AC Generator

Introduction

A generator is a machine that converts mechanical energy into electrical energy by electromagnetic induction. AC power systems result in better design and use of equipment than older electronic equipment powered by Direct Current (DC), which have inverters for ac power and dynamotors for supplying higher voltage dc power.

These components are very heavy compared to their relative power outputs. They are not reliable and increase maintenance costs and time. In today’s aircraft, the same ac-powered equipment obtains various ac voltages and dc power by using simple transformers and transformer-rectifiers. These components are lightweight, simple, and reliable. Modern aircraft use the three-phase,120-/208-volt, 400-hertz (Hz) ac power system.

The number of magnetic poles and rotor revolutions per minute (rpm) determine the voltage and frequency of the generator. With a fixed number of poles, constant frequency requires constant rotor rpm.

An ac generator-rotating field has 12 poles with adjacent poles of opposite polarity. Each pair of poles produces one cycle per revolution; therefore, each revolution produces six cycles. The output frequency of the generator varies in direct proportion to the engine drive speed. A generator operating at 6,000 rpm is operating at 100 revolutions per second or at 600 Hz.

The 120-/208-volt, 400-Hz, three-phase ac power system has many advantages over the 28-volt dc system. It requires less current than the 28-volt dc system because of higher voltage and a ground neutral system. The current required is a fraction of that required for the same power in a 28-volt dc system. This permits the use of smaller aircraft wiring, saving weight.

The ac generator and many of the system’s control and protection components are lighter. Twelve kilowatts is the practical limit to the size of an aircraft dc generator. Aircraft now have ac generators with ratings up to 90 kilovolt-ampere (kVA).

Basic Generator

Generators used to produce an alternating current are called AC generators or alternators. The simple generator constitutes one method of generating an alternating voltage.

It consists of a rotating loop, marked ABCD, placed between two magnetic poles, N and S. The ends of the loop are connected to two metal slip rings (collector rings), S1 and S2. Current is taken from the collector rings by brushes B1 & B2.

BASIC AC GENERATOR

Fleming's Right Hand Rule

This rule says that is you stretch thumb, index finger and middle finger of your right hand perpendicular to each other, then thumbs indicates the direction of motion of the conductor, index finger indicates the direction of magnetic field i.e. N - pole to S - pole, and middle finger indicates the direction of flow of current through the conductor.

FLEMING'S RIGHT HAND RULE

Let's us consider, the rectangular loop of conductor is ABCD which rotates inside the magnetic field about its own axis.

When the loop rotates from its vertical position to its horizontal position, it cuts the flux lines of the field. As during this movement two sides, i.e. AB and CD of the loop cut the flux lines there will be an emf induced in these both of the sides (AB & CD) of the loop. As the loop is closed there will be a current circulating through the loop.

The direction of the current can be determined by Fleming’s right hand Rule.

Now if we apply this right hand rule, we will see at this horizontal position of the loop, current will flow from point A to B and on the other side of the loop current will flow from point C to D.

Now if we allow the loop to move further, it will come again to its vertical position, but now upper side of the loop will be AB and lower side will be CD (just opposite of the previous vertical position). At this position the

tangential motion of the sides of the loop is parallel to the flux lines of the field. Hence there will be no question of flux cutting and consequently there will be no current in the loop.

If the loop rotates further, it comes to again in horizontal position. But now, said AB side of the loop comes in front of N pole and CD comes in front of S pole, i.e. just opposite to the previous horizontal position as shown in the figure beside.

Here the tangential motion of the side of the loop is perpendicular to the flux lines, hence rate of flux cutting is maximum here and according to Fleming's right hand rule, at this position current flows from B to A and on other side from D to C.

Now if the loop is continued to rotate about its axis, every time the side AB comes in front of S pole, the current flows from A to B and when it comes in front of N pole, the current flows from B to A. Similarly, every time the side CD comes in front of S pole the current flows from C to D and when it comes in front of N pole the current flows from D to C.

If we observe this phenomena in different way, it can be concluded, that each side of the loop comes in front of N pole, the current will flow through that side in same direction i.e. out the conductor and similarly each side of the loop comes in front of S pole, current through it flows in same direction i.e. into of the conductor.

The output from the generator is collected from slip ring and AC output can be achieved.

