DC Generators MCQ Quiz - Objective Question with Answer for DC Generators - Download Free PDF

Lap winding

Lap winding is a type of armature winding in which the coil terminals are connected in such a way that the number of parallel paths is equal to the number of poles (for simplex winding) or a multiple of it (for multiplex windings).

This type of winding is commonly used in high-current, low-voltage applications like large DC generators and motors.

The number of parallel paths in a lap winding is given by:

A = P × m

where, A = Number of parallel paths

P = Number of poles

m = Multiplicity of winding (1 for simplex, 2 for duplex, 3 for triplex, etc.)

Calculation

Given, P = 4 

m = 2

The number of parallel paths is given by:

A = 4 × 2

A = 8

Concept:

• Faraday’s laws of electromagnetic induction whenever the flux of magnetic field through the area bounded by a closed conducting loop changes, an emf is produced in the loop.
• The emf is given by  \(\xi =- \frac{d\phi}{dt}\) ---- (1)
• The law described by equation 1 is called Faraday’s laws of electromagnetic induction.
• The flux may be changed in a number of ways-
  • One can change the magnitude of the magnetic field B at the site of the loop,
  • the area of the loop or,
  • the angle between the area-vector dS and the magnetic field B.
• The emf so produced drives an electric current through the loop. If the resistance of the loop is R, the current is 
• \(i=\frac{\xi}{R} = -\frac{1d\phi }{Rdt} \)
• The emf developed by a changing flux is called induced emf and the current produced by this emf is called induced current.
   

Explanation: 

• A DC generator is the type of electrical generator that converts mechanical energy into direct current electricity
• DC generators produce electrical power based on the principle of faraday’s law of electromagnetic induction.
• Based on this law, when a conductor moves in a magnetic field, the magnetic lines of force are cut. This leads to an electromagnetic force induction in the conductor.
• The direction of the induced current (given by Fleming’s right-hand rule) changes as the direction of movement of the conductor changes.

Additional Information

•  Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points.
  • V = IR
• When an electric current passes through a conductor for some time, the conductor gets heated up. Thus, heating of a conductor due to the flow of electric current through it is known as Joule's law of heating
  • H = I²RT

Dynamically induced EMF: When the conductor is rotating and the field is stationary, then the emf induced in the conductor is called dynamically induced EMF.

Static induced EMF: When the conductor is in stationary and the field is changing (varying) then then the emf induced in the conductor is called static induced EMF.

Dynamically induced EMF: When the conductor is rotating and the field is stationary, then the emf induced in the conductor is called dynamically induced EMF.

Static induced EMF: When the conductor is in stationary and the field is changing (varying) then then the emf induced in the conductor is called static induced EMF.

• Separately excited dc generator requires an independent dc external source for energizing the field winding.
• The field windings in a separately excited generator connected in series with dc external source.
• The equivalent circuit representation under steady-state condition is shown in Figure.

The equations under steady-state are:

V_f = I_fR_t

R_t = R_f + R_ext

V_a = E + I_LR_a

I_L = I_a

Where V_f = External dc source

I_f = Field winding current

R_t = Net resistance across external source side

R_f = Field winding resistance

V_a = Voltage across load

E = Generated emf

I_L = Load current

I_a = Armature current

R_a = Armature resistance

• When the field current is held constant and the armature is rotating at a constant speed, the induced emf in an ideal separately excited dc generator is independent of the armature current.
• As the load current IL increases, the terminal voltage decreases as indicated by a solid line. If the armature reaction is neglected, decrement in V_a should be linear and equal to the voltage drop across R_a and carbon brushes.
• However, if the generator is operated at the knee point in the magnetization curve the armature reaction causes a further drop in terminal voltage.

Classification of DC machine (generator/motor)

• A DC generator works on the principle of Electro Magnetic Induction.
• When a DC current passes through a long straight conductor a magnetizing force and a static magnetic field are developed around it.
• If the wire is then wound into a coil, the magnetic field is greatly intensified producing a static magnetic field around itself and forming the shape of a bar magnet giving a distinct North and South pole.
• Then by either moving the wire or changing the magnetic field a voltage and current can be induced within the coil and this process is known as Electromagnetic Induction, which is the basic principle of operation of DC generators.

Faraday’s first law of electromagnetic induction states that whenever a conductor is placed in a varying magnetic field, emf is induced which is called induced emf. If the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current.

