Electrical Machines MCQ Quiz - Objective Question with Answer for Electrical Machines - Download Free PDF

Last updated on May 14, 2025

Latest Electrical Machines MCQ Objective Questions

Electrical Machines Question 1:

If a 6-pole induction motor operates on a 60 Hz supply, its synchronous speed is _____________.

  1. 7200 rpm
  2. 1200 rpm
  3. 2400 rpm
  4. 3600 rpm

Answer (Detailed Solution Below)

Option 2 : 1200 rpm

Electrical Machines Question 1 Detailed Solution

Concept

The synchronous speed of an induction motor is given by:

\(N_s={120f\over P}\)

where, Ns = Synchronous speed

f = Frequency

P = No. of poles

Calculation

Given, P = 6

f = 60 Hz

\(N_s={120\times 60\over 6}\)

Ns = 1200 rpm

Electrical Machines Question 2:

How are the segments of the commutator insulated from each other?

  1. Using thin layers of mica
  2. Using air gaps
  3. Using thick layers of rubber
  4. Using metal sheets

Answer (Detailed Solution Below)

Option 1 : Using thin layers of mica

Electrical Machines Question 2 Detailed Solution

Function of the commutator in the DC machine:

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  • A commutator consists of a set of copper segments, fixed around the part of the circumference of the rotating machine, or the rotor, and a set of spring-loaded brushes fixed to the stationary frame of the machine.
  • The main function of the commutator in the DC machine is to collect the current from the armature conductor as well as supply the current to the load using brushes. And also provides uni-directional torque for the DC motor. 
  • The commutator can be built with a huge number of segments in the edge form of hard-drawn copper.
  • The segments in the commutator are protected by the thin mica layer.

Electrical Machines Question 3:

Which statements about voltage drop in an alternator are correct?

I. The voltage drop due to armature resistance (IaRa) is in phase with the armature current (Ia).

II. At unity power factor, the voltage drop IaRa directly subtracts from the generated EMF (E).

III. For a leading power factor load, IaRa contributes to an increase in terminal voltage (V).

IV. Armature resistance has no effect on voltage regulation.

  1. Statements III and IV only are correct. 
  2. Statements I and II only are correct.
  3. Statements II and III only are correct.
  4. Statements I and IV only are correct.

Answer (Detailed Solution Below)

Option 2 : Statements I and II only are correct.

Electrical Machines Question 3 Detailed Solution

Armature reaction in an alternator

In an alternator, armature reaction refers to the effect of the armature current on the magnetic field of the machine. This interaction results in a distortion of the magnetic field, leading to changes in the terminal voltage.

The voltage drop due to armature reaction is minimized when the power factor is unity, meaning the load is purely resistive. In such a case, the armature reaction has the least impact on the voltage, resulting in the minimum voltage drop.

There are three causes of voltage drop in the alternator.

  • Armature circuit voltage drop due to resistance
  • Armature reactance
  • Armature reaction


The first two factors always tend to reduce the generated voltage, and the third factor may tend to increase or decrease the generated voltage. The nature of the load affects the voltage regulation of the alternator.

Explanation

  • Statement I is correct. The voltage drop due to armature resistance (IaRa) is in phase with the armature current (Ia) because resistance and current are always in phase in an AC circuit.
  • Statement II is correct. At unity power factor, the voltage drop IaRa directly subtracts from the generated EMF (E) because the current and voltage are in phase, so the resistive drop subtracts directly from EMF.
  • Statement III is incorrect. The terminal voltage is given by: V = E - IaRaIaRa is a resistive drop and always reduces terminal voltage, regardless of the power factor.
  • Statement IV is incorrect. Armature resistance does affect voltage regulation, as it causes a voltage drop under load.

Electrical Machines Question 4:

What does the flux created by the load current in the primary winding, which links only with the primary winding, represent?

  1. Induced flux
  2. Magnetic flux
  3. Leakage flux
  4. Mutual flux

Answer (Detailed Solution Below)

Option 3 : Leakage flux

Electrical Machines Question 4 Detailed Solution

The correct answer is option 3.

