Copper Copper lead lead Thermocouple Heater (resistance wire) FIGURE 8-258. Simplified diagram of a thermocouple meter.
flow through the coil. Therefore, the inclined-coil, iron-vane meter can be used to measure either alternating current or direct current. The aluminum disk and the drag magnets provide electromagnetic damping.
Like the moving iron-vane meter, the inclinedcoil type requires a relatively large amount of current for full-scale deflection and is seldom used in high-resistance low-power circuits.
As in the moving iron-vane instruments, the inclined-coil instrument is wound with few turns of relatively large wire when used as an ammeter and with many turns of small wire when used as a voltmeter.
Thermocouple Meter If the ends of two dissimilar metals are welded together and this junction is heated, a d.c. voltage is developed across the two open ends. The voltage developed depends on the material of which the wires are made and on the difference in temperature between the heated junction and the open ends.
In one type of instrument, the junction is heated electrically by the flow of current through a heater element. It does not matter whether the current is alternating or direct because the heating effect is independent of current direction. The maximum current that can be measured depends on the current rating of the heater, the heat that the thermocouple can stand without being damaged, and on the current rating of the meter used with the thermocouple. Voltage can also be measured if a suitable resistor is placed in series with the heater. In meter applications, a D'Arsonval meter is used with a resistance wire heater, as shown in figure 8-258.
As current flows through the resistance wire, the heat developed is transferred to the contact point and develops an e.m.f. which causes current to flow through the meter. The coil rotates and causes the pointer to move over a calibrated scale.
The amount of coil movement is dependent on the amount of heat, which varies as the square of the current. Thermocouple meters are used extensively in a.c. measurements.
Varmeters Multiplying the volts by the amperes in an a.c.
circuit gives the apparent power: the combination of the true power which does the work and the reactive power which does no work and is returned to the line. Reactive power is measured in units of vars (volt-amperes reactive) or kilovars (kilovolt-amperes reactive, abbreviated KVAR). When properly connected, wattmeters measure the reactive power. As such, they are called varmeters.
The illustration in figure 8-259 shows a varmeter connected in an a.c. circuit.
Wattmeter Electric power is measured by means of a wattmeter. Because electric power is the product of current and voltage, a wattmeter must have two elements, one for current and the other for voltage, as indicated in figure 8-260. For this reason, wattmeters are usually of the electrodynamometer type.
The movable coil with a series resistance forms the voltage element, and the stationary coils constitute the current element. The strength of Var meter Load Voltage coil i Reactor мої • Current I coil FIGURE 8-259. A varmeter connected in an
a.c. circuit.
Potential coil Current coils Load FICURE 8-260. Simplified electrodynamometer wattmeter circuit.
the field around the potential coil depends on the amount of current that flows through it. The current, in turn, depends on the load voltage applied across the coil and the high resistance in series with it. The strength of the field around the current coils depends on the amount of current flowing through the load. Thus, the meter deflection is proportional to the product of the voltage across the potential coil and the current through the current coils. The effect is almost the same (if the scale is properly calibrated) as if the voltage applied across the load and the current through the load were multiplied together.
If the current in the line is reversed, the direction of current in both coils and the potential coil is reversed, the net result is that the pointer continues to read up-scale. Therefore, this type of wattmeter can be used to measure either a.c.
or d.c. power.
FREQUENCY METERS Alternating-current electrical equipment is designed to operate within a given frequency range.
In some instances the equipment is designed to operate at one particular frequency, as are electric clocks and time switches. For example, electric clocks are commonly designed to operate at 60
c.p.s. If the supply frequency is reduced to 59
c.p.s., the clock will lose one minute every hour.
Transformers and a.c. machinery are designed to operate at a specified frequency. If the supply frequency falls more than 10 percent from the rated value, the equipment may draw excessive current, and dangerous overheating will result.
It is, therefore, necessary to control the frequency of electric power systems. Frequency meters are employed to indicate the frequency so that corrective measures can be taken if the frequency varies beyond the prescribed limits.
Frequency meters are designed so that they will not be affected by changes in voltage. Because
a.c. systems are designed to operate normally at one particular frequency, the range of the frequency meter may be restricted to a few cycles on either side of the normal frequency. There are several types of frequency meters, including the vibrating-reed type, the fixed-coil and movingcoil type, the fixed-coil and moving-disk type, and the resonant-circuit type. Of these types, the vibrating-reed frequency meter is used most often in aircraft systems, and is discussed in some detail.
Vibrating-Reed Frequency Meter The vibrating-reed type of frequency meter is one of the simplest devices for indicating the frequency of an a.c. source. A simplified diagram of one type of vibrating-reed frequency meter is shown in figure 8-261.
