-:DC Generator:-
Unit-2
By Shri Palash Das
Lecturer in Electrical Engineering
Bundwan Polytechnic.
Q.1 Explain working principle of DC generator.
The generator operates on the principle of the production of dynamically induced emf i. e. whenever flux is cut by the conductor, dynamically induced emf is produced in it according to the laws of electromagnetic induction, which will cause a flow of current in the conductor if the circuit is closed.
For production of dynamically induced emf, three things are necessary a magnetic field, a conductor and motion of the conductor with respect to the field. In de generators the field is produced by the field magnets which are stationary. Permanent magnets are used for very small capacity machines and electromagnets are used for large machines to create magnetic flux. The conductors are situated on the periphery of the armature being rotated by the prime mover.
______. ……. ________........._________
[Note:
What is generator and what is motor?
There are two types of d.c. machines, the d.c. generator and the d.c. motor The d.c. generator converts mechanical energy into electrical energy. The dc motor converts electrical energy into mechanical energy. The d.c. generator is based on the principle that when a conductor is rotated in a d.c. magnetic field, a voltage will be generated in the conductor.]]
Q 2. Describe the construction of dc generator in brief.
Ans:
BASIC STRUCTURE OF ELECTRIC MACHINES
A rotating electric machine has two main parts, stator and rotor, separated by the air gap.The stator of the machine does not move and normally is the outer frame of the machine.
The rotor is free to move and normally is the inner part of the machine
Both stator and rotor are made of ferromagnetic materials. Slots are cut on the inner periphery of the stator and the outer periphery of the rotor. Conductors are placed in the slots of the stator or rotor. They are interconnected to for windings.
Armature winding: The winding in which voltage is induced is called the armature winding
Field winding: The winding through which a current is passed to produce the main flux is called the field winding
Permanent magnets are used in some machines to provide the main flux of the machine.
Detailed answer
A d.c. generator consists of three main parts [Fig]
Magnetic-field system 2. Armature 3. Commutator and brushgear .
Magnetic-field system:
The magnetic-field system is the stationary (fixed) part of the machine It produces the main magnetic flux. The outer frame or yoke is a hollow cylinder of cast steel or rolled steel. An even number of pole cores are bolted to the yoke. The yoke serves the following two purposes:
It supports the pole cores and acts as protecting cover to the machine (b) It forms a part of the magnetic circuit.
Since the poles project inwards they are called salient poles. Each pole core has a pole shoe having a curved surface. The pole shoe serves two purposes:
It supports the field coils.
It increases the cross-sectional area of the magnetic circuit and reduces its reluctance.
The pole cores are made of sheet steel laminations that are insulated from each other and riveted together. The poles are laminated to reduce eddy-current loss. Each pole core has one or more field coils (windings) placed over it to produce a magnetic field. The field coils (or exciting coils) are connected in series with one another such that when the current flows through the coils, alternate north and south poles are produced in the direction of rotation.
Armature::
The rotating part of the d.c. machine is called the armature. The armature consists of a shaft upon which a laminated cylinder, called armature core, is mounted. The armature core has grooves or slots on its outer surface. The laminations are insulated from each other and tightly clamped together. In small machines the laminations are keyed directly to the shaft. In large machines they are mounted on a spider. The purpose of using laminations is to reduce eddy-current loss.
The insulated conductors are put in the slots of the armature core. The conductors are wedged and bands of steel wire are fastened round the core to prevent them flying under centrifugal forces. The conductors are suitably connected. This connected arrangement of conductors is called armature winding. types of windings are used-wave and lap
Commutator and brushgear::
Alternating voltage is produced in a coil rotating in a magnetic field. To obtain direct current in the external circuit a commutator is needed. The commutator, which rotates with the armature, is made from a number of wedge-shaped hard drawn copper bars or segments insulated from each other and from the shaft.The segments from a ring around the shaft of the armature. Each commutative segment is connected to the ends of the armature coils.
Current is collected from the armature winding by means of two or more carbon brushes mounted on the commutator. Each brush is supported in a metal box called a brush box or brush holder. The pressure exerted by the brushes on the commutator can be adjusted and is maintained at a constant value by means of springs. Current produced in the armature winding is passed on to the commutator and then to the external circuit by means of brushes.
Q 3. Classify dc machine and then classify dc generator.
Ans: Classification of Electrical Machines By Constructional Features. Electrical machines may be classified by constructional features and their sub-division into power output ranges and speed ranges can be done.
