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Department of Electrical and Computer Engineering

EE20A - Electromechanical Energy EE20A - Electromechanical Energy ConversionConversion

DC MachineDC Machine

Construction of DC Machines Construction of DC Machines

Commutator

Features of DC Machine Features of DC Machine

Field Winding

DC Machines- DC Machines- Direction of Power Flow and Losses Direction of Power Flow and Losses

DC Machines- DC Machines- Direction of Power Flow and Losses Direction of Power Flow and Losses

DC Machines AnalysisDC Machines AnalysisSymbols that will be used.

= flux per pole p = no. of poles z = total number of active conductors on the armature a = no. of parallel paths in the armature winding

          Aside: Lap Winding -> a = p                 Wave Winding -> a = 2

n = speed of rotation of the armature in rpmwm = speed in radians per second

DC Machines ConnectionsDC Machines Connections

E

-

+Field

F F

Armature

b) Separately Excited

E

RaIf +

--

+

VT

a)

DC Machines ConnectionsDC Machines Connections

E

-

+

Field

F F

Armature

c) Series

E

-

+

Field

F F

Armature

d) Shunt

A

A

DC Machines ConnectionsDC Machines Connections

E

-

+

Field FF

Armaturee) Cummulative Compound

A

A

S S

E

-

+

Field FF

Armatured) Differential Compound

A

A

S S

DC Machines ConnectionsDC Machines Connections

E

-

+

Field FF

Armature

g) Short Shunt

A

A

S S

E

-

+

Field FF

Armature

f) Long Shunt

A

A

S S

EMF EquationEMF Equation

When the rotor rotates in the field a voltage is developed in the armature.

- the flux cut by one conductor in one rotation = p

-

-

- therefore in n rotations, the flux cut by one conductor = np

EMF EquationEMF Equation

EMF induced in the armature windings

TORQUE EQUATIONTORQUE EQUATION

EaIa=Tem - In the DC machine losses areexpressed as rotational losses due to friction and windage (F&W).

- The torque equation can then be rewritten as:-

  SHAFT OUTPUT TORQUE = (Te - TF&W)

DC GeneratorDC Generator

LIfIaI

fR

LVf

I

aRaIaELV

ap

60Nz

aE

E

Ra I f+

--

+

VTR f

LOAD

I a

Note: VT = VL

i.e. Terminal Voltage is the Load Voltage

OPEN CIRCUIT CHARACTERISTICSOPEN CIRCUIT CHARACTERISTICS

The Open Circuit characteristic is a graph relating Open-Circuit Armature voltage of a D.C. Generator versus its field current when the machine is driven at it’s rated speed

DCSource

A

FL

F

L

ZZ

Z

Field Current

Fie

ld R

eg

ula

tor

R f Ea

R a

AA

A

V

OPENCIRCUIT

Diagram showing motor connections for the open circuit test, separately excited

The D.C. Generator field is excited by a separate D.C. source and the current is varied using a generator Field Regulator (a potential divider).

OPEN CIRCUIT CHARACTERISTICSOPEN CIRCUIT CHARACTERISTICS

R a

Z

R f

ZZ

Ea

AA

A

AField Current

V VT

R ext970 ohm s

Diagram showing the D.C. Generator as a self-excited shunt machine

Rext is set to its maximum value. The D.C. Generator is driven at

rated its speed. Rext is decrease to a lower value so that the

machine self-excites ( i.e.. Develop an e.m.f). 

EXTERNAL CHARACTERISTIC OF SHUNT GENERATOREXTERNAL CHARACTERISTIC OF SHUNT GENERATOR

This is a graph relating terminal voltage and the load current of a D.C. Generator when driven at its rated speed with the field current maintained at its normal no-load value.

R a

Z

R f

ZZ

Ea

AA

A

AField Current

VR ext

970 ohm s

A

LOAD

Load Current

T erm inalVo ltage

Diagram showing connections for load test.

Summary

Losses Rotational andf

R 2

fI

-- Windings(Armature)-- Windings(Field )

aR 2

aI Losses

aEn

pza60aRaI -V

n Speed

a60pzn

aE

aRaI -VaE

Note: For motor

(Windage and Friction)