Electrical Fundamentals II: AC Generators

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Questions and Answers

What is one of the main advantages of a permanent magnet generator (PMG)?

Needs brushes for operation

Low maintenance requirements (correct)

Operates only in voltage mode

Requires constant power supply to the field

Which aircraft components typically receive power from a permanent magnet alternator (PMA)?

Battery charging system

Landing gear system

Flight control surfaces

Ignition exciter and FADEC (correct)

In context of brushless alternators, what does the exciter field do?

Induces voltage into the armature (correct)

Induces voltage into the output field winding

Generates power for the load directly

Creates electrical current for auxiliary systems

What type of magnetic field do brushless alternators primarily utilize?

Permanent magnetic field Signup and view all the answers

How is the output from the exciter rectified in a brushless alternator?

Through the generator control unit (GCU) Signup and view all the answers

What determines the AC output power of a permanent magnet generator?

Speed of rotation Signup and view all the answers

What feature do brushless alternators eliminate that is often problematic at high altitudes?

Brush arcing Signup and view all the answers

In which operation mode can permanent magnet alternators (PMAs) function?

Both voltage mode and current mode Signup and view all the answers

What is the significance of phase sequence in three-phase connections?

It influences the direction of rotation of AC motors. Signup and view all the answers

How is the output frequency of an alternator calculated?

With the formula $f = \frac{N \times P}{120}$. Signup and view all the answers

What output frequency can be expected from an alternator with 8 poles driven at 6000 rpm?

400 Hz Signup and view all the answers

What adjustment is made to the hydraulic pump in a Constant Speed Drive (CSD)?

Its displacement is controlled by a governor. Signup and view all the answers

At what speed must a 6 pole alternator be driven to achieve a 400 Hz output frequency?

8000 rpm Signup and view all the answers

What is the primary advantage of using a three-phase connection?

It provides higher transmission efficiency. Signup and view all the answers

What does the Integrated Drive Generator (IDG) combine?

Constant Speed Drive and AC Generator. Signup and view all the answers

In a three-phase system, what is the effect if the phase sequence is changed to A, C, B?

The direction of the motors will reverse. Signup and view all the answers

What does the vertical axis of a sine wave represent?

Magnitude and direction of current or voltage Signup and view all the answers

When does the induced voltage of a simple AC generator reach its maximum positive value?

At 90º position Signup and view all the answers

What happens to the induced voltage as the armature rotates from 180º to 270º?

It increases to a maximum negative value Signup and view all the answers

What characterizes the waveform of a single-phase alternator?

It produces a continuously alternating voltage Signup and view all the answers

What is true regarding the full cycle of a sine wave representing AC?

It consists of an equal amount of positive and negative values Signup and view all the answers

What occurs at the initial position (0º) of the armature in a simple AC generator?

Conductors are not cutting through any magnetic lines of flux Signup and view all the answers

How are the stator windings connected in a single-phase alternator?

In series Signup and view all the answers

What is the primary characteristic of two-phase alternators?

Windings are at right angles to each other. Signup and view all the answers

During which segment does the induced voltage in the simple AC generator begin to decrease?

From 90º to 180º Signup and view all the answers

In a two-phase, three-wire alternator, what is the relationship between the voltages at points A, B, and C?

C is 1.414 times the voltage of either A or B. Signup and view all the answers

Which statement about a two-phase alternator's windings is accurate?

One winding is always inactive when the other is at maximum flux. Signup and view all the answers

What does the generator control unit (GCU) primarily monitor and regulate?

The main generator's output Signup and view all the answers

What is the output voltage phase relationship in a two-phase alternator?

Phase C lags Phase A by 45°. Signup and view all the answers

Why are two-phase alternators seldom seen in actual use?

Their efficiency is lower than that of three-phase alternators. Signup and view all the answers

What happens in the GCU if additional output is required from the generator?

It increases the amount of current to the exciter field winding. Signup and view all the answers

What is the purpose of the dotted wire in a two-phase, three-wire alternator?

To connect B1 to A2 for output voltage generation. Signup and view all the answers

Which component is NOT part of the brushless alternator assembly?

Voltage regulator Signup and view all the answers

What characteristic describes a sinusoidal (sine) wave?

It varies equally around a fixed level. Signup and view all the answers

What output configuration provides additional advantages in a two-phase alternator?

Multiple load connections across phases. Signup and view all the answers

What type of alternators are typically used in aircraft AC generators?

Three-phase or polyphase alternators. Signup and view all the answers

How does the induced electromotive force (EMF) relate to the magnetic field?

It is proportional to the rate at which lines of flux are cut. Signup and view all the answers

What effect does a higher exciter output have on the alternator's performance?

It increases the current through the main generator field. Signup and view all the answers

What defines the flow of electrons in alternating current (AC)?

Electrons flow first in one direction, then in the opposite direction. Signup and view all the answers

What is the primary winding configuration used in the three-phase output stator of the brushless alternator?

