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Welcome toLectures on D.C.Machine : 

2/11/2009 Welcome toLectures on D.C.Machine

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2/11/2009 D. C. Generator

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2/11/2009 Portable D. C. Generator


2/11/2009 DIRECT CURRENT MACHINES DC machine Construction The major advantages of DC machines are: easy speed and torque regulation. The stator of the DC motor has poles, which are excited by DC current to produce magnetic fields. The rotor has a ring-shaped laminated iron-core with slots. Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees. N S Stator with with poles Brush Rotor Field

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2/11/2009 D. C. Generator Principle of Operation When a conductor rotates in a magnetic field, dynamically induced e.m.f is produced in it according to Faraday’s law of electromagnetic induction

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2/11/2009 Generated EMF Let

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2/11/2009 Hence, according to Faraday’s Laws of electromagnetic induction,

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2/11/2009 For Wave wound Generator No. of Parallel Paths = 2 No. of Conductors (in series) in 1 path = Z/2

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2/11/2009 For Lap wound Generator No. of Parallel Paths = P No. of Conductors (in series) in 1 path = Z/P

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2/11/2009 In General, Generated EMF

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2/11/2009 Also, For a given D.C. Machine, Z, P and A are constants Hence, Putting

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2/11/2009 Types of D.C. Generators According to the way in which their fields are excited, generators are classified into : Separately excited D.C. Generator Self- excited D.C. Generator

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2/11/2009 Separately excited D.C. Generators are those whose field magnets are energized from an independent source of D.C. Current Ra = Armature Resistance

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2/11/2009 ii) Self excited D.C. Generators are those whose field magnets are energized by the currents produced by the generators themselves. There are 3 types of self-excited D.C. Generators named according to the manner in which their field coils are connected to the armature. These are : Shunt wound D.C. Generator Series wound D.C. Generator Compound wound D.C. Generator

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2/11/2009 Shunt wound D.C. Generator Rsh – Shunt Field Resistance Ra - Armature Resistance

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2/11/2009 ii) Series wound D.C. Generator

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2/11/2009 iii) Compound wound D.C. Generator Short Shunt

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2/11/2009 Long Shunt

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2/11/2009 Ex.1 A D.C. Shunt Generator delivers 450A at 230V and the resistance of the shunt field and armature are 50O and 0.03 O respectively. Calculate the generated EMF.

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2/11/2009 Ex.2 An 8 - pole D.C. Shunt Generator with 778 wave wound armature conductors running at 500 rpm supplies a load of 12.5O resistance at terminal voltage of 250V. The armature resistance is 0.24O and the field resistance is 250O . Find the armature current, the generated EMF and the flux per pole.