Electrical Technology

September 1

2017 Website: www.electricaledu.com Electrical Engg.(MCQ) Question and Answer for the students of SSC(JE), PSC(JE), BSNL(JE), WBSEDCL, WBSETCL, WBPDCL, CPWD and State and Central Service Examination

1st year 2nd semester

Electrical Technology

Contents

Unit-1(Difference sources of Energy)

i. Conventional sources and Non-conventional sources

ii. Advantages of electrical energy

iii. Uses of electrical energy

iv. Electric charge

v. Electric current

vi. Electric potential

vii. Power: Electrical energy

viii. Resistance

ix. Inductance

x. Capacitance

D.C. Circuits

i. Ohms law

ii. Factors at which the resistance of a conductor depends

iii. Resistivity

iv. Temperature co-efficient of resistance

v. Series combination of resistance

vi. Parallel combination of resistance

vii. Parameter

viii. Circuit

ix. Electrical network

x. Active network

xi. Passive network

xii. Node

xiii. Branch

xiv. Loop

xv. Kirchhoff’s current law

xvi. Kirchhoff’s voltage law

xvii. Sign of e.m.f.

xviii. Sign of IR drops

xix. Equivalent resistance

xx. Branch current

A.C. Circuits

i. Alternating voltage

ii. Wave form of voltage

iii. Wave form of current

iv. Cycle,Time, period, Frequency, Average value, R.M.S. value, Form factor,Peak factor, Phase, Phase

difference

v. Vector Diagram

vi. A.C through pure resistive circuit

vii. A.C. through pure inductive circuit

viii. A.C. through pure capacitive circuit

ix. R-L series circuit

x. R-C series circuit

xi. R-L-C series circuit

xii. Impedance, Reactance, Inductive reactance, Capacitance reactance

xiii. Power factor,Type of power factor

xiv. Power triangle

xv. Impedance table

Unit-II (Electromagnetism)

i. Definition, Magnetic field around a straight current carrying conductor

ii. Right hand gripping rule

iii. Magnetic field around a solenoid

iv. Force on a conductor placed in the magnetic field

v. Force between parallel current carrying conductor

vi. Magneto-motive force (mmf)

vii. Magnetic field intensity

viii. Magnetic flux

ix. Magnetic flux density

x. Permeability

xi. Reluctance

xii. Permeance

xiii. Comparison between magnetic circuit and electric circuit

xiv. Magnetic hysteresis, Hysteresis loop, Hysteresis loss

Electromagnetic induction

i. Faradays law

ii. Lenz’s law

iii. Self inductance and Mutual inductance

iv. Eddy current and Eddy current loss

v. Fleming’s Right hand rule and Fleming’s Left hand rule Electrical Machines

i. Working principle of D.C. generator ii. Type of D.C. Generator iii. Parts of D.C. Generator iv. Working principle of D.C. motor v. Back e.m.f. vi. Types of D.C. motor vii. Application of D.C motor viii. Working principle of transformer ix. E.m.f. equation x. For an ideal transformer

xi. Voltage transformation ratio xii. Rating of transformer xiii. Classification of transformer xiv. Application of transformer

Storage cell i. Difference between primary and secondary cell ii. Name of active material used in lead acid cell iii. Chemical reaction during discharging iv. Chemical reaction during charging v. Indication of full charge battery

Unit-III (Electrical power supply systems) i. Comparison between A.C. and D.C. transmission system ii. Draw the layout of power system or Draw the single line diagram of supply system from generating

station to consumers iii. Idea of Generation, Transmission and Distribution iv. Types of power station or generating station v. Name of thermal power station in West Bengal vi. Hydel power station vii. Some non-conventional sources viii. Single phase system ix. Three phase system x. Star connection xi. Delta connection

Domestic power supply i. Service connection for house wiring ii. Types of house wiring iii. Conductor used iv. Fuse v. Some common name of fuse vi. Earthing, necessity of earthing vii. Type of earthing viii. Connection diagram of two way switch

Measuring and testing of instruments i. Ammeter ii. Voltmeter iii. Circuit diagram for single phase energy meter iv. Meter constant v. Specification of energy meter vi. Multi meter vii. Megger viii. Application of Megger ix. Type of instruments

Unit-1(Difference sources of Energy)

Conventional sources: Traditional sources are known as conventional source. Such as coal, petrol, diesel, nuclear

fuel etc.

