Module 3 Electrical Fundamentals II - AC Generators PDF

Module 3 Electrical Fundamentals II Topic 3.17: AC Generators  INTRODUCTION On completion of this topic you should be able to: 3.17.1 Describe the action of the rotation of a loop in a magnetic field and the waveform produced...

Module 3 Electrical Fundamentals II Topic 3.17: AC Generators  INTRODUCTION On completion of this topic you should be able to: 3.17.1 Describe the action of the rotation of a loop in a magnetic field and the waveform produced 3.17.2 Describe the operation and construction of: Revolving armature AC generators Revolving field AC generators Continued... 30-03-2024 Slide No. 2  INTRODUCTION On completion of this topic you should be able to: 3.17.3 Describe the construction and operation of the following AC generators: Single phase Two phase Three phase Calculate the following in three phase AC generator systems: Line voltage and current Phase voltage and current Power 3.17.4 Describe the advantages and uses of three phase star and delta connections 3.17.5 Describe the operation and construction of permanent magnet generators 30-03-2024 Slide No. 3  INTRODUCTION Most electrical power used in aircraft is AC AC generator – most important means of producing electrical power Typically called alternators Vary greatly in size depending upon load to which they supply power Hydroelectric plants - tremendous in size – Megawatts and very high voltage Typical automobile alternator – small size - 100 to 200 watts 12 volts Many terms and principles covered in this topic – same as those in DC generators 30-03-2024 Slide No. 4  BASIC AC GENERATORS All electrical generators, DC or AC, depend upon principle of magnetic induction. An EMF is induced in a coil as a result of: A coil cutting through a magnetic field A magnetic field cutting through a coil As long as there is relative motion between a conductor and a magnetic field, a voltage will be induced in conductor. Field – part of a generator that produces magnetic field. Armature – part in which the voltage is induced. 30-03-2024 Slide No. 5  BASIC AC GENERATORS For relative motion to take place between conductor and magnetic field, all generators must have two mechanical parts - a rotor and a stator. Rotor is the part that rotates – Stator is the part that remains stationary. In a DC generator, the armature is always the rotor. In alternators (AC generators), the armature may be either the rotor or stator. 30-03-2024 Slide No. 6  ALTERNATOR TYPES Two types of alternators: Revolving-armature – where rotor is armature and stator is field Revolving-field – where rotor is field and stator is armature 30-03-2024 Slide No. 7  REVOLVING-ARMATURE ALTERNATOR Similar in construction to DC generator Armature rotates in stationary magnetic field Low power rating and typically not used 30-03-2024 Slide No. 8  REVOLVING-ARMATURE ALTERNATOR Slip rings and brushes are required to pass entire load current at output voltage. Rotating field alternators are better suited as brushes and slip rings only have to carry field current – typically at much lower voltage. Armature, brushes, and slip rings are difficult to insulate. Arc-over and short circuits can result at high voltages. High-voltage alternators are usually of the rotating field type. 30-03-2024 Slide No. 9  REVOLVING-FIELD ALTERNATOR Stationary armature winding (stator) Direct connection to armature allows and a rotating field winding (rotor). use of large cross-section conductors. Advantage – armature connected directly to load without sliding Majority of alternators fitted to aircraft contacts. are rotating field type. 30-03-2024 Slide No. 10  PERMANENT MAGNET GENERATORS Known as: PMGs Permanent Magnet Generators Permanent Magnet Alternators Engine dedicated alternators Typically use rotor of high-energy rare earth permanent magnets. Typically, AC output power is proportional to speed of rotation. 30-03-2024 Slide No. 11  PERMANENT MAGNET GENERATORS As there is NO requirement for power to be supplied to field, there is NO requirement for brushes or slip rings. Very little maintenance requirement. Typically provide power on gas turbine engines for: Ignition exciter FADEC (Full Authority Digital Engine Control) Other gas turbine engine accessories Depending on electronics, PMAs may be operated in either voltage mode (open circuit) or current mode (closed circuit). 30-03-2024 Slide No. 12  BRUSHLESS ALTERNATOR Used in large jet-powered aircraft – usually Consists of three separate fields: air cooled. Permanent magnetic field Have no current flow between brushes or Exciter field slip rings. Main output field Very efficient at high altitudes where brush arcing is often a problem. 