30 Questions
What is the main advantage of using a permanent magnetic generator (PMG) over a brushed generator?
PMGs require less maintenance due to the lack of brushes and slip rings.
Why are brushless alternators more efficient at high altitudes compared to brushed alternators?
Brushless alternators do not experience brush arcing, which is a common problem at high altitudes.
What is the purpose of the exciter field in a brushless alternator?
To induce voltage into the main output field.
What is the role of the permanent magnets in a brushless alternator?
They provide the initial magnetic flux to start the generator.
Which of the following is NOT a key component of a brushless alternator?
Brushes and slip rings
What is the primary purpose of a brushless alternator in a gas turbine engine?
To provide power to the ignition exciter.
What is the key difference between a revolving armature type alternator and a revolving field type alternator?
The rotor is the field in the revolving armature type, while the rotor is the armature in the revolving field type.
Why is the rotating armature configuration generally not used to supply electric power in large quantities?
The rotating armature configuration requires slip rings and brushes to connect the armature output voltage and current to the load.
What is the key similarity between a DC generator and a revolving-armature alternator?
Both have a rotating armature and a stationary magnetic field.
What is the main advantage of the revolving field type alternator over the revolving armature type?
The revolving field type does not require slip rings and brushes.
Which type of alternator is generally used to supply electric power in large quantities?
Revolving field type
What is the purpose of the slip rings in a revolving-armature alternator?
To connect the armature output voltage and current to the load.
What happens when the rotor poles are opposite the windings of Phase B in the alternator?
Maximum voltage is induced in Phase B
How much rotor rotation corresponds to one quarter of a cycle in the alternator?
90°
What is the relationship between the voltages induced in Phase A and Phase B in the two-phase alternator?
They are 90° out of phase
What is the purpose of the B1-A2 connection made internally in the two-phase three-wire alternator?
To provide a new output voltage
What effect does connecting one end of B1 to one end of A2 have on the alternator's output?
Results in a new output voltage, C, larger than A or B
What is unique about the output connections in a two-phase, three-wire alternator compared to a standard alternator?
Only three connections are brought out from the stator
When the waveform is below the time axis, what is said to be flowing in a negative direction?
Current
What is the total number of degrees in a complete cycle?
360
At the initial position of 0, what is happening to the armature conductors?
They are moving parallel to the magnetic field
As the armature rotates from 90 to 180, what happens to the induced voltage?
It decreases from a maximum positive value to zero
What is the direction of the induced voltage as the armature rotates from 180 to 270?
Negative
What type of alternator produces a single, continuously alternating voltage?
Single-phase alternator
What is the relationship between the line voltage and phase voltage in a wye-connected three-phase system?
The line voltage is 1.73 times the phase voltage.
What is the relationship between the line current and phase current in a wye-connected three-phase system?
The line current is equal to the phase current.
How is the neutral connection used in a wye-connected three-phase system?
The neutral connection is used to supply single-phase voltage.
What is the significance of the 'wye' connection in a three-phase system?
The windings appear as the letter 'Y' without the neutral connection.
What is the purpose of the $ ext{sqrt}{3}$ factor in the relationship between line and phase voltages in a wye-connected three-phase system?
To convert from phase to line voltage.
What is the main advantage of a wye-connected three-phase system over a delta-connected system?
Ability to supply single-phase loads.
Study Notes
Permanent Magnetic Generators (PMGs)
- No requirement for power supply to the field, eliminating the need for brushes or slip rings
- Can be designed to operate in voltage mode (open circuit) or current mode (closed circuit)
- Crucial components for providing power to ignition exciter, FADECs, or other accessories on gas turbine engines
Brushless Alternators
- Used in large jet-powered aircraft
- No current flow between brushes or slip rings, making them efficient at high altitudes
- Consist of three separate fields: permanent magnetic field, exciter field, and main output field
- Permanent magnets induce voltage into the armature, carrying current to a generator control unit (GCU)
Alternator Types
- Revolving armature type: rotor is the armature, stator is the field
- Revolving field type: rotor is the field, stator is the armature (used almost exclusively in aircraft)
Revolving-Armature Alternator
- Similar to DC generator construction
- Armature rotates in a stationary magnetic field
- AC output is brought to the load unchanged via slip rings
- Found only in low-power alternators, not used to supply electric power in large quantities
- Disadvantages: requires strong construction, slip rings, and brushes
Two-Phase Three-Wire Alternator
- Three connections brought out from the stator
- Electrically equivalent to a larger diagram
- Two-phase alternator connected in this manner is called a two-phase, three-wire alternator
Alternator Phase Types
- Single-Phase Alternators: produce a single, continuously alternating voltage
- Three-Phase Alternators: three-phase AC voltage is produced by three separate windings
- 3-Ø Star and Delta Connections: windings connected together to form a star (wye) connection or delta connection
Star (Wye) Connection
- Three-phase alternator leads connected together to form a star connection
- Neutral connection brought out to a terminal for single-phase load supply
- Single-phase voltage available from neutral to each phase (A, B, C)
- Line voltage is the vector sum of individual phase voltages
- Line and phase currents are the same (equal)
Learn about the different types of alternators - revolving armature type and revolving field type, and understand the waveforms produced by each. This quiz covers the rotation and waveform production of three-phase alternators.
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
