AC Generator Theory

AC Generator Theory: The AC Generator

• AC generators, also known as alternators, are crucial for producing electrical power in aircraft which rely mostly on AC power

Alternator Size and Examples

• AC generators are usually called alternators, come in various sizes based on the load they power
• Hydroelectric plant alternators, like those at Wivenhoe Dam, produce around 240 MW at high voltage
• Automotive alternators are small, weighing a few pounds, and output 100-200 W of power at 12V

Principles of Operation

• All electrical generators, DC or AC, use magnetic induction to generate electricity
• An EMF gets induced in a coil when it cuts through a magnetic field

Relative Motion and Components

• Voltage is induced in a conductor with relative motion to a magnetic field
• The field is the part of a generator which produces the magnetic field
• The armature is the part where the voltage is induced
• Generators require a rotor (rotating part) and a stator (stationary part)
• In DC generators, the armature is always the rotor, in alternators, the armature can be either the rotor or stator

Alternator Types

• There are two primary types of alternators:
  • Revolving armature type: the rotor is the armature, and the stator is the field
  • Revolving field type: the rotor is the field, and the stator is the armature, and is predominantly used in aircraft

Revolving-Armature Alternator Construction

• The revolving-armature alternator build is similar to a DC generator, with the armature rotating in a stationary magnetic field

EMF Conversion

• In DC generators, the EMF generated in the armature windings is converted from AC to DC using a commutator
• In alternators, the generated AC is directly supplied to the load via slip rings

Application and Limitations

• Rotating armatures are only found in low power alternators and are not suited to supply large quantities of electrical power

Note:

• AC output is via slip rings

Rotating-Armature Configuration Disadvantages

• The revolving armature needs a very strong build
• Slip rings and brushes connect the armature output to the load, require large size to carry entire load current
• The design makes it difficult to insulate against high AC output voltage, also leads to arc-over and short circuits

Rotating-Field Alternators

• High-voltage (115V AC) alternators are usually the rotating-field type
• Rotating-field alternators suit high-power applications since their brushes and slip rings only carry field current (low DC voltage/current values)

Revolving-Field Alternator Design

• A revolving-field type alternator features a stationary armature winding/stator and Lightweight rotating field winding/rotor
• The armature links directly to the load without sliding contacts in the load circuit
• Large cross-section conductors are usable in the armature, which provides low resistance and eliminates centrifugal load concerns

Permanent Magnet Generators (PMGs)

• Permanent Magnet Generators are also engine-dedicated alternators or permanent magnet alternators

PMG Components

• A PMG includes a high-energy rare-earth permanent magnet rotor which rotates within a steel stator core wound with high-temperature, insulated copper windings

PMG Output

• Provide an AC output as frequency and power proportional to the speed of rotation

Brushless Alternators

• Brushless AC alternators, common in large jet aircraft, don't need brushes or slip rings
• Brushless alternators are efficient at high altitudes, eliminating brush arcing issues

Brushless Alternator Operation

• Brushless alternators induce current into the field coil using an exciter

Brushless Alternator Components

• A brushless alternator has a magnetic field, an exciter field, and a main output field

Brushless Alternator Function

• Permanent magnets provide magnetic flux for generator start-up before field current
• Magnetism creates voltage in an armature that provides current to the generator control unit (GCU) AC is rectified and supplied to the exciter field winding
• The exciter field causes voltage in the exciter output winding, creating DC via silicon diodes, DC flows through the output field winding, inducing voltage in the main output coils

Mounting and Connections

• The permanent magnet, exciter output winding, diodes, and output field winding mount on the generator shaft
• Three-phase output stator windings are in slots within the alternator housing's laminated frame
• Brushless alternator main output stator winding ends connect as a Y, with the neutral winding brought outside with the three-phase windings

GCU Operation

• Generator Control Units monitor and regulate the generator output
• GCUs control current flow into the exciter field
• A higher exciter output boosts current through the main generator field winding, increasing alternator output
• Brushless alternators use permanent magnets, eliminating field flashing and current transfer to a rotating assembly via brushes

Sinusoidal Sine Wave Representation

• The sine wave pictures the value of induced EMF at each instant of time during a 360° rotation of the loop
• The sinusoidal sine wave shows induced EMF for a single coil rotated through a uniform magnetic field at a constant speed

Alternating vs Direct Current

• Alternating current flows first in one direction and then the opposite direction. Alternating current is opposite in behavior to direct current
• DC amperage is constant

