ee-mc1- - construction of d.c. machine

7/28/2019 Ee-mc1- - Construction of d.c. Machine

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Construction of

DC Machine


2/19

Construction of DC Machines

Commutator

SUB: EM/C-1 ( UNIT-1 ) TOPIC: CONSTRUCTION OF D.C MACHINE


3/19

Features of DC Machine

Field Winding



4/19

DC Machine Construction

Fig.4 DC motor stator with poles visible

(4 pole machine).



5/19


Fig.5 Rotor of a DC motor.


6/19


Fig.6 Cutaway view of a DC

motor.


7/19

DC Machines-Direction of Power Flow and Losses


8/19

DC Machines-Direction of Power Flow and Losses


9/19

DC Machines Analysis

Symbols that will be used.

= flux per polep = 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 rpm

wm = speed in radians per second


10/19

DC Machines Connections

E

-

+Field

F F

Armature

b) Separately Excited

E

Ra If +

--

+

VT

a)


11/19


E

-

+

Field

F F

Armature

c) Series

E

-

+

Field

F F

Armature

d) Shunt

A

A


12/19


E

-

+

Field FF

Armaturee) Cummulative Compound

A

A

S S

E

-

+

Field FF

Armature

d) Differential Compound

A

A

S S


13/19


E

-

+

Field FF

Armatureg) Short Shunt

A

A

S S

E

-

+

Field FF

Armature

f) Long Shunt

A

A

S S


14/19

EMF Equation

When the rotor rotates in the field a voltage is

developed in thearmature.

- the flux cut by one conductor in one rotation

= p

-

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


15/19

EMF Equation

EMF induced in

the armature

windings


16/19

DC Generator

LI

fIaI

fRLV

fI

aRaIaELV

a

p

60Nz

aE

E

Ra

If

+

--

+

VT

Rf

L

O

A

D

Ia

Note: VT = VLi.e. Terminal Voltage is the Load Voltage


17/19

OPEN 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 its rated speed

DCSource

A

FL

F

L

ZZ

Z

Field Current

FieldReg

ulator

Rf Ea

Ra

AA

A

V

OPEN

CIRCUIT

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


18/19

OPEN CIRCUIT CHARACTERISTICS

Ra

Z

Rf

ZZ

EaAA

A

A Field Current

V VT

Rext

970 ohms

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).


19/19

EXTERNAL 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.

Ra

Z

Rf

ZZ

Ea

AA

A

AField Current

VR

ext

970 ohms

A

L

OA

D

Load Current

Terminal

Voltage

Diagram showing connections for load

ee-mc1- - construction of d.c. machine

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