Faraday's second law of electromagnetic induction states that the magnitude of emf induced in the coil is equal to the rate of change of flux that linkages with the coil. The flux linkage of the coil is the product of number of turns in the coil and flux associated with the coil.

These laws are related to the emf of a generator.

Concept:

There is the relation between the electrical and mechanical angle of a DC machine is given as 

Electrical angle = \(p\over 2\) ×  (mechanical angle). ---------(1)

where 

P is the no of poles

Calculation:

Given

Electrical angle =  2 ×  (mechanical angle). 

Compare the equation with 1

\(p\over 2\) = 2

P = 4

Hence,

Pole Pair = 2

Based on the connection of armature and field windings DC generators can be classified as:

The correct answer is option 1):(Laminations)

Concept:

• The armature core of a D.C machine is made up of  Laminations
• The armature is built up in cylindrical high-grade silicon steel in the form of lamination. 
• So when lamination is used, the circular path of Eddy Current is terminated. 
•  The damage  due to heating is reduced by lamination
• Eddy Current Loss is the reason for the lamination of the armature of the DC Motor is used to minimize it.

Separately Excited DC Generator:

• These are the generators whose field magnets are energized by some external DC source, such as a battery.
• A circuit diagram of a separately excited DC generator is shown in the figure below. The symbols below are:

Series Wound Generator: In these types of generators, the field windings are connected in series with armature conductors, as shown in the figure below.

Shunt Wound DC Generators:

• In these types of DC generators, the field windings are connected in parallel with armature conductors, as shown in the figure below.
• In shunt wound generators the voltage in the field winding is the same as the voltage across the terminal.

Short Shunt Compound Wound DC Generator: Short Shunt Compound Wound DC Generators are generators where only the shunt field winding is in parallel with the armature winding, as shown in the figure below.

Long Shunt Compound Wound DC Generator: Long Shunt Compound Wound DC Generators are generators where the shunt field winding is in parallel with both series field and armature winding, as shown in the figure below.

• Interpoles are similar to the main field poles and located on the yoke between the main field poles.
• They have windings in series with the armature winding.
• The inter poles are tapering in shape i.e. having a broad base and less pole shoe area to reduce the extra air gap flux under trailing pole tip
• The air gap under inter poles is more than the air gap under the main field poles to avoid the saturation

Interpoles in DC machine has basically two functions:

• Automatic neutralization of cross magnetization due to armature reaction
• To counter and cancel reactance voltage in the coil undergoing commutation

Important Point:

In a DC machine, two kinds of magnetic fluxes are present (armature flux and main field flux). The effect of armature flux on the main field flux is called as armature reaction.

The effect of armature reaction can be reduced by following methods

a) By using compensating winding

b) By using commutation poles or inter-poles

c) By reducing the cross section of pole pieces

The correct answer  is option 2): (as a full wave rectifier output)

Concept:

Based on Faraday's law of Induction the EMF produced inside any generator is alternating in nature.

\(E_{emf} =\) -N\(dϕ \over dt\)

N is the number of loops.

\(dϕ \over dt\) is the chang flux.

DC Generator will have Commutator.

The commutator is also known as Mechanical Rectifier (Rectifier is a component that converts AC to DC). Thus we get DC voltage out of DC Generator.

EMF generated in the DC generator is similar to a full wave rectifier output

Armature emf in a DC generator is given by,

\({E_g} = \frac{{\phi ZNP}}{{60A}}\)

ϕ is flux per pole

Z is the number of conductors

N is speed

P is poles

A is the number of parallel paths

For the lap winding, A = P

For the wave winding, A = 2

• In the series dc machine, there is one field winding wound over the main poles with fewer turns and a large cross-sectional area.
• Series winding is meant to be connected in series with the armature and naturally to be designed for rated armature current.
• So, there will be practically no voltage or very small voltage due to the residual field under no-load condition (I_a = 0).
• However, the field gets strengthened as the load will develop rated voltage across the armature with reverse polarity, is connected and terminal voltage increases.
• Variation in load resistance causes the terminal voltage to vary. The terminal voltage will start falling, at saturation armature reaction effect becomes pronounced at large load current.
• Hence, series generators are not used for delivering power at a constant voltage. Series generator found application in boosting up the voltage in d.c transmission system.