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Magnetism and Shielding Shielding/Spark Suppression Sparking from the brushes of generators and motors now matter how slight results in the propagation of electromagnetic waves which interfere with other electronic and radio equipment. The braided screen is then grounded to...

Magnetism and Shielding Shielding/Spark Suppression Sparking from the brushes of generators and motors now matter how slight results in the propagation of electromagnetic waves which interfere with other electronic and radio equipment. The braided screen is then grounded to the main earth system, usually only once to prevent earth loops. The interference originating from the various sources can be eliminated quite ef- fectively through the use of screening and suppression. Screening involves the enclosure of a generator in a continuous metallic casing and the screening of the output supply cables in a continuous metallic tubing or conduit to prevent direct radiation. To prevent interference being conducted along the distribution cables, the screened output supply cables are terminated in filters or suppression units. These units consists of Chokes (Inductors) and Condensers (Capacitors) of suita- ble electrical rating. Oct13 / Technical Training Corresponding with Part-66 Cat: A B1 B2 3.10 - 18 Copyright by SR Technics 1 For training purposes only Figure 25: Shielding Inside Mobile Phone Screening Screening performs a similar function to bonding in that it provides a low resist- ance path for voltages producing unwanted interference to ground. Sources of electromagnetic interference include, generators, engine ignition equipment, switches, relays, and fluorescent lamps. Usual methods employed to protect against this interference includes screened cables and coaxial cables. Screened cables are usually constructed of a braided metal sheath running around the full length of the cable. They are usually grounded to the main earth system, normally at one end only to prevent earth loops. Figure 26: Coaxial Cable At the higher frequencies Coaxial cables are used as skin effect reduces the effi- ciency of screened cables. An effective Bonding System and Static Dischargers improves the effectiveness of the screening. Figure 28: Static Dischargers/Bonding Strap Figure 27: Shielded Wire 2 DC Motor/Generator Theory 3 DC Generators Figure 1: Generator Operation Energy for the operation of most electrical equipment in an airplane depends upon the electrical energy supplied by a generator. A generator is any machine, which converts mechanical energy into electrical en- ergy by electromagnetic induction. A generator, which produces direct-current energy, is called a DC generator. DC Generator Operation This basic generator principles are explained in the chapter which describes the generation of AC voltage. In a simple generator a coil is rotating in a magnetic field. Because lines of magnetic force are cut by the coil a voltage is induced in the con- ductor. To use the voltage generated in the loop for producing a current flow in an external circuit, some means must be provided to connect the loop of wire in series with the external circuit. Such an electrical connection can be effected by opening the loop of wire and connecting its two ends to two metal rings, called slip rings, against which two metal or carbon brushes ride. The brushes are connected to the exter- nal circuit. By replacing the slip rings of the basic AC generator with two half-cylinders, called a commutator, a basic DC generator is obtained.In the illustration the black side of the coil is connected to the black segment and the white side of the coil to the white segment of the commutator. The segments are insulated from each other. The two stationary brushes are placed on opposite sides of the commutator and are so mounted that each brush contacts each segment of the commutator as the latter revolves simultaneously with the loop. The rotating parts of a DC generator (coil and commutator) are called the arma- ture. The generation of an EMF by the loop rotating in the magnetic field is the same for both AC and DC generators, but the action of the commutator produces a DC volt- age. This generation of a DC voltage is described as follows for the various posi- tions of the loop rotating in a magnetic field. 4 DC Generator Operation Cont. Figure 2: Generator Operation The loop in position A is rotating clockwise, but no lines of force are cut by the coil sides and no EMF is generated. The black brush is shown coming into contact with the black segment of the com- mutator, and the white brush is just coming into contact with the white segment. In position B the flux is being cut at a maximum rate and the induced EMF is max- imum. At this time, the black brush is contacting the black segment and the white brush is contacting the white segment. The deflection of the meter is toward the right, indicating the polarity of the output voltage. At position C the loop has completed 180° of rotation. Again, no flux lines are be- ing cut and the output voltage is zero. The important condition to observe at position C is the action of the segments and brushes. The black brush at the 180° angle is contacting both black and white seg- ments on one side of the commutator, and the white brush is contacting both seg- ments on the other side of the commutator. After the loop rotates slightly past the 180° point, the black brush is contacting only the white segment and the white brush is contacting only the black segment. Because of this switching of the commutator elements, the black brush is always in contact with the coil side moving downward, and the white brush is always in contact with the coil side moving upward. Though the current actually reverses its direction in the loop in exactly the same way as in the ac generator, commutator action causes the current to flow always in the same direction through the external circuit or meter. A graph of one cycle of operation is shown. The generation of the EMF. for posi- tions A,B and C is the same as for the basic ac generator, but then, commutator action reverses the current in the cycle. The process of commutation is sometimes called rectification, since rectification is the converting of an ac voltage to a dc volt- age. At this instant that each brush is contacting two segments on the commutator (po- sitions A, C, and E), a direct short circuit is produced. If an EMF were generated in the loop at this time, a high current would flow in the circuit, causing an arc and thus damaging the commutator. For this reason, the brushes must be placed in the exact position where the short will occur when the generated EMF is zero. This position is called the neutral plane. 5 Generator Classifications Figure 4: Self-excited Generator Generators are classified according to the method by which their magnetic circuits are energized, and the following three classes are normally recognized- Permanent magnetic generators. Separately-excited generators, in which electromagnets are excited by current obtained form a separate source of d.c. Self-excited generators, in which electromagnets are excited by current produced by the machines them- selves. These generators are further classified by the manner in which the fixed windings, i.e. the electromagnetic flied and armature windings, are interconnected. In aircraft DC power supply systems, self-excited shunt-wound generators are em- ployed and the following details are therefore related only to this type. Figure 3: Separately-excited Generator 6 Self-Excited Shunt-Wound Generators End Frame Assemblies Shunt-wound generators are one of the three types in the self-excited class of ma- These assemblies are bolted one at each end of yoke and house the armature chine and as already noted are used in aircraft DC power supply systems. shaft bearings. The drive end frame provides for the attachment of the generator to the mounting pad of the engine of gearbox drive and the commutator and frame provides a mounting for the brush-gear assembly and, in the majority of cases, The term "shunt-wound" is derived from the fact that the high resistance field wind- also provides for the attachment of a cooling air duct. ing is connected across or in parallel with the armature as shown below. The ar- Inspection and replacement of brushes is accomplished by removing a strap which mature current divides into two branches, one formed by the field winding, the normally covers apertures in the commutator end frame. other by the external circuit. Since the field winding is of high resistance, the ad- vantage gained of having maximum current flow through the external circuit and Figure 5: DC Generator expenditure of unnecessary electrical energy within the generator is avoided. Generator Construction A typical self-excited shunt-wound four-pole generator, which is employed in a cur- rent type of turbo-prop civil transport aircraft, is illustrated in the figure below. It is designed to provide an output of 9 kilowatts at a continuous current of 300 am- peres (A) over the speed range of 4,500 to 8,500 rev/min. In its basic form the construction follows the pattern conventionally adopted and consists of five principal assemblies; namely, the yoke, armature, two end frames and brush-gear assembly. The Yoke The yoke forms the main