Leakage flux

  • The flux created by the load current in the primary winding, which links only with the primary winding, represents leakage flux.
  • This flux doesn't pass through the core to link with the secondary winding and instead, flows through the winding insulation and insulating oil. This leakage flux contributes to the primary leakage reactance. 
  • This flux does not contribute to energy transfer between windings and is considered a form of energy loss.


Magnetic flux

  • ​Magnetic flux is defined as the number of magnetic field lines passing through a given closed surface. It provides the measurement of the total magnetic field that passes through a given surface area.


Mutual flux

  • Mutual flux is the magnetic flux that links both the primary and secondary windings of a transformer, contributing to its operation. 


Induced flux

  • Induced flux refers to the change in magnetic flux through a loop of wire, which results in the generation of an electromotive force (EMF) or voltage. This process is a fundamental principle in electromagnetic induction.

Electrical Machines Question 5:

In distribution transformers, the explosion vent is designed to:

  1. improve the cooling system
  2. control oil pressure
  3. release pressure in case of excessive buildup
  4. monitor transformer oil quality

Answer (Detailed Solution Below)

Option 3 : release pressure in case of excessive buildup

Electrical Machines Question 5 Detailed Solution

The correct answer is option 3.

Function of the explosion vent in the distribution transformer:

  • When an internal fault occurs within the transformer, such as a short circuit or insulation breakdown, it can generate high-pressure gases.
  • These gases can rapidly increase the pressure inside the transformer tank. 
  • The explosion vent is designed to release this excess pressure when it reaches a predetermined level, typically around 5 psi. 
  • By releasing the pressure, the explosion vent prevents the transformer tank from bursting or being severely damaged. 
  • It also helps to prevent potential hazards, such as fire or injury, associated with a transformer explosion. 

Top Electrical Machines MCQ Objective Questions

In the method of speed control of induction motor by inducing emf in the rotor circuit, if the injected voltage is in phase opposition to the induced rotor emf, then:

  1. the rotor resistance decreases
  2. the rotor resistance increases
  3. the rotor reactance decreases
  4. the rotor reactance increases

Answer (Detailed Solution Below)

Option 2 : the rotor resistance increases

Electrical Machines Question 6 Detailed Solution

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Rotor emf injection method:

For below-rated speeds: In this method, injected emf has the same frequency of rotor slip frequency and that emf is 180° out of phase with rotor emf.

F1 U.B N.J 31-07-2019 D 5

E2R is resultant emf in the rotor

E2R = E2 – E1

\(T \propto \frac{{sE_{2R}^2}}{{{R_2}}}\)

R2 is rotor resistance

T is the torque

s is the slip

Here, the value of rotor emf becomes less. To maintain constant torque, the value of slip will increase. Therefore, the speed will be decreased.

At this condition, effective rotor resistance increases.

For above-rated speeds: In this method, injected emf has the same frequency of rotor slip frequency and that emf is in phase with rotor emf.

F1 U.B N.J 31-07-2019 D 6

E2R is resultant emf in the rotor

E2R = E2 + E1

\(T \propto \frac{{sE_{2R}^2}}{{{R_2}}}\)

R2 is rotor resistance

T is the torque

s is the slip

Here, the value of rotor emf becomes more. To maintain constant torque, the value of slip will decrease. Therefore, the speed will be increased.

At this condition, effective rotor resistance decreases.

The capacitor commonly used for ceiling fan motors has a value of 2.3 μF. The type of capacitor is used __________.

  1. Paper capacitor
  2. Electrolyte capacitor
  3. Parallel plate with Mica dielectric
  4. None of these

Answer (Detailed Solution Below)

Option 1 : Paper capacitor

Electrical Machines Question 7 Detailed Solution

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  • The single-phase induction motor is not self-starting. Hence, it requires an auxiliary means or equipment to start the motor.
  • Mechanical methods are impractical and, therefore the motor is started temporarily converting it into the two-phase motor.
  • Commonly used starting methods for a ceiling fan is a permanent capacitor or single value capacitor motor.
  • The permanent capacitor motor also has a cage rotor and the two windings named as main and auxiliary windings.
  • The capacitor C is permanently connected in the circuit both at the starting and the running conditions.
  • It is also called as a single value capacitor motor as the capacitor is always connected in the circuit.
  • A paper capacitor is used in the permanent capacitor motor as an electrolytic capacitor cannot be used for the continuous running operation of the ceiling fan.
  • The cost of the paper capacitor is higher, and the size is also large as compared to the electrolytic capacitor of the same ratings.