AC Waveform

• The most common AC waveform is a sine waveform

Electron Flow and Potential Difference

• More force is produced to drive electrons, and hence a greater potential difference, when a conductor cuts flux lines quickly
• This is what dictates the sine wave peaking when the wires are perpendicular to/across face of the magnetic field

Sine Wave Cycles

• Each cycle of a sine wave has two identical voltage shapes
• Above the horizontal line variations are positive alternation to show current movement in one direction
• Below the horizontal line variations are negative alternation to show current movement in the opposite direction because terminal voltage polarities reversed

Amplitude and Symmetry

• Amplitude is the magnitude of the maximum value of each alternation from zero, and are the same
• A sine wave is a symmetric waveform, meaning it varies equally around a level and represents voltage or current

Sine Wave Properties

• The sine wave is an alternating/both positive and negative waveform
• It is the AC waveform, and connected to circular rotation
• Electrons first move in a direction than another
• Voltage and current continuously vary

Sine Wave Representation

• A sine wave that represents current or voltage on a graph
• Depicted using two axis
  • Vertical Axis: magnitude and direction of voltage or current
  • Horizontal Axis: time/angle of rotation in degrees

Waveform Direction

• Waveform above the time axis: current flows in a positive direction, while the opposite is true when its below
• Full sine wave cycle: requires 360 degrees, with half being positive and half negative

Armature rotation

• Armature conductors are moving parallel to the, not cutting through them at the initial position of 0°
  • no voltage gets induced
• Armature rotates from 0° to 90°
  • conductors are cutting through more lines of flux
  • induced voltage builds the peak positive direction

Rotation Beyond 90 Degrees

• Armature continues to rotate from 90 to 180°
  • armature cuts fewer lines of flux and induced voltage drops from maximum positive value to zero
• Armature continues to rotate from 180° to 270°
  • conductors cut more lines of flux but in the opposite direction
  • voltage is induced in negative direction, and building up to a maximum at 270°

Alternator Phase Types: Single-Phase Alternators

• A single-phase alternator produces a single continuously alternating voltage
• Stator/armature windings connect in series to add up the individual voltages to create a single-phase AC voltage

Understanding Phases

• The term 'phase' may be thought of as meaning voltage, as in single voltage
• In circuit analysis, phase becomes Ø symbol

Single-Phase Alternator Usage

• Single-phase alternators have various applications, often for light loads
• Power in homes, shops, and ships comes from single-phase power

Power Generation

• Single-phase power alternators make single-phase power - but not all single-phase power comes from single-phase alternators

Two-Phase Alternators

• Two-phase alternators have two or more single-phase windings around the stator at a symmetrical spacing, so the AC voltage in one is 90° out of phase
• When one winding cuts the maximum flux lines, the other cuts none
• A two-phase alternator builds two separate single-phase voltages, and each voltage is completely independent from the each other, which leads to certain obtained advantages

Winding Configuration & Output

• Two phases physically placed at 90° to each other in a two-pole, two-phase alternator create 90° separated outputs
• Two phase alternator produces two single-phase voltages, working with basic principles, though the rotor and stator set ups differ

Rotor Configuration

• The two-phase alternator's rotor and produces two single-phase voltages independently of the each other.

Stator Configuration

• The stator comes with two single-phase windings away from each other
• Each is has two-phase winding/two coils, so the voltages add

Rotor Pole Positioning & Voltage Induction

• In a two-phase alternator the rotor poles are opposite the windings of Phase A
  • Maximum voltage induced in Phase A, while the voltage induced in Phase B is zero

Rotor Rotation Impact

• Rotor rotates counterclockwise away from the A windings and approaches the B windings.Voltage induced in Phase A drops and in Phase B the voltage rises from zero
• When, rotor poles are aligned with Phase B windings (right-hand schematic), Phase B voltage is at maximum, Phase A voltage is at zero
• These corresponds to 90° rotation, with voltages in phase A and B at 90° out of phase for 1 cycle

Two-Phase Three-Wire Alternator

• A two-phase, three-wire alternator results from internal connection of B1-A2 when wiring the stator, having three output connections from four separate wires taken from the A and B stator windings

Voltage Output and Advantages

• The new sine-wave voltage output (C) exceeds Phase A or B
• This comes from the adding of Phase A-B instantaneous values making point C appear midway giving a C lag of 45° to signal a lead in B
• Three potential types of load connections can be made with three-wire connection
  • A and B (across each phase)
  • across both phases, C
• Voltage C is 1.414 times line voltage
• Additional multi-outputs increase advantages and capabilities over single-phase alternator but remains uncommon in practice

Three Phase Alternators

• aircraft alternators commonly use three-phase or polyphase circuits
• three single-phase windings spaced, such the one phase has a voltage induced by 120° from the other two.

