L-30(NKD)(ET) ((EE)NPTEL) - Module 8 Three-phase Induction...

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Module 8 Three-phase Induction Motor Version 2 EE IIT, Kharagpur
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Lesson 30 Construction and Principle of Operation of IM Version 2 EE IIT, Kharagpur
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In the previous, i.e. first, lesson of this module, the formation of rotating magnetic field in the air gap of an induction motor (IM), has been described, when the three-phase balanced winding of the stator is supplied with three-phase balanced voltage. The construction of the stator and two types of rotor squirrel cage and wound (slip-ring) one, used for three-phase Induction motor will be presented. Also described is the principle of operation, i.e. how the torque is produced. Keywords : Three-phase induction motor, cage and wound (slip-ring) rotor, synchronous and rotor speed, slip, induced voltages in stator winding and rotor bar/winding. After going through this lesson, the students will be able to answer the following questions: 1. How would you identify the two types (cage and wound, or slip-ring) of rotors in three-phase induction motor? 2. What are the merits and demerits of the two types (cage and wound, or slip-ring) of rotors in IM? 3. How is the torque produced in the rotor of the three-phase induction motor? 4. How does the rotor speed differ from synchronous speed? Also what is meant by the term ‘slip’? Construction of Three-phase Induction Motor This is a rotating machine, unlike the transformer, described in the previous module, which is a static machine. Both the machines operate on ac supply. This machine mainly works as a motor, but it can also be run as a generator, which is not much used. Like all rotating machines, it consists of two parts stator and rotor. In the stator (Fig. 30.1), the winding used is a balanced three-phase one, which means that the number of turns in each phase, connected in star/delta, is equal. The windings of the three phases are placed (electrical) apart, the mechanical angle between the adjacent phases being [ ], where p is no. of poles. For a 4-pole (p = 4) stator, the mechanical angle ° 120 p / ) 120 2 ( ° × Version 2 EE IIT, Kharagpur
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between the winding of the adjacent phases, is ° = ° = ° × 60 2 / 120 ] 4 / ) 120 2 [( , as shown in Fig. 29.4. The conductors, mostly multi-turn, are placed in the slots, which may be closed, or semi-closed, to keep the leakage inductance low. The start and return parts of the winding are placed nearly , or ° 180 ) 180 ( β ° apart. The angle of short chording ( ) is nearly equal to , or close to that value. The short chording results in reducing the amount of copper used for the winding, as the length of the conductor needed for overhang part is reduced. There are also other advantages. The section of the stampings used for both stator and rotor, is shown in Fig. 30.2. The core is needed below the teeth to reduce the reluctance of the magnetic path, which carries the flux in the motor (machine).
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This note was uploaded on 08/03/2010 for the course ELECTRICAL EE212 taught by Professor Shetty during the Spring '10 term at International Institute of Information Technology.

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L-30(NKD)(ET) ((EE)NPTEL) - Module 8 Three-phase Induction...

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