30-03-2024 Slide No. 13  BRUSHLESS ALTERNATOR 30-03-2024 Slide No. 14  BRUSHLESS ALTERNATOR Permanent magnets furnish magnetic Exciter field then induces voltage into flux to start generator producing output. exciter output winding. Induces voltage into armature – current Output from exciter is rectified and to generator control unit (GCU). resulting DC flows to output field winding. In GCU – AC is rectified and sent to exciter field winding. Voltage is then induced into main output coils. 30-03-2024 Slide No. 15  BRUSHLESS ALTERNATOR Mounted on generator shaft and rotate as a unit: Permanent magnet Exciter output winding Exciter rectifier – 6 diodes Output field winding Three phase output stator windings – Wye wound (Y) in laminated frame. 30-03-2024 Slide No. 16  BRUSHLESS ALTERNATOR GENERATOR CONTROL UNIT – GCU GCU: Monitors and regulates main generator's output Controls amount of current that flows into exciter field If additional output required: GCU increases amount of current to exciter field winding Increases exciter output Higher exciter output increases current through main generator field Alternator output increases 30-03-2024 Slide No. 17  SINUSOIDAL (SINE) WAVE Single coil rotating 360° through uniform Coil moving perpendicular to lines of flux – magnetic field at a constant speed. faster cut rate – higher EMF induced. Visual representation of changing Coil moving parallel to lines of flux – NO amplitude and polarity of induced EMF. lines of flux cut – NO EMF induced. Induced EMF – proportional to rate at which lines of flux are cut. 30-03-2024 Slide No. 18  SINUSOIDAL (SINE) WAVE A symmetrical waveform that varies equally around a fixed level. Can be either a representation of voltage or current. An alternating (swings both positive and negative) waveform. Most commonly identified as the alternating current (AC) waveform. Bears a direct relationship to circular rotation. 30-03-2024 Slide No. 19  ALTERNATING CURRENT With AC, electrons flow first in one direction, then in the other. Both current and voltage vary continuously. Graphic representation for AC is a sine wave, which can represent current or voltage. There are two axes used to depict a sine wave – Vertical and Horizontal. Vertical axis – represents the magnitude and direction of current or voltage. Horizontal axis – represents time, or angle of rotation in degrees. Waveform above ‘time axis’ – positive – waveform below ‘time axis’ – negative. A complete cycle occurs in 360º, half of which is positive and half negative. 30-03-2024 Slide No. 20  SIMPLE AC GENERATOR As armature rotates through magnetic field – induced voltage varies At initial position (0º) Conductors moving parallel to magnetic field Not cutting through any magnetic lines of flux – NO voltage is induced As armature rotates from 0 to 90º Conductors cut through more & more lines of flux Induced voltage builds to a maximum in positive direction 30-03-2024 Slide No. 21  SIMPLE AC GENERATOR As generator continues to rotate from 90º to 180º Armature cuts less and less lines of flux Induced voltage decreases from a maximum positive value to zero As armature continues to rotate from 180º to 270 º Conductors cut more and more lines of flux, but in opposite direction Voltage is induced in negative direction building up to a maximum at 270º 30-03-2024 Slide No. 22  SIMPLE AC GENERATOR Full sine wave production as armature rotated through 360º. 30-03-2024 Slide No. 23  SIMPLE AC GENERATOR SINUSOIDAL WAVEFORM Note polarity change during 360 degree rotation. 30-03-2024 Slide No. 24  SINGLE PHASE ALTERNATOR Generator that produces a single, continuously alternating voltage. Stator (armature) windings are connected in series. Individual voltages, therefore, add to produce a single-phase AC voltage. Found in many applications – most often used when loads are relatively light. Single Ø – Used in homes, shops, etc. to operate portable tools, appliances, etc. Basic alternator with single-phase (1Ø) output 30-03-2024 Slide No. 25  TWO PHASE ALTERNATORS Windings of two phases are physically at right angles (90°) to each other. Each output is a single-phase voltage, just as if other did not exist. Each winding made up of two windings connected in series so their voltages add. Windings electrically separated from each other. When one winding is cutting maximum flux lines, other is cutting no flux lines. Rotor is identical to that used in single-phase alternator. 