Note : The direction of the current can be determined by the Fleming's right hand rule, the direction of the current is the conventional direction of the current i.e Positive terminal to the Negative terminal. However there are certain book which consider the the flow of electrons as the direction of flow of current i.e. from the negative terminal to the positive terminal in such cases the direction of the current in a generator can be found out by Left Hand Rule (not the Fleming Left Hand Rule). The Left Hand Rule is similar to the Fleming Right Hand Rule except that the direction of the current gets reversed.

WORKING OF A GENERATOR

Working of AC Generator

As we have seen above that the generator produced an alternating current and with the help of slip rings an AC output can be achieved.

Let us see how

CASE 1

As we know now thats the first half of the revolution current flows always along AB which is connected to slip ring 1 and CD which is connected to to slip ring 2. The brush X is connected to slip ring 1 and the brush Y is connected to the slip ring 2.

So the total current Flow path will be from brush X to slip ring 1 and in the path ABCD to the slip ring 2 then to the brush Y and through the load back to the brush X. The Direction of current flow is shown in the diagram.

Case 2

In the next half revolution,current flows always along DC which is connected to the slip ring 2 and BA which is connected to the slip ring 1. The brush X is like before connected to slip ring 1 and the brush Y is connected to the split ring 2.

So the total current Flow path will be from brush Y to slip ring 2 and in the path DCBA to the slip ring 1 then to the brush Y and through the load back to the brush X. The Direction of current flow is shown in the diagram.

Hence, the current in the load resistance in both cases are opposite to each other. Hence the output is AC.

WORKING OF AC GENERATOR

OUTPUT OF AC GENERATOR

Construction of Alternator

AC generators, or alternators, are based on the principles that relate to the simple AC generator and consist of

Rotor
Stator

However, in a practical AC generator the magnetic field is rotated rather than the conductors from which the output is taken i.e field exciters are rotating and the armature coil is stationary.

Hence the stator of an alternator is not meant to serve path for magnetic flux. Instead, the stator is used for holding armature winding. The stator core is made up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses

The magnetic field is usually produced by a rotating electromagnet (the rotor ) rather than a permanent magnet as it has a number of advantages like :

the conductors are generally lighter in weight than the magnetic field system and are thus more easily rotated
the conductors are more easily insulated if they are stationary
the currents which are required to produce the rotating magnetic field are very much smaller than those which are produced by the conductors.

The Electromagnetic used can be excited by on of the following method

A direct connected, direct current generator

This system consists of a DC generator fixed on the same shaft with the AC generator. A variation of this system is a type of alternator which uses DC from the battery for excitation, after which the alternator is self-excited.

By transformation and rectification from the AC system

This method depends on residual magnetism for initial AC voltage buildup, after which the field is supplied with rectified voltage from the AC generator.

Integrated brushless type

This arrangement has a direct current generator on the same shaft with an alternating current generator. The excitation alternator with circuit is completed through silicon rectifiers rather than a commutator and brushes. The rectifiers are mounted on the generator shaft and their output is fed directly to the alternating current generator main rotating field

Types of Rotor

There are two types of rotor used in an AC generator / alternator:

Salient pole type

Salient pole type rotor is used in low and medium speed alternators. This type of rotor consists of large number of projected poles (called salient poles), bolted on a magnetic wheel. These poles are also laminated to minimize the eddy current losses. Alternators featuring this type of rotor are large in diameters and short in axial length.

Cylindrical Type

Cylindrical type rotors are used in high speed alternators, especially in turbo alternators. This type of rotor consists of a smooth and solid steel cylinder having slots along its outer periphery. Field windings are placed in these slots.

CYLINDRICAL ROTOR VS SALIENT ROTOR

Single Phase AC Generator

The stator consists of five coils of insulated heavy gauge wire located in slots in the high-permeability laminated core. These coils are connected in series to make a single stator winding from which the output voltage is derived.

The two-pole rotor comprises a field winding that is connected to a DC field supply via a set of slip rings and brushes. As the rotor moves through one complete revolution the output voltage will complete one full cycle of a sine wave.

The major difference between an alternator and a DC generator is the method of connection to the external circuit; that is, the alternator is connected to the external circuit by slip rings, but the DC generator is connected by a commutator.

By adding more pairs of poles to the arrangement it is possible to produce several cycles of output voltage for one single revolution of the rotor. The frequency of the output voltage produced by an AC generator is given by:

F = PN/60

where, F is the frequency of the induced e.m.f. (in Hz), P is the number of pole pairs, and N is the rotational speed (in rev/min).

