Basic Electrical Engineering MCQs #SET1

01) Two 100W, 200V lamps are connected in series across a 200V supply. The total power consumed by each lamp will be in Watts :-
A) 25 Watts
B) 50 Watts
C) 100 Watts
D) 200 Watts

02) Two electric bulbs have transparent filament of same thickness. It one of them gives 60 W & other gives 100 W, then-
A) 60 W lamp filament has shorter length
B) 100 W lamp filament has longer length
C) 60 W lamp filament has longer length
D) 60 W & 100 W lamp filament have equal length

03) Bulb in street lighting are connected in
A) Parallel
B) Series
C) Series parallel
D) end of end

04) A piece of copper and another piece of germanium are cooled from room temperature to 80K. The resistance of
A) each of them increases.
B) each of them decreases.
C) copper increases and germanium decreases.
D) copper decreases and germanium increases.

05) If a large resistance is connected parallel a small resistance then net resistance will be
A) Greater than the large resistance
B) Smaller than the small resistance
C) Smaller than the greater but larger than the smaller resistance
D) None of the above option

06) The speed of a DC motor is
A) Directly proportional to flux and inversely proportional to back emf.
B) Directly proportional to both back emf and flux.
C) Directly proportional to back emf and flux.
D) Directly proportional to back emf and inversely proportional flux.

07) In which braking back emf exceeds supply voltage?
A) Regenerative.
B) Dynamic.
C) Plugging.
D) None of these.

08) Third pin of a 3 pin plug is thicker and longer due to
A) for designing purpose.
B) for protection purpose.
C) all pins are of same size.
D) none of these.

09) Power factor can be defined as
A) both option B and C.
B) cosine of angle between voltage and current.
C) ratio of resistance versus impedance.
D) sine of voltage and current.

10)A 60 Hz frequency voltage would case a bulb to turn on and off
A) 60 times per second.
B) 120 times per second.
C) 70 times per second.
D) 80 times per second.


Differences Between Alternator and Generator

Differences Between Alternator and Generator

Alternator and Generator both are electro-mechanical machines which converts mechanical input to electrical output i.e., electrical energy.Though output of the both machines electrical power, there are some key differences between alternator and generator are there in terms of design type of power etc.

What is the difference between an alternator and a generator?

Differences Between Alternator and Generator

Alternator vs Generator

Alternator Generator
Alternator generates only AC power in every condition. Generator can generates both AC power or DC power
But using commutator we convert to DC power.
(Can say mostly used for DC power generation.)
Alternator generates higher output than Generator. Generator generates lower output than Alternator.
Alternator generates alternating EMF at its output terminal. Generator generates constant EMF at its output terminal.
But internally it is alternating in nature.
Alternator has stationary Armature and rotating magnetic field. Generator has stationary magnetic field and rotating
Armature for high voltage output but in case of low voltage
output rotating armature and rotating magnetic field are used.
It works on faraday law of electromagnetic induction. It also works on faraday law of electromagnetic induction.
Alternator has DC field which generates constant magnetic
field in Armature.
Generator has permanent magnet which generates constant
magnetic field in stator.
In alternator we take supply from its stator. In generator we take supply from rotor.
It is a major difference between alternator and generator.
difference between alternator and generator wikipedia
advantage of alternator vs generator
alternator vs generator efficiency

Differences Between Gravity Control and Spring Control

Comparison of Gravity Control and Spring Control

Gravity control or Spring control both are comes under the category of "controlling torques" of an electrical measurement instrument.Before going have a comparison between gravity control and spring control let us have a look at why we need controlling torque in a measurement instrument?

1) Controlling torque produces a force equal and opposite to the deflecting force in order to make the deflection of pointer at a definite magnitude. If this system is absent then the pointer will swing beyond its final steady position for the given magnitude and deflection will become indefinite.

2) It brings the moving system back to zero position when the force which causes the movement of the moving system is removed. It will never come back to its zero position in the absence of controlling system. We have some advantages of spring control over gravity control.

