UNIT II

DC MACHINES
BY
L.SHEELA
Maxwell’s Cork screw Rule :

Hold the cork screw in

your right hand and rotate
it in clockwise in such a
way that it advances in the
direction of current. Then
the direction in which the
hand rotates will be the
direction of magnetic lines
 Fleming’s left hand rule

Used to determine the direction of force

acting on a current carrying conductor
placed in a magnetic field .
The middle finger , the fore finger and
thumb of the left hand are kept at right
angles to one another .
The middle finger represent the direction
of current
The fore finger represent the direction
of magnetic field
The thumb will indicate the direction of
force acting on the conductor .
This rule is used in motors.
Fleming’s Right hand rule
Used to determine the direction of emf induced in a
conductor
The middle finger , the fore finger and thumb of
the left hand are kept at right angles to one another.
The fore finger represent the direction of magnetic
field
The thumb represent the direction of motion of the
conductor
The middle finger will indicate the direction of the
inducted emf .
This rule is used in DC Generators
Lenz’s Law

The direction of induced emf is given by Lenz’s law .

According to this law, the induced emf will be acting in such a
way so as to oppose the very cause of production of it .

e = -N (dØ/dt) volts
DC MACHINES

 A DC machine is an electro-mechanical energy

conversion device.
 It can convert Mechanical power into
Electrical Power. When output electrical power
is DC , it is called DC Generator.
 When it converts DC electrical power into
mechanical power , it is known as DC Motor
Constructional Features of DC
Machines
 Body or Magnetic Frame or Yoke
 Pole Core and Pole Shoes
 Field or Exciting coils
 Armature Core
 Armature Winding
 Commutator
 Brushes
 End Housing and Bearings
 Shaft
Bearing Brush Body / Yoke
Pulley End Housing

Shaft

Armature
Commutator
Brush Field Core
holder Field Winding

MAIN CONSTRUCTIONAL FEATURES

 YOKE
ARMATURE

+
SHAFT

FIELD POLE
BRUSH & COIL
- COMMUTATOR
MAIN CONSTRUCTIONAL FEATURES
1.Magnetic Frame or Yoke
 The outer cylindrical frame to which main poles
and inter poles are fixed and by means of the
machine is fixed to the foundation is called YOKE
 It serves two purposes:
 a) It provides mechanical protection to the
inner parts of the machines.
 b) It provides a low reluctance path for the
magnetic flux.
 The yoke is made of cast iron for smaller machines
and cast steel or fabricated rolled steel for larger
machines.
2. Pole Core and Pole Shoes
 The pole core and pole shoes are fixed
to the yoke by bolts. They serves the
following purpose :
 a) They support the field or
exciting coils.
 b) They distribute the magnetic flux on
the armature periphery more
uniformly.
 c) The pole shoes have larger X-
section, so, the reluctance of the
magnetic path is reduced. The pole
core and pole shoes are made of
laminated steel assembled by riveting POLE CORE
together under hydraulic pressure.
3. Field or Exciting Coils
 Field coils or exciting coils are used to
magnetize the pole core. Enameled
copper wire is used for the
construction of these coils.
 When direct current is passed through
these coils/ winding, it sets up the
magnetic field which magnetize the
pole core to the reqd. flux.
4.Armature Core
 Armature is a rotating part of the DC machine, reversal
of flux takes place, so hysteresis losses are produced.
To minimize this loss, silicon steel is used for the
construction.
 The rotating armature cuts the main magnetic field ,
therefore an e.m.f is induced in the armature core. This
e.m.f circulates eddy currents in the core which results
in eddy current loss in it.
 The armature core is laminated to reduce the eddy
current loss.
 Armature core serves the following purposes:
 a) It houses the conductors in the slots.
 b) It provides an easy path for magnetic flux
 5. Armature Winding
 The no. of conductors in form of coils
placed in the slots of the armature and
suitably inter connected are called
winding
 This is the armature winding where
conversion of power takes place i.e. in ARMATURE WINDING
case of generator , mechanical power is
converted into electrical power and in
case of a motor, electrical power is
converted into mechanical power.
 Depending upon the types of inter
connection. of coils , the winding can be
classified into two types;
Contd..
i) Lap Winding;
 The conductors/coils are connected in such a way that
no of parallel paths are equal to no. of poles. If machine
has ‘P’ no. of poles and ‘Z’ no. of conductors, then there
will be ‘P’ no. of parallel paths. And each path will have
‘Z/P’ no of conductors in series. Also the no. of brushes are
equal to no. of parallel paths. Out of which half of the
brushes will be positive and remaining will be negative.
ii) Wave Winding;
 The conductors are so connected that they are
divided into two parallel paths only , irrespective of the no.
of poles. If machines has ‘Z’ no. of conductors, there will
be only two parallel paths and each will be having ‘Z/2’ no.
of conductors connected in series with only two brushes.
 COPPER SEGMENT
RISER
MICA INSULATION
6.Commutator END RING

ADJUSTING NUT
 It is the most important part of a DC machine and
serves the following purpose :-
i) It connects the rotating armature conductors to
the stationary external circuit through the brushes.
METAL SLEEVE
 ii) It converts altering current induced in the
SHAFT
armature conductors into unidirectional current in
the external load circuit in generating action and it
converts alternating torque into unidirectional torque
produced in the armature in motoring action.
 The commutator is of cylindrical shape and is made of
wedge shaped hard drawn copper segments. The
segments are insulated from each other by a thin
sheet of mica. The segments are held together by
means of two V-shaped rings that fit into the V-
grooves cut into the segments. Each armature coil is
connected to the commutator segment through riser.

