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.
The effect of the additive series field is that of increased field flux with increased load. The extent of the increased field flux depends on the degree of saturation of the field as determined by the shunt field current. Thus, the terminal voltage of the generator may increase or decrease with load, depending on the influence of the series field coils. This influence is referred to as the degree of compounding.
A flat-compound generator is one in which the no-load and full-load voltages have the same value; whereas an under-compound generator has a full-load voltage less than the no-load value, and an over-compound generator has a full-load voltage which is higher than the no-load value.
Changes in terminal voltage with increasing load depends upon the degree of compounding.
If the series field aids the shunt field, the generator is said to be cumulative-compounded (B of figure 9-17).
If the series field opposes the shunt field, the machine is said to be differentially compounded, or is called a differential generator.
Compound generators are usually designed to be overcompounded. This feature permits varied degrees of compounding by connecting a variable shunt across the series field. Such a shunt is sometimes called a diverter. Compound generators are used where voltage regulation is of prime importance.
Differential generators have somewhat the same characteristics as series generators in that they are essentially constant-current generators. However, they generate rated voltage at no load, the voltage dropping materially as the load current Cumulative compounding 100% Differential connection Output voltage compounding Series connection Output current or load 100% FIGURE 9-18. Generator characteristics.
Neutral 240V 120V FIGURE 9-19. Three-wire generator.
increases. Constant-current generators are ideally suited as power sources for electric arc welders and are used almost universally in electric arc welding.
If the shunt field of a compound generator is connected across both the armature and the series field, it is known as a long-shunt connection, but if the shunt field is connected across the armature alone, it is called a short-shunt connection. These connections produce essentially the same generator characteristics.
A summary of the characteristics of the various types of generators discussed is shown graphically in figure 9-18.
Three-Wire Generators Some d.c. generators, called three-wire generators, are designed to deliver 240 volts, or 120 volts from either side of a neutral wire (figure 9-19). This is accomplished by connecting a reactance coil to opposite sides of the commutator, with the neutral connected to the midpoint of the reactance coil. Such a reactance coil acts as a lowloss voltage divider. If resistors were used, the IR loss would be prohibitive unless the two loads were perfectly matched. The coil is built into some generators as part of the armature, with the midpoint connected to a single slip ring which the neutral contacts by means of a brush. In other generators, the two connections to the commutator are connected, in turn, to two slip rings, and the reactor is located outside the generator. In either case, the load unbalance on either side of the neutral must not be more than 25 percent of the rated current output of the generator. The three-wire generator permits simultaneous operation of 120-volt lighting circuits and 240-volt motors from the same generator.
Armature Reaction Current flowing through the armature sets up electromagnetic fields in the windings. These new fields tend to distort or bend the magnetic flux between the poles of the generator from a straight line path. Since armature current increases with load, the distortion becomes greater with an increase in load. This distortion of the magnetic 00000A Rotation офо a Neutral plane 000000 000000 A Field excited, armature unexcited Rotation B Armature excited, field unexcited 0000 Rotation 000000 C Both field and armature excited FIGURE 9-20. Armature reaction.
feld is called armature reaction and is illustrated in figure 9-20.
Armature windings of a generator are spaced in such a way that, during rotation of the armature, there are certain positions when the brushes contact two adjacent segments, thereby shorting the armature windings to these segments. Usually, when the magnetic field is not distorted, there is no voltage being induced in the shorted windings, and, therefore, no harmful results occur from the shorting of the windings. However, when the field is distorted, a voltage is induced in these shorted windings and sparking takes place between the brushes and the commutator segments. Consequently, the commutator becomes pitted, the wear on the brushes becomes excessive, and the output of the generator is reduced. To correct this condition, the brushes are set so that the plane of the coils which are shorted by the brushes is perpendicular to the distorted magnetic field, which is accomplished by moving the brushes forward in the direction of rotation. This operation is called shifting the brushes to the neutral plane, or plane of commutation. The neutral plane is the position where the plane of the two opposite coils is perpendicular to the magnetic field in the generator. On a few generators, the brushes can be shifted manually ahead of the normal neutral plane to the neutral plane caused by field distortion. On nonadjustable brush generators, the manufacturer sets the brushes for minimum sparking.
Interpoles may be used to counteract some of the effects of field distortion, since shifting the brushes is inconvenient and unsatisfactory, especially when the speed and load of the generator are changing constantly. An interpole is a pole placed between the main poles of a generator.
For example, a four-pole generator has four interpoles, which are north and south poles, alternately, as are the main poles. A four-pole generator with interpoles is shown in figure 9-21.