NOTE : The above formula can also be written as

F = PN/120

here the P is the number of pole not the number of pole pairs (two poles make a pair), N is the rotation speed in rev/min and F is the frequency in HZ.

SINGLE PHASE GENERATOR AND ITS OUTPUT

Two Phase Generator

By adding a second stator winding to the single-phase AC generator shown in Fig. we can produce an alternator that produces two separate output voltages which will differ in phase by 90°.

This arrangement is known as a two-phase AC generator. When compared with a single-phase AC generator of similar size, a two-phase AC generator can produce more power. The reason for this is attributable to the fact that the two-phase AC generator will produce two positive and two negative pulses per cycle whereas the single-phase generator will only produce one positive and one negative pulse.

Thus, over a period of time, a multi-phase supply will transmit a more evenly distributed power and this, in turn, results in a higher overall efficiency.

TWO PHASE GENERATOR AND ITS OUTPUT

Three Phase Generator

The three-phase AC generator has three individual stator windings, as shown in Fig . The output voltages produced by the three-phase AC generator are spaced by 120° as shown in Fig.

Each phase can be used independently to supply a different load or the generator outputs can be used with a three phase distribution system.

In a practical three-phase system the three output voltages are identified by the colours red, yellow, and blue or by letters A, B, and C respectively.

THREE PHASE GENERATOR AND ITS OUTPUT

Three-Phase Generation and Distribution

Three-phase generation and distribution can be in one of the following combinations

Star Connection

In star-connected three-phase distribution system one end of all the coils (point 2) is connected together to a common point called as Neutral (N) and the other point (point 1) is the output end.

The relationship between the line and phase voltages can be determined from the phasor diagram. This diagram shows the relative directions of the three alternating phase voltages (VPh) and the voltages between the lines (VL).

It is important to note that three line voltages are 120° apart and that the line voltages lead the phase voltages by 30°.

In Star connection the Line Current is equal to Phase Current since there is a single flow current path between the line, but the Line Voltage is √3 times times the Phase Voltage.

STAR CONNECTION

Delta Connection

In Delta-connected three-phase distribution system start (point 1) of the first coil is connected to the end (point 2) of the second coil. Similarly the start (point 1) of the second coil is connected to the end (point 2) of the third coil and start (point 1) of the third coil is connected to the end (point 2) of the first coil.

In this arrangement the three line currents are 120° apart and that the line currents lag the phase currents by 30°.

In Delta connection the line Voltage is equal to phase Voltage since the Voltage is measured between same points, but the Line Current is √3 times times the Phase Current.

DELTA CONNECTION

Aircraft Generator

Aircraft ac generators range in size from the tachometer instrument generator up to the 90,000 volt-ampere generators. Generators are categorized as either brush-type or brushless. Regardless of weight, shape, or rating, practically all of these generators have the following common characteristics:

• The stator (stationary armature winding) provides the ac output.

• The ac generator field (rotor) is a rotating magnetic field with fixed polarity.

• Regulating the rpm of the rotating magnetic field controls the voltage frequency.

• Controlling the strength of the magnetic field regulates the voltage.

Brushless Alternator

This design is more efficient because there are no brushes to wear down or to arc at high altitudes. This generator consists of a pilot exciter, an exciter, and the main generator system. The need for brushes is eliminated by using an integral exciter with a rotating armature that has its AC output rectified for the main AC field, which is also of the rotating type.

The brushless generator is a more complex device but has significantly increased reliability coupled with reduced maintenance requirements.

The brushless generator can be divided into three main sections:

Permanent magnet generator
Rotating field
Three-phase output

The AC generator uses a brushless arrangement based on a rotating rectifier and permanent magnet (PMG). The output of the PMG rectifier is fed to the voltage regulator which provides current for the primary exciter field winding. The primary exciter field induces current into a three-phase rotor winding.

The output of this winding is fed to the shaft-mounted rectifier diodes which produce a pulsating DC output which is fed to the rotating field winding. It is important to note that the excitation system is an integral part of the rotor and that there is no direct electrical connection between the stator and rotor.

The output of the main three-phase generator is supplied via current transformers (one for each phase) that monitor the load current in each line. An additional current transformer can also be present in the neutral line to detect an out-of-balance condition (when the load is unbalanced an appreciable current will flow in the generator neutral connection).

