Electricals 9

DC Generator are classified on the basis of their winding assembling. Means how to stator and armature winding connected it may be series or parallel or both. here you learn and understand Types of DC Generator | series dc generator | shunt dc generator | compound dc generator.

Types of DC Generator with diagram

Generators are classified as follows:
1) Parmanent Magnet DC Generator
2) Sepratly Excited DC Generator
3) Self Excited DC Generator

Self Excited DC Generator are 3 types:
1. Shunt wound generator
2. Series wound generator
3. Compound DC Generator

Compound DC Generator are 2 types:
(1) Short shunt
(2) Long shunt


Detail of the Types of DC Generator | series dc generator | shunt dc generator | compound dc generator:


1) Permanent Magnet DC Generator:
   Parmanent magnet is used for flux.
   (you can read detail we already posted)

2) Sepretly Excited DC Generator:

DC Generator in which field winding is supplied from an external DC source (EX. battery) is called a separately excited DC generator.
Figure shows the connections of a separately excited generator. The voltage output depends upon the speed of rotation of armature and the field current (Eg = φZNP/60 A). Larger the field current and speed, larger is the generated e.m.f.

Separately excited DC generators are rarely used in practice.Normally of self excited type  used.



Armature current, Ia = IL
Terminal voltage, V = Eg - IaRa
Electric power developed = EgIa
Power delivered to load = EgIa - I R = I E - I R = VIa





3) Self Excited DC Generator

3.1) Shunt-Wound DC generator

When the field winding of a generator is connected in parallel with the generator armature, the generator is called a shunt-wound generator .
The excitation current in ashunt-wound generator is dependent upon the output voltage and the field resistance. Normally, field excitation is maintained between 0.5 and 5 percent of the total current output of the generator.




Shunt field current, Ish = V/Rsh
Armature current, Ia = IL + Ish
Terminal voltage, V = Eg - IaRa
Power developed in armature = EgIa
Power delivered to load = VIL

There are four internal Types of Losses in DC Generator that decrease efficiency of a DC generator.
1) Copper losses
2) Eddy-current losses
3) Hysteresis losses
4) Mechanical losses

Types of Losses in DC Generator

1) 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 (armature curent square into armature resistance)

Field: If^2*Rf (field current square into field resistance)


2) Eddy-Current Losses:

As the armature rotates within the field, it cuts the lines of flux at the same time that the copper
coils of wire that are wound on the armature cut the lines of flux. Since the armature is made
of iron, an EMF is induced in the iron, which causes a current to flow. These circulating
currents within the iron core are called eddy-currents

How to reduce:
To reduce eddy-currents, the armature and field cores are constructed from laminated (layered)
steel sheets. The laminated sheets are insulated from one another so that current cannot flow
from one sheet to the other.

3) Hysteresis losses :

Hysteresis losses occur when the armature rotates in a magnetic field. The magnetic domains
of the armature are held in alignment with the field in varying numbers, dependent upon field
strength. The magnetic domains rotate, with respect to the particles not held in alignment, by
one complete turn during each rotation of the armature. This rotation of magnetic domains in
the iron causes friction and heat. The heat produced by this friction is called magnetic hysteresis
loss.

How to reduce:
To reduce hysteresis losses, most DC armatures are constructed of heat-treated silicon steel,
which has an inherently low hysteresis loss. After the heat-treated silicon steel is formed to the
desired shape, the laminations are heated to a dull red and then allowed to cool. This process,
known as annealing, reduces hysteresis losses to a very low value.

4) Mechanical losses :

Rotational or mechanical losses can be caused by bearing friction, brush friction on the
commutator, or air friction (called windage), which is caused by the air turbulence due to
armature rotation. Careful maintenance can be instrumental in keeping bearing friction to a
minimum. Clean bearings and proper lubrication are essential to the reduction of bearing friction.
Brush friction is reduced by assuring proper brush seating, using proper brushes, and maintaining
proper brush tension. A smooth and clean commutator also aids in the reduction of brush
friction.

construction of dc generator and dc motor is similar . A dc motor can be used as dc generator without any constructional changes or vice versa.There are some main part such as  Armature, Rotor, Stator, Field, commutator, brush and yoke.

Construction of dc machine
Construction of dc generator
Construction of dc motor

Construction and parts of dc motor or dc generator:

1. Armature
2. Rotor
3. Stator
4. Field
5. commutator
6. brush

fig.1 DC Generator construction

Armature:

Armature is rotating part of machine, It is conductor coil which cuts magnetic flux of magnet and generate electrical energy at output terminal
The purpose of the armature is to provide the energy conversion in a DC machine (refer to Figure ).

In a DC generator, the armature is rotated by an external mechanical force, such as a steam turbine and wind turbine . This rotation induces a voltage ( as per faraday law) and current flow in the armature. Thus, the armature converts mechanical energy to electrical energy.

In a DC motor, the armature receives voltage from an outside electrical source and converts electrical energy into mechanical energy in the form of torque.

Rotor:

Rotor is moving part.
The purpose of the rotor is to provide the rotating element in a DC machine (refer to Figure 2). In a DC generator, the rotor is the component that is rotated by an external force. In a DC motor, the rotor is the component that turns a piece of equipment. In both types of DC machines, the rotor is the armature.

Stator:

stator is fixed part.
The stator is the part of a motor or generator that is stationary (refer to Figure 1). In DC machines, the purpose of the stator is to provide the magnetic field. The stator in Figure 1 is provided by permanent magnet (generally electromagnets are used).



