AC Generators.pdf

AC Generators (3.17) Learning Objectives 3.17.1 Describe the rotation of loop in a magnetic field and the waveform produced by an AC generator (Level 2). 3.17.2.1 Explain the operation and construction of a revolving armature type AC generator and it's relative disadvantages (Level 2). 3.17.2.2 Expl...

AC Generators (3.17) Learning Objectives 3.17.1 Describe the rotation of loop in a magnetic field and the waveform produced by an AC generator (Level 2). 3.17.2.1 Explain the operation and construction of a revolving armature type AC generator and it's relative disadvantages (Level 2). 3.17.2.2 Explain the operation and construction of a revolving field type AC generator (Level 2). 3.17.3 Describe constructional features and typical uses of single phase, two-phase and three-phase alternators (Level 2). 3.17.4 Differentiate between three-phase star and delta connections and their relative advantages (Level 2). 3.17.5 Describe the operation and construction of permanent magnet generators (Level 2). 3.17.6.1 Explain the purpose of a constant speed drive unit (S). 3.17.6.2 Describe the construction of an integrated drive generator (S). 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 235 of 284 AC Generator Theory The AC Generator Most of the electrical power used aboard aircraft is AC. As a result, the AC generator is the most important means of producing electrical power. AC generators, generally called alternators, vary greatly in size depending on the load to which they supply power. For example, the alternators in use at hydroelectric plants, such as Wivenhoe Dam (west of Brisbane), are tremendous in size, generating around 240 MW at very high voltage levels. Another example is the alternator in a typical automobile, which is very small by comparison. It weighs only a few pounds and produces between 100 and 200 W of power, usually at a potential of 12 V. Example of a small automotive alternator Many of the terms and principles covered in this topic will be familiar to you. They are the same as those covered in the topic on DC generators. You are encouraged to refer back as needed, and to refer to any other source that will help you master this topic. Regardless of size, all electrical generators, whether DC or AC, depend on the principle of magnetic induction. An EMF is induced in a coil as a result of a coil cutting through a magnetic field or a magnetic field cutting through a coil. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 236 of 284 As long as there is relative motion between a conductor and a magnetic field, a voltage will be induced in the conductor. The part of a generator that produces the magnetic field is called the field. The part in which the voltage is induced is called the armature. For relative motion to take place between the conductor and the magnetic field, all generators must have two mechanical parts: a rotor and a stator. The rotor is the part that rotates; the stator is the part that remains stationary. In a DC generator, the armature is always the rotor. In alternators, the armature may be either the rotor or the stator. Three-phase alternator rotation and waveform produced The following sections discuss the different types of alternators and the relevance of the waveforms produced. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 237 of 284 Alternator Types There are two types of alternators: 1. Revolving armature type – the rotor is the armature and the stator is the field. 2. Revolving field type – the rotor is the field and the stator is the armature; used almost exclusively in aircraft. Alternator types 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 238 of 284 Revolving-Armature Alternator The revolving-armature alternator is similar in construction to the DC generator in that the armature rotates in a stationary magnetic field. Rotating armature alternator In the DC generator, the EMF generated in the armature windings is converted from AC to DC by means of the commutator. In the alternator, the generated AC is brought to the load unchanged by means of slip rings. The rotating armature is found only in alternators of low power rating and is generally not used to supply electric power in large quantities. Note: The AC output is via slip rings. Disadvantages of the Rotating-Armature Configuration The revolving armature must be very strongly constructed to withstand centrifugal loads. A rotating-armature alternator requires slip rings and brushes to connect the armature output voltage and current to the load. The brushes and slip rings must be large enough to pass the entire load current. They are difficult to insulate against the relatively high alternator AC output voltage, resulting in arc-over and short circuits. