3.17 AC Generators Flashcards
What does the term ‘Alternating Current’ (AC) describe?
The flow of charge that changes direction periodically, causing the voltage level to reverse with the current.
What induces a current in a wire in an AC generator?
Spinning a loop of wire inside a magnetic field.
What can be used to rotate the wire in an AC generator?
A wind turbine, steam turbine, flowing water, or mechanical drive from a prime mover such as a gas turbine or reciprocating engine.
What happens when the loop of wire is at 12 and 6 o’clock positions?
It cuts no magnetic lines of force, resulting in no induced EMF.
At what angle does the loop of wire induce an EMF?
At 90° as it cuts the lines of flux.
How does the density of the magnetic field affect induced EMF?
The higher the density, the greater the value of induced EMF.
What is the nature of AC?
Bi-directional.
How is AC usually represented?
By a sinusoidal waveform.
What is the mathematical formula for a sine waveform in AC?
EMF = amplitude × sin(2πft)
What do AC waveforms vary according to?
The method of generation.
What is the most common type of AC waveform?
Sine waveform.
What do the axes represent in a sine waveform?
Amplitude on the y-axis and time on the x-axis.
How is amplitude sometimes referred to in the context of AC waveforms?
Voltage.
What is a key feature of the AC waveform regarding its cycles?
It has the same amplitude in the positive and negative half cycles.
What type of signals do AC waveforms represent?
Time-dependent signals or periodic signals.
Fill in the blank: The AC waveform is ______ dependent.
time
What are the two main parts of an AC generator?
Rotor and stator
What is the principle of operation of a permanent magnet generator?
The engine driven rotor is a permanent magnet that, when rotated, cuts the stationary output windings producing an alternating voltage and current output
What are the applications of permanent magnet generators?
Used as Tachogenerators and Ram Air Turbines (RAT) on aircraft
What type of generator is similar to a DC generator but uses slip rings instead of a commutator?
Rotating armature generator
What is a key disadvantage of rotating armature generators?
Relatively low limit output due to arc over and shorts from high voltages
What generates heat in rotating armature generators that limits output?
Combination of current load (demand) and friction on the brushes
In a rotating field generator, where is the field winding located?
On the rotor
In a rotating field generator, where is the armature located?
In the stator
What type of voltage is applied to the rotating field in a rotating field generator?
Low voltage DC
What is the function of the stator in a rotating field generator?
Holds the windings that are cut by the rotating magnetic field
What is generated in the armature of a rotating field generator as a result of the cutting action?
AC power
What is a benefit of using a rotating field generator compared to a rotating armature generator?
Requires smaller, less costly slip ring connections
How many contacts do DC rotors typically need?
Two contacts
What is the maximum voltage level that can be wound in the stationary armature of a rotating field generator?
Up to tens of thousands of volts
What is the main type of generator that uses a rotating armature?
DC generators
What is the advantage of rotating field generators for AC generation?
Especially advantageous at voltages of 11 kV or more
Fill in the blank: The rotating field generator switches the output and field around. Instead of _______ the stator, the rotor is magnetised.
magnetising
What does the term ‘single-phase’ refer to in generators?
The number of unique phases of the output windings of a generator
What is a single-phase generator?
A generator that produces a single, continuously alternating voltage
Where are the stator windings of a single-phase generator connected?
In series and inserted in slots cut in the inner surface of the laminated stator
How is the rotor of a single-phase generator driven?
By a prime mover carrying the exciter field windings fed from a DC source
In which applications are single-phase generators commonly used?
For relatively light loads, homes, portable tools, and small appliances
True or False: All single-phase power comes from single-phase generators.
False
What characterizes a two-phase generator?
Two or more single-phase windings spaced symmetrically around the stator
What is the phase difference between the outputs of each winding in a two-phase generator?
90°
What are the challenges associated with two-phase generators?
Control complexity, vibrational and uneven heating issues, and higher costs compared to three-phase systems
What is the primary advantage of using three-phase generators?
More economical for higher ratings and better performance
What is the phase difference between the voltages in a three-phase generator?
120°
What is the configuration of the windings in a three-phase generator?
Three single-phase windings spaced so that their voltages run 120° out of phase
How do the outputs of the three phases in a three-phase generator behave?