housing of the generator, and is designed to carry the electromagnet system made up of the four field windings and pole pieces. It also provides for the attachment of the end frame assemblies. The windings are pre-formed coils of the required ampere-turns, wound and con- nected in series in such a manner that when mounted on the pole pieces, the po- larity of the field produced at the poles by the coil current is alternately North and South. The field windings are suitably insulated and are close fit on the pole pieces which are bolted to the yoke. The faces of the pole pieces are subjected to varying mag- netic fields caused by rotation of the armature, giving rise to induced EMF which in turn produces eddy currents through the pole pieces causing local heating and power wastage. To minimize these effects the pole pieces are of laminated con- struction; the thin soft iron laminations being oxidized to insulate and to offer high electrical resistance of the induced EMF. 7 Armature Assembly Contact between brushes and commutator is maintained by the pressure exerted The armature assembly comprises the main shaft (which may be solid or hollow) by the free ends of adjustable springs anchored to posts on the brush holders. core and main winding commutator and bearings; the whole assembly being stat- The brushes are fitted with short leads or "pigtails" of flexible copper braid moulded ically and dynamically balanced. into the brush during manufacture. In the generator shown, the shaft is hollow and internally splined to mate with The free ends of the pigtails terminate in spade or plate type terminals, which are splines of a drive shaft, which passes through the entire length of the armature connected to the appropriate main terminals of the generator via the brush holders shaft. and connecting links. Armature windings are made up of a number of individual identical coils, which fit, into slots at the outer edges of steel laminations, which form the core of the arma- Spark Suppression ture. The coils are made from copper strip and as security against displacement Sparking at the brushes of a generator, no matter how slight, results in the propa- by centrifugal force, steel wire (in some cases steel strip) is bound round the cir- gation of electromagnetic waves, which interfere with the reception of radio sig- cumference of the armature. The ends of each coil are brought out to the commu- nals. tator and silver brazed to separate segments, the finish of one coil being The interference originating in generators may be eliminated quite effectively by connected to the same segment as the beginning of another coil. screening and suppression. The complete winding thus forms a closed circuit. The windings are invariably vacuum-impregnated with silicone varnish to maintain Screening involves the enclosure of a generator in a continuous metallic casing insulation resistance under all conditions. and the sheathing of output supply cables in continuous metallic tubing or conduit to prevent direct radiation. In common with most aircraft generators, the commutator is of small diameter to To prevent interference being conducted along the distribution cable system, the minimize centrifugal stressing, and is built up of long, narrow copper segments screened output supply cables are terminated in filter or suppressor units. These corresponding in number to that of the field coils (a typical figure is 51 coils). units consist of chokes and capacitors of suitable electrical rating built into metal The segment surfaces are swept by brushes which are narrow and mounted in cases located as close to a generator as possible. pairs (usually four pairs) to maintain the brush contact area per segment - an es- Independent suppressor units are rather cumbersome and quite heavy, and it is sential pre-requisite for effective commutation. therefore the practice in the design of current types of generator to incorporate in- Brush-Gear Assembly ternal suppression systems. These systems do not normally contain chokes, but consist simply of suitably rated The brush-gear assembly is comprised of the brushes and the holding equipment capacitors which are connected between generator casing (earth) and terminals. necessary for retaining the brushes in the correct position, and at the correct angle with respect to magnetic neutral axis. The use of internal suppression systems eliminates the necessity for screened output supply cables and conduits thereby making for a considerable saving in the Brushes used in aircraft generators are the electrographite type made from artifi- overall weight of a generator installation. cial graphite. The graphite is produced by taking several forms of natural carbons, grinding them in fine powder, blending them together and consolidating the mixture into the de- sired solid shape by mechanical pressure in an electric furnace. These brushes posses both the robustness of carbon and the lubricating proper- ties of graphite. In addition they are very resistant to burning by sparking, they cause little commutator wear and their heat conductivity enables them to withstand overloads. 8 Figure 6: DC Generator 9 DC Motors. DC Motors Figure 7: DC Motor An electric motor is a device which changes electric power to mechanical energy: that is, its function is opposite to that of a generator. Electric motors and generators are very much alike in construction, and some gen- erators are actually used as motors under some conditions. This is true of engine- driven generators that are used as motors to start the engine. Certain turbojet engines employ starter-generators, which are constructed with special series field windings for use in starting. These field windings are only en- ergized during starting, and their purpose is to provide a very high starting torque. Usually it is not good practice to use a generator as a motor, because certain fea- ture which make a generator more efficient will have the opposite effect when the unit is used as a motor. Electric motors are classified in many ways; the number of different types of mo- tors is so great, however, that it would be impossible to describe them with simple classifications. There are a few basic feature which are common to all DC motors, and these will help to indicate the type of motor to be used for a specific purpose. DC motors are described in part by the type of internal winding they have. There are series-wound, shunt-wound, and compound-wound motors, named ar- mature winding. Motors of all types are usually rated according to horsepower. Usually the data plate will also show the voltage and amperage. Additional infor- mation on DC motors includes RPM, type of duty, and some other points descrip- tive of the motor design. Electric motors are used in aircraft, missiles, and spacecraft for many purposes. Among the many units and systems requiring electric motors are engine starters, cowl flaps, intercooler or heat-exchanger shutter or control valves, heat-control valves, landing gear, flaps, trim tabs, flight controls, fuel pumps, hydraulic pumps, vacuum pumps, controllable propellers, gyro-stabilizing units, navigation devices, and tracking devices. 10 Principles of DC Motors Figure 8: Force Acting on a Conductor Force Acting on a Conductor The operation of a DC motor depends on the principle that a current carrying con- ductor placed in, and at right angles to a magnetic field tends to move at right an- gles to the direction of the field. The magnetic field between a north and south pole of a magnet is shown in figure (A). The lines of force, comprising the field extend from the north pole to the south pole. A cross section of a current carrying conductor is shown in figure (B). The plus sign in the wire indicates that the electron flow is away from the observer. The direction of the flux loops around the wire is counter-clockwise, as shown. This follows from the left-hand flux rule which states that if the conductor is grasped in the left hand with the thumb extended in the direction of the current flow, the fingers will curve around the conductor in the direction of the magnetic flux. If the conductor (carrying the electron flow away from the observer) is placed be- tween the poles of the magnet, as in figure (C), both fields will be distorted. Above the wire the field is weakened, and the conductor tends to move upward. The force exerted upward depends on the strength of the field between the poles and on the Figure 9: Right-Hand Motor Rule strength of the current flowing through the wire. If the current through the conductor is reversed, as in figure (D), the direction of the flux around the wire is reversed. The field below the conductor is now weakened, and the conductor tends to move downward. A convenient method of determining the direction of motion of a current carrying conductor in a magnetic field is by the use of the right-hand motor rule. Practical DC motors depend for their operation on the interaction between the field flux and a large number of current carrying conductors. 