30-03-2024 Slide No. 26  TWO PHASE THREE WIRE ALTERNATOR Note schematic diagram in bottom left corner. Only three connections have been brought out from stator – electrically same as four. Instead of connection at output terminals, B1-A2 connection made internally. Called a two-phase, three-wire alternator. 30-03-2024 Slide No. 27  TWO PHASE THREE WIRE ALTERNATOR Wire (dotted) now connects one end of B1 to one end of A2. Provides a new output voltage – sine wave voltage, C, larger than either A or B. Result of adding instantaneous values of phase A and phase B. C appears half way between A and B and lags A by 45° and leads B by 45°. 30-03-2024 Slide No. 28  TWO PHASE THREE WIRE ALTERNATOR Three-wire connection makes three different possible load connections: A and B (across each phase), and C (across both phases). Output at C is always 1.414 times voltage of either phase. Multiple outputs are additional advantages of two Ø alternator over single Ø. Two Ø alternator is seldom seen in actual use. 30-03-2024 Slide No. 29  THREE PHASE ALTERNATORS Three phase or polyphase – typically used in all aircraft AC generators. Three x single Ø windings spaced 120° apart – outputs 120° out of phase. May be connected in: Wye (Y) / Star, or Delta 30-03-2024 Slide No. 30  STAR CONNECTIONS Single-phase voltage available from neutral to A / B / C phases. In a star connected system, line current is equal to phase current. 𝐼𝐿𝐼𝑁𝐸 = 𝐼𝑃𝐻𝐴𝑆𝐸 Line voltage is a combination of two(2) phase voltages 𝑉𝐿𝐼𝑁𝐸 = 3 𝑥 𝑉𝑃𝐻𝐴𝑆𝐸 𝑜𝑟 1.73 𝑉𝑃𝐻𝐴𝑆𝐸 30-03-2024 Slide No. 31  THREE Ø DELTA WINDING Three phase stator connected with phases connected end-to-end or Delta. With Delta connection: Line voltage is equal to phase voltage : 𝑉𝐿𝐼𝑁𝐸 = 𝑉𝑃𝐻𝐴𝑆𝐸 Line current is combination of 2 phase currents: 𝐼𝐿;𝐼𝑁𝐸 = 3 𝑥 𝐼𝑃𝐻𝐴𝑆𝐸 𝑜𝑟 1.73𝐼𝑃𝐻𝐴𝑆𝐸 30-03-2024 Slide No. 32  POWER IN 3 PHASE SYSTEMS Formula for calculating power consumed in a three Ø circuit: 𝑃 = 3 𝑉𝐼 𝐶𝑜𝑠𝜃 Where V and I are assumed to be line values and Cos θ is the power factor. 𝑰𝑳𝑰𝑵𝑬 𝒙 𝑽𝑳𝑰𝑵𝑬 𝒙 𝟏.𝟕𝟑𝟐 To calculate power in kilo volt-amps (kVA): 𝒌𝑽𝑨 = 𝟏𝟎𝟎𝟎 𝑰𝑳𝑰𝑵𝑬 𝒙 𝑽𝑳𝑰𝑵𝑬 𝒙 𝟏.𝟕𝟑𝟐 𝒙𝑪𝒐𝒔𝜽 To calculate power in kilowatts (kW): 𝒌𝑾 = 𝟏𝟎𝟎𝟎 Remember that line values are connected from phase to phase – one line. AC generators (as with most AC generators) – power typically rated in kVA. 30-03-2024 Slide No. 33  POWER IN 3 PHASE SYSTEMS What is the power rating of this aircraft AC generator? To calculate power output (capability) in kilo volt-amps (kVA): 𝑰𝑳𝑰𝑵𝑬 𝒙 𝑽𝑳𝑰𝑵𝑬 𝒙 𝟏. 𝟕𝟑𝟐 𝒌𝑽𝑨 = 𝟏𝟎𝟎𝟎 =69 kVA 30-03-2024 Slide No. 34  PHASE SEQUENCE When rotor rotates, if voltages in each winding reach positive peak values in order of A, B, C. – Phase rotation is ABC. If rotor driven in opposite direction – phase sequence would be A,C,B. When connected to a three Ø load, phase sequence is important. Particularly rotating machinery (AC motors) as direction of rotation can be affected. 30-03-2024 Slide No. 35  ADVANTAGES AND APPLICATIONS OF THREE PHASE CONNECTIONS 30-03-2024 Slide No. 36  ALTERNATOR FREQUENCY CONTROL Output frequency of alternator depends upon: Speed of rotation of the rotor, and Number of poles To determine frequency: 𝑁𝑥𝑃 𝑓= 120 Where: f = Frequency P = Number of poles N = Speed in rpm 30-03-2024 Slide No. 37  ALTERNATOR FREQUENCY CONTROL What is the output frequency of an alternator with 8 poles driven at 6,000 rpm? 400 Hz At what speed must a 6 pole alternator be driven to have a 400 Hz output? 8000 rpm 30-03-2024 Slide No. 38  CONSTANT SPEED DRIVE – CSD Maintains constant frequency as engine speed varies. Engine-driven hydraulic pump supplies hydraulic motor – drives AC generator. Displacement of pump controlled by governor – senses generator rotational speed. Typical CSD and AC Generator fitted to engine accessory drive. 30-03-2024 Slide No. 39  INTEGRATED DRIVE GENERATOR – IDG Constant Speed Drive (CSD) and AC Generator in one unit. 30-03-2024 Slide No. 40  CONCLUSION Now that you have completed this topic, you should be able to: 3.17.1 Describe the action of the rotation of a loop in a magnetic field and the waveform produced 3.17.2 Describe the operation and construction of: Revolving armature AC generators Revolving field AC generators Continued.... 30-03-2024 Slide No. 41  CONCLUSION Now that you have completed this topic, you should be able to: 3.17.3 Describe the construction and operation of the following AC generators: Single phase Two phase Three phase Calculate the following in three phase AC generator systems: Line voltage and current Phase voltage and current Power 3.17.4 Describe the advantages and uses of three phase star and delta connections 3.17.5 Describe the operation and construction of permanent magnet generators 30-03-2024 Slide No. 42  This concludes: Module 3 Electrical Fundamentals II Topic 3.17: AC Generators