Mastering Interview Questions and Answers for Generators

Table of Contents

1. Introduction
2. What is a Generator?
3. Working Principle of a Generator
4. Important Parts of a Diesel Engine
5. Main Parts of an AC Generator
6. Difference Between AC and DC Generators
7. Faraday's Laws of Electromagnetic Induction
8. Types of Losses in DC Machines
9. Short Circuit Ratio of a Generator
10. Types of Generators
11. Separately Excited Generators
12. Self-Excited Generators
13. Series, Shunt, and Compound Generators
14. Constant and Variable Losses
15. Role of a Commutator
16. Function of Brushes
17. Armature Reaction
18. Commutation and Its Improvement
19. Advantages of Parallel Operation of Generators
20. Automatic Transfer Switch (ATS)
21. Function of Turbocharger in DG Set
22. Function of Fuel Injector in DG Set
23. Importance of Starting Motor in DG Set

Introduction

Generators are commonly used machines that convert mechanical energy into electrical energy. They play a crucial role in various industries and power systems by providing a reliable source of electricity. In this article, we will explore the working principle of generators, discuss the different types of generators, and delve into the important components and functions of generators.

What is a Generator?

A generator is a machine that converts mechanical energy into electrical energy. It operates based on Faraday's laws of electromagnetic induction. By utilizing magnetic fields and conductors, generators produce electricity that can be used to power electrical appliances and systems.

Working Principle of a Generator

The principle of operation of a generator is based on the phenomenon of electromagnetic induction. When a conductor cuts through magnetic flux, an electromotive force (EMF) is induced in the conductor. This induced EMF causes a current to flow if the conductor circuit is closed. The direction of the induced EMF and the resulting current is determined by Fleming's right-hand rule.

Important Parts of a Diesel Engine

In a generator powered by a diesel engine, several important parts contribute to its efficient functioning. These parts include:

1. Alternator: The alternator is responsible for converting mechanical energy from the diesel engine into electrical energy.
2. Engine Base Frame: The engine base frame provides stability and support to the diesel engine.
3. Fuel Tank: The fuel tank stores the diesel fuel that powers the engine.
4. Control Panel: The control panel allows operators to monitor and control the generator's operation.
5. Circuit Breaker: The circuit breaker protects the generator from electrical overload by interrupting the flow of current.
6. Exhaust System: The exhaust system directs the emission of exhaust gases from the diesel engine.
7. Radiator Cooling System: The radiator cooling system prevents the engine from overheating by dissipating excess heat.
8. Lubrication System: The lubrication system ensures smooth operation of the diesel engine by reducing friction between moving parts.
9. Turbocharger: A turbocharger compresses the air flowing into the diesel engine, increasing its power output.

Main Parts of an AC Generator

An AC (alternating current) generator consists of two main parts:

1. Stator: The stator is the stationary part of the generator that houses the armature windings. These windings produce the magnetic fields required for the generation of electricity.
2. Rotor: The rotor is the rotating part of the generator that contains the field windings. The rotation of the rotor induces electromagnetic forces in the stator, resulting in electrical generation.

Difference Between AC and DC Generators

The main difference between AC and DC generators lies in the configuration of their magnetic fields and armatures. In an AC generator, the magnetic field is rotating while the armature remains stationary. In contrast, a DC generator has a stationary magnetic field and a rotating armature.

Faraday's Laws of Electromagnetic Induction

Faraday's laws of electromagnetic induction form the basis of generator operation. The first law states that when a conductor cuts magnetic flux, an electromotive force (EMF) is induced in the conductor. The magnitude of the induced EMF is equal to the rate of change of flux linkage.

Types of Losses in DC Machines

In a DC generator or motor, losses can be classified into three types:

1. Copper Losses: These losses occur due to the resistance of the windings, leading to power dissipation.
2. Iron or Core Losses: These losses occur in the armature and are due to the rotation of the armature in the magnetic field of the poles. They include hysteresis loss and eddy current loss.
3. Mechanical Losses: These losses are caused by friction and windage in the machine.

Short Circuit Ratio of a Generator

The short-circuit ratio (SCR) of a generator is the ratio of the field current required to produce the rated voltage on open circuit to the field current required to circulate the rated current on short circuit.

Types of Generators

Generators can be classified based on their method of field excitation. The two main types are separately excited DC generators and self-excited DC generators.

Separately Excited Generators

A separately excited generator is a DC generator whose field magnet winding is supplied from an independent external DC source, such as a battery. This type of generator offers more control over the field excitation and is commonly used in applications that require precise voltage regulation.