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 239 of 284 Rotating-Field Alternators High-voltage alternators (115-V AC) are usually of the rotating-field type. Rotating-field alternators are better suited for higher power applications because their brushes and slip rings are only required to carry field current (at relatively low values of DC voltage and current). The revolving-field type alternator has a stationary armature winding (stator) and a light-weight rotating field winding (rotor). The armature is connected directly to the load without sliding contacts in the load circuit. Large cross-section (low resistance) conductors can be used in the armature because it is not subject to centrifugal loads. Rotating field alternator 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 240 of 284 Permanent Magnet Generators Permanent Magnet Generators (PMGs) are also called engine-dedicated alternators or permanent magnet alternators. A PMG consists of a high-energy rare-earth permanent magnet rotor that rotates within a steel stator core wound with high-temperature insulated copper windings. Typically, it provides an AC output with a frequency and power proportional to the speed of rotation. Permanent magnetic generators As there is no requirement for power to be supplied to the field, there is no requirement for brushes or slip rings. Depending on the conditioning electronics downstream, PMGs can be designed to operate in either voltage mode (open circuit) or current mode (closed circuit). They are crucial components for providing power to the ignition exciter, FADECs (Full Authority Digital Engine Control) or other accessories on gas turbine engines. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 241 of 284 Brushless Alternators The AC alternators used in large jet-powered aircraft are of the brushless type and are usually air cooled. Since the brushless alternators have no current flow between brushes or slip rings, they are very efficient at high altitudes, where brush arcing is often a problem. As discussed earlier, alternator brushes are used to carry current to the rotating electromagnet. However, in a brushless alternator, current is induced into the field coil through an exciter. A brushless alternator consists of three separate fields: a permanent magnetic field, an exciter field and a main output field. Schematic of a brushless generator The permanent magnets furnish the magnetic flux to start the generator, producing an output before field current flows. The magnetism produced by these magnets induces voltage into an armature that carries the current to a generator control unit, or GCU. Here, the AC is rectified and sent to the exciter field winding. The exciter field then induces voltage into the exciter output winding. The output from the exciter is rectified by six silicon diodes, and the resulting DC flows through the output field winding. From here, voltage is induced into the main output coils. The permanent magnet, exciter output winding, six diodes and output field winding are all mounted on the generator shaft and rotate as a unit. The three-phase output stator windings are wound in slots that are in the laminated frame of the alternator housing. The main output stator winding ends of a brushless alternator are connected in the form of a Y, and in the case of the diagram above, the neutral winding is brought to the outside of the housing along with the three-phase windings. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 242 of 284 The GCU actually monitors and regulates the main generator's output by controlling the amount of current that flows into the exciter field. For example, if additional output is needed, the GCU increases the amount of current flowing to the exciter field winding, which in turn increases the exciter output. A higher exciter output increases the current flowing through the main generator field winding, thereby increasing alternator output. Since brushless alternators utilise a permanent magnet, there is no need to flash the field. In addition, the use of a permanent magnet eliminates the need to carry current to a rotating assembly through brushes. Sinusoidal Sine Wave The sinusoidal sine wave shows the value of induced EMF at each instant of time during a 360° rotation of the loop. A sinusoidal sine wave is a representation of induced EMF for a single coil rotated through a uniform magnetic field at a constant speed. Alternating current (AC), unlike direct current (DC), flows first in one direction and then in the opposite direction. DC amperage is constant. The most common AC waveform is a sine (or sinusoidal) waveform. When a conductor is cutting lines of flux quickly, it produces a greater force to drive electrons, and hence a greater potential difference. This is represented by the sine wave peaking when the wires are moving directly across (perpendicular to) the face of the magnetic field. Aviation Australia Sinusoidal wave form Each cycle of the sine wave consists of two identically shaped variations in voltage. The variations that occur during the time considered the positive alternation (above the horizontal line) indicate current movement in one direction. The direction of current movement is determined by the generated terminal voltage polarities. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 243 of 284 The variations that occur during the time considered the negative alternation (below the horizontal line) indicate current movement in the opposite direction because the generated voltage terminal polarities have reversed. The distance from zero to the maximum value of each alternation is the amplitude. The amplitude of the positive alternation and the amplitude of the negative alternation are the same. A sine wave is a symmetrical waveform that varies equally around a fixed level and can be a representation of either voltage or current. The sine wave is an alternating (swings both positive and negative) waveform and is most commonly identified as the AC waveform. It bears a direct relationship to circular rotation. With AC, electrons flow first in one direction, and then in the other. Both current and voltage vary continuously. The graphic representation for AC is a sine wave, which can represent current or voltage. Two axes are used to depict a sine wave. The vertical axis represents the magnitude and direction of current or voltage. The horizontal axis represents time or angle of rotation in degrees. Aviation Australia Graph of AC signal When the waveform is above the time axis, current is said to be flowing in a positive direction. When the waveform is below the time axis, current is said to be flowing in a negative direction. A complete cycle occurs in 360°, half of which is positive and half negative. As the armature rotates through the magnetic field, at the initial position of 0°, the armature conductors are moving parallel to the magnetic field. They are not cutting through any magnetic lines of flux. Therefore, no voltage is induced. As the armature rotates from 0° to 90°, the conductors cut through more and more lines of flux. Induced voltage builds to a maximum in the positive direction. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 244 of 284 Graphs showing 270 degrees of rotation As the generator continues to rotate from 90° to 180°, the armature cuts fewer and fewer lines of flux. The induced voltage decreases from a maximum positive value to zero. The armature continues to rotate from 180° to 270°. The conductors cut more and more lines of flux, but in the opposite direction. The voltage is induced in the negative direction, building up to a maximum at 270°. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 245 of 284 Alternator Phase Types Single-Phase Alternators A generator that produces a single, continuously alternating voltage is known as a single-phase alternator. The stator (armature) windings are connected in series. The individual voltages, therefore, add together to produce a single-phase AC voltage. Aviation Australia Single phase output from a generator The definition of phase as you learned it while studying AC circuits may be less useful here. Remember, 'out of phase' meant 'out of time'. Now, it may be easier to think of the word phase as meaning voltage, as in single voltage. The need for a modified definition will be easier to see as we go along. Phase is often replaced with Ø in mathematical circuit analysis. Thus, for example, 3-Phase can be represented as 3-Ø. Single-phase alternators are found in many applications. They are most often used when the loads being driven are relatively light. The reason for this will be more apparent as we get into multiphase alternators (also called polyphase). Power that is used in homes, shops and ships to operate portable tools and small appliances is singlephase power. Single-phase power alternators always generate single-phase power. However, all single-phase power does not come from single-phase alternators. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 246 of 284 Two-Phase Alternators Two-phase alternators have two or more single-phase windings spaced symmetrically around the stator so that the AC voltage induced in one is 90° out of phase with the other. These windings are electrically separate from each other so that when one winding is cutting the maximum number of flux lines, the other is cutting no flux lines. A two-phase alternator is designed to produce two completely separate voltages. Each voltage, by itself, may be considered a single-phase voltage. Each is generated completely independent of the other. Certain advantages are gained. Aviation Australia Two-phase output In a simplified two-pole, two-phase alternator, the windings of the two phases are physically at right angles (90°) to each other. You would expect the outputs of each phase to be 90° apart, which they are. The graph shows the two phases 90° apart, with A leading B. There will always be 90° between the phases of a two-phase alternator. This is by design. The two-phase alternator works via the same principle as a single-phase alternator. Each output is a single-phase voltage, just as if the other did not exist. The rotor is identical to that used in the single-phase alternator. The stator consists of two single-phase windings completely separated from each other. Each singlephase winding is made up of two coils connected in series so their voltages add. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 247 of 284 In the left-hand schematic, the rotor poles are opposite the windings of Phase A. Therefore, the voltage induced in Phase A is maximum, and the voltage induced in Phase B is zero. As the rotor continues rotating counterclockwise, it moves away from the A windings and approaches the B windings. As a result, the voltage induced in Phase A decreases from its maximum value, and the voltage induced in Phase B increases from zero. In the right-hand schematic, the rotor poles are opposite the windings of Phase B. Now the voltage induced in Phase B is maximum, whereas the voltage induced in Phase A has dropped to zero. Notice that a 90° rotation of the rotor corresponds to one quarter of a cycle. The waveform picture shows the voltages induced in phase A and B for one cycle. The two voltages are 90° out of phase. The two outputs, A and B, are independent of each other. Two-Phase Three-Wire Alternator Now, look at the smaller schematic diagram in bottom left corner. Aviation Australia Two-phase three-wire output Only three connections have been brought out from the stator. Electrically, this is the same as the large diagram above. Instead of being connected at the output terminals, the B1-A2 connection was made internally when the stator was wired. A two-phase alternator connected in this manner is called a two-phase, three-wire alternator. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 248 of 284 The large schematic shows four separate wires brought out from the A and B stator windings. This is the same as in previous alternator. Notice, however, that the dotted wire now connects one end of B1 to one end of A2. The effect of making this connection is to provide a new output voltage. This sine-wave voltage, C in the picture, is larger than either A or B. It is the result of adding the instantaneous values of phase A and phase B. For this reason it appears exactly half way between A and B. Therefore, C must lag A by 45° and lead B by 45°, as shown in the small vector diagram. The three-wire connection makes possible three different load connections: A and B (across each phase), and C (across both phases). The output at C is always 1.414 times the voltage of either phase. These multiple outputs are additional advantages of the two-phase alternator over the single-phase type. The two-phase alternator is seldom seen in actual use. Three-Phase Alternators A three-phase or polyphase circuit is used in most aircraft alternators. The three-phase alternator, as the name implies, has three single-phase windings spaced such that the voltage induced in any one phase is displaced by 120° from the other two. Three-phase output A schematic diagram of a three-phase stator showing all the coils becomes complex, and it is difficult to see what is actually happening. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 249 of 284 View A of the simplified schematic above shows all the windings of each phase lumped together as one winding. The rotor is omitted for simplicity. The voltage waveforms generated across each phase appear on a graph as phase-displaced 120° from each other. The three-phase alternator is made up of three single-phase alternators whose generated voltages are out of phase by 120°. The three phases are independent of each other. 3-Ø Star and Delta Connections Rather than having six leads coming out of the three-phase alternator, the same leads from each phase may be connected together to form a star connection or more commonly referred to as a wye (Y) connection. It is called a wye connection because, without the neutral, the windings appear as the letter Y, sometimes, sideways or upside-down. I LINE = I PHASE A A A A 260V 100A AV = 450V 100A 260V 100A C E LINE = 1.73 x E PHASE AV = 450V 100A B B 260V 100A AV = C WYE-Connected Winding Values C B 450V 100A C N Aviation Australia Star or Y connection The neutral connection is brought out to a terminal when a single-phase load must be supplied. Single-phase voltage is available from neutral to A, neutral to B, and neutral to C. In a three-phase, Y-connected alternator, the total voltage, or line voltage, across any two of the three line leads is the vector sum of the individual phase voltages. Each line voltage is 1.73 times one of the phase voltages. Because the windings form only one path for current flow between phases, the line and phase currents are the same (equal). A three-phase stator can also be connected so that the phases are connected end-to-end; it is now delta-connected (because it looks like the Greek letter delta, Δ). 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 250 of 284 In the delta connection, line voltages are equal to phase voltages, but each line current is equal to 1.73 times the phase current. Aviation Australia Delta connection Both the wye and the delta connections are used in alternators. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 251 of 284 Formula for Calculating Power in 3-Ø Circuits To calculate power consumed in a 3-Ø circuit, use: P = √ 3 × V × I × Cosineθ where V and I are assumed to be line values and cos θ is the power factor. To calculate power in kilo volt-amps (kVA; Apparent Power): P ower (kV A) = I Line × V Line × 1.732 1000 To calculate power in kilowatts (kW; True Power): P ower (kW ) = I Line × V Line × 1.732 × pf 1000 Remember that line values are as connected from phase to phase – one line. AC generators are typically power rated in kVA. To calculate the power rating of the following AC generator: Aviation Australia WYE connection example 200 × (115 × 1.732) × 1.732 P ower (kV A) = 1000 P ower (kV A) = 69 kV A 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 252 of 284 Phase Sequence When the rotor rotates, if the voltages in each winding reach their positive peak values in order of A, B, C, then the phase rotation is ABC. If Phases B and C are transposed, then the phase rotation becomes ACB. This causes three-phase motors to rotate in the reverse direction. Aviation Australia Phase sequence of a three-phase signal When connected to a 3-Ø load, the phase sequence is very important, particularly with rotating machinery (AC motors), as direction of rotation can be affected. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 253 of 284 Advantages and Applications of Three-Phase Connections Star Delta Similar ends connected Dissimilar ends connected For balanced or unbalanced loads More suited to balanced loads V line = √3 V phase V line = V phase I line = I phase I line = √3 I phase Two values of voltage available One common voltage available Common connection available for earthing No common earthing point V line leads V phase by 30° I line leads I phase by 30° Suited to long-distance power transmissions Suited to locally operated machinery Alternator Frequency Control The output frequency of alternator voltage depends on the speed of rotation of the rotor and the number of poles. The faster the rotational speed, the higher the frequency. The lower the rotational speed, the lower the frequency. The more poles there are on the rotor, the higher the frequency is for a given speed. Factors determining alternator AC frequency 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 254 of 284 When a rotor has rotated through an angle such that two adjacent rotor poles (a north and a south pole) have passed one winding, the voltage induced in that winding has varied through one complete cycle. For a given frequency, the more pairs of poles there are, the lower the speed of rotation. A two-pole generator must rotate at four times the speed of an eight-pole generator to produce the same frequency of generated voltage. The frequency of any AC generator in hertz (Hz), which is the number of cycles per second, is related to the number of poles and the speed of rotation, as expressed by the equation: NP f = 120 P is the number of poles N is the speed of rotation in revolutions per minute (rpm) 120 is a constant to allow for the conversion of minutes to seconds and from poles to pairs of poles. Example The output frequency of an alternator with eight poles driven at 6000 rpm is 400 Hz. A six-pole alternator must be driven at 8000 rpm to have a 400 Hz output. 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 255 of 284 AC Generator Aircraft Connections Constant Speed Drive To provide a constant frequency as engine speed varies and maintain a uniform frequency between multiple generators, most AC generators are connected to a Constant Speed Drive unit, or CSD. Although CSDs come in a variety of shapes and sizes, their principle of operation is essentially the same. The drive unit consists of an engine-driven axial-piston variable-displacement hydraulic pump that supplies fluid to an axial-piston hydraulic motor. The motor then drives the generator. The displacement of the pump is controlled by a governor which senses the rotational speed of the AC generator. The governor action holds the output speed of the generator constant and maintains an AC frequency at 400 Hz, plus or minus established tolerances. The term CSD is most commonly applied to hydraulic transmissions found on the accessory drives of gas turbine engines, such as aircraft jet engines. On modern aircraft, the CSD is often combined with a generator into a single unit known as an integrated drive generator (IDG). Image by Daderot CSD for Boeing 727 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 256 of 284 Integrated Drive Generator Some modern jet aircraft produce AC with a generator called an Integrated Drive Generator, or IDG. An IDG differs from a CSD in that it contains both the CSD and the AC generator in the same housing. This is an example of a modern integrated drive generator (IDG). The term generator is still used even though IDGs are actually brushless alternators. Example of an IDG 2024-02-15 B-03b Electrical Fundamentals CASA Part 66 - Training Materials Only Page 257 of 284