Understanding Aircraft Generators and Alternators

Understanding Aircraft Generators and Alternators

Table of Contents:

1. Introduction
2. AC Generators in Modern Aircraft
3. Types of AC Generators 3.1 Rotating Armature Type 3.2 Rotating Field Type
4. Differences Between AC and DC Generators
5. Advantages and Disadvantages of Rotating Armature AC Generators 5.1 Pros 5.2 Cons
6. Advantages of Rotating Field AC Generators
7. Construction of AC Generators 7.1 Rotor and Stator 7.2 Field Windings 7.3 Armature Windings
8. Power Output and Heat Dissipation
9. Rating of AC Generators
10. Polyphase AC Generators 10.1 Three-Phase Generators 10.2 Advantages of Three-Phase Systems
11. Star and Delta Connections 11.1 Star Connection 11.2 Delta Connection
12. Conclusion

AC Generators in Aircraft: A Comprehensive Guide

Introduction:

AC generators play a crucial role in powering the electrical systems of modern aircraft. These generators provide alternating current (AC) that is supplied by alternators. In this article, we will explore the different types of AC generators, their advantages and disadvantages, and the construction and operation of these essential components.

AC Generators in Modern Aircraft:

Most modern aircraft utilize AC generators as their primary source of electrical power. These generators are commonly known as alternators and are responsible for supplying AC to various electrical systems on the aircraft. Unlike direct current (DC) generators, the rotating part in AC generators can either be the armature or the field.

Types of AC Generators:

There are two main types of AC generators: the rotating armature type and the rotating field type. The rotating armature type is similar in construction to a DC generator, where the armature rotates in a stationary magnetic field. In this type, the generated AC is taken from the rotating assembly through slip rings. However, rotating armature AC generators have several disadvantages, including heavy and difficult-to-insulate rotating coils, high resistance across the brushes, and challenging cooling of the armature coils. Due to these limitations, the rotating armature type is typically used only in small output AC generators and not commonly used for supplying aircraft AC systems.

On the other hand, most AC generators used in aircraft employ the rotating field type. These generators have a stationary armature and a rotating field, similar to DC alternators but without the rectifier. The rotor, also known as the rotating part, energizes the field windings either through a permanent magnet or with DC from a separate source. One advantage of the rotating field type is that it requires only a low current to be fed through slip rings to the field windings, significantly reducing arcing problems associated with brush gear.

Construction of AC Generators:

AC generators consist of two main parts: the rotor and the stator. The rotor, also called the rotating part, energizes the field windings and rotates inside the stationary stator. The field windings can be energized using a permanent magnet or DC from an external source. The stator, which remains stationary, includes the armature windings connected in series to supply the output. These windings are positioned opposite each other and spaced at 120 degrees in three-phase generators, the most common type used in aircraft.

Polyphase AC Generators:

Polyphase or multiphase AC generators have multiple sets of single-phase armature windings. The most common type is the three-phase generator, which has three sets of armature windings. The three phases, named A, B, and C, are mechanically arranged at 120 degrees to each other. The currents and voltages generated in this configuration have the same frequency but are out of phase with each other by 120 degrees. This three-phase system offers several advantages, including a greater power-to-weight ratio, simplicity and efficiency in AC motors, and easier parallel connection of generators.

Star and Delta Connections:

In three-phase generators, the outputs of the phases are commonly connected together using either the star or delta method. In a star-connected generator, the three phases are joined at one end to form a neutral point, while the connections at the opposite end are called the line connections. The neutral point is grounded and carries any out-of-balance current between the phase loads. On the other hand, a delta-connected generator forms a closed mesh with the ends of the three phases joined together. This connection, although not commonly used in aircraft power systems, is suitable for rotational speed sensing equipment.

Conclusion:

AC generators are vital components of the electrical systems in modern aircraft. Understanding the different types of generators, their advantages and disadvantages, and the construction and operation of these devices is crucial for aviation professionals. By utilizing polyphase systems and proper connections, aircraft can efficiently and reliably generate AC power for various onboard systems.