How can a load be shifted from one DC shunt generator to another running in parallel?

  1. Adjust their field rheostat
  2. Insert a resistance in the armature circuit
  3. Adjust the speed of the prime mover
  4. Use an equaliser connection

Answer (Detailed Solution Below)

Option 1 : Adjust their field rheostat

Electrical Machines Question 8 Detailed Solution

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  • When two generators are operating in parallel, the load may be shifted from one shunt generator to another merely by adjusting the field excitation
  • If generator 1 is to be shut down, the whole load can be shifted onto generator 2 provided it has the capacity to supply that load
  • In that case, the field current of generator 1 gradually reduces to zero

 

Important Points:

For stable parallel operation, the most suitable type of DC generator is a shunt generator as it has slightly drooping characteristics. If there is any tendency for a generator to supply more or less than its proper share of load it changes system voltage which certainly opposes this tendency. This restores the original division of load. Thus the shunt generators automatically remain in parallel, once they are paralleled.

The characteristics of dc shunt generator are shown below

SSC JE Electrical 36 20Q BSPHCL JE EE Part 1 Hindi - Final images Q1

Flemings right hand rule is used to find the

  1. Direction of rotation
  2. Direction of flux
  3. Direction of emf
  4. Direction of torque

Answer (Detailed Solution Below)

Option 3 : Direction of emf

Electrical Machines Question 9 Detailed Solution

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Fleming right-hand thumb rule:

When a conductor such as a wire attached to a circuit moves through a magnetic field, an electric current is induced in the wire due to Faraday's law of induction

Fleming's right-hand rule (for generators) shows the direction of induced current when a conductor attached to a circuit moves in a magnetic field.

  • The thumb is pointed in the direction of the motion of the conductor relative to the magnetic field.
  • The first finger is pointed in the direction of the magnetic field. (north to south)
  • Then the second finger represents the direction of the induced or generated current within the conductor (from the terminal with lower electric potential to the terminal with higher electric potential, as in a voltage source)

 

F2 Shubham Madhu 07.08.20 D3

Finger

Right-hand rule

Left-hand rule

Thumb

The direction of motion of conductor (input)

The direction of the conductor (output)

Forefinger

Magnetic field

Magnetic field

Middle finger

The direction of induced emf (output)

The direction of current (input)

 

Conclusion:

The direction of induced emf is known by Flemings right-hand thumb rule

Induction generators deliver power at ______ power factor

  1. Lagging
  2. Leading
  3. Unity
  4. Zero

Answer (Detailed Solution Below)

Option 2 : Leading

Electrical Machines Question 10 Detailed Solution

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Induction generator always works with leading power factor since it will take large amount of reactive power to produce sufficient amount of working flux so that armature reaction is always magnetizing hence it will work always with leading pf.

F3 Savita Engineering 21-7-22 D4

Important Point

  • induction generator is basically an induction motor, which runs above the synchronous speed
  • when it acts as a generator it will supply the active power back to source, but for this supply of active power it needs reactive power as input to keep its winding excited
  • in case if the induction motor is connected to the grid, it will draw the required reactive power for the excitation of windings, but if it is standalone system (ie. not connected to grid) then a capacitor bank will be always connected, and this will provide leading reactive power to keep the winding excited for the process of mechanical to electrical energy conversion
  • since the reactive power is supplied by the capacitor, the induction generator is operating in leading power factor

F1 U.B Madhu 29.01.20 D11

The three characteristics shown in the given graphs, represent which of the following motors?

  1. Three-phase induction motor
  2. Three-phase synchronous motor
  3. DC series motor
  4. DC shunt motor

Answer (Detailed Solution Below)

Option 3 : DC series motor

Electrical Machines Question 11 Detailed Solution

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In a DC motor, T ∝ ϕIa

In series motor, ϕ ∝ Ia

⇒ T ∝ (Ia)2

In shunt motor, ϕ is constant

⇒ T ∝ Ia

The characteristic of DC series and shunt motor are shown below.