11 Basic DC Motor Figure 10: DC Motor Operation A coil of wire through which current flows will rotate when placed in a magnetic field. This is the technical basis governing the construction of a DC motor. The figure shows a coil mounted in a magnetic field in which it can rotate. However, if the connecting wires from the battery were permanently fastened to the terminals of the coil and there was a flow of current, the coil would rotate only until it lined itself up with the magnetic field. Then, it would stop, because the torque at that point would be zero. A motor, of course, must continue rotating. It is necessary, therefore, to design a device that will reverse the current in the coil just at the time the coil becomes par- allel to the lines of force. This will create torque again and cause the coil to rotate. If the current-reversing device is set up to reverse the current each time the coil is about to stop, the coil can be made to continue rotating as long as desired. One method of doing this is to connect the circuit so that, as the coil rotates, each contact slides off the terminal to which it connects and slides on to the terminal of opposite polarity. In other words, the coil contacts switch terminals continuously as the coil rotates, preserving the torque and keeping the coil rotating. The coil terminal segments are labelled A and B. As the coil rotates, the segments slide onto and past the fixed terminals of brushes. With the arrangement, the direction of current in the side of the coil next to the north seeking pole flows toward the reader, and the force acting on that side of the coil turns it downward. The part of the motor which changes the current from one wire to another is called the commutator. 12 DC Motor Operation The commutator, likewise, contains a large number of segments instead of only two. When the coil is positioned as shown in A, current will flow from the negative ter- minal of the battery to the negative (-) brush, to segment B of the commutator, The armature in a practical motor is not placed between the poles of a permanent through the loop to segment A of the commutator, to the positive (+) brush, and magnet but between those of an electromagnet, since a much stronger magnetic then, back to the positive terminal of the battery. field can be furnished. The core is usually made of mild or annealed steel, which can be magnetized strongly by induction. The current magnetizing the electromag- By using the right-hand motor rule, it is seen that the coil will rotate counter-clock- net is from the same source that supplies the current to the armature. wise. The torque at this position of the coil is maximum, since the greatest numbers of A simple motor of the type described does not deliver a smooth flow of power be- lines of force are being cut by the coil. cause the torque is high when the armature is at right angles to the field poles and there is no torque at the moment the armature is in line with the field poles. When the coil has rotated 90° to the position shown in B, segments A and B of the commutator no longer make contact with the battery circuit and no current can flow In order to deliver smooth power, the armature is provided with additional coils so through the coil. that there will always be a high torque. At this position, the torque has reached a minimum value, since a minimum num- ber of lines of force are being cut. However, the momentum of the coil carries it beyond this position until the seg- Counter EMF and Net EMF ments again make contact with the brushes, and current against enters the coil; A conductor moving across a magnetic field will have an EMF induced within itself. this time, though, it enters through segment A and leaves through segment B. Since the conductors in the armature of a motor are cutting across a magnetic field However, since the positions of segments A and B have also been reversed, the as the armature rotates, an EMF is produced in the conductors and this EMF op- effect of the current is as before, the torque acts in the same direction, and the coil poses the current being applied to the armature from the outside source. continues its counter-clockwise rotation. This inducted voltage is called counter EMF, and it acts to reduce the amount of On passing through the positions shown in C, the torque again reaches maximum. current flowing in the armature. Continued rotation carries the coil again to a position of minimum torque, as in D. The net EMF is the difference between the applied EMF and the counter EMF. At this position, the brushes no longer carry current, but once more the momentum An engine-driven generator unit, such as an auxiliary power plant, gives an excel- rotates the coil to the point where current enters through segment B and leaves lent example of the action of the counter EMF in a generator. When battery voltage through A. is applied to the generator, it acts as a motor to start the engine. Further rotation brings the coil to the starting point and, thus, one revolution is When the engine starts and begins to run at normal speed, the counter EMF pro- completed. duced in the armature becomes greater than the applied battery voltage. The cur- The switching of the coil terminals from the positive to the negative brushes occurs rent then flows in the opposite direction and charges the battery. twice per revolution of the coil. Counter EMF plays a large part in the design of a motor. The torque in a motor containing only a single coil is neither continuous nor very Motors must be designed to operate efficiently on the net EMF, which is only a effective, for there are two positions where there is actually no toque at all. fraction of the applied EMF; hence, the resistance of the armature coils must be To overcome this, a practical DC motor contains a large number of coils wound on relatively low. the armature. These coils are so spaced that, for any position of the armature, Before a motor gains speed, the current through the armature is determined by the there will be coils near the poles of the magnet. This makes the torque both con- applied EMF and the armature resistance. tinuous and strong.  Since the armature resistance is low, the current is very high. 13 As the speed of the motor increases, the counter EMF builds up and opposes the Compound Wound DC Motors applied EMF thus reducing the current flow through the armature. Compound motors have one set of field coils in parallel with the armature circuit, This explains the facts that there is a large surge of current when a motor is first and another set of field coils in series with the armature circuit. This type of motor started and that the current then rapidly falls off to a fraction of its initial value. is a compromise between shunt and series motors. It develops an increased start- With some electric-motor installations the starting current is so high that it would ing torque over that of the shunt motor, and has less variation in speed than the overheat and damage the wiring or the armature, and so resistance must be in- series motor. serted into the circuit until the motor has gained speed. Shunt, series, and compound motors are all DC motors designed to operate from The resistance may be automatically cut out as the speed of the motor increases, constant-potential variable-current DC sources. or it may be controlled manually. Figure 11: Types of DC Motors Types of Direct Current Motors The construction of a DC motor is essentially the same as that of a DC generator. The DC generator converts mechanical energy into electrical energy back into me- chanical energy. A DC generator may be made to function as a motor by applying a suitable source of a direct voltage across the normal output electrical terminals. There are various types of DC motors, depending on the way the field coils are connected. Each has characteristics that are advantageous under given load conditions. Such loads are found in machine shop drives. They include lathes, milling machines, drills, planers, shapers, and so forth. Series Wound DC Motors Series motors have the field coils connected in series with the armature circuit. This type of motor, with constant potential applied, develops variable torque but its speed varies widely, under changing load conditions. That is, the speed is low un- der heavy loads, but becomes excessively high under light loads. Series motors are commonly used to drive electric cranes, hoists, winches, and certain types of vehicles (for example, electric trucks). Series motors are used extensively to start internal combustion engines. Shunt Wound DC Motors These DC motors have their armature coils wound in parallel with their field coils. They provide for relatively constant speed operation over a range of different size loads. However, they produce only a low starting torque. 