The current whose frequency is to be measured flows through the coil and exerts maximum attraction on the soft-iron armature twice during each cycle (A of figure 8-261). The armature is attached to the bar, which is mounted on a flexible support. Reeds of suitable dimensions to have natural vibration frequencies of 110, 112, 114, and so forth up to 130 c.p.s. are mounted on the bar (B of figure 8-261). The reed having a frequency of 110 c.p.s. is marked "55" cycles; the one having a frequency of 130 c.p.s. is marked "65" c.p.s.; the one having a frequency of 120
c.p.s. is marked "60" c.p.s., and so forth.
In some instruments the reeds are the same lengths, but are weighted by different amounts at the top so that they will have different natural rates of vibration.
When the coil is energized with a current having a frequency between 55 and 65 c.p.s., all the reeds are vibrated slightly; but the reed having a 385
Coil Soft-iron. armature 55 cycles 60 cycles 65 cycles — Reed _ Reeds Bar Current whose frequency Flexible is to be measured support Bar A B Circuit Reeds 55 56 57 58 60 61 62 63 64 65 Cycles per second Vibrating reed Indicator dial FIGURE 8-261. Simplified diagram of a vibrating-reed frequency meter.
natural frequency closest to that of the energizing current (whose frequency is to be measured) vibrates through a larger amplitude. The frequency is read from the scale value opposite the reed having the greatest amplitude of vibration.
An end view of the reeds is shown in the indicator dial (C of figure 8-261). If the energizing current has a frequency of 60 c.p.s., the reed marked "60" c.p.s. will vibrate the greatest amount, as shown.
CHAPTER 9
AIRCRAFT GENERATORS AND MOTORS D. C. GENERATORS 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 energy by electromagnetic induction. A generator designed to produce alternating-current energy is called an a.c. generator, or alternator; a generator which produces directcurrent energy is called a d.c. generator. Both types operate by inducing an a.c. voltage in coils by varying the amount and direction of the magnetic flux cutting through the coils.
For airplanes equipped with direct-current electrical systems, the d.c. generator is the regular source of electrical energy. One or more d.c.
generators, driven by the engine, supply electrical energy for the operation of all units in the electrical system, as well as energy for charging the battery.
The number of generators used is determined by the power requirement of a particular airplane.
In most cases, only one generator is driven by each engine, but in some large airplanes, two generators are driven by a single engine. Aircraft equipped with alternating-current systems use electrical energy supplied by a.c. generators, also called alternators.
Theory of Operation In the study of alternating current, basic generator principles were introduced to explain the generation of an a.c. voltage by a coil rotating in a magnetic field. Since this is the basis for all generator operation, it is necessary to review the principles of generation of electrical energy.
When lines of magnetic force are cut by a conductor passing through them, voltage is induced in the conductor. The strength of the induced voltage is dependent upon the speed of the conductor and the strength of the magnetic field. If the ends of the conductor are connected to form a complete circuit, a current is induced in the conductor. The conductor and the magnetic field make up an elementary generator. This simple generator is illustrated in figure 9-1, together with the components of an external generator circuit which collect and use the energy produced by the simple generator. The loop of wire (A and B of figure 9-1) is arranged to rotate in a magnetic field. When the plane of the loop of wire is parallel to the magnetic lines of force, the voltage induced in the loop causes a current to flow in the direction indicated by the arrows in figure 9-1. The voltage induced at this position is maximum, since the wires are cutting the lines of force at right angles and are thus cutting more lines of force per second than in any other position relative to the magnetic field.
As the loop approaches the vertical position shown in figure 9-2, the induced voltage decreases because both sides of the loop (A and B) are approximately parallel to the lines of force and Pole piece Armature N Slip ring Load Brush FIGURE 9-1. Inducing maximum voltage in an elementary generator.
FIGURE 9-2. Inducing minimum voltage in an elementary generator. FIGURE 9-4. Inducing a minimum voltage in the opposite direction.
the rate of cutting is reduced. When the loop is To use the voltage generated in the loop for vertical, no lines of force are cut since the wires producing a current flow in an external circuit, are momentarily traveling parallel to the magnetic some means must be provided to connect the loop lines of force, and there is no induced voltage. of wire in series with the external circuit. Such an As the rotation of the loop continues, the number electrical connection can be effected by opening of lines of force cut increases until the loop has the loop of wire and connecting its two ends to rotated an additional 90° to a horizontal plane. As two metal rings, called slip rings, against which shown in figure 9-3, the number of lines of force two metal or carbon brushes ride. The brushes cut and the induced voltage once again are maxi- are connected to the external circuit.