They are independent of each other
Fill in the blank: A _______ generator produces multiple AC voltages.
multi-phase
What is one main reason two-phase generators are not commonly used today?
Complexity and cost compared to three-phase generation
What type of circuit is primarily used in aircraft generators?
Three-phase or polyphase circuit
True or False: The windings in a two-phase generator are electrically connected.
False
What are the two types of connections in three-phase circuits?
Star Connection and Delta Connection
What is a Star Connection?
A four-wire system where one lead from each phase connects to a common junction
What is a Delta Connection?
A three-wire system where phases are connected end-to-end
What is the main advantage of using a three-phase power supply over a single-phase power supply?
It can transmit three times the power with just three conductors
In a balanced Star Connection, how does the line voltage relate to the phase voltage?
Line voltage is 1.73 (√3) times the phase voltage
What happens to line current and phase current in a Star Connection?
Line current is equal to phase current
What are the typical AC voltages found in aircraft systems?
26 V AC, 115 V AC, 200 V AC
What is a key characteristic of the Delta Connection?
Line voltages are equal to phase voltages
In a balanced Delta Connection, how does the line current relate to the phase current?
Line current is 1.73 times the phase current
What is a major advantage of a Star Connection?
Used for high voltage applications
What is a major advantage of a Delta Connection?
More torque and efficient for heavy-duty applications
Fill in the blank: In a three-phase Delta system, Phase Voltage = _______
Line Voltage
Fill in the blank: In a three-phase Star system, √3 x Phase Voltage = _______
Line Voltage
True or False: The Star Connection can distribute load evenly among phases.
True
What is the effect of adding a single-phase device to a three-phase distribution system?
It creates an unbalanced condition in the generator
What occurs if too many single-phase loads are added to one phase of a generator?
Overheating will occur
What does the output of a generator depend on?
The velocity of the rotor
How does a Star Connection affect insulation requirements?
Requires less insulation than Delta Connection
What is the benefit of the ability to earth the neutral point in a Star Connection?
Improves safety and stability
What is the implication of the Delta Connection allowing for one phase to be damaged?
Remaining two phases can still deliver power
What factor contributes to the simplicity and low construction cost of Delta Connections?
Fewer conductors are needed
What is the relationship between line and phase currents in a Star Connection?
Line current = Phase current
What happens to the generator’s capacity if one phase becomes damaged in a Delta Connection?
Capacity is reduced to 57.7% of normal
What is the significance of the common neutral point in a Star Connection?
Allows for unbalanced loading and dual voltage applications
What is a Permanent Magnet Generator (PMG)?
A modern high-output brushless generator also known as a Brushless AC Generator
What issues do Permanent Magnet Generators address?
Problems related to slip rings and brushes in carrying exciter current to the rotating field
What are the advantages of a brushless generator?
- Lower maintenance cost
- High stability and consistency of output
- Better performance at high altitudes
How do brushless generators transfer current?
By using electromagnetic induction to transfer current from stationary components to rotating components
What type of armature do brushless generators typically use?
A three-phase, star-connected armature
What is the voltage across a single phase of a brushless generator?
115 V
What is the voltage across two of the main output terminals?
200 V
How is the circuit connected for aircraft circuits requiring 115 V single-phase power?
Between one main phase and ground
What are the three separate generators within a PMG?
- Permanent magnet generator
- Exciter generator
- Main generator
What role does the permanent magnet play in a PMG?
It induces an alternating voltage into the stationary PMG three-phase armature winding
What does the Generator Control Unit (GCU) do?
Rectifies AC armature current and regulates the exciter field current
What frequency does the GCU typically work with?
1200 Hz
What happens if more generator output is required?
The GCU increases the exciter field current
What is the output voltage generated by the main generator armature?
115 V across a single phase and 200 V across two phases
What are the typical labels for the output terminals of an aircraft AC generator?
- A, B, C and N (Star Windings)
- T1, T2, T3, T4, T5, T6 (Delta Windings)
Fill in the blank: The brushless system induces voltage into the rotor using _______.
[magnetic flux lines]
True or False: The main generator armature is connected to the output terminals of the generator.
True
What does the exciter field induce?
An alternating voltage into the exciter armature
True or False: The main field weakens the main armature’s output when more output is required.
False
What is the function of the rotating rectifier in a PMG?
Changes AC to DC and sends a current to the main generator field
