1) A DC generator produces direct current through electromagnetic induction. When a conductor moves through a magnetic field, an electromotive force (EMF) is induced in the conductor. 2) The basic components of a DC generator are magnetic poles and conductors that rotate within the magnetic field. 3) In a single loop DC generator, an EMF is induced in the sides of a rotating rectangular conductor loop as it cuts through the magnetic flux lines. The loop is connected to brushes to output a direct current.

1. Principle of DC Generator
• A dc generator produces direct power. Both of these
generators produce electrical power, based on same
fundamental principle of Faraday’s law of electromagnetic
induction.
• when an conductor moves in a magnetic field it cuts magnetic
lines force, due to which an emf is induced in the conductor.
• The most basic two essential parts of a generator are
a) a magnetic field and
b) conductors which move inside that magnetic field.
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2. Single Loop DC Generator
Rectangular loop of conductor is ABCD which rotates inside the
magnetic field about its won axis ab.
As during this movement two sides, i.e. AB and CD of the loop
cut the flux lines there will be an emf induced in these both of
the sides
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3. The loop is opened and connect it with a split ring as shown in
the figure below.
Single Loop DC Generator
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4. It is seen that in the first half of the revolution current flows always
along ABLMCD i.e. brush no 1 in contact with segment a.
In the next half revolution, in the figure the direction of the induced
current in the coil is reversed.
But at the same time the position of the segments a and b are also
reversed which results that brush no 1 comes in touch with that
segment b.
Hence, the current in the load resistance again flows from L to M.
Single Loop DC Generator
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5. The position of the brushes is so arranged that the change over
of the segments a and b from one brush to other takes place
when the plane of rotating coil is at right angle to the plane of
the lines of force.
Single Loop DC Generator
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6. Construction of DC Generator
A DC Generator has the following parts
1) Yoke
2) Pole of Generator
3) field winding
4) Armature of dc generator
5) Brushes of generator
6) Bearing
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7. Yoke of DC Generator
(i) It holds the magnetic pole cores of the generator and acts
as cover of the generator.
(ii) It carries the magnetic field flux.
• In small generator, yoke are made of cast iron.
• But for large construction of DC generator, where weight of
the machine is concerned, lighter cast steel or rolled steel is
preferable for constructing yoke of dc generator.
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8. Pole cores and pole shoes of DC Generator
There are mainly two types of construction available.
One: Solid pole core, where it made of a solid single piece of cast
iron or cast steel.
Two: Laminated pole core, where it made of numbers of thin, plates
of annealed steel.
The construction of magnetic poles basically comprises of two parts
namely,
the pole core and
the pole shoe, stacked together and then attached to the yoke.
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9. These two structures are assigned for different purposes,
the pole core is of small cross sectional area and its function is to
just hold the pole shoe over the yoke,
whereas the pole shoe having a relatively larger cross-sectional
area spreads the flux produced over the air gap
Pole cores and pole shoes of DC Generator
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10. Armature Core of DC Generator
• The purpose of armature core is to hold the armature
winding and provide low reluctance path for the flux
• Although a dc generator provides direct current but induced
current in the armature is alternating in nature.
• That is why, cylindrical or drum shaped armature core is
build up of circular laminated sheet.
• In every circular lamination, slots are either die – cut or
punched on the outer periphery and the key way is located
on the inner periphery
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11. Armature Winding of DC Generator
• Armature winding are generally formed wound.
• Various conductors of the coils are insulated from each other.
• The conductors are placed in the armature slots, which are
lined with tough insulating material.
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12. Commutator of DC Generator
• The commutator plays a vital role in dc generator.
• It collects current from armature and sends it to the load as
direct current.
• It actually takes alternating current from armature and
converts it to direct current and then send it to external load.
• It is cylindrical structured and is build up of wedge – shaped
segments of high conductivity, hard drawn or drop forged
copper.
• Each segment is insulated from
the shaft
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13. Brushes of DC Generator
• The brushes are made of carbon.
• These are rectangular block shaped.
• The only function of these carbon brushes of dc generator is
to collect current from commutator segments.
• The brushes are housed in the rectangular box shaped brush
holder.
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14. • If the airgap is of uniform length, the e.m.f. generated in a
conductor remains constant while it is moving under a pole face,
• and then decreases rapidly to zero when the conductor is midway
between the pole tips of adjacent poles.
DC generator
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15. • Three coils, 1–1′, 2–2′ and 3–3′, are
arranged in the slots so that their end
connections overlap one another
• we find that the direction of the e.m.f.
generated in conductor 1 is towards the
paper whereas that generated in
conductor 1′ is outwards from the paper.
• The distance between coil sides 1 and 1′
is called a pole pitch.
• In practice, the coil span must be a
whole number and is approximately
equal to
DC generator
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16. Armature windings can be divided into two groups, depending
upon the manner in which the wires are joined to the
commutator, namely:
1. Lap windings.
2. Wave windings.
• In lap windings the two ends of any one coil are taken to
adjacent segments
• In wave windings the two ends of each coil are bent in
opposite directions and taken to segments some distance
apart
• if a machine has p pairs of poles
• No. of parallel paths with a lap winding = 2p
• and No. of parallel paths with a wave winding = 2
Armature winding
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17. Armature winding
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18. • When an armature is rotated through one revolution, each
conductor cuts the magnetic flux emanating from all the N poles
and also that entering all the S poles.
• if Φ is the useful flux per pole, in webers, entering or leaving
the armature, p the number of pairs of poles and Nr the speed
in revolutions per minute
Calculation of e.m.f. generated in an armature winding
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19. Calculation of e.m.f. generated in an armature winding
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20. If Z is the total number of armature conductors, and c the
number of parallel paths through winding between positive and
negative brushes
The number of conductors in series in each of the parallel paths
between the brushes remains practically constant; hence total
e.m.f. between brushes is
Average e.m.f. per conductor × no. of conductors in series per
path
Calculation of e.m.f. generated in an armature winding
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21. A four-pole wave-connected armature has 51 slots with 12
conductors per slot and is driven at 900 r/min. If the useful flux
per pole is 25 mWb, calculate the value of the generated e.m.f.
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22. Total number of conductors = Z = 51 ×
12 = 612; c = 2; p = 2;
N = 900 r/min; Φ=0.025 Wb
Using expression , we have
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23. An eight-pole armature is wound with 480 conductors. The
magnetic flux and the speed are such that the average e.m.f.
generated in
each conductor is 2.2 V, and each conductor is capable of
carrying a
full-load current of 100 A. Calculate the terminal voltage on no
load,
the output current on full load and the total power generated
on
full load when the armature is
(a) lap-connected;
(b) wave-connected.
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24. 24

25. 25

26. Copper Losses
• Copper loss is the power lost as heat in the windings; it is caused by the flow of
current through the coils of the DC armature or DC field.
• This loss varies directly with the square of the current in the armature or field and
the resistance of the armature or field coils.
Armature: Ia
2 Ra
Field: If
2 Rf
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2
0 
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27. Armature reaction
shows the distribution of flux when
there is no armature current, the flux in
the gap being practically radial and
uniformly distributed.
shows the distribution of the flux set
up by current flowing through the
armature winding
shows the resultant distribution of
the flux due to the combination of
the fluxes
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28. • Over the trailing halves of the pole faces the cross flux is in
opposition to the main flux, thereby reducing the flux density,
whereas over the leading halves the two fluxes are in the
same direction, so that the flux density is strengthened.
• One important consequence of this distortion of the flux is
that the magnetic neutral axis is shifted through an angle θ
from AB to CD
Armature reaction
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