing machine, etc. The speed of an induction motors is almost constant but slightly falls with the increase in load. The starting current is 5 times higher than the full load current. Interchanging any two-phase connection to the motor or starter can reverse the direction of rotation of these motors. The working principles and different parts of induction motors are given below.

i. Working Principle

When stator winding of a three-phase induction motor is connected across the three phase electric supply, a rotating magnetic field will be produced. This rotating magnetic field, when links with the short-circuited rotor bars, an electro-motive-force (E.M.F.) will be induced in the rotor bars. This induced electro-motive-force will circulate a heavy eddy current in the rotor bars and produced its own magnetic field. By Lenz’s law this induced current will oppose the cause producing it i.e. relative motion of the stator field. The combined interaction of both the magnetic fields (Stator and Rotor) will produce a net resultant moving force (Torque) in the rotor. As the current in the rotor bars is set up entirely by effect of electromagnetic induction from the stator, hence the name “Induction Motor”.

a) Stator: As the name indicates, it is the stationary part of the induction motor and made of silicon steel strips of thickness, varying from 0.3 to 1.35 mm.These strips are combined together, which are called laminated strips and the combination is known as laminated core. These laminated stampings/strips are slotted to receive the winding. These slots may be of open or semi-closed types, to facilitate the winding. The core and end covers are fitted in the cast iron frame, so that external mechanical injury may not take place to stator winding.

The same stator can be used for single-phase induction motors. The stator carries three-phase winding and is fed from three-phase electric supply. The stator is wound for a definite number of poles and depends upon the speed of motor. Greater the number of poles lesser will be the rotor speed and vice-versa. This speed can be calculated by the formulae, N s = 120f/p. Where N s, is the synchronous speed of the stator field, f, is the supply frequency, which is normally 50 cycles per second, and P is the number of poles. The synchronous speed for a 2-pole motor is 3000 r.p.m. and for a 4-pole motor it is 1500 r.p.m.Generally the speed of rotor is slightly less (up-to 5% of synchronous speed) than the speed of stator field. The difference between the speed of stator field and rotor is known as slip and it should be as low as possible.

b) Rotor: The rotor is the rotating part of the motor and made of silicon steel strips. The thickness of these strips varies from 0.3 to 1.35 mm, as in case of stator. These strips are clamped together to form rotor core, called as laminated core. This laminated core is slotted to totally closed type, to receive rotor winding. In large capacity motors thick aluminium bars are inserted and are short-circuited with end rings. Nowadays, melted aluminium is filled in these slots, which works as short-circuited winding. This winding is known as squirrel cage winding. As we know plain induction motor is having poor starting torque and draws heavy starting current. So, such types of problems can be over come by modifying the rotor slightly by providing double cage in the rotor

The rotor has been provided with double slots to accommodate two rotor windings. A low resistance copper bars are deeply embedded in the slots and brass or aluminium bars are inserted near the surface. This type of winding is known as double cage winding. The whole assembly is keyed on the shaft of the motor.

c) End Covers: As the name indicates these covers are used to cover the ends of the motor and are made of cast iron. These end covers are fitted with the stator frame with the help of nuts and bolts. The ball bearings are fitted in the end covers to keep the rotor exactly in the centre of the stator, so that it can move freely.

d) Shaft and Bearings: Mostly ball bearings are used in large capacity motors where as the bush bearings is used in small capacity motors because the noise level is high in ball bearing as compared to bush bearings. The main purpose of bearings is to keep the rotor exactly in centre and ensure free movement for the rotor. The shaft is a long circular bar, made of mild steel. The rotor assembly and cooling fan is securely keyed to the shaft of the motor.

e ) Fan: In general, the fan is used to cool down the temperature. When the motor runs on load, the heat is produced in the motor winding as well as in the core due to copper and iron losses respectively. So, the fan serves the purpose of transferring the heat from inside to outside of the motor by forced air circulation.It sucks the air from the atmosphere through the air ducts and discharge back to atmosphere after cooling the winding and core of the motor. In large capacity motors these fans may be fitted on the rotor shaft near the winding whereas in small capacity motors these may be fitted outside the end covers. A mild steel or cast iron cover is used to cover the fan to avoid chances of accidents.

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