14 Characteristics of DC Motors Figure 13: Series Motor DC motors are series-wound, shunt-wound, or compound-wound, depending upon the arrangement of the field windings with respect to the armature circuit. In a series motor the field coils are connected in series with the armature as shown. Since all the current used by the motor must flow through both the field and the armature, it is apparent that the flux of both the armature and the field will be strong. The greatest flow of current through the motor will take place when the motor is being started; hence; the starting torque will be high. A motor of this type is very useful in installations in which the load is continually Figure 14: applied to the motor and in which the load is heavy when the motor starts. In aircraft, series motors are used to operate engine starters, landing gear, cowl flaps, and similar equipment. In each case the motor must start with a fairly heavy load; the high starting torque of the series motor is particularly well suited to this condition. If a series motor is not connected mechanically to a load, the speed of the motor will continue to increase for as long as the counter EMF is substantially below the applied EMF. The speed may increase far above the normal operating speed of the motor, and this may result in the armature flying apart because of the centrifu- gal force developed by the rapid rotation. A series motor should always be con- nected mechanically to a load to prevent it from "running away". Figure 12: Per-mag Motor 15 The reason for the increase in speed when a series motor is not driving a load may Figure 15: Characteristics be understood if the behaviour of the field in such a motor is considered. As the speed of the motor increases, the counter EMF increases. As the counter EMF in- creases, however, the field current decreases. Remember that the field is in series with the armature and that since the counter EMF causes the armature current to decrease, it must necessarily cause a de- crease in the field current. This weakens the fields so that the counter EMF cannot build up sufficiently to oppose the applied voltage. A current continues to flow through both the armature and the field, and the resulting torque increases the ar- mature speed still further. This increase of speed will continue until the centrifugal force tears the armature apart, or, as is the case with very small motors, the friction and other losses in the motor balance the armature torque. In a shunt motor the field coils are connected in parallel with the armature. The shunt field must have sufficient resistance to limit the field current to that required for normal operation because the counter EMF of the armature will not act to re- duce the field current. Since the voltage applied to the field at operating speed will be practically the same as the voltage applied to the motor as a whole, regardless of counter EMF, the resistance of the field must be many times the resistance of the armature. This is usually accomplished by winding the field coils with many turns of fine wire. The result of this arrangement is that the motor will have a low starting torque be- cause of a weak field. The reason for the weak field is that the armature, owing to its low resistance, draws most of the current when the motor is first starting. As the armature of a shunt motor gains in speed, the armature current will de- crease because of counter EMF, and the field current will increase. This will cause a corresponding increase in torque until the counter EMF is almost equal to the applied EMF, at which time the motor is operating at its normal speed. This speed is almost constant for all reasonable loads. When a load is applied to a shunt motor, there is a slight reduction in speed which causes the counter EMF to decrease and the net EMF across the armature to in- crease. Since the resistance of the armature is low, a slight rise in net EMF will cause a comparatively large increase in armature current, which in turn increase the torque. This prevents a further decrease in speed and actually holds the speed to a point only slightly less than the no-load speed. 16 The current flow increases to a level sufficient to hold the speed against the in- Figure 17: Characteristics creases load. Because of the ability of the shunt motor to maintain an almost con- stant speed under a variety of loads, it is often called a constant speed motor. Shunt motors are used when the load is small at the start and increases as the mo- tor speed increases. Typical of such loads are electric fans, centrifugal pumps, and motor-generator units. When a motor has both a series field and a shunt field it is called a compound mo- tor. This type of motor combines the feature of series and shunt motors; that is, it has a strong starting torque like the series motor but will not over-speed when the load is light. This is because the shunt winding maintains a field which allows the counter EMF to increase sufficiently to balance the applied EMF. When the load on a compound motor is increased, the speed of the motor will decrease more than it does in a shunt motor, but it provides speed sufficiently constant for many practical applica- tions. Compound motors are used to operate machines subject to a wide variety of loads. In aircraft they are used to drive hydraulic pumps which may operate from a no- load condition to a maximum-load condition.  Neither a shunt motor nor a series motor would satisfactorily fulfil these require- ments. Figure 16: Characteristics 17 Practical DC Motor Construction Figure 18: DC Motor The major parts in a practical motor are the armature assembly, the field assem- bly, the brush assembly, and the end frame. Armature Assembly The armature assembly contains a laminated, soft-iron cores, coils, and a commu- tator, all mounted on a rotating steel shaft. Laminations made of stacks of soft iron, insulated from each other, form the arma- ture core. Solid iron is not used , since a solid-iron core revolving in the magnetic field would heat and use energy needlessly. The armature windings are insulated copper wire, which are inserted slots insulated with fiber paper to protect the windings. The ends of the windings are connected to the commutator segments. Wedges or steel bands hold the windings in place to prevent them form flying out of the slots when the armature is rotating at high speeds. The commutator consists of a large number of copper segments insulated from each other and the armature shaft by pieces of mica. Insulated wedge rings hold the segments in place. Field Assembly The field assembly consists of the field frame, the pole pieces, and the field coils. The field frame is located along the inner wall of the motor housing. It contains lam- inated soft steel pole pieces on which the field coils are wound. A coil, consisting of several turns of insulated wire, fits over each pole piece and, together with the pole, constitutes a field pole. Some motors have a few as two poles, others as many as eight. Brush Assembly The brush assembly consists of the brushes and their holders. The brushes are usually small blocks of graphite carbon. 18 Starter Generators Starter Generators Figure 20: Starter Generators Most small turbine engines are equipped with starter generators rather than sep- arate starters and generators. This effects an appreciable weight saving, as both starters and generators are quite heavy and they are never used at the same time. The armature of a starter generator is splined to fit into a drive pad on the engine, rather than being connected through a clutch and drive jaws as starters are. Starter generators are equipped with two or three sets of field windings. Below we have a schematic of a typical starter generator. The generator consists of an armature and a series field around the interpoles and a shunt winding for generator control. A series motor field is wound around the pole shoes inside the field frame and the end of this winding is connected to the C-terminal. For starting current from the battery or external power unit flows through the series winding and the armature._ As soon as the engine starts the start relay disconnects this winding and connects the generator circuit to the aircraft electrical system. The starter generator unit shown pictorially and schematically is basically a shunt generator with an additionally heavy series winding. This series winding is electrically connected to produce a strong field and a result- ing high torque for starting. Starter generator units are desirable from an economical standpoint since one unit performs the function of both starter and generator. Additionally the total weight of the starting system components is reduced and few- er parts are required. While acting as a starter the unit makes no practical use of its shunt field winding. A source of 24Vdc and 1500amperes is usually required for starting. When operating as a generator the shunt, compensating and commutating wind- ings are used. The series field is used only for starting purposes. The shunt field is connected in the conventional voltage control circuit for genera- tor voltage control. Compensating and commutating (interpoles) windings provide almost sparkless commutation from no load to full load. 19 AC Theory 20 AC Theory Introduction Figure 1: Alternating current, or AC is current which flows first in one direction and then in the other at regular intervals. The diagram below shows the conventional current flow (positive to negative) 'downwards' through the resistor. If we reversed the connections to the battery (Di- agram below) the current would flow 'upwards' through the resistor. If we continue to reverse the connections at regular intervals with a device such as a changeover switch, we would produce a type of AC. Assume the changeover switch is operated at 2 second intervals and that the changeover can occur instantly. Draw graphs, on the axes provided (bottom diagram below), of how you think the voltage and current will vary with time. Remember the battery voltage (3V) and the resistance value (1). Graphs, which show how current or voltage varies over a period of time, are known as waveforms. 