Three Phase Simplified

• It shows each phase windings lumped together as one rotor omitted for clarity

Waveform Graph

• The voltages on waveforms generated across each phase displace one another by 120°
  • alternator consists made of three single-phase alternators
  • the voltages being generated out of phase 120 degrees and each phase separate from each other

Star vs Delta Connections

• The loads on leads coming from a three output alternator make the leads be connected to either form a star or wye (Y) connection
• Known as a wye because it resembles the letter Y (sideways or upside down) which does not require for windings appearance to not have neutral

Star or Y Connection

• brought out to terminal phase loads are supplied
• Single-phase voltage is available from neutral to A, neutral to B, and neutral to C

Voltage relationship

• Total voltage, or line voltage, is Vector sum of the individual phase voltages when dealing, and can be found in a Y connected alternator
• the value of each line voltage is 1.73 times the phase voltages
• the phases of the flow of current is in only one path, the line and phase being equal
• The delta connectedness (looks like a Greek delta, Δ)
• Used in some stators connected phase and end

Key Properties of Delta Connections

• line voltage = phase voltages
• line current = 1.73 x phase current

Formula for Calculating Power in Three-Phase Circuits

• Formula: P = √3 × V × I × Cosined
• V and I are line values, cosine 0 is the power factor

Apparent Power Calculation

• use the formula Power (kVA) = (I Line × V Line × 1.732) / 1000, Apparent Power describes kilo-volt amps

True Power Calculation

• True Power describes kilowatts (kW), and use the formula Power (kW) = (I Line × V Line × 1.732 × pf) / 1000

AC Generators Note

• AC generators typically have power ratings measured in kVA
• AC Generator Example can be found by working through connection sample in article

Phase Rotation Impact

• Rotor rotates and the phase sequence becomes A, B,C when the voltages reach their peak positive values.
• If Phases B and C gets transposed, leads to a phase alteration, with rotation in reverse

Three Phase Important Factors

• important when 3phase are connected particularly with rotating machinery and direction of rotation

Star and Delta applications chart

• Star:

• Similar ends connected

• For balanced or unbalanced loads

• V line = √3 V phase

• I line = I phase

• Two values of voltage available

• Common connection available for earthing

• V line leads V phase by 30°

• Suited to long-distance power transmissions

• Delta:

• Dissimilar ends connected

• More suited to balanced loads

• V line = V phase

• I line = √3 I phase

• One common voltage available

• No common earthing point

• I line leads I phase by 30°

• Suited to locally operated machinery

Alternator Frequency Control

• Alternator voltage frequency output is depended on rotor rotation, lower number of poles, and faster rotational speed

Rotor and Voltage Cycle

• Voltage induced in that winging has one cycle after its rotated angle and poles passed through it
• Less rotation: more poles
• 2- pole generation: spins 4 times the speed for an 8 pole to get voltage generation output
• Speed of AC generator is in hertz/cycles expressed as:
  • f=(NP)/120 (p is poles, N is speed) to convert minutes to seconds and poles to pairs

AC Generator Aircraft Connections : Constant Speed Drive

• Most AC generators get connected to a constant speed drive in order to provide a constant frequency as well a uniform between multiple generators

CSD Unit Notes

• Drive has governor, engine driving hydraulic motor and pump
• Governor regulates displacement , sensing AC rotation to hold constant speed in output at frequency 400 Hz

Constant speed and turbines

• Accessory drives on gas turbines has CSD, modern builds are integrated generator/IDG

Integrated Drive Generator

• Modern jets make AC power with IDG,s different from CSD (integrated/generator)

Integrated note

• Still uses the generator even when brushless in alternation

AC Motor Theory I: Advantages of AC Motors

• AC Motors cost less than DC Motors and lack brushes/commutators than eliminates the issue of wear and dangerous sparking

AC Motor Applications

• AC suited for speed app because determined by AC volts to motor
• Vary in sizes, shapes, and design
• Has single and poly-phase options

Motor Notes

• Motors come in single phases
• Induction (single-phase/poly) energizes inductions rotors
• AC Motors are synchronous and constant energized by the DC voltage