Non-conventional sources: Non-traditional sources are known as non-conventional source. Such as solar energy,

wind energy, tidal energy and bio-gas etc.

Advantages of electrical energy:

(i) It is transportable over a long distance.

(ii) It is pollution less.

(iii) It can be converted into any other form of energy.

(iv) It can be stored for future use.

Uses of electrical energy:

(i) Lighting

(ii) Heating

(iii) Electrical motor

(iv) Electroplating

(v) Electronics

(vi) Welding

Basic concepts of electrical quantities

Electric charge: Electric charge is the property that causes to experience a force when close to other charged body.

It is denoted by Q and unit is Coulomb (C). It is two types:

(i) Positive charge

(ii) Negative charge

Electric current: The flow of free electron from higher potential to lower potential is known as current. It is

denoted by I and unit is Ampere (A).

Electric potential: The amount of work done per unit charge is known as electric potential. It is denoted by V and

unit is Volt. i.e. V = 𝑊

𝑄

Power: The energy consumed per unit time is known as power. It is denoted by P and unit is Watt (W)

Electrical energy: The energy consumed in a given time is known as electrical energy. It is denoted by E and unit is

kilo-watt-hour (kWh).

Resistance: It is the property of material by virtue of which it opposes the flow of electric current through it. It is

denoted by R and unit is Ohm (Ω).

Inductance: It is the property of a material by virtue of which it opposes any change of current passing through it.

It is denoted by L and unit is Henry (H).

Capacitance: It is the capacity of a condenser to store electrical energy in its dielectric medium. It is denoted by C

and unit is farad (F) or micro-farad (µF).

D.C. supply A.C. supply

1. Its polarity cannot change with time Its polarity changes with time

2. It can be stored It cannot be stored

3. Example of DC supply is battery or cell Example of AC supply is generator

4. DC symbol is AC symbol is

Instruments To be measured Unit

Ammeter Current Ampere (A)

Voltmeter Voltage Volt (V)

Wattmeter Power Watt (W), kilo-watt (kW)

Energy meter Energy kWh

Multi meter Current, voltage, resistance --

Parameter Symbol Unit

Resistance (R)

Ohm (Ω) or kilo-ohm (kΩ)

Inductance (L)

Henry (H) or mili-henry (mH)

Capacitance (C)

Farad (F) or micro-farad (µF)

D.C. Circuits

Ohms law: The potential difference (V) between the end of the conductor is directly proportional to the current (I)

flowing through the conductor, keeping the temperature and other physical condition unchanged.

i.e. V α I

or, V=IR

Where, R is the resistance.

Note: Ohms law is not applicable to the non-linear device such as diode, transistor, SCR, electrolytes, arc lamp,

semi-conductor etc.

Law of resistance: The resistance of a conductor material is directly proportional to the length of the conductor

and inversely proportional to the cross-sectional area of the conductor at constant temperature.

i.e. R α 𝑙

𝑎

or R = 𝜌𝑙𝑎

Where, ρ is the specific resistance or resistivity

Factors at which the resistance of a conductor depends:

(i) Length of conductor (Rαl)

(ii) Cross-sectional area of the conductor (Rα1

𝑎)

(iii) Working temperature: The resistance of pure metals or alloys increases with the rise of temperature.

But in case of semi-conductor, insulator and electrolytes decreases the resistance with the rise of

temperature.

(iv) Nature of current: Due to skin effect the resistance of the conductor is higher in A.C. than that of D.C.

