Synchronous Generator MCQs

Welcome to Electrical Engineering XYZ MCQs post on synchronous generators. If you’re looking to test your knowledge or gain a deeper understanding of synchronous generators, you’ve come to the right place. Synchronous generators, also known as alternators, play a crucial role in converting mechanical energy into AC electrical energy. In this post, we’ve curated a collection of multiple-choice questions (MCQs) that will challenge your understanding of synchronous generator principles, operation, and key concepts. Whether you’re a student, an engineer, or simply interested in the topic, these MCQs will help you assess your grasp on synchronous generator fundamentals. Get ready to dive into the world of synchronous generators and expand your knowledge through this engaging MCQ post on Synchronous Generators.

Contents

1 Synchronous Generator is also known as
2 The stator of an alternator rarely employs
3 A turbo-alternator uses
4 The synchronous reactance of alternator is due to
5 The voltage regulation of an alternator for PF of 0.8 lagging is X at unity PF
6 The magnitude of EMF generated by an alternator depends on
7 When the alternator operates at a unity load power factor the effect of armature reaction is
8 The frequency of EMF generated per revolution in an alternator is equal to
9 The stator of an alternator is wound for ____________ on the rotor
10 Voltage regulation of an alternator is larger than that of a DC generator because of
11 A two pole generator in 50 Hz grid should be turned at

Synchronous Generator is also known as

Synchronous Generator is also known as:

Alternator
DC Generator
Both of these
None of these

Correct answer: 1. Alternator

Explanation: An alternator is a type of synchronous generator that converts mechanical energy into electrical energy. It is commonly used in power plants, automotive applications, and other industries to generate alternating current (AC). The term “alternator” is often used interchangeably with “synchronous generator” because both refer to the same device.

On the other hand, a DC generator, also known as a dynamo, is a different type of generator that produces direct current (DC). Unlike an alternator, which generates AC, a DC generator uses a commutator and brushes to convert mechanical energy into DC.

Therefore, when considering the given options, the correct choice is “Alternator.”

The stator of an alternator rarely employs

Closed type slots
Wide open type slots
Semi-closed type slots
None of these

Correct answer: 1. Closed type slots

A turbo-alternator uses

A turbo-alternator uses:

Rotating AC armature winding
Salient-pole field structure
Non-salient-pole field structure
None of the above

Correct answer: 3. Non-salient-pole field structure

Explanation: Turbo alternators are high speed alternators that posses non-salient rotors. This is done primarily because of two reasons.

At high speeds salient pole rotors have large windage losses
It is quite hard to build salient pole construction, since at high speeds they can’t withstand centrifugal speeds

The synchronous reactance of alternator is due to

The synchronous reactance of alternator is due to:

Armature reaction
DC Field excitation
Leakage flux
None of the above

Correct answer: 1. Armature reaction

Explanation: The synchronous reactance of an alternator is primarily influenced by armature reaction. Armature reaction refers to the magnetic field produced by the flow of alternating current in the stator windings of the alternator. This current produces a magnetic field that interacts with the rotor’s magnetic field, resulting in the distortion of the magnetic field in the air gap.

The interaction between the rotor’s magnetic field and the distorted stator magnetic field causes a phenomenon called armature reaction, which affects the synchronous reactance. Synchronous reactance represents the opposition offered by the alternator’s windings to the flow of alternating current and is an essential parameter in analyzing the performance of synchronous machines like alternators.

The other options listed in the question are not directly responsible for the synchronous reactance:

DC field excitation: While the field excitation current is necessary for establishing the rotor’s magnetic field, it does not directly contribute to the synchronous reactance. The field excitation current primarily influences the strength and shape of the rotor magnetic field, which, in turn, affects the alternator’s voltage output and reactive power capability.
Leakage flux: Leakage flux refers to the magnetic flux that does not link with the windings of the alternator. Although leakage flux can have an impact on various aspects of alternator performance, it does not directly contribute to the synchronous reactance. Leakage flux affects parameters such as voltage regulation, losses, and overall efficiency of the alternator.