Self-Excited Generators

A self-excited generator is a DC generator whose field magnet winding is supplied current from the output of the generator itself. There are three types of self-excited generators:

1. Series Generator: The field winding is connected in series with the armature winding, causing the whole armature current to flow through the field winding as well as the load.
2. Shunt Generator: The field winding is connected in parallel with the armature winding, allowing the terminal voltage of the generator to be applied across it.
3. Compound Generator: A compound wound generator has two sets of field windings on each pole. One is connected in series with the armature winding, while the other is connected in parallel. This configuration allows for a combination of series and shunt excitation.

Constant and Variable Losses

In DC machines, losses can be categorized as constant losses and variable losses. Constant losses include iron losses, mechanical losses, and shunt field losses. Variable losses refer to copper losses, which occur due to the currents in the windings.

Role of a Commutator

The commutator in a DC generator plays a significant role in facilitating the collection of current from the armature conductors. It converts the alternating current induced in the armature conductors into a unidirectional current in the external load circuit.

Function of Brushes

Brushes in a generator serve the purpose of leading current from the rotating loop or winding to the external stationary load. They ensure efficient electrical contact and provide a low-resistance path for the flow of current.

Armature Reaction

Armature reaction refers to the effect of the magnetic field produced by the armature current on the distribution of flux under the main poles of a generator. It can weaken the main flux and cause cross-magnetization or distortion. Armature reaction can result in reduced generated voltage and sparking at the brushes.

Commutation and Its Improvement

Commutation refers to the reversal of current in the coils as they pass the brushes in a DC generator. Good commutation is essential to avoid sparking and ensure smooth operation. Techniques such as resistance commutation and commutation EMF are employed to improve commutation and minimize sparking.

Advantages of Parallel Operation of Generators

Parallel operation of generators offers several advantages, including continuity of service, increased efficiency, ease of maintenance and repair, and the ability to increase plant capacity. It also ensures a non-availability of a single large unit.

Automatic Transfer Switch (ATS)

An automatic transfer switch (ATS) is a device that transfers power from a standard source, such as a utility, to emergency power, such as a generator, in the event of a power failure. The ATS senses the power interruption and signals the engine panel to start the generator. When the standard source is restored, the ATS transfers power back and shuts down the generator.

Function of Turbocharger in DG Set

In a DG (diesel generator) set, the turbocharger plays a crucial role in compressing the air flowing into the diesel engine. This increases the density of the air and allows the engine to squeeze more air into the cylinder. More air means more fuel can be added, resulting in increased power output.

Function of Fuel Injector in DG Set

The fuel injector in a DG set atomizes the liquid fuel and sprays the required amount of fuel into the combustion chamber of the engine. It is responsible for delivering the fuel in a fine mist to ensure efficient combustion and power generation.

Importance of Starting Motor in DG Set

The starting motor is essential for internal combustion engines, such as those used in DG sets. It converts electrical energy into mechanical energy, starting the engine in motion before the ignition phase of the Otto cycle. The battery is used to power the starting motor and initiate the engine's operation.

Highlights

• Generators convert mechanical energy into electrical energy.
• Faraday's laws of electromagnetic induction govern generator operation.
• AC and DC generators differ in their magnetic field and armature configurations.
• Diesel engines used in generators have multiple important parts for efficient functioning.
• Commutation and commutators play a crucial role in DC generator operation.
• Parallel operation of generators offers advantages in terms of continuity of service and increased plant capacity.
• Turbochargers and fuel injectors enhance the performance of DG sets.
• Starting motors are vital for initiating the operation of DG sets.

FAQs

Q: What is the difference between an AC generator and a DC generator? A: The main difference lies in the configuration of their magnetic fields and armatures. AC generators have a rotating magnetic field and a stationary armature, while DC generators have a stationary magnetic field and a rotating armature.

Q: How does a generator convert mechanical energy into electrical energy? A: Generators operate based on the principles of electromagnetic induction. When a conductor cuts through magnetic flux, an electromotive force (EMF) is induced in the conductor, leading to the generation of electrical energy.

Q: What are the advantages of parallel operation of generators? A: Parallel operation allows for continuity of service, increased efficiency, easier maintenance and repair, and the ability to increase plant capacity. It also provides redundancy and ensures a non-availability of a single large unit.

Q: Why is commutation important in a DC generator? A: Commutation is crucial for a DC generator to ensure the reversal of current in the short-circuited coil without sparking. It enables the smooth flow of current in the external load circuit.

Q: What is the function of a turbocharger in a DG set? A: Turbochargers compress the air flowing into a diesel engine, increasing its power output by allowing more air to be squeezed into the cylinder. This facilitates efficient combustion and enhances the overall performance of the DG set.

Q: How does a fuel injector work in a DG set? A: Fuel injectors atomize the liquid fuel and spray it in the required amount into the combustion chamber of the engine. This ensures efficient combustion and optimal power generation in a DG set.

Q: Why is a starting motor important in a DG set? A: The starting motor is necessary to initiate the motion of the engine before the ignition phase of the Otto cycle. It converts electrical energy into mechanical energy, allowing the engine to power itself.