Generators (Jeppesen Powerplant 8-A)

Covers material in the Jeppesen Powerplant Textbook, 8-1 through 8-20

Define Generator

A __**Generator**__ converts mechanical energy into electrical energy by electromagnetic induction

When is maximum voltage induced into a generator?

When the __**maximum number of magnetic flux lines are being cut**__…This happens when the wire loop (armature) is perpenducular to the magnet, or 90 degrees

What direction does current flow as it is induced into the wire loop of a generator?

Both directions, the direction of current flow changes as the wire loop spins. This is Alternating Current

What is a commutator?

The copper bars on the end of a generator armature to which the rotating coils are attached.

What does a commutator do? How does it “recitfy” AC to DC?

The commutator transmits the voltage induced into the spinning armature into the stationary carbon brushes. As the armature rotates in a DC generator, the commutator is designed so that one brush always contacts the portion of the loop moving downward through the flux lines and the other brush maintains contact with the portion of the loop moving upward. Commutator action produces pulsing DC voltage that fluctuates from zero to its maximum, twice every revolution

Define Ripple

The variation in DC voltage that occurs through the use of a commutator. In layman’s terms, the small bumps in DC voltage between peaks.

Why is a low amount of ripple preferable? How is ripple lowered?

The lower the ripple, the closer the output voltage resembles pure DC voltage; To decrease the variation between mix and max values of voltage, more __**windings**__, or __**loops**__, can be added to the armature.

What are the components of a __**DC Generator**__?

* Field Frame
* Armature
* Commutator
* Brushes

What are the 2 functions of a Generators Field Col

1. To complete the magnetic circuit between the pole
2. To act as a mechanical support for the other parts of the generator

What are the components that make up the electromagnet in the generators field frame?

The wire coil, called a __**Field Coil**__

The core, called the __**Field Pole**__

What are __**Salient Poles**__? What is their purpose?

* Salient poles are protruding poles in the stator of a generator or motor that __**reduce the air gap between the stator and rotor**__.
* The purpose of salient poles is to create a stronger magnetic field in the air gap, which increases the efficiency and power output of the machine.

Define __**Armature**__ and explain its function

The armature is the rotating component of a generator that contains a series of wire loops. When the armature rotates, it cuts through the magnetic field, which induces an electromotive force (EMF) in the wire loops. This EMF generates electrical power in the generator.

What are the components of an armature?

What is a __**Drum-Type Armature**__?

A type of armature used in some generators where the wire loops are wound around the surface of a cylindrical drum-shaped core. There is no electrical connection between the coils and the core.

This type of armature is commonly used in smaller generators where compactness is important.

Label the figure provided (Figure 8-12)

What material is used between the segments of a commutator?

Mica

What is __**lap winding**__, and why is it done?

Lap winding is a winding technique used in armatures where each coil side overlaps the adjacent coil side. This technique is used to increase the amount of current that can flow through the armature and to reduce the armature's resistance, which results in increased power output.

What are brushes for a commutator made of?

Brushes for a commutator are typically made of __**carbon**__. This material has good electrical conductivity, low friction, and a high resistance to wear

Define what a __**carbon brush**__ is and explain its purpose

* A carbon brush is a component used to transfer electrical current between the stationary wiring and the rotating commutator or slip ring.
* The purpose of a carbon brush is to deliver electrical current to the rotating component without damaging it or causing excessive wear.

How do carbon brushes compensate for wear?

How can a technician tell if a carbon brush is worn or not?

A few methods are…

* Holes in the brush, indicating the wear limit
* Lines drawn on the brush, indicating the wear limit
* Measurement dimensions of the brush in the MM

What are the __**three types of DC Generators**__ (Just list them)

1. __**Series-Wound**__
2. __**Shunt-Wound**__
3. __**Compound-Wound**__

Define __**Series-Wound**__ In relation to DC Generator design

* The armature, field coils, and load are wired in series.
* This type is not used in aviation, as the voltage is not constant and varies with the load.

Define __**Shunt-Wound**__ In relation to DC Generator design

* The armature, field coils, and load are wired in parallel.
* This type of generator provides a constant voltage to the load, and is more commonly used in aviation

Define __**Compound-Wound**__ In relation to DC Generator design

* This type of generator design combines the series and shunt winding, taking advantage of both systems’ favorable characteristics.
* The benefit of a compound-wound generator is that it provides a __**more stable output voltage**__ compared to a shunt-wound generator. This is achieved by adding a series field winding to the shunt-wound generator, which improves its voltage regulation.

What is __**Field Saturation?**__

What is/ define __**Magnetomotive Force?**__

What is a __**Starter Generator**__? Explain the Pros/Cons

* A __**Starter Generator**__ is a starter and a generator combined in one unit.
* This system saves weight (two systems in one replaces two separate heavy systems) but is not necessarily as efficient at starting or generating as the independent standalone units.