Therefore, the correct answer to the given multiple-choice question is “Armature reaction.” The synchronous reactance of an alternator is primarily influenced by the effects of armature reaction, while the other options listed (DC field excitation and leakage flux) are not directly responsible for determining the synchronous reactance.

The voltage regulation of an alternator for PF of 0.8 lagging is X at unity PF

The voltage regulation of an alternator for PF of 0.8 lagging is X at unity PF. Here X implies

Smaller than
Greater than
The same as
None of the above

Correct answer: 2. Greater than

Explanation: The voltage regulation of an alternator refers to the change in voltage between no load and full load conditions. It is typically expressed as a percentage and is calculated by subtracting the no-load voltage from the full-load voltage, divided by the no-load voltage and multiplied by 100.

In the given question, it states that the voltage regulation of the alternator is X at unity power factor (PF). Unity power factor means that the load on the alternator is purely resistive, resulting in a power factor of 1.

Since the power factor is given as 0.8 lagging for which the voltage regulation is X, and unity power factor (PF = 1) is resistive, the voltage regulation at unity power factor will be greater than X. This is because inductive loads (lagging power factor) cause more voltage drop compared to resistive loads (unity power factor). Therefore, the voltage regulation at unity power factor will be higher or greater than the voltage regulation at a power factor of 0.8 lagging.

The magnitude of EMF generated by an alternator depends on

The magnitude of EMF generated by an alternator depends on:

Rotor speed
Flux per pole
Number of poles
All of these

Correct answer: 4. All of these

When the alternator operates at a unity load power factor the effect of armature reaction is

When the alternator operates at a unity load power factor the effect of armature reaction is:

Wholly distorting
Wholly demagnetizing
Wholly magnetizing
Partly distorting, partly demagnetizing

Correct answer: 1. Wholly distorting

The frequency of EMF generated per revolution in an alternator is equal to

The frequency of EMF generated per revolution in an alternator is equal to:

Number of poles
Number of pair of poles
Twice the number of poles
Thrice the number of poles

Correct answer: 2. Number of pair of poles

Explanation: An alternator consists of a rotating rotor and a stationary stator. The rotor is equipped with electromagnets or permanent magnets, which create a rotating magnetic field as the rotor spins. The stator, on the other hand, contains a set of conductive windings or coils.

When the magnetic field of the rotor cuts across the stator windings, an alternating current (AC) is induced in the stator coils. The frequency of this alternating current corresponds to the rate at which the magnetic field is changing or rotating.

The frequency of the induced EMF is directly related to the speed of the rotor and the number of magnetic poles. It is given by the formula:

Frequency = (Number of poles × Speed in revolutions per second) / 120

However, it’s important to note that the number of poles mentioned in the formula refers to the number of pairs of poles rather than the actual count of individual poles. In an alternator, each pole consists of a north pole and a south pole. Therefore, the number of pairs of poles is equal to the number of individual poles divided by 2.

So, to calculate the frequency of the generated EMF per revolution, we use the formula:

Frequency = (Number of pairs of poles × Speed in revolutions per second) / 120

Based on this explanation, the correct answer to the multiple-choice question would be “Number of pair of poles.” The frequency of the EMF generated per revolution in an alternator is equal to the number of pairs of poles.

The stator of an alternator is wound for ____________ on the rotor

The stator of an alternator is wound for ____________ on the rotor:

The same number of poles as
More number of poles than
Less number of poles than
None of the above

Correct answer: 1. The same number of poles as

Voltage regulation of an alternator is larger than that of a DC generator because of

Large leakage reactance
Large armature resistance
Complex effects of armature reaction
None of these

Correct answer: 3. Complex effects of armature reaction

A two pole generator in 50 Hz grid should be turned at

A two pole generator in 50 Hz grid should be turned at:

1500 RPM
3000 RPM
6000 RPM
9000 RPM

Correct answer: 1. 3000 RPM

Explanation: Speed = 120 * f / P

Speed = 120 * 50 / 2 = 3000 RPM

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