Electrical machines as per their power outputs may be classified as
small size electrical machines with an output up to 0.6 kW.
medium size electrical machines with power outputs ranging from 0.6 kW to 250 kW.
large size electrical machines with outputs exceeding 250 kW but not beyond about 5,000 kW.
Electrical machines as per their operating speeds may be classified as
(i) low speed machines- speed ranges from 250 to 400 rpm.
(ii) medium speed machines-speed ranges frora 400 to 1,500 rpm.
high speed machines-speed more than 1,500 rpm.
DC generator according to excitation of field system maybe classified as
Separately excited DC generator
Self excited DC generator
Self excited DC generator are further classified as
Shunt wound or shunt generator
Series wound or series generator
Compound wound compound wound generator
Compound wound DC generator further divided as cumulative compound wound dc generator and differentially compound wound DC generator.
Separately excited d.c. machine
As the name implies, the field coils are energised by a separate d.c. source. The connections showing the separately excited d.c. machines are given in Fig.
Shunt wound d.c. generator: A machine in which the field coils are connected in parallel with the armature is called a shunt machine. Since the shunt field receives the full output voltage of a generator or the supply voltage of a motor, it is generally made of large number of turns of fine wire carrying a small field current. Figure shows the connection diagrams of d.c. shunt generator.
Series wound d.c. generator:
A d.c. machine in which the field coils are connected in serie is called a series machine. The series field winding carries the armatur current since the armature current is large, the series field winding consist of turns of wire of large cross-sectional area. Figure shows the connection of dc generator.
Compound wound d.c. generator:
A d.c. machine having both shunt and series fields is called a compound machine.
Each field pole of the machine carries two windings. The shunt winding has many tums of fine wise and the series winding has few turns of large cross-sectional area. The compound machine may be connected in two ways. If the shunt field is connected in parallel with the armature alone the machine is called the short- shunt compound machine. Such a machine is shown in Fig.
Generally compound generators are also of the two types, known as
cumulative compound wound and differential compound wound generators.
In cumulative compound wound generators, series field assists the shunt field, as shown in Fig. whereas in differential compound wound generators, series field opposes the shunt field, as shown in Fig.
Q.5 :Deduce the EMF equation of the generator.
E.M.F. EQUATION OF D.C. MACHINE
As the armature rotates, a voltage is generated in its coils. In case of a generator, the e.m.f. of rotation is called the generated emf (or armature e.m.f.) and
Er=Eg
Let useful flux per pole is Ф webers(Wb)
Total number of poles= P
Total number of conductors in the armature=Z
Speed of rotation of armature = n in revolutions per second (r.p.s.)
A number of parallel paths through the armature between brushes of opposite polarity = A
Number of armature conductors in series for each parallel path = Z/A
Since the flux per pole is Ф each cond,uctor cuts a flux in one revolution. PФ Web
Generated voltage per conductor =
flux cut per revolution in PФ/ time taken for one revolution in seconds
Since n revolutions are made in one second, one revolution will be made in 1/n second. Therefore the time for one revolution of the armature is 1/n second.
The average voltage generated per conductor Eav= PФ/(1/n) volts =P Ф n volt.The generated voltage E is determined by the number of armature conductors in series in any one path between the brushes.
Therefore the total voltage generated E = (average voltage per conductor) x (number of conductors in series per path)
that is,
E= PФn xZ/A
E= PФn Z/A
Equation is called the e.m.f. equation of a d.c. machine.
For lap winding A=P i.e equal to number of poles
For wave winding A=2
EXAMPLE 6.1. A 4-pole, wave-wound armature has 720 conductors and is rotated at 1000 rev/min. If the useful flux is 20 mWb, calculate the generated voltage. Ans:480 volt
SOLUTION. Here P=4, A=2, Z=720
N = 1000 rpm.
EXAMPLE 6.2. An 8-pole lap-connected armature has 40 slots with 12 conductors per slot generates a voltage of 500 V. Determine the speed at which it is running if the flux per pole is 50 mWb. Ans:1250RPM
SOLUTION. Here P-8. For lap winding A = P
A=8
Total number of conductors = (number of slots) x (conductors per slot)
Z=40×12=480
E=500 volts, =50 mWb=50 x 1000 Wb
EXAMPLE 6.3. A d.c. generator has an armature emf of 100 V when the useful per pole is 20 mWb and the speed is 800 rpm. Calculate the generated e.m.f. (a) with the same flux and a speed of 1000 rpm., (b) with a flux per pole of 24 mWb and a speed of 900 r.p.m.
Source: Electric Machine by Ashfaq Husain
A course in electrical technology ( Electric Machine volume- II) by JB Gupta
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