The document discusses the principles of direct current (DC) generators, including the conversion of mechanical energy into electrical energy through electromagnetic induction according to Faraday's law. It describes the components and operation of a DC machine, including the armature, commutator, and various winding configurations (lap, wave, frogleg) to achieve desired output characteristics. Additionally, it covers the types of field windings (series and shunt) and their effects on the machine's performance.

1. BAB 1- DC GENERATOR

2. ELECTRICAL
MACHINE
GENERATOR
DC
Self exited
Separately
exited
AC
1  3 
MOTOR
DC
series shunt Compound
AC
1  3 
TRANSFORMER

3.  In a generator, conductors forming an electric
circuit are made to move through a magnetic
field.
 By Faraday’s law an e.m.f. is induced in the
conductors and thus a source of e.m.f. is
created.
 A generator converts mechanical energy into
electrical energy

4. E -The induced e.m.f.
B -the flux density (teslas),
L -the length of conductor in the magnetic field (m),
V -the conductor velocity, (m/s).

5. Fleming’s Right-hand rule
(often called the geneRator rule)
which states:
Let the thumb, first finger and
second finger of the
right hand be extended such that
they are all at right
angles to each other (as shown in
Figure). If the first
finger points in the direction of the
magnetic field and the
thumb points in the direction of
motion of the conductor
relative to the magnetic field, then
the second finger will
point in the direction of the induced
e.m.f.

6. If the conductor moves at an angle θ◦ to the
magnetic field (instead of at 90◦ as assumed
above) then
E=Blv sin θ volts

8. The left-hand side is moving in
an upward direction
(check using Fleming’s right-
hand rule), with length l
cutting the lines of flux which
are travelling from left to
right. By definition, the induced
e.m.f. will be equal to
Blv sin θ and flowing into the
page

9. The right-hand side is moving
in a downward direction
(again, check using Fleming’s
right-hand rule),
with length l cutting the same
lines of flux as above.
The induced e.m.f. will also be
equal to Blv sin θ but
flowing out of the page.
Therefore the total e.m.f. for the loop conductor
=2Blv sin θ

10. Now consider a coil made up of a number of turns N
The total e.m.f. E for the loop conductor is now given by:
E = 2NBlv sin θ

11. Problem 1.
A rectangular coil of sides 12 cm and 8 cm is rotated in a magnetic
field of flux density1.4T, the longer side of the coil actually cutting
this flux. The coil is made up of 80 turns and rotates at
1200 rev/min.
(a) Calculate the maximum generated e.m.f.
(b) If the coil generates 90 V, at what speed will the coil rotate?

13. b) Since E =2NBlv sin θ

14. The action of a commutator

25. DC generator
Stator
Yoke
Pole
Field winding
Rotor
(Armature
Iron core
Armature
winding
Commutator

26. The arrangement shown in Fig. 1.5 (a) is called a
‘two-segment’commutator and the voltage is applied
to the rotating segments by stationary brushes,
(usually carbon blocks), which slide on the
commutator material, (usually copper), when rotation
takes place.

27. In practice, there are many conductors on the rotating part of a d.c.
machine and these are attached to many commutator segments.
A schematic diagram of a multi segment commutator is shown in Fig.
1.5(b).

28. Poor commutation results in sparking at the trailing
edge of the brushes.
This can be improved by using interpoles (situated between each
pair of main poles), high resistance brushes, or using brushes
spanning several commutator segments

29. The basic parts of any d.c. machine are shown in
Fig. below, and comprise:
(a) a stationary part called the stator having,
(i) a steel ring called the yoke, to which are attached
(ii) the magnetic poles, around which are the

30. (iii) field windings, i.e. many turns of a conductor wound round
the pole core; current passing through this conductor creates an
electromagnet

31. (b) a rotating part called the armature mounted in bearings housed in
the stator and having
(iv) a laminated cylinder of iron or steel called the core, on
which teeth are cut to house the
(v) armature winding, i.e. a single or multiloop
conductor system, and
(vi) the commutator

32. Rotor of a dc motor Stator with poles visible.
Construction of DC
machine

33. Stator: non-moving coil
Rotor: rotating part
Armature coil
Brushes
Rotor is the rotating part -
armature
Stator is the stationary part - field