What is __**Armature Reaction**__

When the electromagnetic fields produces in the windings of an armature bend the lines of magnetic flux between the poles of a generator, causing a __**distortion in the magnetic field**__.

What happens when the magnetic field __**is not**__ distorted? What happens when the magnetic field __**is**__ distorted

* When a carbon brush passes over a commutator, the brushes contact two adjacent segments at the same time, thereby shorting the segments together.
* When the magnetic field is not distorted, no voltage is induced in the shorted windings and no harmful results occur
* When the magnetic field is distorted, voltage is induced in the shorted windings, and arcing occurs between the brushes and the commutator segments

What is “__**shifting the brushes**__”?

* The process of moving the brushes forward in the direction of rotation
* This is done in an effort to set the plane of the coils being shorted __**perpendicular**__ to the distorted magnetic field

What is an __**Interpole**__?

Another field pole that is placed between the main poles.

What does an __**Interpole**__ do?

* Cancels the electromagnetic fields produces by the armature windings and minimizes distortion.
* The strength of the Interpole varies with the load placed on the generator, so the distortion is kept at a minimum during all operational circumstances.

How are generators rated?

* According to their power output
* (For example, a generator may be rated “300 amps at 28.5 volts”)

What is a generators “__**Coming In Speed**__”?

The speed at which the generator begins to produce its normal voltage

What do the B, A, and E markings on a generators terminals?

* __**B-Positive armature lead**__
* __**E-Negative armature lead**__
* __**A-Positive shunt winding lead**__

What are the 3 parts of a __**Three-Unit Regulator**__

1. Voltage Regulator
2. Current Limiter
3. Reverse Current Cutout

Describe how a __**Three-Unit-Regulator**__ works

A 3 unit regulator is an electrical control system used to regulate the output voltage of an aircraft generator. It consists of three components: __**a voltage regulator, a current limiter, and a reverse current cutout.**__

* The voltage regulator senses the generators output voltage. When this voltage is too high, the normally close contacts open through the use of a relay, which diverts the path of current through a resistor. This resistor reduces the voltage sent to the shunt field, which decreases the voltage output of the generator.
* The current limiter limits the maximum output current through the same system as the voltage regulator, but is wired in series as to sense current. When current is too high, it is diverted through a resistor that decreases the strength of the shunt field and, concequently, decreases the current output of the generator.
* The reverse current cutout prevents battery discharge through the generator when it is not operating through the use of a normally open contact point. When generator voltage flows through a coil, it closes the RCCO contact, allowing the batteries and systems to charge/operate as normal. However, when battery voltage tries to backflow through the system, the reverse direction of electricity pushes the contact back open, cutting out this reverse current.

What are the 3 types of generator voltage regulation (not the 3 parts of a three-unit regulator)

1. 3 Unit Voltage Regulator
2. Solid State Voltage Regulator
3. Carbon Pile Voltage Regulator

What is motoring?

* When the battery discharges through the generator armature due to generator voltage falls below battery voltage. This causes the generator to spin.
* This process is prevented by the Reverse Current Cutout circiut

When (what type of generator) are Carbon-Pile Regulators used?

Heavy-Duty generators that require a high field current

What is “Flashing the Field”? Why would this have to be done?

"__**Flashing the field**__" is the process of energizing the field winding of a generator to restore its magnetic field. This may be necessary if the generator has lost its residual magnetism due to extended periods of disuse or if it has been repaired.

What is a Growler? What does it do?

A growler is a tool used to test the armature of a generator for short circuits or other faults. It consists of an iron core surrounded by a coil of wire. When the coil is energized, it produces a magnetic field that induces a voltage in the armature windings. This allows the tester to detect any faults in the armature by identifying areas where the voltage induced is lower than expected.

What is done to test an armature on a growler?

Place the armature on an energized growler and then place the test lamp probes on adjacent segments of each armature coil. The lamp should light with each set of commutator bars. Failure of the test lamp to illuminate indicates an open circuit in that coil.

When cleaning a commutator, you should never use…?

Emery cloth or other conductive material

What is Undercutting?

The process of removing the mica insulation between the bars of some commutators

What percent of total rated generator output is permitted for continuous electrical load?

Continuous electrical load on a generator should not exceed 80% of its rated output. This is to ensure that the generator operates within safe limits and does not overheat or become damaged.

Why are generator master switches used?

* To control the electrical power supply to the main electrical bus of an aircraft.
* They allow the operator to selectively connect or disconnect the generator from the electrical system, which is important for safety and maintenance reasons.
* If a generator is malfunctioning or needs to be serviced, the operator can use the master switch to disconnect it from the system and avoid any potential damage or safety hazards.