L-29(NKD)(ET) ((EE)NPTEL)

L-29(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 29 Rotating Magnetic Field in Three-phase Induction Motor Version 2 EE IIT, Kharagpur
In the previous module, containing six lessons (23-28), mainly, the study of the single- phase two-winding Transformers – a static machine, fed from ac supply, has been presented. In this module, containing six lessons (29-34), mainly, the study of Induction motors, fed from balanced three-phase supply, will be described. In this (first) lesson of this module, the formation of rotating magnetic field in the air gap of an induction motor, is described, when the three-phase balanced winding of the stator is supplied with three-phase balanced voltage. The balanced winding is of the same type, as given in lesson no. 18, for a three-phase ac generator. Keywords : Induction motor, rotating magnetic field, three-phase balanced winding, and balanced voltage. After going through this lesson, the students will be able to answer the following questions: 1. How a rotating magnetic field is formed in the air gap of a three-phase Induction motor, when the balanced winding of the stator is fed from a balanced supply? 2. Why does the magnitude of the magnetic field remain constant, and also what is the speed of rotation of the magnetic field, so formed? Also what is meant by the term ‘synchronous speed’? Three-phase Induction Motor A three-phase balanced winding in the stator of the Induction motor (IM) is shown in Fig. 29.1 (schematic form). In a three-phase balanced winding, the number of turns in three windings, is equal, with the angle between the adjacent phases, say R & Y, is (electrical). Same angle of (elec.) is also between the phases, Y & B. ° 120 ° 120 A three-phase balanced voltage, with the phase sequence as R-Y-B, is applied to the above winding. In a balanced voltage, the magnitude of the voltage in each phase, assumed to be in star in this case, is equal , with the phase angle of the voltage between 120 . ° Version 2 EE IIT, Kharagpur

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Rotating Magnetic Field The three phases of the stator winding (balanced) carry balanced alternating (sinusoidal) currents as shown in Fig. 29.2. R R Y Y B B Axis of phase R Axis of phase B Axis of phase Y θ Fig. 29.2: The relative location of the magnetic axis of three phases. 120 ° 120 ° 120 ° t I i m R ω cos = ) 120 ( cos ° = t I i m Y ) 240 ( cos ) 120 ( cos ° = ° + = t I t I i m m B Please note that the phase sequence is R-Y-B. is the maximum value of the phase currents, and, as the phase currents are balanced, the rms values are equal ( m I B Y R I I I = = ). Three pulsating mmf waves are now set up in the air-gap, which have a time phase difference of from each other. These mmf’s are oriented in space along the magnetic axes of the phases, R, Y & B, as illustrated by the concentrated coils in Fig. 29.2. Please note that
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L-29(NKD)(ET) ((EE)NPTEL) - Module 8 Three-phase Induction...

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