34.  More loops of wire = higher rectified voltage
 In practical, loops are generally placed in slots of an iron core
 The iron acts as a magnetic conductor by providing a low-reluctance path for magnetic
lines of flux to increase the inductance of the loops and provide a higher induced
voltage.
 The commutator is connected to the slotted iron core.
 The entire assembly of iron core, commutator, and windings is called the armature.
 The windings of armatures are connected in different ways depending on the
requirements of the machine.
Loops of wire are wound around slot in a metal core DC machine armature

35.  Lap Wound Armatures
◦ are used in machines designed for low voltage and high
current
◦ armatures are constructed with large wire because of high
current
◦ Eg: - are used is in the starter motor of almost all automobiles
◦ The windings of a lap wound armature are connected in
parallel. This permits the current capacity of each winding to
be added and provides a higher operating current
◦ No of current path, C=2p ; p=no of poles

36.  Wave Wound Armatures
◦ are used in machines designed for high voltage and low current
◦ their windings connected in series
◦ When the windings are connected in series, the voltage of each
winding adds, but the current capacity remains the same
◦ are used is in the small generator in hand-cranked
megohmmeters
◦ No of current path, C=2

37.  Frogleg Wound Armatures
◦ the most used in practical nowadays
◦ designed for use with moderate current and moderate
armatures voltage
◦ the windings are connected in series parallel.
◦ Most large DC machines use frogleg wound
armatures.
Frogleg wound armatures

38.  Most DC machines use wound electromagnets to
provide the magnetic field.
 Two types of field windings are used :
◦ series field
◦ shunt field

39.  Series field windings
◦ are so named because they are connected in series with the armature
◦ are made with relatively few windings turns of very large wire and have a
very low resistance
◦ usually found in large horsepower machines wound with square or
rectangular wire.
◦ The use of square wire permits the windings to be laid closer together,
which increases the number of turns that can be wound in a particular
space

40. Square wire permits more turns than round wire in the same area
Square wire contains more surface than round wire
– Square and rectangular wire can also be made physically smaller
than round wire and still contain the same surface area

41.  Shunt field windings
◦ is constructed with relatively many turns of small wire,
thus, it has a much higher resistance than the series field.
◦ is intended to be connected in parallel with, or shunt, the
armature.
◦ high resistance is used to limit current flow through the
field.

42.  When a DC machine uses both series and shunt fields, each
pole piece will contain both windings.
 The windings are wound on the pole pieces in such a manner
that when current flows through the winding it will produce
alternate magnetic polarities.

43. Winding
Lap
C=2p
Wave
C=2
Separately
Excited
Frogleg
Self excited
armature field
series shunt compound

45.  The magnetic field produced by the stator poles induces
a voltage in the rotor (or armature) coils when the
generator is rotated.
 This induced voltage is represented by a voltage source.
 The stator coil has resistance, which is connected in
series.
 The pole flux is produced by the DC excitation/field
current, which is magnetically coupled to the rotor
 The field circuit has resistance and a source
 The voltage drop on the brushes represented by a
battery

46. DC Machine Equivalent Circuit
1. Permanent magnet
2. Separately excited
3. Self-excited

47. DC Machine Equivalent Circuit
1. Permanent magnet
• The poles are made of permanent magnets.
• No field winding required.
• Small size.
• Disadvantage is low flux density, so low torque.

48. DC Machine Equivalent Circuit
2. Separately excited
The field flux is derived from a separate power source
independent of the generator itself.
B
Field
winding
Armature
winding

49. DC Machine Equivalent Circuit
3. Self-excited
• Shunt machine
The field flux is derived
by connecting the field
directly across the
terminals of the
generator.
B

50. DC Machine Equivalent Circuit
3. Self-excited
Series machine
• field are connected
in series with
armature
B

51. DC Machine Equivalent Circuit
3. Self-excited
• Cumulatively compounded
• Differentially compounded
B
B
B
B

52. DC Machine Equivalent Circuit
3. Self-excited
Compounded dc generator
• both a shunt and a series field
are present

53. DC Machine Equivalent Circuit
Compounded dc motor
• both a shunt and a
series field are present
3. Self-excited