mum. The direction of cutting, however, is in By replacing the slip rings of the basic a.c.
the opposite direction to that occurring in figures generator with two half-cylinders, called a com- 9-1 and 9-2, so the direction (polarity) of the mutator, a basic d.c. generator (figure 9-6), is induced voltage is reversed. obtained. In this illustration the black side of the As rotation of the loop continues, the number of coil is connected to the black segment and the lines of force having been cut again decreases, white side of the coil to the white segment. The and the induced voltage becomes zero at the segments are insulated from each other. The two position shown in figure 9-4, since the wires A and stationary brushes are placed on opposite sides B are again parallel to the magnetic lines of force. of the commutator and are so mounted that each If the voltage induced throughout the entire brush contacts each segment of the commutator 360° of rotation is plotted, the curve shown in as the latter revolves simultaneously with the figure 9-5 results. This voltage is called an alter- loop. The rotating parts of a d.c. generator (coil nating voltage because of its reversal from positive and commutator) are called an armature.
to negative values-first in one direction and then The generation of an e.m.f. by the loop rotating in the other. in the magnetic field is the same for both a.c. and
d.c. generators, but the action of the commutator produces a d.c. voltage. This generation of a d.c.
voltage is described as follows for the various + Voltage Maximum
- 360°. -1 Cycle - Voltage 90° 180° 270° 360° Minimum FIGURE 9-3. Inducing maximum voltage in the Maximum opposite direction. FIGURE 9-5. Output of an elementary generator.
FIGURE 9-6. Basic d.c. generator.
positions of the loop rotating in a magnetic field, with reference to figure 9-7.
The loop in position A of figure 9-7 is rotating clockwise, but no lines of force are cut by the coil sides and no e.m.f. is generated. The black brush is shown coming into contact with the black segment of the commutator, and the white brush is just coming into contact with the white segment.
In position B of figure 9-7, the flux is being cut at a maximum rate and the induced e.m.f. is maximum. 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 of figure 9-7, the loop has completed 180° of rotation. Again, no flux lines are being 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 segments on one side of the commutator, and the white brush is contacting both segments 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 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 moying upward. Though the current actually reverses its direction in the loop in exactly the same way as in the a.c. 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 in figure 9-7. The generation of the e.m.f. for positions A, B, and C is the same as for the basic
a.c. generator, but at position D, commutator D Induced EMF
- 1 Revolution FIGURE 9-7. Operation of a basic d.c. generator.
action reverses the current in the external circuit, and the second half-cycle has the same waveform as the first half-cycle. The process of commutation is sometimes called rectification, since rectification is the converting of an a.c. voltage to a d.c.
voltage.
At the instant that each brush is contacting two segments on the commutator (positions A, C, and E in figure 9-7), a direct short circuit is produced. If an e.m.f. 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 e.m.f. is zero. This position is called the neutral plane.
The voltage generated by the basic d.c. generator in figure 9-7 varies from zero to its maximum value twice for each revolution of the loop. This variation of d.c. voltage is called "ripple," and may be reduced by using more loops, or coils, as shown in A of figure 9-8. As the number of loops is increased, the variation between maximum and minimum values of voltage is reduced (B of figure 9-8), and the output voltage of the generator approaches a steady d.c. value. In A of figure 9-8 the number of commutator segments is increased in direct proportion to the number of loops; that is, there are two segments for one loop, four segments for two loops, and eight segments for four loops.
The voltage induced in a single-turn loop is small. Increasing the number of loops does not increase the maximum value of generated voltage, but increasing the number of turns in each loop will increase this value. Within narrow limits, the output voltage of a d.c. generator is determined by the product of the number of turns per loop, the total flux per pair of poles in the machine, and the speed of rotation of the armature.
An a.c. generator, or alternator, and a d.c.
generator are identical as far as the method of generating voltage in the rotating loop is concerned. However, if the current is taken from the loop by slip rings, it is an alternating current, and the generator is called an a.c. generator, or alternator. If the current is collected by a commutator, it is direct current, and the generator is called a d.c. generator.
Construction Features of D. C. Generators Generators used on aircraft may differ somewhat in design, since they are made by various manufacturers. All, however, are of the same general construction and operate similarly. The major parts, or assemblies, of a d.c. generator are a field frame (or yoke), a rotating armature, and a brush assembly. The parts of a typical aircraft generator are shown in figure 9-9.