Wye (Y) wound Signup and view all the answers

In a star-connected system, how does the line current compare to the phase current?

Line current is equal to phase current. Signup and view all the answers

What is the relationship between line voltage and phase voltage in a delta-connected system?

Line voltage is equal to phase voltage. Signup and view all the answers

What is the correct formula to calculate power consumed in a three-phase circuit?

P = 3VICosθ Signup and view all the answers

How do you calculate power in kilowatts (kW) from line values in a three-phase system?

kW = 1000ILINE × VLINE × Cosθ Signup and view all the answers

In a star connection, how is line voltage defined in terms of phase voltage?

VLINE = 1.732 × VPHASE Signup and view all the answers

What does the power factor Cosθ represent in the power calculation formula for a three-phase circuit?

It represents the phase difference between voltage and current. Signup and view all the answers

In a three-phase system, how is the line current defined in a delta connection?

ILINE = 3 × IPHASE Signup and view all the answers

What is the significance of the factor 1.732 in the power calculation formulas pertaining to three-phase systems?

It relates to the number of phases. Signup and view all the answers

Study Notes

Module 3: Electrical Fundamentals II - Topic 3.17: AC Generators

AC generators, also known as alternators, are crucial for producing electrical power in aircraft.
Alternator sizes vary greatly depending on the load they supply.
Large alternators, like those in hydroelectric plants, are tremendous in size and produce megawatts of power at very high voltage. In contrast, smaller alternators, like automotive alternators, produce 100-200 watts at 12 volts.
The fundamental principle behind all AC generators (and DC generators) is magnetic induction.
An electromotive force (EMF) is induced in a coil when it cuts through a magnetic field or when a magnetic field cuts through a coil.
For relative motion to occur between a conductor and a magnetic field, alternators need two components: a rotor and a stator. The rotor rotates, and the stator remains stationary.
In a DC generator, the armature always acts as the rotor. In AC generators, the armature can be either the rotor or the stator.

Types of Alternators

Revolving-armature: The rotor is the armature (the component where voltage is induced), and the stator is the field (the component producing the magnetic field). These are typically low power and not commonly used.
Revolving-field: The rotor is the field, and the stator is the armature. This is the more common type of alternator used in aircraft.
The revolving-field alternator has an advantage of directly connecting the armature to the load, eliminating the need for sliding contacts and brushes, beneficial at higher altitudes.
With the revolving armature, slip rings and brushes are required to pass current, which makes it difficult to insulate at high voltages and prone to arc-over and short circuits.

Other AC Generator Types

Permanent Magnet Generators (PMGs): These alternators are dedicated to specific engines and utilize high-energy rare-earth permanent magnets in the rotor for the field. Their output is proportional to the speed of rotation of the rotor.
PMGs offer the advantage of low maintenance because no power is required for the field, eliminating the need for brushes or slip rings.
Brushless Alternators: These are frequently utilized in large jet aircraft, usually air cooled and highly efficient, especially at high altitudes where brush arcing would be problematic.
This type of alternator uses a permanent magnet for the exciter field, a separate exciter field winding, a rectifier, and a main output field winding that gets its voltage from the exciter.

AC Generator Control Unit (GCU)

The GCU monitors and regulates the output of the main generator and controls the amount of current flowing into the exciter field.
When more generator output is required, the GCU increases the current directed to the exciter field winding. This results in a higher exciter output, enhancing current in the main generator field, and correspondingly rising alternator output.

AC Waveforms

The output from an alternator is a sinusoidal wave, a symmetrical wave that varies around a fixed level, typically representing voltage or current.
The lines of flux cutting through the conductor determine the amplitude and polarity of the induced EMF.

Basic Three-Phase Alternator Considerations

Three-phase alternators are the standard in aircraft. -They employ three separate windings with 120-degree phase shifts between outputs.
These windings can then be connected in a wye (Y) or star configuration or a delta configuration.
Power calculations for three-phase circuits use the relationship P = √3VI CosΦ, where V and I represent line values and Cose is the power factor.
Typically, the power of AC generators (as with most alternators) is measured in kVA.
Different winding configurations (star or delta) have unique electrical characteristics, affecting voltage, current, and power, and are relevant for how they are connected to the load.

Alternator Frequency Control

Alternator output frequency depends on the speed of the rotor and the number of poles.
The formula to determine frequency is f = N x P / 120, where f = Frequency, N = Speed in rpm, and P = Number of Poles.

Constant Speed Drive (CSD) and Integrated Drive Generator (IDG)

The constant speed drive (CSD) is an arrangement to maintain a consistent frequency even when the engine speed changes, by using an engine-driven hydraulic pump/motor system.
The integrated drive generator (IDG) combines the CSD and the AC generator into a single unit.

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Description

This quiz covers essential concepts related to AC generators, known as alternators, which are vital in producing electrical power for aircraft and other applications. Key topics include the differences in alternator sizes, the principles of magnetic induction, and the roles of the rotor and stator in generating electromotive force (EMF). Test your knowledge on these fundamental electrical concepts.