Understanding Aircraft Alternators: How Do They Work?

Table of Contents

1. Introduction
2. Types of Direct Current Generators and Alternators
   1. Series Wound Field
   2. Shunt Wound Field
   3. Compound Wound Field
3. Series Wound DC Generator
   1. Working Principle
   2. Output Voltage Variation
4. Shunt Wound DC Generator
   1. Working Principle
   2. Control of Output Voltage
5. Compound Wound DC Generator
   1. Working Principle
   2. Compensation for Voltage Drop
6. Self-Exciting DC Generator
   1. Residual Magnetism
   2. Flashing the Field
7. Loss of Residual Magnetism
   1. Field Flashing
8. Alternators vs. DC Generators
   1. Advantages of Alternators
   2. Conversion of AC to DC
9. Diode Rectifiers
10. Conclusion

Types of Direct Current Generators and Alternators

Direct current (DC) generators and alternators are essential for supplying electrical power to aircraft systems. There are three main types of field winding connections for these generators: series, shunt, and compound. This article will explore each type in detail and discuss their working principles, output voltage variations, and control mechanisms. Additionally, we will compare DC generators to alternators, highlighting the advantages of the latter and the use of diode rectifiers for converting AC to DC.

Series Wound DC Generator

In a series wound DC generator, the magnetic field is produced by stationary electromagnets consisting of field coils wound around the pole pieces of the field permanent magnet. The armature, field coils, and external circuit are all connected in series. As a result, the output or terminal voltage of the generator varies with the applied load. Initially, with no load, the terminal voltage is very low due to the weak magnetic field. However, as the load current increases, the field strength and voltage in the armature winding also increase. There is a point of field saturation where further increases in load current do not affect the voltage.

Shunt Wound DC Generator

Unlike the series wound generator, a shunt wound DC generator has its field winding connected in parallel with the armature. The field current in a shunt generator is independent of the load current. The shunt field windings have a large number of turns and require a relatively small current to produce the necessary field flux. As the generator starts, the residual magnetism in the field poles induces an electromotive force (EMF) in the armature, causing a current to flow in the field coil. This increases the field strength, leading to a greater induced EMF and further increasing the field current. The terminal voltage reaches its rated value rapidly, and within the normal operating range, the voltage drop due to load current is relatively small.

Compound Wound DC Generator

A compound wound DC generator is equipped with both series and shunt field windings. The series field is added to compensate for the voltage drop that occurs in a shunt generator as the load is increased. By increasing the strength of the magnetic field with an appropriate number of windings, the compound wound generator can achieve an almost constant voltage output. It overcomes the limitations of a shunt generator and provides a stable voltage supply for loads requiring constant voltage.

Self-Exciting DC Generator

A self-exciting DC generator does not require an external electrical supply to the field on startup. The residual magnetism present in the field pole pieces when the machine is static is sufficient to initiate the generation process. As soon as the generator is rotated, a voltage is produced, causing a current to flow in the field coil. This increases the magnetic field's strength, further enhancing the voltage and field current until the output voltage reaches its controlled value. However, if the residual magnetism is lost or destroyed, the generator won't build up the output voltage. In such cases, the field needs to be flashed by passing a current through it in the normal direction.

Alternators vs. DC Generators

Alternators have replaced DC generators as the primary source of electrical power in modern light aircraft. In an alternator, the functions of the rotating and stationary components are reversed compared to a DC generator. The magnet with its field windings is on the rotating assembly (rotor), while the heavy armature windings are on the stationary assembly (stator). The field current is fed onto the rotor through brushes and slip rings. Alternators offer several advantages over DC generators, including lighter weight, a more stable output at low engine speed, and the absence of commutator-related problems. To convert the AC output of an alternator to DC, diode rectifiers are used.

Diode Rectifiers

Diode rectifiers are solid-state devices used to convert the AC output of an alternator to DC. They allow current to flow unrestricted in one direction (towards their arrowhead) but block flow in the other direction. By using diode rectifiers, the need for a commutator in the generator is eliminated. This results in a lighter and more reliable electrical system, as there are no moving parts or issues with brush wear.

In conclusion, understanding the different types of DC generators and alternators is vital for efficient aircraft system power supply. Series, shunt, and compound wound generators each offer unique characteristics in terms of output voltage variation and control. Alternators, with their reversed design and diode rectifiers, provide numerous advantages over traditional DC generators. By harnessing the power of these electrical systems, aircraft can enjoy a stable and reliable source of electrical power.

Highlights

• DC generators and alternators are integral to supplying electrical power to aircraft systems.
• There are three main types of field winding connections: series, shunt, and compound.
• Series wound DC generators have a varying output voltage based on the applied load.
• Shunt wound DC generators maintain a relatively constant output voltage despite load variations.
• Compound wound DC generators offer compensation for voltage drops in a shunt generator, resulting in a more stable output.
• Self-exciting generators utilize residual magnetism to initiate the generation process, but can also be flashed if the magnetism is lost.
• Alternators have replaced DC generators in modern light aircraft due to their lighter weight and more stable output.
• Diode rectifiers are used to convert the AC output of alternators to DC, eliminating the need for a commutator.

FAQs

Q: What is the main difference between a series wound DC generator and a shunt wound DC generator? A: The main difference lies in the connection of the field winding. In a series wound generator, the field coils, armature, and external circuit are all connected in series. In a shunt wound generator, the field winding is connected in parallel with the armature.

Q: Can the output voltage of a shunt wound DC generator be controlled? A: Yes, the output voltage of a shunt wound generator can be controlled by placing a variable resistance in series with the field coil. This allows for adjustment of the current flowing in the field circuit and, consequently, the generator's output voltage.

Q: How does a compound wound DC generator compensate for voltage drops? A: A compound wound generator utilizes both series and shunt field windings. The series field is added to increase the strength of the magnetic field when the load current is increased, compensating for the voltage drop in a shunt generator.

Q: Is a commutator necessary in modern aircraft electrical systems? A: No, with the advent of diode rectifiers, which convert AC to DC, the need for a commutator in the generator is eliminated. Diode rectifiers are solid-state devices that offer a more reliable and maintenance-free solution.

Q: Why have alternators replaced DC generators in aircraft systems? A: Alternators offer several advantages over DC generators, including lighter weight, more stable output at low engine speeds, and no commutator-related problems. These factors make alternators the preferred choice for modern aircraft electrical power supply.