Field Frame The field frame is also called the yoke, which is the foundation or frame for the generator. The frame has two functions: It completes the magnetic circuit between the poles and acts as a mechanical support for the other parts of the D E generator. In A of figure 9-10, the frame for a two-pole generator is shown in a cross-sectional view. A four-pole generator frame is shown in B of figure 9-10.
In small generators, the frame is made of one · piece of iron, but in larger generators, it is usually Revolutions. made up of two parts bolted together. The frame has high magnetic properties and, together with the pole pieces, forms the major part of the magnetic circuit. The field poles, shown in figure 9-10, FIGURE 9-8. Increasing the number of coils are bolted to the inside of the frame and form a reduces the ripple in the core on which the field coil windings are mounted.
voltage. The poles are usually laminated to reduce eddy 390
Connector lugs Field Drive end frame frame Air scoop Brush Sealed connector ball bars „bearings Commutator Steel ring- Commutator Drive shaft end frame Brush and holder Screw Field winding Pole shoe Field frame Armature Drive end FIGURE 9-9. Typical 24-volt aircraft generator.
current losses and serve the same purpose as the iron core of an electromagnet; that is, they concentrate the lines of force produced by the field coils. The entire frame including field poles, is made from high-quality magnetic iron or sheet steel.
A practical d.c. generator uses electromagnets instead of permanent magnets. To produce a magnetic field of the necessary strength with permanent magnets would greatly increase the physical size of the generator.
The field coils are made up of many turns of insulated wire and are usually wound on a form which fits over the iron core of the pole to which it is securely fastened (figure 9-11). The exciting current, which is used to produce the magnetic field and which flows through the field coils, is obtained from an external source or from the generated d.c. of the machine. No electrical connection exists between the windings of the field coils and the pole pieces.
Most field coils are connected in such a manner that the poles show alternate polarity. Since there is always one north pole for each south pole, there must always be an even number of poles in any generator.
Note that the pole pieces in figure 9-10 project from the frame. Because air offers a great amount of reluctance to the magnetic field, this design reduces the length of the air gap between the poles and the rotating armature and increases the efficiency of the generator. When the pole pieces are made to project as shown in figure 9-10, they are called salient poles.
Armature The armature assembly consists of armature coils wound on an iron core, a commutator, and associated mechanical parts. Mounted on a shaft, it rotates through the magnetic field produced by the field coils. The core of the armature acts as an iron conductor in the magnetic field and, for this reason, is laminated to prevent the circulation of eddy currents.
There are two general kinds of armatures: the ring and the drum. Figure 9-12 shows a ring-type armature made up of an iron core, an eightsection winding, and an eight-segment commutator. This kind of armature is rarely used; most generators use the drum-type armature.
A drum-type armature (figure 9-13) has coils 391
placed in slots in the core, but there is no electrical connection between the coils and core. The use of slots increases the mechanical safety of the armature. Usually, the coils are held in place in the slots by means of wooden or fiber wedges.
The connections of the individual coils, called coil ends, are brought out to individual segments on the commutator.
Flux Frame FIGURE 9-11. A field coil removed from a field pole.
ing flanges fitted with bolts. Rings of mica insulate Pole the segments from the flanges. The raised portion Field of each segment is called a riser, and the leads winding from the armature coils are soldered to the risers.
When the segments have no risers, the leads are soldered to short slits in the ends of the segments.
The brushes ride on the surface of the commutator, forming the electrical contact between the armature coils and the external circuit. A flexible, braided-copper conductor, commonly called a pigtail, connects each brush to the external circuit.
Flux Frame The brushes, usually made of high-grade carbon and held in place by brush holders insulated from the frame, are free to slide up and down in their holders in order to follow any irregularities in the surface of the commutator. The brushes are usually adjustable so that the pressure of the brushes on the commutator can be varied and the position Pole of the brushes with respect to the segments can be adjusted.
'Field The constant making and breaking of conwinding nections to the coils in which a voltage is being B FIGURE 9-10. A two-pole and a four-pole frame assembly.
N Commutators Figure 9-14 shows a cross-sectional view of a typical commutator. The commutator is located at the end of an armature and consists of wedgeshaped segments of hard-drawn copper, insulated from each other by thin sheets of mica. The seg- FIGURE 9-12. An eight-section, ring-type ments are held in place by steel V-rings or clamp- armature.
Commutator Slots Shaft oils FIGURE 9-13. A drum-type armature.
induced necessitates the use of material for brushes which has a definite contact resistance.