Three-Phase Rotating Fields

• operates by rotating the magnetic field to result in connectable stator winding, upper left represents windings and right rep y connected stator

Diagram notes

• individual phase connected points and spacings make 120 degrees
• Left hand rule in polarity
• Voltage and terminals

Three Phase Rotations Analysis

• Results voltage points at the given polarities
• At point 1 magnets fields at max
• Point 2 volts feel higher aids the 1A field

Rotations and Volts facts

• Points: max voltage at field
• AID field: magnetic (creates rotates)
• Rotations clock
• One rotation = rotated field

Rotor Movement

• To explain rotor, we cant mount magnetic bar in rotor where if it doesn't turn

Alignment factors to keep in mine

• Aligned towards stator
• Fields draw closer as it rotates

Motor Shaft Note

• Through the pivot: same as
• This speed : sync
• The speed is known
• operating motor shaft is attached

AC motors: factos

• Simplifications to show field for mechanical motion
• Using magnets doesn't produce enough
• Motors methods

Speed Of AC Motors

• The magnetic field rotates in speed in relevance to current supply (supply field rotation/motor shaft relation) Ns= f*120/ number

• The equation of notes are phases, poles and cause

Synchronous Motors

• ACs that operate at almost speed

Asynchronous Motors

• Asynchronous is anything less than a speed/slip (5% and not build quality.)

AC Motor Speed Control

• They are fixed.

Practically notes regarding AC Motors

• Using converters and separation windings are the 2 options for AC
• Dual needs the Frequency

Converter Notes regarding A/C unit parts

• three - phase motor well
• One sensitive frequency(capacitive only mains"

Direction of Rotation

• It's determined by depends the direction of rotation and reverse the terms by rewinding of the phase

The Direction Notes to think about

• 3 to to rewind
• Select with to select/phase( some rewinds 90)

Induction Motors: induction motor

• Popular kind for rugged build and rotor, not connected to volts source ex
  • Refrigerator compressors
  • Bend Griner
  • table sAws
• Simple (2) stator & or

Induction Motor Stator: stator types

• pattern in arragned windings
• Alternating passes will induce current in stator

notes for direction interaction

• induction means the current can only happens the Stator to rotor

Squirrel- Cage, motor rotor: squirrel cage

• In slotted cylinders (heavy copper metal rings)
• Because low voltage gap.
• If contains the (coils(called wourded rotor"
• Same basic principle of Stator action

squirrel/ cage facts notes to consider when checking facts later as a studying

Squirrel Cage Motor: facts rotor

• made of bars for conductions with metal (copper, aluminum is typical)
• Reassemble the cage shape and bar welded materials.

the facts what to consider in detail as you learn/ reread

-

• The Cylinder make slot cylinder and windings
• Heavy copper metal or brass and no insulation

Slanted Rotator

• Ensures smooth and acceleration during startup
• Increases inductance but increases/decreases force and energy during out put.

Wounded Rotator

• Use in induction for when the starting requirements severe. Is the starting be adjusted. Adjustments in one phase resistance will allow this

At Speed, consider these facts

• windings shorted" the Sq cage at speed"

Induction Motor

• The stator is a small gap. (lenzs low is EMF" will the changing field(induction motor" for it. Stator will have force/ rotor EMF the( canceling rotation) the stator rotor action)

Rotating field is state

• It's Impossible to state for. rotor at (stator speed" without rotation with induced( no stator/ rotors, or the voltage and rotors must be than it ) the rotations/ load/ speed depend force

The Rotation Notes

• The rotating force to cutting the rotor
• Load is the force needed. And heavier = slow rotors or high power levels

You also have : slip

• and loads
• needs a power that constant so constant or motors with low resistance

and speed

Slip: the terms

• 3000,1000 motors depending original design" 3,6 poles vs. #
• Seen never reaches design/ Torque =0 or operant's operate"
• the great- the: formula to calculate and the N is
• The notes to be found

Synchronus Motors: constant electricity

• Same construction as but a motor
• Synchronous. are always constrants load and drive DC synchronus or thousands- phase"
• The Design basics: three

Synchronous Motor

• Rotor rotation

The steps

• To How it the power that:

  • Is Energized as and that on rotating

• Energize 3 phase ac but cant is it to the and AC, so : •

Stator or cage

• The rotor : will is one rotor, a and the has the • To:
• Will the a at speed
• Is will is and field is
• device is sync force

Synchronies electric motors or motor Pole rotation

One The ac

AC with