Advantages of spring control | Disadvantages of gravity control

Gravity Control Spring Control
Adjustable small weight is used which produces
the controlling torque.
Two hair springs are used which exert controlling torque.
Controlling torque can be varied. Controlling torque is fixed.
The performance is not temperature dependent. The performance is temperature dependent.
The scale is nonuniform. The scale is uniform.
The controlling torque is proportional to sinA (A is angle). The controlling torque is proportional to 0.
The readings can not be taken accurately. The readings can be taken very accurately.
The system must be used in vertical position only. The system need not be necessarily in vertical position.
Proper levelling is required as gravity control. The levelling is not required.
Simple, cheap but delicate. Simple, rigid but costlier compared to gravity control.
Rarely used for indicating and portable instruments. Very popularly used in most of the instruments.

Controlling System: Gravity vs Spring Control
Advantages of spring control over gravity control
What is advantages of spring control over gravity control
What is gravity control,spring control?
Controlling torque in measuring instruments

What is Transfer Function In Control System ?

Transfer Function of Control System

Definition: Mathematically it is defined as the ratio of Laplace transform of output (response) of the system to the Laplace transform of input (excitation or driving function), under the assumption that all initial conditions are zero.
Symbolically system can be represented as shown in the below figure where input is represented as r(t) and output is represented as c(t) and the transfer function of the system is C(s)/R(s).

C(s) ---- Laplace transform of output.
R(s) ---- Laplace transform of input.
Transfer Function G(s)= C(s)/R(s)

Advantages and Features of Transfer Function

The various features of the transfer function are,

i) It gives mathematical models of all system components and hence of the overall system. Individual analysis of various components is also possible by the transfer function approach.

ii) As it uses a Laplace approach, it converts time domain equations to simple algebraic equations.

iii) It suggests operational method of expressing equations which relate output to input.

iv) The transfer function is expressed only as a function of the complex variable ‘s'. It is not a function of the real variable, time or any other variable that is used as the independent variable.

v) It is the property and characteristics of the system itself.  Its value is dependent on the parameters of the system and independent of the values of inputs. Transfer function is to be obtained for a pair of input and output and then it remains constant for any selection of input as long as output variable is same. it helps in calculating the output for any type of input applied to the system.

vi) Once transfer function is known, output response for any type of reference input can be calculated.

vii) It helps in determining the important information about the system i.e. poles, zeros, characteristic equation etc..

viii) it helps in the stability analysis of the system.

ix) The system differential equation can be easily obtained by replacing variable 's' by d/dt.

x) Finding inverse, the required variable can be easily expressed in the time domain.
this is much more easy than to analyse the entire system in the time domain.

Disadvantages of Transfer Function

The few limitations of the transfer function approach called approach are,

i) Only applicable to linear time invariant systems.

ii) It does not provide any information concerning the physical structure of the system. From transfer function, physical nature of the system whether it is electrical, mechanical, thermal or hydraulic, cannot be judged.

iii) Effects arising due to initial conditions are totally neglected. Hence initial conditions loose their importance.

Procedure to Determine the Transfer Function of a Control System

1) Write down the time domain equations for the system by introducing different variables in the system.

2) Take the Laplace transform of the system equations assuming all initial conditions to be zero.

3) Identify system input and output variables.

4) Eliminating introduced variables, get the resultant equation in terms of input and output variables.

Star to Delta and Delta to Star Conversion (Wye-Delta Transformations)

Star to Delta and Delta to Star Conversion

(Wye-Delta Transformations)

It is easy to calculate equivalent resistance of resistors which are in either parallel or series.Sometimes we may face difficulty in solving equivalent value. There we make use of star to delta or delta to star conversion. It is a powerful tool to simplify the complexity of the network. By applying this we can measurably reduce the problem solving time.

How To Identify Where Star Delta Transformation Is Required? 

  • Delta, or Δ (also known as the 'Pi' or π) configuration, and
  • Star connected network which has the symbol of the letter 'Y' (also known as the  'T') configuration.
  • In the below picture you can observe the connection of delta or 'Pi' configuration, star or 'T' configuration.
Delta Configuration

Star Configuration.