Also, this material must be such that the friction between the commutator and the brush is low, to prevent excessive wear. For these reasons, the material commonly used for brushes is high-grade carbon. The carhon must be soft enough to prevent undue wear of the commutator and yet hard enough to provide reasonable brush life. Since the contact resistance of carbon is fairly high, the brush must be quite large to provide a large area of contact. The commutator surface is highly polished to reduce friction as much as possible.
Oil or grease must never be used on a commutator, and extreme care must be used when cleaning it to avoid marring or scratching the surface.
Tightening nut .. Iron ring Mica V-ring
•Front V-ring Commutator bars
- Commutator bar Mica Mica insulation - Iron shell between bars Back V-ring Slots with mica inner and outer rings for insulation FIGURE 9-14. Commutator with portion removed to show construction.
TYPES OF D. C. GENERATORS There are three types of d.c. generators: serieswound, shunt-wound, and shunt-series or compound wound. The difference in type depends on the relationship of the field winding to the external circuit.
Series-Wound D. C. Generators The field winding of a series generator is connected in series with the external circuit, called the load (figure 9-15). The field coils are composed of a few turns of large wire; the magnetic field strength depends more on the current flow rather than the number of turns in the coil. Series generators have very poor voltage regulation under changing load, since the greater the current through the field coils to the external circuit, the greater the induced e.m.f. and the greater the terminal or output voltage. Therefore, when the load is increased, the voltage increases; likewise, when the load is decreased, the voltage decreases.
The output voltage of a series-wound generator may be controlled by a rheostat in parallel with the field windings, as shown in A of figure 9-15.
Since the series-wound generator has such poor regulation, it is never employed as an airplane Field rheostat ‚oad Field coils To load Arm B FIGURE 9-15. Diagram and schematic of a series-wound generator.
generator. Generators in airplanes have field windings which are connected either in shunt or in compound.
Shunt-Wound D. C. Generators A generator having a field winding connected in parallel with the external circuit is called a shunt generator, as shown in A and B of figure
9-16. The field coils of a shunt generator contain many turns of small wire; the magnetic strength is derived from the large number of turns rather than the current strength through the coils. If a constant voltage is desired, the shunt-wound generator is not suitable for rapidly fluctuating loads. Any increase in load causes a decrease in the terminal or output voltage, and any decrease in load causes an increase in terminal voltage;
since the armature and the load are connected in series, all current flowing in the external circuit passes through the armature winding. Because of the resistance in the armature winding, there is a voltage drop (IR drop = current X resistance).
As the load increases, the armature current increases and the IR drop in the armature increases.
The voltage delivered to the terminals is the difference between the induced voltage and the voltage drop; therefore, there is a decrease in terminal voltage. This decrease in voltage causes a decrease in field strength, because the current in the field coils decreases in proportion to the decrease in terminal voltage; with a weaker field, the voltage is further decreased.
When the load decreases, the output voltage increases accordingly, and a larger current flows in the windings. This action is cumulative, so the output voltage continues to rise to a point called field saturation, after which there is no further increase in output voltage.
The terminal voltage of a shunt generator can be controlled by means of a rheostat inserted in series with the field windings as shown in A of figure 9-16. As the resistance is increased, the field current is reduced; consequently, the generated voltage is reduced also. For a given setting of the field rheostat, the terminal voltage at the armature brushes will be approximately equal to the generated voltage minus the IR drop produced by the load current in the armature; thus, the voltage at the terminals of the generator will drop as the load is applied. Certain voltage-sensitive devices are available which automatically adjust the field rheostat to compensate for variations in 394
Field rheostat Field coil Arm To load Shunt circuit Main circui FIGURE 9-16. Shunt-wound generator.
load. When these devices are used, the terminal voltage remains essentially constant.
Compound-Wound D. C. Generators A compound-wound generator combines a series winding and a shunt winding in such a way that the characteristics of each are used to advantage.
The series field coils are made of a relatively small number of turns of large copper conductor, either circular or rectangular in cross section, and are connected in series with the armature circuit.
These coils are mounted on the same poles on which the shunt field coils are mounted and, therefore, contribute a magnetomotive force which influences the main field flux of the generator. A diagrammatic and a schematic illustration of a compound-wound generator is shown in A and B of figure 9-17.
If the ampere-turns of the series field act in the same direction as those of the shunt field, the combined magnetomotive force is equal to the sum of the series and shunt field components.
Load is added to a compound generator in the same manner in which load is added to a shunt generator, by increasing the number of parallel paths across the generator terminals. Thus, the decrease in total load resistance with added load is accompanied by an increase in armature-circuit and series-field circuit current.
Series field coil Shunt field coil To load Arm Load • Compound wound B FIGURE 9-17. Compound-wound generator.