21  Basic Maintenance Training Manual The Elementary Generator With the loop moving in this direction (black down, white up) we are assuming the polarity of the generated voltage to be positive. The elementary generator utilises the principle that electricity is produced when- ever a conductor passes through a magnetic field. Finish the diagram below by drawing in the remainder of the waveform for a com- As the armature loop rotates, electricity is produced. plete revolution of the loop. This is 'picked up' by contact between the slip rings and brushes and passed to the Keep in mind that between positions C and D, the white part of the loop will be external circuit. moving downwards and the black part upwards, different from when the loop was The 'center zero' meter can then be used to monitor output at each stage. moving between A and B. Also keep in mind that between positions D and A, the white part of the loop will The diagram below shows the armature loop in various positions; the remainder of be moving downwards and the black part moving upwards. the generator's components has been left out for clarity. Between B and C the generated voltage is reducing, until at C it reduces to zero. Assume the armature loop rotates clockwise within the magnetic field. In position A both the black and white parts of the loop are moving parallel to the magnetic This is because at this point no field is being cut. field, it doesn't cut through it, so no voltage is generated. Between C and D the voltage increases in a negative direction and by the time it As the loop rotates towards position B (90 later) maximum voltage is generated. returns to position A reduces again to zero. Notice that the black part of the loop is moving downwards through the field and The name we give to this kind of waveform is a SINE WAVE. the white part is moving upward. Figure 2: N S N S N S N S Electron Electron Flow Flow A Generator B C D Terminal Voltage A B C D E + Generator Terminal 0 180˚ 270˚ 360˚ 0˚ 90˚ Voltage - 22 Period Figure 3: When you are analysing an AC sine wave, it is often necessary to know exactly how much time is required to generate one complete cycle. The time required to produce one complete cycle is called the period of the waveform. The period of a sine wave is shown in the Fig below. The period is usual measured in seconds although other units of time can be used. Furthermore, the period is often represented by the letter T as shown. If a generator produces 1 cycle of output voltage in 1 second, the output sine wave has a period of 1 second. However, if 4 cycles are produced in 1 second, the out- put sine wave will have a period of ¼ of a second (T = 0.25 seconds). It is important to remember, that the period is the time of one cycle, and not the total time required to generate a given number of cycles. Time of a cycle is also expressed in angular notation. Remember that an armature is rotated through 360 degrees, to produce a complete sine wave. The output volt- age started at zero and increased to maximum at 90 degrees. This portion is equal to ¼ of one revolution. From maximum back to zero completes one alternation, which is ½ of one cycle. The other ½ cycle is the other alteration. Angular motion is measured in radians. A radian is approximately 57.3 degrees. A circle (360 degrees) contains exactly 2 radians. 23 The Cycle Figure 4: When the loop of our elementary generator rotated through 360 degrees, the gen- erated voltage and current completed one cycle. In other words, a cycle is a complete set of positive and negative values. Frequency The frequency of an alternating current or voltage is a measure of the number of cycles that occur in 1 second of time. In mathematical terms this is: A frequency of one cycle per second is known as 1 HERTZ (Hz). Frequency is the reciprocal of time. This relationship is shown in the following equation: F = 1/T The diagram below shows 25 cycles occurring in half a second. What is the frequency of the waveform? This value of 50 Hz is interesting in that it is the standard frequency of the domestic electricity supply. The standard frequency for aircraft electrical supplies is 400 Hz. 24 Frequency Ranges Figure 5: Frequencies that range from just a few hertz to many millions of hertz are widely used in the electronics industry. As mentioned before 50 hz is the standard frequency of the domestic electricity supply, but in many electronic applications, much higher frequencies are required. This is because high frequencies are needed to carry information or intelligence. The higher frequencies are easier to convert into electromagnetic (radio) waves. The higher frequencies can be transmitted more easily or long distances. Mechanical AC generators cannot produce these higher frequencies. Mechanical generators cannot rotate at the very high speeds required to produce frequencies such as 10Khz. To produce a frequency equal to 10Khz requires a waveform with a period of (1 divided by 10,000). The generator would have to turn at the rate of 600,000 revolutions per minute (RPM). Therefore, electronic generators are used to produce the required frequencies which would be necessary. Electronic circuits do not require moving parts and are easily capable of producing frequencies many times greater than 10Khz. When working with frequencies that extend up to many million of hertz, you must work with very large numbers. However, these large numbers can be reduced to a manageable size by using var- ious metric prefixes and position notation (powers of ten). The metric prefixes most commonly used for this purpose are defined in the table shown. The negative prefixes are included in the table because they are commonly used in electronics. However, the negative prefixes are not used to represent frequency. Remember, time and frequency are closely related. 25 Max or Peak Value RMS values are the ones most usually quoted when values of alternating current or voltage are being referred to. The maximum value of a waveform is the highest value it reaches in either a pos- Maximum values are used when the amount of conductor insulation required for itive or negative direction. If we compare the two waveforms in the diagram below electrical equipment is being considered. we can see that the DC is at a maximum whenever it is switched on, whereas the AC is at maximum only once per half cycle. All the other instantaneous values are Figure 6: Sine Wave less than peak values. Peak to Peak Value This value is twice the maximum value since it is measured between maximum positive and maximum negative (See Peak to Peak Value diagram). Effective or RMS Value The EFFECTIVE value of a sine wave is usually referred to as the RMS or ROOT MEAN SQUARE value. This refers to the mathematical process used to find the value. Simply, it is the amount of heat that a particular current can produce in a circuit, provides a convenient method of finding the effective or RMS value. If a DC current of 1 amp flows through a resistor to raise its temperature by  100° C, a sine wave AC of maximum value 1 amp would only raise its temperature by 70.7 °C. From this we can see that the sine wave AC. is only 0.707 times as effective as the 1 amp DC. This is the AC maximum value. Therefore: I RMS= 0.707 x I PEAK Or: I PEAK = I RMS / 0.707 = I RMS x 1.414 In this instance we have quoted the RMS value with respect to current I. The same would apply to voltage. VRMS = VPEAK x 0.707 26 Average Value The average value of an alternating current is obtained by averaging all the instan- taneous values over half a cycle. The average value of a half cycle of sine wave AC is 0.637 of the maximum value. VAVERAGE = VPEAK x 0.637 Although the meter deflection depends on average values, the scales are normally calibrated in RMS. So bear in mind whenever a DC Meter is used to measure AC values with a recti- fier network, the Meter responds to average values but is calibrated to read RMS. The last topic we are going to look at is the relationship between RMS and average values. For a DC waveform the average value will of course be the same as the maximum value. Figure 10: Sine Wave 27 Types of AC Waveforms Triangular waves are used as electronic signals and are seldom used to provide electrical power. Although the sine wave is the most basic and widely used AC waveform, it is not the only type of waveform that is used in electronics. Figure 11: Periodic Waveforms In fact, many different types of AC waveforms are used and these waveforms may have very simple or extremely complex shapes. The Square wave The Fig below shows two different types of non-sinusoidal waveforms, which rep- resent either current or voltage. In each case, only one cycle of the waveform is shown. Waveform Fig 11(a) is commonly referred to as a square wave. The name was selected because its positive and negative alternations are square in shape. The square shape of each alternation indicates that the voltage, or current, imme- diately increases to its maximum or peak value, at one polarity and remains there throughout that alternation. Then the voltage waveform immediately changes its polarity, or the current waveform reverses its direction. Notice that the waveform jumps to a peak value almost instantly, and remains there for the duration of the second alternation. When continuous train of these square waves is produced, the voltage or current simply continues to fluctuate back and forth between its peak values. Not all square waves are symmetrical, as shown in Fig 11(a). In some cases, the positive half may be wider or narrower (longer or shorter time duration) than the negative half. Also, some square waves may have a positive peak value that is higher or lower (amplitude variations), than its negative peak value. Although electrical power can be generated as square waves, the square wave is more useful as an electronic signal. The square wave is used to represent elec- tronic data because its characteristics can be easily varied. The Triangular wave The waveform shown in Fig 11(b) is called triangular wave because its positive and negative alternations are triangular in shape. Notice that during the positive alternation the waveform rises at a linear rate from zero to a peak value and then decreases linearly back to zero. Then, on the negative alternation its polarity, or direction, reverses. Triangular waves may have peak values that are higher or lower than those shown in Fig 11(b). In other words, the positive and negative alternations may not always form a per- fect triangle which has three equal sides. 