Field:

The purpose of the field (winding) in a DC machine is to provide a magnetic field for producing either a voltage (generator) or a torque (motor) (refer to Figure 1). The field in a DC machine is
produced by either a permanent magnet or an electromagnet. Normally, electromagnets are used
because they have an increased magnetic strength, and the magnetic strength is more easily varied
using external devices. In Figure 2, the field is provided by the stator.

Yoke:
Yoke is frame and outer covering body  made up of iron metal.

Comutator:

AC to DC Voltage converter.
The commutator converts the AC voltage generated in the rotating loop(ARMATURE) into a DC voltage. It also serves as a means of connecting the brushes to the rotating loop(ARMATURE).
In a simple one-loop generator, the commutator is made up of two semi-cylindrical pieces of a smooth conducting material, usually copper, separated by an insulating material, as shown in below Figure . Each half of the commutator segments is permanently attached to one end of the rotating loop(ARMATURE), and the commutator rotates with the loop(ARMATURE).

commutator with carbon brush

Commutator is the electrical device which perform mechanically conversion of electric current from AC to DC. Commutator is made up of two semi-cylindrical pieces of a smooth conducting material, usually copper, separated by an insulating material.

Commutator Action is called as commutation. Means AC to DC conversion.



Commutation Action : In DC Generator

• The commutator converts the AC voltage generated in the rotating loop into a DC voltage. It also serves as a means of connecting the brushes to the rotating loop.
• The purpose of the brushes is to connect the generated voltage to an external circuit. In order to do this, each brush must make contact with one of the ends of the loop.
• Since the loop or armature rotates, a direct connection is impractical. Instead, the brushes are connected to the ends of the loop through the commutator. which the brushes make contact with each end of the loop.

Working principle of dc generator
Theory of operation of dc generator and construction and working principle of dc generator.

A basic DC generator has four basic parts:
(1) a magnetic field (2) a single conductor or loop
(3) a commutator and (4) brushes.

A basic DC generator working

Multiple Choice Questions of Electric Current and Ohm's Law (31-45)

Multiple Choice Questions of Electric Current and Ohm's Law (46-60)

Multiple Choice Questions of Electric Current and Ohm's Law (16-30):

16.Conductance is expressed in terms of

(A) ohm / m

(B) m / ohm

(C) mho / m

(D) mho.


17. Which of the following could be the value of resistivity of copper?

(A) 1.7 x 10-8 ohm-cm

(B). 1.7 x 10-6 ohm-cm

(C). 1.6 x 10-5 ohm-cm

(D). 1.7 x 10-4 ohm-cm


18. A copper wire of length l and diameter d has potential difference V applied at its two ends. The drift velocity is vd. If the diameter of wire is made d/3, then drift velocity becomes

(A) 9 vd

(B) vd / 9

(C)vd /3

(D)vd.


19.Two resistances R1 and R2 give combined resistance of 4.5 ohms when in series and 1 ohm when in parallel. The resistances are

(A)3 ohms and 6 ohms

(B)3 ohms and 9 ohms

(C)1.5 ohms and 3 ohms

(D)1.5 ohms and 0.5 ohms.



20. We have three resistances of values 2 Ω, 3 Ω and 6 Ω. Which of the following combination will give an effective resistance of 4 Ω?

(A) All the three resistances in parallel

(B) 2 Ω resistance in series with parallel combination of 3 Ω and 6 Ω resistance

(C) 3 Ω resistance in series with parallel combination of 2 Ω and 6 Ω resistance

(D) 6 Ω resistance in series with parallel combination of 2 Ω and 3 Ω resistance.


21. Three equal resistors connected in series across a source of emf together dissipate 10 watts of power. What would be the power dissipated in the same resistors when they are connected in parallel across the same source of emf?

(A) 10 watts

(B) 30 watts

(C) 90 watts

(D) 270 watts.


22.Current I in the figure is




(A)1.5A

(B)0.5A

(C)3.5A

(D)2.5A


23.Four identical resistors are first connected in parallel and then in series. The resultant resistance of the first combination to the second will be

(A) 1 / 16 times

(B) 1 / 4 times

(C) 4 times

(D) 16 times.

24.Twelve wires of same length and same cross-section are connected in the form of a cube as shown in figure below. If the resistance of each wire is R, then the effective resistance between P and Q will be


(A)R

(B) 5 / 6 R

(C) 3 / 4 R

(D) 4 / 3 R.


25. When P = Power, V = Voltage, I = Current, R = Resistance and G = Conductance, which of the following relation is incorrect?

(A) V = √ (PR)

(B). P= V2G

(C) G= P / I2

(D) I =√ (P / R)


26.The unit of electrical conductivity is

(A) mho / metre

(B) mho / sq. m

(C) ohm / metre

(D) ohm / sq. m.


27.Which of the following bulbs will have the least resistance ?

(A) 220 V, 60 W

(B) 220 V, 100 W

(C) 115 V, 60 W

(D) 115 V, 100 W.


28.The ratio of the resistance of a 100 W, 220 V lamp to that of a 100 W, 110 V lamp will be nearly

(A) 4

(B)2

(C) 1 / 2

(D) 1 / 4


29.The resistance of a 100 W, 200 V lamp is

(A) 100 ohm

(B) 200 ohm

(C) 400 ohm

(D) 1600 ohm.


30.Two 1 kilo ohm, 1/2 W resistors are connected in series. Their combined resistance value and wattage will be

(A) 2 kΩ, 1/2 W

(B) 2 kΩ, 1 W

(C) 2 kΩ, 2 W

(D) l kΩ, 1/2 W.



Here you can also read More important MCQs

Basic Electric MCQs 1 TO 15
Basic Electric MCQs 16 TO 30
Transformer MCQs 1 TO 20