Alternators (Jeppesen Powerplant 8-B)

Covers Material from the Jeppesen Powerplant Textbook, pages 8-21 through 8-27

Why is Ac current used on large aircraft?

Smaller wires can be used

What spins on an alternator?

The magnet or the coil

How is AC current rectified by an alternator?

By the use of six solid-state diodes, rather than by a commutator like a generator.

What are the primary components of an alternator?

The Rotor, The Stator, the rectifier, and the brush assembly.

The Interlacing _____ of a rotor form the poles for the rotating magnetic field

Fingers

What does it mean when an alternator is referred to as "3 phase”?

* __**"3 phase" means that the alternator has three separate windings or sets of coils that produce alternating current (AC) electrical output.**__
* These windings are arranged so that the electrical currents they produce are out of phase with one another, meaning that they peak and trough at different times in the AC cycle. By combining these three AC outputs, the alternator can produce a relatively constant and smooth supply of electrical power

What are the components of an alternator brush assembly?

Two brushes and their respective brush springs and brush holders

Why are brushes on an alternator more efficient/better than those on a generator?

Because the brushes ride on the smooth surface of the slip rings, rather than on the intermittent surface of a generators commutator

How is voltage output controlled on an alternator?

By varying the DC field current (shunt field)

What are the 3 types of alternator voltage regulators?

1. __**Vibrator Controls**__


1. Used on low output alternators, not efficient
2. __**Transistorized Voltage Regulator**__


1. This system uses both vibrating points and transistors for voltage control
3. __**Solid-State Voltage Regulator**__


1. Semi-conductors replace moving parts, which make this system very efficient, reliable, and generally not serviceable.

What is an ACU and what is it's function?

* __**Alternator Control Unit**__
* Used to remove the alternator from the bus when an overvoltage condition exists

2 likely problems if an alternator is not producing electrical power

1. Shorted or open diode in the rectifying circuit
2. Open circuit in the field windings

What Is a brushless alternator?

* __**An alternator that has no current flow between brushes or slip rings, and only uses induction to transmit current to/from the spinning rotor.**__
* This system means that brushless alternators are very efficient at high altitudes, where brush arcing is typically a problem.

What are the 3 separate fields in a brushless alternator

1. __**Permanent magnetic field**__
2. __**Exciter Field**__
3. __**Main Output Field**__

How does a brushless alternator work?

In a brushless alternator, the __**Permanent Magnetic Field**__ is created by a set of magnets that are mounted on the rotor. As the rotor spins, the magnets create a magnetic field that passes over the stator windings, inducing an electrical current in them. This electrical current is typically small, but it is used to power the exciter field.

__**The Exciter Field**__ is a winding on the stator that is powered by the small electrical current generated by the permanent magnetic field. As current flows through the exciter field, it creates a magnetic field that passes over the rotor, inducing an electrical current (that is larger than the permanent magnet could make) in the __**Exciter Output**__ windings.

The __**Main Output Field**__ is the winding on the rotor that generates the majority of the electrical output of the alternator. It is connected to the exciter output through a set of diodes that rectify the AC Exciter output to DC, which is then used to power the main output field. The main output field creates a large magnetic field that induces a much larger electrical current in the __**Main Output Stator**__ windings than the permanent magnet could. This electrical current is the final output of the alternator and can be used to power aircraft electrical systems.

Overall, the combination of the permanent magnetic field, the exciter field, and the main output field creates a self-sustaining system that generates electrical power without the need for brushes or a commutator. The system is controlled by electronic voltage regulators that monitor the output and adjust the excitation current as necessary to maintain a stable supply of electrical power. The GCU is usually connected to the airplane's electrical system and may communicate with other avionics systems to provide data and status information.

How are AC alternators rated?

In __**volt-amps**__, which is a measure of the apparent power being produced.

What determines the frequency produced by an AC system?

* __**The number of poles**__
* __**The speed of the rotor**__

What is the equation to determine the frequency of an AC generator/alternator?

__**F= P x N /120**__

\
Where…

* **F = Frequency**
* **P = The Number of Poles**
* **N = The speed in RPM**

What is/What does a constant speed drive do?

A constant speed drive (__**CSD**__) is a device that __**maintains a constant output speed of a generator or alternator**__, regardless of engine speed or load. This systems allows the generator/alternator to maintain a constant AC frequency of 400 Hz

What is/ what does an integrated drive generator do? How does it work?

The IDG is a high-output generator and CSD unit all in the same housing.

What must be done if an IDG or a CSD is disconnected in flight?

They cannot be reconnected in flight; only ground personnel can place them back online.