28 RLC Circuits 29 Resistance in AC Circuits Power in AC Resistive Circuits Many AC circuits consist of 'pure resistance' only, and for such circuits the same The power used in an AC circuit is the average of all the instantaneous values of rules and laws apply as for DC circuits. power or "heating effect' in a complete cycle. Ohm's law, Kirchoff's laws, and the circuit rules for voltage, current and power can All the corresponding instantaneous values of voltage and current are multiplied be used exactly as in DC circuits. together to find the instantaneous values of power, which are then plotted for the corresponding time, to form a power wave. Current and Voltage in Resistive Circuits The average of this power wave is the actual power used in the circuit. (Diagram below). When an AC power supply is connected across a resistor, the voltage (V) and the current (I) rise and fall together in both a positive and negative direction. For "in-phase" voltages and current waves, all the instantaneous powers are above the zero axis and the entire power wave is above the zero axis. The voltage increases to a maximum in one direction, decreases to zero increases to a maximum in the opposite direction and again decreases to zero to complete Thus during the first half cycle of V and I, the power wave increases in a positive a cycle. direction from zero to a maximum and then decreases to zero, as do the V and I waves. The current follows the voltage exactly. During the next half cycle the power wave again increases in a positive direction As the voltage increases, the current increases. to maximum and then back to zero. When the voltage decreases the current decreases, and at the moment the volt- At this time V and I increase, then decrease in a negative direction. age changes polarity, so does the current. Because of this, the voltage and current waves are said to be Note that if a new axis is drawn through the power wave halfway between its max- "in-phase" (Diagram below). imum and minimum values, the power wave frequency is twice that of the voltage and current waves. To summarise, in a resistive circuit the voltages and currents are  Also notice that some of the instantaneous values of the power wave are less than "in-phase" whenever they are of the same frequency and pass through zero at the those of the current and voltage values. same time, both going in the same direction. This is because when two numbers less than one are multiplied together, the result Figure 1: is smaller than the original numbers,  e.g. 0.5V x 0.5A = 0.25W. The axis of the power wave represents the average value of power in a resistive circuit, since the shaded areas above the axis are equal to those below. Average power is the actual power used in any AC circuit. The average power is equal to half the maximum, positive power value. This value can also be found by multiplying the RMS values of V and I together, i.e. 2 x 0.707 x 4 x 0.707 = 4 approx. This is the same value as indicated for average power in Diagram below. The reason the answer is approximate is that 0.707 is only an approximation for the RMS value of a sine wave. 30 AC Inductive Circuits Effects of Frequency on Inductive Reactance If the frequency of the applied AC voltage is low, the current has more time to rise This section explains how inductance affects AC circuits. before the voltage starts dropping than it would if the frequency was high. We will begin by examining the effect that inductance has in opposing current flow. The lower the frequency the higher the current in an inductive circuit. The opposition offered by an inductance to current flow is known as "inductive re- The opposition to current flow offered by an inductance, known as inductive reac- actance". tance, depends not only on inductance but also on frequency. The formula used to calculate inductive reactance is given below: Inductive Reactance The Diagram below shows the relationship between voltage and current the in- stant after the switch is closed. XL = 2 ΠFL ohms Assume that this happens at precisely the time that the voltage passes through zero in a positive direction. where F = frequency in Hertz Initially the current will rise as the voltage rises but there will be a delay because L = Inductance in henries of the inductance present in the circuit. The delay will prevent the current from reaching the same value as it would in a Π = a constant, 3.14 DC circuit. XL = Inductive Reactance in ohms This is because, before it can reach its maximum value the AC voltage would start dropping. From this we can see that in a circuit containing inductance, a DC voltage will pro- As with resistance XL has been expressed in ohms. duce a higher current than an AC voltage of the same value. This is because it is the opposition to current flow. Figure 6: Figure 7: 31 True and Apparent Power Power Factor The power factor of a circuit is the ratio between true power in watts and apparent True Power power in volt amperes. In any AC circuit the true power consumed by a load will be found by using one of the following formulae: - TRUE POWER (WATTS) POWER FACTOR = ------------------------------------------------------ TRUE POWER = I2R = V2 / R watts or Vr x Ir APPARENT POWER (VOLT AMPERES) You can see that if it was possible for a circuit to have no resistance whatsoever then the true power would be zero or infinity. Let's examine what the power factor in a resistive circuit is. Use the values shown in the Diagram below. Apparent Power In a purely resistive circuit the true power is always equal to the apparent power The “apparent” or “wattless” power consumed by a circuit is found simply by mul- so the power factor will always be 1 or unity. tiplying the r.m.s. values of current and voltage together. The result is expressed In a purely inductive circuit the power factor will always be zero. in volt amperes or VA. We should be able to see from what's been said that power factor seems to be re- lated to the phase angle of the circuit. APPARENT POWER = V x I volt amperes or V/A When the phase angle is zero, as in a purely resistive circuit, the power factor is unity. Alternating current devices are normally rated in VA or kVA.  When the phase angle is 90 degrees, as in a purely inductive circuit, the power This is because although a device might consume only a small amount of power, factor is zero. large currents could be involved. When a circuit contains elements of both resistance and inductance the phase an- gle will be somewhere between zero and 90 degrees and the power factor will be Reactive Power somewhere between unity and zero. The actual values will depend on the exact proportions of resistance and induct- This is the calculated power to do with the reactive element either a capacitor or ance. an inductor, Reactive Power = I2 x XL or XC VARs 32 Notes/Comments Capacitors (Review) The diagram below shows the essential elements which affect capacitance. 1. Inductive reactance is the opposition to current flow that inductance offers in The plates are made of metal and the material that separates the plates is known a circuit. as the dielectric. It is calculated using the formula 2 FL.  It is made of an insulating material. Inductive reactance is measured in ohms. Three main factors determine the value of capacitance: - 2. In a purely inductive circuit the current lags the voltage by 90 degrees. 3. The phase angle between the current and voltage in a circuit containing both resistance and inductance is affected by the amount of resistance and induc- PLATE AREA (A) tive reactance in the circuit. The more “resistive” the circuit, the smaller will be the phase angle. The more “inductive” the circuit, the closer the phase angle will be to 90 de- DISTANCE BETWEEN PLATES (d) grees. 