COMMUTATOR
7. Brushes

 Brushes are made of high grade

carbon. They form the connecting link
between armature winding and the
external circuit.
 Thebrushes are held in particular
position around the commutator by
brush holders.
8. End Housing and Bearings

 End Housing : They are attached to the ends END HOUSING

of main frame and support bearing . The
front housing supports the bearing and the
brush assembly whereas rear housing
supports the bearing only.
 Bearing : The function of the bearing is to
reduce friction between the rotating and
stationary parts of the machines. These are
fitted in the end housings. Generally, high
carbon steel is used for the construction of
the bearings.
9. Shaft
 The function of shaft is to
transfer mechanical power to
the machine or from the
machine .
 Shaft is made of mild steel
with maximum breaking
strength. All the rotating parts SHAFT

like armature core,

commutator, cooling fan etc.
are keyed to the shaft.
DC Generator

S
ME



CH

A B
AN

S N _
ICA

+
LI

Te
NP

 
UT

  
Tm ELECTRICAL
 
LOAD

N 
 Working Principle of Generator
 The field windings are shown as excited from external source. The
polarity of electro-magnetic field will depend upon the direction of
field current.
 The armature carries conductors in side the slots. Two brushes are
placed at the right angle to the main field axis. The brushes are
stationary whereas armature is free to rotate.
 When the armature is rotated in the magnetic field, an e.m.f will
be induced in the armature conductors. The direction of the
induced e.m.f can be found by applying Fleming’s Right Hand
Rule.
 The direction of induced e.m.f will depend upon the direction of
rotation of armature , if polarity of field poles to be kept
unchanged. When load is connected across the armature
terminals , the current will flow through the armature circuit.
 The direction of current will be same as that of induced e.m.f. The
armature will now be considered as electro-magnet and its
 MAGNETIC FIELD

B
A A 
Q P
B

LOAD 0o
 MAGNETIC FIELD

B
A 
Q P
+ B
_
e

LOAD 30o

t
 MAGNETIC FIELD

B
A 
Q P
+ B _
e

LOAD 60o

t
 MAGNETIC FIELD

A
B


Q A B P
+ _
e

LOAD 90o

t
 MAGNETIC FIELD

A B

BA 
Q P
+ A _
e

LOAD 120o

t
 MAGNETIC FIELD

A
BA 
Q P
+ A _
e

LOAD 150o

t
 MAGNETIC FIELD

A
B A 
Q P
+ A
e

LOAD 180o

t
 MAGNETIC FIELD

A
B 
Q P
+ A
_
e

LOAD 210o

t
 MAGNETIC FIELD

B 
Q P
+ A _
e

LOAD 240o

t
 MAGNETIC FIELD

B
A


Q B A P
+ _
e

LOAD 270o

t
 MAGNETIC FIELD

B A

AA

Q P
+ B _
e

LOAD 300o

t
 MAGNETIC FIELD

B
A A 
Q P
+ B _
e

LOAD 330o

t
 MAGNETIC FIELD

B
A A 
Q P
B
e

LOAD 360o

t
EMF Equation of DC Generator
Let, Ø= flux per pole in weber

Z = Total number of conductor

P = Number of poles

A = Number of parallel paths

N =armature speed in rpm

Eg = emf generated in any on of the parallel path

Flux cut by 1 conductor in 1 revolution =P*φ

Flux cut by 1 conductor in 60 sec = P φ N /60
Avg emf generated in 1 conductor = PφN/60
Number of conductors in each parallel path = Z /A

Eg = PφNZ/60A
 For lap winging,
A=P, Eg = ΦZN/ 60
For wave winding,
A=2, Eg = ΦZN/ 60 *(P/2)
Types of DC Generators
DC generators are generally classified according to their method of
excitation .
Separately excited DC generator
Self excited D C generator
Separately excited DC
generator

 Field winding is excited by separate DC supply

Field winding is excited by separate DC supply
Armature current Ia = load current IL
Terminal voltage V = Eg – Ia Ra – Vbrush , Vbrush
is drop across brush, low value, so neglected.
Generated emf Eg = V + Ia Ra + Vbrush
Generator field winding is supplied from the
armature of the generator itself.
Excitation occurs due to presence of residual flux in
the poles.