(nearly about 15% to 20%)

Resistivity: The resistivity of a conductor may be defined as the resistance offered by unit length of the conductor

and having unit cross-section of the conductor. Its unit is ohm-meter (Ωm)

Temperature co-efficient of resistance: The temperature co-efficient of resistance of a conductor is the change in

resistance per ohm per degree change in temperature counting from 00. It is denoted by alpha (α).

Let, R0 = resistance of the material at 00

R1 = “ “ “ “ 𝑡10C

R2 = “ “ “ “ 𝑡20C

Then we can write,

R1= R0(1+α0t)

and

R2= R1[1+α(t2 – t1)]

Series combination of resistance: Let the three pure resistances R1, R2 and R3 are connected in series across a d.c.

voltage source V.

V = VR1 + VR2 + VR3

= IR1 + IR2 + IR3

= I(R1 + R2 + R3) …………….(i)

If the equivalent resistance is Req then,

V = IReq ………………..(ii)

Therefore, IReq = I(R1 + R2 + R3)

And

Req = R1 + R2 + R3

Parallel combination of resistance: Let the three resistances R1, R2 and R3 are connected in parallel across a d.c.

voltage V. Let the source current is I and branch current are I1, I2 and I3 respectively.

Since the voltage drop in each branch is same

Then, V = VR1 = VR2 = VR3

And I = I1 + I2 + I3 .................(i)

If the equivalent resistance is Req

Then, V = IReq ………………(ii)

Or 𝑉

𝑅𝑒𝑞 =

𝑉

𝑅1 +

𝑉

𝑅2 +

𝑉

𝑅3

Or

1

𝑅𝑒𝑞 =

1

𝑅1 +

1

𝑅2 +

1

𝑅3

Parameter: The different elements of an electrical circuit such as resistance, inductance and capacitance are

known as parameter.

Circuit: It is the conducting path through which the electric current flows or tends to flow.

Electrical network: A combination of different elements such as resistance, inductance and capacitance are

connected in any manner is called an electrical network.

Active network: The network which contains one or more sources of e.m.f. is called Active network.

Passive network: The network which has no source of e.m.f. is called Passive network.

Node: It is the junction point of two or more branches of the network.

Branch: It is the part between two junctions.

Loop: It is the closed path of the circuit.

Kirchhoff’s current law: In any electrical network the algebraic sum of the currents meeting at a point is zero.

Assuming incoming current to be positive (+ve) and outgoing current to be negative (-ve)

Therefore, I1 + (-I2) + (-I3) + (+I4) + (-I5) = 0

Or, I1 + I4 = I2 + I3 + I5

Or, Incoming current = outgoing current

Therefore, ∑ 𝐈 = 0

Kirchhoff’s voltage law: The algebraic sum of the products of current and resistance plus the algebraic sum of

e.m.f. in any closed network is zero.

∑ I0R0 + ∑ e. m. f = 0

Sign of e.m.f.:

A rise in potential should be +ve

A fall in potential should be -ve

As we go from the –ve terminal of a battery to its +ve terminal, there is a rise in potential. Hence the sign should

be +ve. Similarly if we go from the +ve terminal to –ve terminal, there is a fall in potential. And sign should be –ve.

Sign of IR drops: If we go through a resistor in the same direction as the current, then there is a fall in potential

and sign will be –ve.

If we go through a resistor in the opposite direction to the current then there is a rise in potential and sign will be

+ve.

Applying KVL,

-I1R1 – I2R2 – E2 + I3R3 + E1 – I4R4 = 0

Or, - I1R1 – I2R2 + I3R3 – I4R4 = E2 – E1

Equivalent resistance:

1

𝑅𝑒𝑞 =

1

𝑅1 +

1

𝑅2

Req = 𝑅1𝑅2

𝑅1+𝑅2

Branch current:

I1 = I×𝑅2

𝑅1+𝑅2 and I2 = I×

𝑅1

𝑅1+𝑅2

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