An interpole has the same polarity as the next main pole in the direction of rotation. The magnetic flux produced by an interpole causes the current in the armature to change direction as an armature winding passes under it. This cancels the electromagnetic fields about the armature windings. The magnetic strength of the interpoles varies with the load on the generator; and since field distortion varies with the load, the magnetic field of the interpoles counteracts the effects of the field set up around the armature windings 397
Terminal connections E FIGURE 9-21. Generator with interpoles.
and minimizes field distortion. Thus, the interpole tends to keep the neutral plane in the same position for all loads on the generator; therefore, field distortion is reduced by the interpoles, and the efficiency, output, and service life of the brushes are improved.
Generator Ratings A generator is rated in power output. Since a generator is designed to operate at a specified voltage, the rating usually is given as the number of amperes the generator can safely supply at its rated voltage.
Generator rating and performance data are stamped on the name plate attached to the generator. When replacing a generator, it is important to choose one of the proper rating.
The rotation of generators is termed either clockwise or counterclockwise, as viewed from the driven end. Usually, the direction of rotation is stamped on the data plate. If no direction is stamped on the plate, the rotation may be marked by an arrow on the cover plate of the brush housing. It is important that a generator with the correct direction of rotation be used; otherwise the voltage will be reversed.
The speed of an aircraft engine varies from idle
r.p.m. to takeoff r.p.m.; however, during the major portion of a flight, it is at a constant cruising speed. The generator drive is usually geared to revolve the generator between 18 and 1½ times the engine crankshaft speed. Most aircraft generators have a speed at which they begin to produce their normal voltage. Termed the "coming-in" speed, it is usually about 1,500 r.p.m.
Generator Terminals On most large 24-volt generators, electrical connections are made to terminals marked B, A, and E. (See figure 9-22.) The positive armature lead in the generator connects to the B terminal. The negative armature lead connects to the E terminal.
The positive end of the shunt field winding connects to terminal A, and the opposite end connects to the negative terminal brush. Terminal A receives current from the negative generator brush through the shunt field winding. This current passes through the voltage regulator and back to the armature through the positive brush. Load current, which leaves the armature through the negative brushes, comes out of the E lead and passes through the load before returning to the armature through the positive brushes.
REGULATION OF GENERATOR VOLTAGE Efficient operation of electrical equipment in an airplane depends on a constant voltage supply from the generator. Among the factors which determine the voltage output of a generator, only one, the strength of the field current, can be conveniently controlled. To illustrate this control, refer to the diagram in figure 9-22, showing a simple generator with a rheostat in the field circuit. If the rheostat is set to increase the resistance in the field circuit, less current flows through the field winding and the strength of the magnetic field in which the armature rotates decreases. Consequently, the voltage output of the generator decreases. If the resistance in the field Shunt field Rheostat Load Terminal connections Current flow FIGURE 9-22. Regulation of generator voltage by field rheostat.
ator systems require a different type of regulator, such as the carbon-pile voltage regulator.
Loac FIGURE 9-23. Vibrating-type voltage regulator.
circuit is decreased with the rheostat, more current flows through the field windings, the magnetic field becomes stronger, and the generator produces a greater voltage.
With the generator running at normal speed and switch K open (figure 9-23), the field rheostat is adjusted so that the terminal voltage is about 60 percert of normal. Solenoid S is weak and contact B is held closed by the spring. When K is closed, a short circuit is placed across the field rheostat. This action causes the field current to increase and the terminal voltage to rise.
When the terminal voltage rises above a certain critical value, the solenoid downward pull exceeds the spring tension and contact B opens, thus reinserting the field rheostat in the field circuit and reducing the field current and terminal voltage.
When the terminal voltage falls below a certain critical voltage, the solenoid armature contact B is closed again by the spring, the field rheostat is now shorted, and the terminal voltage starts to rise. The cycle repeats with a rapid, continuous action. Thus, an average voltage is maintained with or without load change.
The dashpot P provides smoother operation by acting as a damper to prevent hunting. The capacitor C across contact B eliminates sparking.
Added load causes the field rheostat to be shorted for a longer period of time and, thus, the solenoid armature vibrates more slowly. If the load is reduced and the terminal voltage rises, the armature vibrates more rapidly and the regulator holds the terminal voltage to a steady value for any change in load, from no load to full load, on the generator.
Vibrating-type regulators cannot be used with generators which require a high field current, since the contacts will pit or burn. Heavy-duty gener- Carbon-Pile Voltage Regulator The carbon-pile voltage regulator depends on the resistance of a number of carbon disks arranged in a pile or stack. The resistance of the carbon stack varies inversely with the pressure applied. When the stack is compressed under appreciable pressure, the resistance in the stack is less. When the pressure is reduced, the resistance of the carbon stack increases, because there is more air space between the disks, and air has high resistance. Pressure on the carbon pile depends upon two opposing forces: a spring and an electromagnet. The spring compresses the carbon pile, and the electromagnet exerts a pull which de- Adjusting knob Insulating tubes Carbon disks Metal A Field plug Armature D FIGURE 9-24. Illustrating the controlling effect of a carbon-pile voltage regulator.