Dc motor

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Welcome to

Lectures on D.C.Machine

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D. C. Generator

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Portable D. C. Generator

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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 polesBrush

Rotor

Field

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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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Generated EMF

Let

Ei.epathsparalleltheofoneanyingeneratedEMFEEMFGenerated

ArmatureinpathparallelanyininducedEMFE

rpminrotationArmatureN

ArmatureinPathsParallelofNo.A

polesGeneratorofNo.P

SlotperConductorsofNo.SlotsofNo.

ConductorsArmatureofNo.TotalZ

WbinFlux/Poleφ

g =

====

×===

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secN

60dt,revolution1forTime

N/60s/secRevolutionofNo.

WbφPdφ,revolution1intorcut/conducFluxNow,

voltdt

dφConductorGenerated/EMFAverage

=∴

==

=

Hence, according to Faraday’s Laws of electromagnetic induction,

volt60

φPN

60/N

φP

voltdt

dφconductorGenerated/EMF

==

=

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For Wave wound Generator

No. of Parallel Paths = 2

No. of Conductors (in series) in 1 path = Z/2

volt120

φZPNvolt

260

φPNpathGenerated/EMF =×= Z

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For Lap wound Generator

No. of Parallel Paths = P

No. of Conductors (in series) in 1 path = Z/P

volt60

φZNvolt

P60

φPNpathGenerated/EMF =×= Z

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In General,

Generated EMF

windinglapforP

windingwavefor2Awhere,

voltA

P

60

φZNE g

==

=

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Also,

voltA

P

π

ωφZvolt

A

PφZ

πN

πEg

=

=

260

2

2

1

For a given D.C. Machine, Z, P and A are constants

Hence, Putting

rps)inis(N

voltsφNk2π

ωφkE

get,we,A

ZPk

aag

a

==

=

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Types of D.C. Generators

According to the way in which their fields are excited, generators are classified into :

i) Separately excited D.C. Generatorii) Self- excited D.C. Generator

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i) Separately excited D.C. Generators are those whose field magnets are energized from an independent source of D.C. Current

DropsBrushIREV ag −−=Ra = Armature Resistance

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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 tothe 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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i) Shunt wound D.C. Generator

Rsh – Shunt Field ResistanceRa - Armature Resistance

sha III +=

DropsBrushRIEV aag −−=

shsh RIV =

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ii) Series wound D.C. Generator

DropsBrush)RR (IEV seaag −+−=

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iii) Compound wound D.C. Generator

Short Shunt

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Long Shunt

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Ex.1 A D.C. Shunt Generator delivers 450A at 230V and the resistance of the shunt field and armature are 50Ω and 0.03 Ωrespectively. Calculate the generated EMF.

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