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Electrical Machines - I

Lecture 27: Mod-01 Lec-27- Lecture-27-DC Shunt Generators

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Lecture Details :

Electrical Machines-I by Prof. Debaprasad Kastha, Department of Electrical Engineering, IIT Kharagpur. For more details on NPTEL visit http://nptel.iitm.ac.in

Course Description :

Fundamentals of single phase transformers : Introduction; Basics of magnetic circuits, Ampere’s law, linear and nonlinear magnetic circuits; Faraday’s law of electromagnet induction - Concept of an ideal transformer, assumptions; ideal transformer at no load and on load, phasor diagram, voltage current and power relations; basic construction of a practical single phase transformer, conceptual differences with an ideal transformer - Constructional details of a practical single phase transformer, core, winding,insulation and cooling methods; circuit model of a practical transformer incorporating an ideal transformer - Analysis of a practical single phase transformer, exact and approximate equivalent circuits, phasor diagram, relation between referred variables and parameters, per unit system of representation - Worked out examples, question - answer session.

Testing and performance of single phase transformers : Experimental determination of single phase transformer equivalent circuit parameters, Open circuit and short circuit tests, per unit parameters; worked out example - Percentage regulation of a single phase transformer, phasor diagram, derivation, per unit representation; worked out example - Efficiency of a single phase transformer, maximum efficiency, all day efficiency; Sumpner’s test for experimental determination of a single phase transformer efficiency - Worked out examples, question - answer session.

Fundamentals of three phase transformers : Bank of three single phase transformers, voltage, current and kVA rating with different connections, open . connection; worked out example - Three phase transformer as a single unit; constructional features, three limb,five limb, core type and shell type constructions; comparison with bank of three single phase transformers - Three phase transformer connections, polarity and terminal convention,vector groups; Y/Y and ./. connections, connection and phasor diagram,per phase equivalent circuit based analysis - Y/.and ./Y connections. Connection and phasor diagrams, per phaseequivalent circuit based analysis; worked out examples - Y/Z and ./Z connections. Connection and phasor diagrams, voltage,current relations.

Operation and performance of three phase transformers : Tap changing transformer - Parallel operation of single and three phase transformers, conditions and connection diagrams, per phase equivalent circuit based analysis with equal and unequal no load voltages, load sharing - Harmonics and switching transients in transformers, effect of transformer connections, inrush current - Worked out examples, question - answer session.

Special purpose transformers : Single and three phase three winding transformer, equivalent circuit, phasor diagram, tests - Single and three phase autotransformers, advantages, equivalent circuit and phasor diagram - Three phase to two phase conversion using Scott connection, connection and phasor diagram, analysis with balanced and unbalanced loads - Worked out examples, question - answer session.

Construction and operating principle of three phase induction machines : Basic constructional features of round rotor polyphase ac machines. Airgap mmf and flux waveforms due to single phase distributed winding - Airgap mmf and flux waveform in round rotor machines due to balanced three phase distributed winding excited by balanced three phase alternating current, rotating magnetic field - Generated voltage and torque expression in three phase round rotor ac machine with distributed winding - Construction and types of three phase induction machines, wound rotor and squirrel cage induction machines; representation of the squirrel cage by a balanced three phase winding - Basic operating principle, motoring and generating mode of operation, generated rotor voltage, synchronous speed, slip speed, per unit slip.

Analysis of three phase induction machines : Exact and approximate per phase equivalent circuit; phasor diagram under no load and loaded condition - Power flow diagram in a three phase induction machine, airgap power, slip power, mechanical power; torque-slip and current-slip characteristics - Starting torque, breakdown slip, breakdown torque, maximum mechanical power, effect of equivalent circuit parameters - Worked out examples, question - answer session.

Testing of three phase induction machines : No load and blocked rotor tests for determining equivalent circuit parameters; losses and efficiency - Induction machine performance computation from circle diagram - Cogging torque and crawling; induction machines with deep bar and double cage rotors - Worked out examples, question - answer session.

Starting and speed control of three phase induction machines : Direct on line starting, stator resistance and reactance based starting, rotor resistance based starting - Y/. and auto transformer based starting; worked out examples - Speed control of induction motors by stator voltage variation and pole changing techniques - Variable frequency speed control of induction machines, v/f control method, slip power control method - Worked out examples, question - answer session.

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