4. Current lags the voltage in an inductive circuit because the back EMF is in DIELECTRIC MATERIAL () anti-phase to the applied voltage. C = . A / d Farads Figure 18: 33 Plate Area Figure 20: The Diagram below shows that a large plate area provides more space for storing positive and negative charges. Therefore the larger the plate area the higher the capacitance. Figure 19: Equivalent or Total Capacitance When we connected resistors in series the total or equivalent resistance of the cir- cuit increased because we lengthened the resistive path. When we connected resistors in parallel the total resistance of the circuit de- creased because we introduced more paths for the current to flow through. Distance Between Plates The equivalent inductance of series and parallel circuits follows similar rules. The influence that the charged plates of a capacitor have on each other is affected Let's examine total capacitance of capacitors in series and parallel. by the distance between them. The further the plates are apart the less the influence they have. We say that the capacitance decreases as the distance between the plates in- creases. Dielectric Material The dielectric material used to separate the plates also influences capacitance. The Diagram below shows two capacitors with identical size plates and the same distance between their plates. The capacitor with a material like mica as a dielectric has a higher capacitance than the one using air. 34 Series Connected Capacitors To calculate the total capacitance of capacitors in series, use the following formula and then transpose for total capacitance Ct. 1 / Ct = 1 / C1+1 / C2 … etc. Parallel Connected Capacitors To calculate the total capacitance of capacitors in parallel, use the following for- mula: Ct = C1 + C2 …etc So, when capacitors are connected in series or parallel the effect on the total ca- pacitance is exactly opposite to the effect for resistors connected in the same way. 35 Capacitive Reactance Effect of Frequency on Capacitive Reactance Capacitive reactance is the opposition to current flow that a capacitor offers. Let's consider operating the reversing switch in the circuit in the previous Diagram We have seen that in a DC capacitive circuit current only flows while charging or back and forth, first quickly and then less quickly. discharging is taking place. When the switch is operated quickly or at high frequency, a higher average current Once the capacitor is charged or discharged, current flow ceases. Since an AC will flow because the current is reversed before it has a chance to fall to a lower voltage alternately changes direction, current will flow back and forth as the capac- level. itor is charged and discharged. When the switch is operated at a lower frequency, the current is allowed to drop to The value of the current that flows is determined by the amount of capacitive re- a lower value before reversal takes place. actance in the circuit. This produces a lower average current. See Diagram below. To examine capacitive reactance further, let's first look at current in a capacitive circuit. We can summarise this by saying; the higher the frequency the higher the current, and therefore the lower the capacitive reactance. Current in a Capacitive Circuit Figure 24: Let's look at the current when the switch in our circuit in Diagram below is posi- tioned to A. At this instant, current flows at a maximum rate to charge the capacitor. The rate will reduce as the capacitor becomes closer to being fully charged, until it finally falls to zero. Should we now reverse the battery terminals by positioning the switch to B, current will flow at a maximum rate in the opposite direction as the capacitor discharges and then charges with the opposite polarity. The current will reduce to zero as be- fore, as the capacitor becomes charged with opposite polarity. Figure 23: 36 Effect of Capacitance on Capacitive Reactance Figure 25: To examine the effect that capacitance has on capacitive reactance it is conven- ient to recall the time constant of a capacitive circuit. The time constant determines the time it takes for charged current to decay. The higher the time constant the longer is the decay period. If the capacitance of a circuit is increased, the time constant increases. See that in Fig. below graph left has the highest time constant, capacitance, and average current. From this we can see that increasing the capacitance will cause the average cur- rent to increase and therefore the capacitive reactance to decrease. Capacitive Reactance Is Inversely Proportional To Capacitance We have now determined that both capacitance and frequency are inversely pro- portional to capacitive reactance. The formula used to calculate capacitive reactance is: Xc = 1 / 2FC where: F is frequency in Hertz, C is capacitance in farads,  is a constant, 3.14 Since Xc represents opposition to current flow, it is expressed in ohms just like re- sistance. 37 Phase Relationship - Current and Voltage in Capacitive Circuits Figure 26: The phase relationship between current and voltage in an AC capacitive circuit is exactly opposite to that of an AC inductive circuit. In a purely inductive circuit the current lags the voltage by 90°, whilst in a purely capacitive circuit the current leads the voltage by 90° A convenient aid to memorising the phase relationship between current and volt- age in purely inductive and capacitive circuits is the word CIVIL: CIVIL Capacitive Current Voltage Current Inductive In a theoretical circuit of pure capacitance, the voltage across the capacitor exists only after current flows to charge the plates. At the instant a capacitor starts to charge, the voltage across its plates is zero and the current flow is maximum. As the capacitor charges the current drops to zero whilst the voltage rises to its maximum value. When full charge is reached the current is zero and the voltage maximum. During discharge the current starts at zero and rises to a maximum in the opposite direction whilst the voltage falls from maximum to zero. From this we can see that the current leads the voltage by 90° as shown in Diagram below. Consider the phase relationship between current and voltage in a circuit that has equal proportions of resistance and capacitive reactance Diagram below. We should bear in mind that the current and voltage in a purely resistive circuit are in phase and in a purely capacitive circuit the current leads the voltage by 90°. It seems reasonable that the phase angle should be leading by an amount between zero and 90°. In this case we have equal proportions of resistance and capacitive reactance and we can assume that the phase angle will be half of 90°, i.e. 45°. 38 39 Series AC Circuits Voltage Whenever current and voltage are being referred to, remember that it's always the Introduction RMS values that are being considered, unless specifically stated otherwise. Series AC circuits may be formed from the various combinations of resistance, in- In a series AC circuit the total voltage V cannot be found by adding the individual ductance, and capacitance. voltages VR, VL and VC together because they will not be in phase with each other, Every circuit contains a certain amount of resistive, inductive and capacitive com- as shown in diagram below. ponents, but in some cases individual components can be so small as to be ig- To determine voltage values in AC series circuits it is necessary to learn about nored. something known as Vector or Phasor representation. You have seen how R, L and C individually affect AC current, phase angle and power. Now we are going to find out how various combinations of R, L and C affect series VS2 = VR2 + VX2 AC circuits. Current VS = √ VR2 + VX2 In a series circuit such as the one shown in the Diagram below there is only one path through which current can flow, regardless of which types of component the Where VX = VL - VC or VC - VL circuit contains. The current will be the same in all parts of the circuit therefore all phase angles will Figure 30: be measured with respect to the circuit current Figure 29: 40 Vector Representation of AC Values Series R and L Circuits A vector quantity is one that has magnitude and direction. Vector Diagrams If we said that an aircraft was travelling due east at 300 knots, we could represent this velocity by a line drawn to scale horizontally on the page as shown in Diagram Let's see in the Diagram below how we represent current and voltage for circuits below. containing pure resistance, inductance, and capacitance, using phasor diagrams. We can represent AC values of voltage, current, and power in a similar way with Since the current and voltage are in phase both the I and the VR vectors are drawn what are known as vectors. horizontally. If we wanted to represent current in an AC series circuit we draw a line whose The VL phasor leads the I phasor by 90°. length represents the value of the current, horizontally from left to right. The voltage phasor is therefore drawn vertically upwards. It is conventional that the horizontal is used for the vector, which represents the value, which is common to all parts of the circuit. The voltage VC lags I by 90°. The voltage phasor is therefore drawn vertically downwards. In a series circuit it is the current, and is often referred to as the "reference" vector. Figure 32: CIVIL In a Capacitive ac circuit, circuit current leads the applied Voltage by 90º. In an Inductive ac circuit, the circuit current lags the applied voltage by 90º. Figure 31: 300 kts 41 3.15 Transformers 42 Transformers - Introduction Figure 1: Typical Transformers and Their Symbols One main advantage that alternating current has over direct current is that voltag- es can be raised or lowered using devices known as transformers. Transformers are used for many commercial and domestic uses. They are used extensively in aircraft systems and can be found: 1. In Lighting circuits. 