Delta (Δ) to Star (Y) Conversion 

In delta to star conversion, the delta connected elements will be converted to star connection in order to simplify the network analysis.Transforming from delta to star introduces one additional node.

Star (Y) to Delta (Δ) Conversion 

In  star to delta conversion, the star connected elements will be converted to delta connection in order to simplify the network analysis.Transforming from star to delta removes one node.

Star-Delta Transformation

Delta-Star Transformation

Star-Delta Transformation / Delta-Star Transformation can be used to any type of electrical component.Star to delta conversion formula have been given above.Delta wye transformation problems with solution pdf will be added (Stay Tuned).

Site Selection of Hydroelectric Power Stations

Factors Affecting Hydroelectric Plant Location

Site selection of Hydroelectric  power plant

Hydroelectric power / Hydro-power plant is one of the eco-friendly power generation systems.It has less running cost and high efficiency (around 50%, compared to thermal plant much better). Site selection of hydroelectric power plant depends on some factors in order to give efficient output in terms of overall performance and economic.

Site selection of Hydroelectric power plant. 
Choice of Site For Hydro-Electric Power Stations

What are the points to be considered while selecting the sites of hydroelectric power plant?

The following points should be taken into account while selecting the site for a hydro-electric power station : 

(i) Availability of water.

Since the primary requirement of a hydro-electric power station is the availability of huge quantity of water,  such plants should be built at a place (e.g., river, canal) where adequate water is available at a good head.

(ii) Storage of water.

There are wide variations in water supply from a river or canal during the year.  This makes it necessary to store water by constructing a dam in order to ensure the generation of power throughout the year.  The storage helps in equalizing the flow of water so that any excess quantity of water at a certain period of the year can be made available during times of very low flow in the river.   This leads to the conclusion that site selected for a hydro-electric plant should provide adequate facilities for erecting a dam and storage of water.

(iii) Cost and type of land.

The land for the construction of the plant should be available at a reasonable price.  Further, the bearing capacity of the ground should be adequate to withstand the weight of heavy equipment to be installed.

(iv) Transportation facilities.

The site selected for a hydro-electric plant should be accessible by rail and road so that necessary equipment and machinery could be easily transported. It is clear from the above mentioned factors that ideal choice of site for such a plant is near a river in hilly areas where dam can be conveniently built and large reservoirs can be obtained.

how to select site for hydroelectric power plant ?
site selection of hydroelectric power plant
steps to site selection of hydroelectric power plant
selection of site for thermal power plant

Line to Line Fault Definition, Fault Current Analysis, Calculation

Line to Line Fault Definition, Fault Current Calculation

What are the points to be covered?
  • What is line to line fault?
  • How line to line fault occurs?
  • Fault current in line to line fault.
  • Calculate  line to line fault current.

What is Meant by Line to Line Fault ?

When two conductors of a 3 phase system are short circuited line to line fault or unsymmetrical fault occurs.This fault is severe compared to symmetrical faults in power system.In order to safeguard power system network analysis of unsymmetrical faults (in this case line to line fault) must be done.

Line to Line Fault Definition, Fault Current Calculation

Fault Current Calculation in Line to Line Fault

Consider a 3-phase system with an earthed neutral.Assume a line-to-line fault between the blue (B) and yellow (Y) lines as shown in figure The conditions created by this fault lead to :
Again taking R-phase as the reference, we have,
Take above result as equation (i)

Take above result as equation (ii)

Note :: From above steps it is clear that the line-to-line fault the zero sequence component of current I0 is equal to zero in line to line fault.

Fault Current in Line-to-Line Fault: Examination of exp. (i) and exp (ii) reveals that sequence impedance should be connected as shown in above figure.It is clear from the figure that :
Phase Voltages. Since the generated e.m.f. system is of positive phase sequence only, the sequence
components of e.m.f. in R-phase are :
Summary: For line-to-line fault (Blue and Yellow lines) :

Above what is line to line fault and how to find line to line current fault current are discussed.