1. CHAPTER 2
DC GENERATOR
Electrical Machines

2. Introduction
A DC generator converts mechanical energy into electrical energy (DC). The working of a DC generator is based on the principle that, when a conductor cuts a magnetic field an e.m.f. is induced in the conductor. A dynamically induced e.m.f. will be produced in the conductor when it will cut the magnetic flux as per the laws of electromagnetic induction
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3. Introduction
If the circuit is closed this induced e.m.f. will cause a current to flow through circuit.
Basically a D.C. machine is an alternating current machine. The e.m.f. generated in the conductor is alternating in nature. With the help of the commutator segments this alternating e.m.f. is converted into a direct one.
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4. Construction of DC Machine
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5. Construction of a DC Machine
Pole
Armature
Yoke
Commutator
Brushes
Interpole
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6. Working Principle of a DC Generator
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7. Working Principle of a DC Generator
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8. Armature Winding There are two types of windings Lap winding
The armature windings are divided into no. of sections equal to the no of poles
Wave winding
A = 2
Used in high voltage low current output.
Requires 2 brushes
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9. Lap Winding
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10. Wave Winding
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11. Coil Terminologies Coil Coil span Commutator pitch Pole pitch
Back pitch
Front pitch
Commutator pitch
Resultant pitch
Full pitch
Fractional pitch
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12. EMF Equation 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
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13. EMF Equation Flux cut by 1 conductor in 1 revolution = P * φ
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
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14. Armature Reaction
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15. Armature Reaction Magnetic neutral axis: It is perpendicular
to the lines of force between the two
opposite adjacent poles.
Leading pole Tip (LPT):
It is the end of the pole which first comes in contact with the armature.
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16. Armature Reaction Trailing pole tip: It is the end of the pole
which comes in contact later with the
armature.
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17. Effect of Armature Reaction
Efficiency of the machine is decreased
Sparks are produced at the brushes.
There is demagnetising effect on. the main poles.
e.m.f induced is less
Self-excited generators some times fail to build up e.m.f.
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18. Remedies of Armature Reaction
Brushes must be shifted to the new position of the MNA
Field winding must be provided with extra turns
Reluctance is increased by making the slots on the tips
The laminated cores of the shoe are staggered
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19. Remedies of Armature Reaction
In big machines the compensating winding at pole shoes are used.
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20. Demagnetising and Cross Magnetising Ampere Turns
Let us consider a DC machine having P poles with Z number of armature conductors.
Let us assume that the armature is having A
parallel paths.
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21. Commutation in DC Generator
The change in direction of current takes place when the conductors are along the brush axis . During this reverse process brushes short circuit that coil and undergone commutation. Due to this sparking is produced and the brushes will be damaged and also causes voltage dropping.
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22. Methods to Improve Commutation
Use of high resistance brush
Brush shifting
Interpoles
Compensating winding
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23. Classification of DC Generator
Separately excited DC machine
Self excited DC machine
Shunt wound DC generator
Series wound DC generator
Compound wound
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24. Separately excited DC machine
In this type of machine, the field of the machine is excited by an independent external source. This source can be a battery, rectifier circuit or a D.C. generator
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25. Separately excited DC machine
V be the field excitation voltage
Rf be the field resistance.
If be the field current
Eg be the generated e.m.f.
in the armature
Ra be the armature resistance
Ia be the armature current
IL be the load current
VL be the load voltage
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26. Series Wound DC Generator
In this type of machine the field winding is connected in series with the armature winding of a DC generator
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27. Series Wound DC Generator
In this type of machine, since the field winding carry the full armature current, the field winding has less number of turns of wire of large cross sectional area. From the machine circuit shown,
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28. Shunt Wound DC Generator
In this type of machine, the field winding is connected in parallel with the armature winding.
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29. Shunt Wound DC Generator
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30. Compound Wound DC Generator
In this type of DC machine the poles are having two separate windings. That is for a compound wound DC machine there are two field windings, namely, the shunt field winding and the series field winding.
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31. Compound Wound DC Generator
Depending upon how these two windings are connected with the armature of the DC machine, they are classified as,
Long shunt compound DC machine.
Short shunt compound DC machine.
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32. Compound Wound DC Generator
Short shunt
Long shunt
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33. Magnetisation Characteristics
This characteristic is called the magnetic characteristic or the Open Circuit Characteristic (O.C.C.) of a separately excited D.C. generator.
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34. Characteristic of a Separately Excited DC Generator
Without Armature reaction
With Armature reaction
With Armature reaction and IR drop
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35. Characteristic of a DC Series Generator
Open Circuit Characteristic
Internal characteristic
The external characteristic
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36. Critical Load Resistance
Line OA represent the resistance of the external load circuit.
Line OB, the critical load resistance
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37. Voltage Build-up in a DC Shunt Generator
The voltage buildup in a DC generator depends on the presence of a residual flux in the poles of the generator.
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38. Characteristic of a DC Shunt Generator
Internal characteristic
External characteristic
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39. Critical Field Resistance
Line OA represent the field resistance
Line OB is called the critical field resistance.
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40. Critical Load Resistance
Line A represent the external load resistance
Line C is called the critical load resistance.
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41. Characteristic of a Compound DC Generator
The external characteristic of a compound generator
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42. Application of DC Generator
DC shunt generator is used for such applications where constant voltage is required.
DC series generators are used as Boosters and also as a supply for arc lamps.
Differential Compound generators are used to supply dc welding machines.
Level compound generators are used to supply power for offices, hostels and Lodges etc.
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43. Application of DC Generator
Over compound generators are used to compensate the voltage drop in Feeders.
Separately excited generator are used as a supply for DC Motors ,it is also used for such applications where a wide range of voltage is required for the testing purposes.
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