2. As power supplies within a variety of Avionic equipment. 3. For providing 26 volt AC Instrument power supplies. 4. As a main component in Transformer Rectifier units. 43 Transformer Principle Figure 2: When an alternating current flows through a coil, an alternating field expands and contracts with it. When the alternating magnetic field cuts through the adjacent turns of the coil a back EMF is produced opposing the change in current. Exercise: What happens if we place two coils close together, but insulated from each other, and pass an alternating current through one of them? Study the Diagram below and comment on what you think is going to happen to the centre zero voltmeter in each of the two cases. Imagine that the voltage is alternating in 'slow motion'. Give reasons for your answer. Comments on Exercise The voltmeter indicates first in one direction and then the other. The field that was moving it not only cut its own turns but also the turns of the other coil. In doing this, an EMF was induced into this coil, first of one polarity as the field ex- pands, and then of opposite polarity as the field contracts. The two coils in our ex- ample formed a simple transformer. So, a transformer transfers electrical power from one coil to another. We call this 'mutual inductance'. 44 Turns Ratio Figure 3: Turns Ratio The diagram below shows a simple transformer with one winding of 100 turns con- nected to a 240 volt AC supply. This winding is called the 'primary' winding. The other winding has 1000 turns and can provide us with a voltage of a different value from the supply voltage. This winding is called the 'secondary' winding. The voltage produced by the sec- ondary winding depends upon the turns ratio. In our example the turns ratio is 10. This is because there are 100 turns on the pri- mary and 1000 turns on the secondary. numbers of turns on secondary Turns ratio = ---------------------------------------- numbers of turns on primary  1000 = ------------ 100 = 10 The secondary voltage VS will equal the primary voltage VP x the turns ratio. In our case 10 x 240 = 2400 volts. Since this transformer causes an increase in voltage it is known as a 'step up' transformer. 45 Phase Relationship Figure 5: A phase shift of 180 degrees takes place across a transformer, as shown in dia- gram below. This is because the EMF induced in the secondary is as a result of a back EMF which is in opposition to the change producing it. Reversal of the output or secondary terminals will bring the primary and secondary voltage in phase. Figure 4: Exercise: Determine from the diagram below what the secondary voltage will be. What do you think would be a suitable name for this type of transformer? Figure 6: 46 Comments on Exercise: Exercise: The secondary voltage is 24 volts. Look at diagrams A and B and determine: In this case the turns ratio was: 1. Which transformer is a step up transformer and which is a step down trans- 10 / 100 = 1 / 10 former? 2. Which transformer has a high secondary voltage and which has a low second- ary voltage? therefore the secondary voltage would be 3. Which transformer has a high secondary current and which has a low second- ary current? 1 / 10 x 240 = 24 volts Figure 7: Since we dropped the voltage, a 'step down' transformer would be a suitable name. Relationship between Primary and Secondary Power A transformer transfers electrical power from the primary to the secondary by mu- tual induction. It transfers this power with very little loss. Assume for the moment it does this with no loss. If we can transfer power from the primary circuit to the secondary circuit with no loss of power we say: PRIMARY POWER = SECONDARY POWER Since power is voltage x current we represent primary power as  Vp x Ip and we represent secondary power as Vs x Is. Since primary power equals secondary power we say: Vp x Ip = Vs x Is Comments on Exercise: 47  Transformer Construction 1. Diagram A shows a step down transformer, diagram B shows a step up Transformers have their windings wound on cores. The cores are made of a suit- transformer. able ferromagnetic material to provide an easy path for the magnetic field. 2. The transformer in diagram has a low secondary voltage and the trans This allows a better magnetic 'linkage' between the primary and secondary wind- former in diagram B has a high secondary voltage ings. Some examples of core construction are shown in diagram below. The transformer in diagram A has a high secondary current and the transformer in Figure 8: diagram B has a low secondary current. It's important to note that the terms 'step up' and 'step down' refer only to the volt- age and not the current. 48 Transformer Losses Hysteresis Transformers transfer power from the primary winding to the secondary winding During each cycle of alternating current, the transformer core is taken through a with very little loss. cycle of magnetising in one direction, demagnetising and then magnetising in the Transformer efficiency is approximately 97 or 98%. other direction. The energy expended in demagnetising the core is 'lost' energy. Using a core ma- terial, such as silicon steel, which can be demagnetised easily, can reduce hyster- Power Out esis losses. Efficiency = ------------------------------------ x 100% Figure 10: Power Out +Power Losses Transformer losses can be divided into IRON and COPPER losses. Iron losses are losses developed in the core and copper losses are developed in the windings. Let's first look at iron losses, which consist of; magnetic leakage, hysteresis and eddy current losses. Magnetic Losses In a practical transformer a small number of the flux lines do not complete their cir- cuit through the core but take shorter paths as shown in diagram below. Note the primary and secondary flux leakage. Magnetic or flux leakage can be reduced by winding the coils one over the other and by careful design of the transformer core. Figure 9: Eddy Current Losses The alternating magnetic field in a transformer can induce 'eddy' currents in the iron core. These eddy currents cause energy to be lost as heat. The eddy currents can be reduced by building up the core with laminations insu- lated with a resin. The effect on eddy currents is shown in diagram below. 49 Figure 11: Copper Losses Copper losses are caused by the resistance of the primary and secondary wind- ings. Current flow in the windings causes energy to be lost in the form of heat. The amount of heat loss will be a function of I2 R. Copper losses can be reduced by ensuring that the resistance of the windings is as low as possible. Effect of Frequency on Transformers The inductive effect of the windings depends upon the frequency of the supply, i.e., INDUCTIVE REACTANCE XL= 2 π F L ( A transformer is designed for use at a particular frequency and any attempt to run it at a lower frequency will increase the current and the transformer is liable to over- heat or even 'burn out'. If operated at frequencies above the rated limit, eddy currents, hysteresis losses and winding reactance increase and transformer efficiency is reduced. 50 Servicing Never allow a current transformer to operate without a secondary load. If the nor- mal load has to be disconnected, provide an alternative load or even a short circuit Transformers require little attention, but air cooled types must be kept free from before allowing primary current to flow. dust or dirt, as this is liable to give rise to overheating. They should never be covered as this will interfere with cooling. Periodic insulation Figure 18: tests between windings and between each winding and the core should be carried out, and the output voltage at various loads should be verified with an accurate voltmeter. Current Transformers Current transformers are used wherever large currents have to be monitored. Aircraft generator feeders, for example, carry currents too large to be connected directly to an ammeter, the current coil of a wattmeter, or control and protection circuits within generator control units. The diagram below shows an ammeter supplied by the secondary of a current transformer. The turns ratio is arranged to give full scale deflection when full load current flows in the primary. If the ammeter gives full scale deflection with 5 amps, and the primary full load cur- rent is 50 amps, a turns ratio of 20 would be required if there are two primary turns. Current transformers having a single turn primary are usually arranged as shown in diagram below, where P represents the primary conductor passing through the centre of a laminated iron ring C. The secondary winding S is wound around the ring. This type of current transformer is known as a bar-primary current transformer. Sealed current transformer assemblies are very common on modern aircraft. They utilise generator feeder cables as the primary. The secondaries are contained within the sealed assembly. The assembly has a plug and socket type connector. A current transformer assembly can contain numerous secondary windings; up to eight is not uncommon. The outputs of current transformer assemblies often provide current information for control and protection circuits inside generator control units. The diagram bel