Phase 2 (chapter 7)

Aircraft Frequency is

400 Hertz

CSD

Constant Speed Drive, helps provide a constant frequency as engine speed varies, and to maintain a uniform frequency between multiple generators connected to a common electrical bus, most AC generators are connected through the CSD.

What does a slip ring do?

Deliver voltage to a aircraft

What is the difference between the 3 types of DC generators?

Depend on how the field winding is connected to the external circuit.

The main electrical power for small aircraft is

Direct current because it is storable and aircraft engines are started through the use of battery power.

Reverse current cutout

Disconnects the battery from the generator when the generator output is lower than the battery output. If the battery were not disconnected, it would discharge through the generator armature when the generator voltage falls below that of the battery.

Eddy Current Losses

Due to opposing magnetic field being produced

Pigtail

Flexible braided copper conductor that connects each brush to a external circuit.

Over compound generator

Full load voltage higher than the no load voltage.

The voltage produced by an alternator is controlled in the same way as in a

Generator- by varying the DC field current

Under-compound generator

Has a full-load voltage less than the no load voltage.

Two Phase Alternators

Have two or more single phase windings spaced symmetrically around the stator, so that the AC voltage induced in one is 90 degrees out of phase with the voltage induced in the other.

The difference between DC alternators and DC generators

In a alternator the magnetic poles rotate and induce voltage into a fixed, or stationary winding.

Three Unit Regulator

Includes a current limiter, a reverse current cutout, and a voltage regulator

Mica

Insulates the segments from the flanges

Flat- compound generators

Is one in which the no load and the full load voltages have the same value

Commutator

Located at one end of the armature and consist of wedge-shaped segments of hard drawn copper.

What do brushes do and where are they?

Located on the slip ring, and they change AC current to DC current. When they are neutral, it will not cause sparking.

The process of increasing and decreasing the field current could be accomplished in

Low output alternators with vibrator type controls that interrupt the field current by opening a set of contacts

What causes a brush to arc?

Low spring tension.

Series-Wound

Made of large wire and few turns. They are not suitable for aircrafts because they have poor voltage regulation capabilities. However, they are suitable for situations where a constant RPM and constant load are applied to the generator. A field rheostat is connected in parallel with the field windings to control the amount of current flowing in the field coils.

Generators are typically designed to be

Over compounded

Brushes

Provide a connection between the generator and the aircraft. Ride on the surface of the commutator and act as a electrical contact between armature coils and a external circuit. A spring provides proper tension. Made out of high grade carbon.

Alternators are usually driven by

Pulley

Carbon Pile Voltage Regulator

Relies on the amount of airspace within a stack of carbon disks to control generator voltage. Pressure is maintained on the disks by a spring while an electromagnet controls spring tension.

3 types of DC generators

Series-wound, shunt-wound, compound-wound

Alternating current alternators can be

Single phase, two phase, three phase, and sometimes even six phase or more.

The three phase, full wave rectifier in an alternator is made up of

Six heavy duty silicon diodes. Three of the diodes are pressed into the slip ring end frame, and the other three are pressed into a heat sink that is electrically insulated from the end frame.

IDG

Some modern jets produce a AC with a generator called a Integrated Drive Generator. The IDG is a high output generator that uses brushes and slip rings to carry DC exciter current to the rotating field. An IDG comprises both the constant speed drive unit and the generator sealed in the same housing.

Growler Test

Specially designed test unit for DC generators and motors and a variety of tests on the armature and field coils is performed

What are the three factors of EMF?

Speed at which lines are cut, Number of loops, strength of the magnetic field.

Generating Systems

The allowable voltage drop in the main power wires coming off the generator to the bus bar is 2 percent of the regulated voltage when the generator is producing it's rated current. The total continues electrical load permitted in a given system is limited to 80 percent of the total rated generator output.

Risers

The raised part on a commutator

The primary components of an alternator include

The rotor, the stator, the rectifier, and the brush assembly

Since the current is small compared to the field current that flows through the emitter collector

There is no arcing at the contacts

Almost all aircraft electrical systems use what kind of alternator?

Three phase alternators

The efficiency and service life of alernator brushes are

Typically better than on DC generators

Most alternators are

3 phase alternators. This means that the stator has three separate coils that are 120 degrees apart.

Shunt-Wound

A generator having a field winding connected in parallel with the external circuit. They contain many turns with small wires. In this, the armature and the load are connected in series; the field windings are connected in parallel with the external load. It decrease in output voltage when a load is increased. Used in smaller aircrafts. When the load decreases, the output voltage increases accordingly, and a larger current flow in the windings. The action is cumulative and, if allowed, the output voltage would rise to a point called saturation.

An alternator rotor consist of

A wire coil wound on an iron spool between two heavy iron segments with interlacing fingers.

three-phase alternator

AKA polyphase, circuit is used in most aircraft alternators. The three phase alternator has three single phase windings spaced so that the voltage induced in each winding is 120 degrees out of phase with the voltage in the other two windings. One lead winding is connected to form a common junction.

What is a Ripple?

Absence of power in the DC cycle

Generator Overhaul

Accomplished any time a generator is determined to be inoperative, or at the same time the aircraft engine is overhauled.

Generators ratings

According to its power output. Since a generator is designed to operate at a specific voltage, the rating is usually given as the number of amperes the generator can safely supply at its rated voltage.

Field Frame

Also known as the yoke. Has two primary functions. Completed the magnetic circuit between the poles, and it acts as a mechanical support for the other parts.

ACU

Alternator Control Unit This drops the alternator from the circuit when a overvoltage condition exist.

Armature reaction

Anytime current flows through a conductor, a magnetic field is produced. Therefore, when current flows through a armature, electromagnetic fields are produced in the windings. These fields tent to distort or bend the lines of magnetic flux between the poles of the generator.

Coming in speed

Around 1,500 RPM

Compound-Wound

Combines a series winding and a shunt winding so that characteristics of each are used. The series field coils consist of a relatively small number of turns made of a large copper conductor, either circular or rectangular in cross section. Series field coils are connected in series with the armature circuit. These coils are mounted in the same poles as the shunt field coils and, therefore, contribute to the magnetizing force, or magnetomotive force, which influences the generators main field flux.

Armature

Consist of the armature coils, the commutator, field windings, and other associated mechanical parts.

Brushless Alternators

Used in large jet powered aircraft. Usually air cooled. Since they have no current between brushes or slip rings, they are very efficient at high altitudes where brush arcing is often a problem. Current is induced into the field coil through an exciter. A brushless alternator consist of three separate fields, a permanent magnetic field, a exiter field, and a main output field.

The transistorized voltage regulator utilized both

Vibrating and transistors for voltage control

AC alternators are rated in

Volt-amps. Which is a measure of the apparent power being produced by the generator.