Characteristics of DC series motor:

SSC JE Electrical 77 15Q Full Test 2 Part4 Electrical Machines Part1 Hindi images q7a

Characteristics of DC shunt motor:

SSC JE Electrical 77 15Q Full Test 2 Part4 Electrical Machines Part1 Hindi images q7b

Identify the machine shown in the circuit.

F2 U.B Madhu 5.11.19 D 4

  1. DC short shunt compound generator
  2. DC short shunt compound motor
  3. DC shunt motor
  4. DC long shunt compound generator

Answer (Detailed Solution Below)

Option 4 : DC long shunt compound generator

Electrical Machines Question 12 Detailed Solution

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

Type of DC Machine

Circuit diagram

Separately excited DC generator

F1 U.B Deepak 31.12.2019 D 6

DC shunt generator

F1 U.B Deepak 31.12.2019 D 7

DC series generator

F1 U.B Deepak 31.12.2019 D 8

DC short shunt compound generator

F1 U.B Deepak 31.12.2019 D 9

DC long shunt compound generator

F1 U.B Deepak 31.12.2019 D 10


Therefore, the machine shown in the question represents a DC long shunt compound generator.

Which of the following generators at load condition offers positive poorest voltage regulation?

  1. Cumulative compounded
  2. Series
  3. Differential compounded
  4. Shunt

Answer (Detailed Solution Below)

Option 3 : Differential compounded

Electrical Machines Question 13 Detailed Solution

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The correct answer is option "3'.

Concept :- For differential compound generator it is positive (Poorest positive regulation among all)

  • The voltage regulation of a generator is defined as the change in the voltage drop from no load to full load to full load voltage.
  • Voltage regulation = (no-load voltage - full load voltage)/full load voltage
  • In the case of a series generator, the field is connected in series with the armature. Any increase in load current causes an increase in the field and hence the terminal voltage rises.
  • Hence it has negative voltage regulation and it has the poorest voltage regulation.

During on-load conditions, the differentially compounded DC generator has the poorest voltage regulation as shown.

5e26d331f60d5d6c153be4e8 16328251006251

During the no-load condition, the DC series generator has the poorest voltage regulation.

5e26d331f60d5d6c153be4e8 16328251006432

Key Points

  •  Voltage regulation of shunt generator is positive
  • For series generator, it  is negative (Poorest negative voltage regulation among all)
  • For over compound generator it is negative
  • For under compound it is positive
  • For, flat compound generator it is zero (lowest Voltage regulation among all)
  • For differential compound generator it is positive (Poorest positive regulation among all)

In case of dc shunt motors, the regenerative braking is employed when the load _________.

  1. has an overhauling characteristic
  2. is variable
  3. is constant
  4. also acts as braking force

Answer (Detailed Solution Below)

Option 1 : has an overhauling characteristic

Electrical Machines Question 14 Detailed Solution

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Regenerative braking: In this type braking back emf Eb is greater than the supply voltage V, which reverses the direction of the motor armature current. The motor begins to operate as an electric generator.

Overhauling motor: A motoring motor is converting electrical energy into mechanical energy. An overhauling motor is being driven by the load and is converting mechanical energy into electricity, acting as a generator.

In case of regenerative braking, dc shunt motor acts a generator and hence regenerative braking is employed when the load has an overhauling characteristic.

Determine the pitch factor for winding: 36 stator slots, 4-poles, coil spans 1 to 8.

  1. cos 200
  2. cos 400
  3. cos 300
  4. cos 800

Answer (Detailed Solution Below)

Option 1 : cos 200

Electrical Machines Question 15 Detailed Solution

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Concept:

Pitch factor for nth harmonic is given by,

\({k_c} = \cos \frac{{n\alpha }}{2}\)

Where α is short pitch angle in degrees

Calculation:

Given-

Total slots = 36,

Number of poles = 4

Coil span = 1 to 8 = 8 - 1 = 7 slots

Now, Slots per pole = 36 / 4 = 9

Number of empty slots = 9 – 7 = 2 slots

\(\therefore \alpha = 180^\circ \times \frac{2}{{9}} = 40^\circ\)

Hence pitch factor can be calculated as

K= cos 20°

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