Process: armature cut the residual flux, small

emf will induced, this produces small field current
in the field winding. Then flux per pole increases.
Then by cumulative process, generator produces its
Series DC generator
 Field winding is connected in series with the
armature.

 Ia = Ise = IL
 Eg = V + Ia Ra + Ia Rse + Vbrush
 V = Eg - Ia Ra - Ia Rse – Vbrush
Vbrush – neglected (low value)
Shunt DC Generator
 Field
winding is connected across the
armature.
 Eg = V + Ia Ra
V = Eg – Ia Ra
 Ish = V /Rsh
 Ia = IL + Ish
Compound DC Generator

 Itconsists of both shunt and series field

winding.
 One winding in series (Rse) and other
winding is in parallel (Rsh) with the
armature.
 Two types
 long shunt compound generator
 short shunt compound generator
Long Shunt DC Compound
Generator
 Shunt field winding is connected across both series
field and armature winding

 Ise = Ia = IL + Ish
 Ish = V / Rsh
 Eg = V + Ia (Ra + Rse) + Vbrush
 V = Eg - Ia (Ra + Rse) – Vbrush
Vbrush – neglected (low value)
Short Shunt Compound DC
Generator
Shunt field winding is connected in parallel with the armature and this
combination is series to series field winding
 Ise = IL
 Ia = Ise + Ish
 Ish = V / Rsh
 Eg = V + Ia Ra + Ise Rse + Vbrush
 V = Eg - Ia Ra - Ise Rse – Vbrush
Vbrush – neglected (low value)
 Voltage across shunt field winding = Ish Rsh
Ish Rsh = Eg – Ia Ra – Vbrush
 Substitute Eg value in the above equation
Ish Rsh = V + Ise Rse
 Shunt field current Ish = (V + Ise Rse)/ Rsh
 Power developed in armature = Eg Ia
 Power delivered to load = V IL
(power formula is same for all types of DC generator )
Performance Characteristics of DC
Generators
DC Motor

S


AD


LO
A B

L
S N _

A
+

IC
AN
Te

H
 

EC


M
DC  
SUPPLY v TL

N 
 Working Principle of DC Motor
 The armature is connected across a supply voltage ‘V’ and the field
windings are excited from the same supply or from any external dc
source. The magnetic polarities due to the current in armature
winding.
 The electro-magnetic torque Te will be developed in the anti-
clockwise direction as opposite poles of armature field and main
field will attract each other. The armature will rotate in anti-
clockwise direction due to Te .
 To reverse the direction of rotation of armature, either the direction
of current in the field winding or armature winding will have to be
reversed. If the direction of currents in both the windings are
reversed, direction of rotation of armature will be unchanged.
 As the mechanical load on the armature i.e. rotor shaft represented
by load torque TL is increased, more and more electro-magnetic
torque will be developed by the armature to balance the mechanical
torque requirements for which the armature will draw more current
from the supply mains.
Types of Excitations
 the dc motors can be classified into two categories ;
 1) Separately excited dc motors
 2) Self excited dc motors.
Separately Excited DC Motors
+
+ Ia
Rf A
F FF

If M E V
Supply
Ra

VDC _ _ AA
+
_
Separately Excited DC Motor
 The field winding is excited from a supply which is not connected to the
armature winding. It may be noted that current flowing through the field
winding is independent of load and is equal to V / R f , where Rf is the
field circuit resistance. The flux produced is proportional to the field
current i.e. Ø  If
Self Excited DC Motors

+
If Ia IL
F A

V
M E SUPPLY

Ra
FF
FF
AA
_
Armature and field windings are connected across a constant
source of supply. The field current If is drawn from the same source
as that of armature current.
 DC series Motor
YY Y
+
+ Ia V = E + Ia Ra + Ia Rse
A ISE IL or

E = V - Ia ( Ra + Rse )

M E V
SUPPLY and
Ra Ia = Ise = IL
AA_
_
 The field winding is connected in series with the armature so that I f = Ia = IL .
Therefore field winding is made up of thick winding wire of less no. of turns as
compared to that of shunt field winding so that armature current can flow through
it without overheating. In case of dc series machine , Ø  If  Ia .. The relationship
between induced e.m.f. and terminal voltage is as follows
 DC Compound Motors
ISh YY Y
 There are two field windings , namely a
+
shunt field winding and a series field
winding. The shunt field winding is Ia IL
connected in parallel with the armature
Z A ISE
and series field winding is connected in
series with the combination .
 Series field winding will carry a large
armature current Ia or IL and therefore it Rsh M E
SUPPLY
V
is made of wire of large cross section
and has a few turns only. The resistance
of series field winding is very small. The
shunt field winding is made up of wires
Ra
of small cross section and has high ZZ AA
resistance. Since the resistance of
shunt field winding is high , the current
flowing through it is very small as
_
compared to that of series field winding

Depending up on the connections of shunt field winding in the combination of

armature and series field winding, dc compound generators can be named as
i) Short shunt compound generators.
ii) Long shunt compound generators.