The generator output is fed to the various aircraft systems and a solid-state regulator. This rectifies the output and sends a regulated direct current to the stator exciter field of the PMG. The regulator maintains the output of the generator at 115 V AC and is normally contained within a generator control unit(GCU)

BRUSHLESS ALTERNATOR

Alternator Constant Speed Drive

Alternators are not always connected directly to theairplane engine like DC generators. Since the various electrical devices operating on AC supplied by alternatorsare designed to operate at a certain voltage and ata specified frequency, the speed of the alternators must be constant; however, the speed of an airplane engine varies. Therefore, the engine, through a constant speed drive installed between the engine and the alternator,drives some alternators. The discussion of a constant speed drive system will be based on such a drive, found on large multi engine aircraft.The constant speed drive is a hydraulic transmission,which may be controlled either electrically or mechanically.

The constant speed drive assembly is designed to deliver an output of 6,000 rpm, provided the input remains between 2,800 and 9,000 rpm. If the input,which is determined by engine speed, is below 6,000rpm, the drive increases the speed in order to furnish the desired output. This stepping up of speed is known as overdrive. In overdrive, an automobile engine will operate at about the same rpm at 60 mph as it does in conventional drive at 49 mph. In aircraft, this principle is applied in the same manner. The constant speed drive enables the alternator to produce the same frequency at slightly above engine idle rpm as it would at takeoff or cruising rpm.

With the input speed to the drive set at 6,000 rpm, the output speed will be the same. This is known as straight drive and might be compared to an automobile in high gear.

However, when the input speed is greater than 6,000 rpm, it must be reduced to provide an output of 6,000 rpm. This is called underdrive, which is comparable to an automobile in low gear. Thus, the large input, caused by high engine rpm, is reduced to give the desired alternator speed. As a result of this control by the constant speed drive,the frequency output of the generator varies from 420cps at no load to 400 cps under full load.

Accordingly, the purpose of the constant speed drive may be stated as the conversion of the varying speed of the jet engine to a constant speed, so that the generator it drives will produce current at 400 Hz within narrow limits. Constant speed drive consists essentially of a hydraulic transmission with mechanical controls governing the output rotation speed. The transmission is capable of either adding to or subtracting from the speed received from the gearbox in order to provide constant output speed to keep the generator on frequency. Mechanical (flyweight) governor action keeps the generator output close to 400Hz.

CSD Functional Description

Each CSD consists essentially of two positive displacement axial slipper piston type hydraulic units, and a mechanical differential, which performs the speed summing function. The hydraulic units are the same in physical size, one unit having a variable hydraulic displacement unit and a variable angle wobbler and the other having a fixed angle wobbler and, therefore a fixed displacement. The hydraulic units rotate independently and are positioned on opposite sides of a common stationary port plate.

The variable displacement hydraulic unit runs at a fixed ratio with respect to the transmission input speed. Because the wobbler angle of the variable displacement unit is continuously variable in both directions (from full positive wobbler angle, to zero angle, to full negative wobbler angle), the displacement, of the variable displacement hydraulic unit, is continuously variable, from zero to full rated displacement in both directions. The fixed displacement hydraulic unit is driven by oil delivered by the variable displacement hydraulic unit. The fixed displacement hydraulic unit, will therefore run at any speed, from zero to full rated speed in either direction. The working pressure between the two hydraulic units is proportional to the torque being transmitted to the generator.

At the lower input speeds, the variable displacement unit acts as a hydraulic pump to supply flow to the fixed unit which is added to the input speed through the differential. At the straight through input speed, torque is transmitted directly through the differential unit and the fixed unit is not rotating. The variable displacement unit wobbler will be slightly offset from the zero angle so that some pumping will be accomplished and leakage loses made up. At input speed above straight through, the variable angle wobbler is set to allow negative displacement of the variable displacement hydraulic unit.

The working pressure, in this case, is manipulated to allow the fixed displacement hydraulic unit to be motored by the differential and subtract from the input speed. The variable displacement unit is acting as a motor. The multiple piston hydraulic unit in the mechanical differential type CSD unit handles only a portion of the power transmitted, therefore it is reduced in size. Since power loss is less in mechanical differentials than for multiple piston type hydraulic units, heat rejection is low resulting in high efficiency

Integrated Drive Generator (IDG)

Provide an elegant solution for supplying constant frequency AC electrical power to the aircraft, which can simplify the design of the complete electrical system. The IDG makes use of a highly reliable continuously variable transmission - the constant speed drive - which converts the variable input speed provided by an aircraft's engine into a constant output speed for the IDG's integral AC generator. This integration of drive and generator provides a proven and reliable solution for constant frequency electrical power.