Week 3 Day 2 24F DC Generators and Controls PDF

AVIA-1030Z38 1  Airframe Vol 1. 9-27 DC Generators and Controls  DC generators transform mechanical energy into electrical energy.  As the name implies, DC generators produce direct current and are typically found on...

AVIA-1030Z38 1  Airframe Vol 1. 9-27 DC Generators and Controls  DC generators transform mechanical energy into electrical energy.  As the name implies, DC generators produce direct current and are typically found on light aircraft.  In many cases, DC generators have been replaced with DC alternators.  Both devices produce electrical energy to power the aircraft’s electrical loads and charge the aircraft’s battery.  Even though they share the same purpose, the DC alternator and DC generator are very different. 2  Airframe Vol 1. 9-27 DC Generators and Controls  DC generators require a control circuit in order to ensure the generator maintains the correct voltage and current for the current electrical conditions of the aircraft.  Typically, aircraft generators maintain a nominal output voltage of approximately 14 volts or 28 volts. 3  Airframe Vol 1. 9-27 Generators  These principles show that voltage is induced in the armature of a generator throughout the entire 360° rotation of the conductor.  The armature is the rotating portion of a DC generator.  As shown, the voltage being induced is AC.  [Airframe Figure 9-40]  Since the conductor loop is constantly rotating, some means must be provided to connect this loop of wire to the electrical loads. 4  Airframe Vol 1. 9-29 Generators  Airframe Figure 9-40 5  Airframe Vol 1. 9-29 Generators  Airframe Figure 9-41 6  Airframe Vol 1. 9-27 Generators  As shown in Airframe Figure 9-41, slip rings and brushes can be used to transfer the electrical energy from the rotating loop to the stationary aircraft loads.  The slip rings are connected to the loop and rotate; the brushes are stationary and allow a current path to the electrical loads.  The slip rings are typically a copper material and the brushes are a soft carbon substance.  It is important to remember that the voltage being produced by this basic generator is AC, and AC voltage is supplied to the slip rings. 7  Airframe Vol 1. 9-27 Generators  Since the goal is to supply DC loads, some means must be provided to change the AC voltage to a DC voltage.  Generators use a modified slip ring arrangement, known as a commutator, to change the AC produced in the generator loop into a DC voltage.  The action of the commutator allows the generator to produce a DC output.  By replacing the slip rings of the basic AC generator with two half cylinders (the commutator), a basic DC generator is obtained. 8  Airframe Vol 1. 9-29 Generators 9 AVIA-1030S1 10  General 12-127 Theory of Operation  In the study of alternating current, basic generator principles were introduced to explain the generation of an AC 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. 11  General 12-128 Theory of Operation  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 General Figure 12-269, 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 General Figure 12-269A) is arranged to rotate in a magnetic field. 12  General 12-128 Theory of Operation  General Figure 12-269 13  General 12-128 Theory of Operation  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 General Figure 12-269.  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 General Figure 12-270, the induced voltage decreases because both sides of the loop (A and B) are approximately parallel to the lines of force and the rate of cutting is reduced. 14  General 12-128 Theory of Operation  General Figure 12-270 15  General 12-128 Theory of Operation  When the loop is vertical, no lines of force are cut since the wires are momentarily traveling parallel to the magnetic lines of force, and there is no induced voltage.  As the rotation of the loop continues, the number of lines of force cut increases until the loop has rotated an additional 90° to a horizontal plane.  As shown in General Figure 12-271, the number of lines of force cut and the induced voltage once again are maximum.  The direction of cutting, however, is in the opposite direction to that occurring in Figures 12- 269 and12-270, so the direction (polarity) of the induced voltage is reversed. 16  General 12-128 Theory of Operation  General Figure 12-271 17  General 12-128 Theory of Operation  As rotation of the loop continues, the number of lines of force having been cut again decreases, and the induced voltage becomes zero at the position shown in General Figure 12-272, since the wires A and B are again parallel to the magnetic lines of force.  If the voltage induced throughout the entire 360° of rotation is plotted, the curve shown in General Figure 12-273 results.  This voltage is called an alternating voltage because of its reversal from positive to negative value — first in one direction and then in the other. 18  General 12-128 Theory of Operation  General Figure 12-272 19  General 12-128 Theory of Operation  General Figure 12-273 20  Airframe Vol 1. 9-27 Generators  In Airframe Figure 9-42, the red side of the coil is connected to the red segment and the amber side of the coil to the amber segment.  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 commutator revolves simultaneously with the loop.  The rotating parts of a DC generator (coil and commutator) are called an armature. 21  Airframe Vol 1. 9-29 Generators  Airframe Figure 9-42 22  Airframe Vol 1. 9-27 Generators  As seen in the very simple generator of Airframe Figure 9-42, as the loop rotates the brushes make contact with different segments of the commutator.  In positions A, C, and E, the brushes touch the insulation between the brushes; when the loop is in these positions, no voltage is being produced.  In position B, the positive brush touches the red side of the conductor loop.  In position D, the positive brush touches the amber side of the armature conductor. 23  Airframe Vol 1. 9-27 Generators  This type of connection reversal changes the AC produced in the conductor coil into DC to power the aircraft.  An actual DC generator is more complex, having several loops of wire and commutator segments.  Because of this switching of commutator elements, the red brush is always in contact with the coil side moving downward, and the amber brush is always in contact with the coil side moving upward. 24  Airframe Vol 1. 9-27 Generators  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.  The voltage generated by the basic DC generator in Airframe Figure 9-42 varies from zero to its maximum value twice for each revolution of the loop.  This variation of DC voltage is called ripple and may be reduced by using more loops, or coils, as shown in Airframe Figure 9-43. 25  Airframe Vol 1. 9-30 Generators  Airframe Figure 9-43. 26  Airframe Vol 1. 9-27 Generators  As the number of loops is increased, the variation between maximum and minimum values of voltage is reduced [Airframe Figure 9-43], and the output voltage of the generator approaches a steady DC value.  For each additional loop in the rotor, another two commutator segments is required.  A photo of a typical DC generator commutator is shown in Airframe Figure 9-44. 27  Airframe Vol 1. 9-30 Generators  Airframe Figure 9-44. 28  General 12-130 Generators In view A of General Figure 12-276A 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 DC 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. 29  General 12-130 Generators An AC generator, or alternator, and a DC 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 AC generator, or alternator. If the current is collected by a commutator, it is direct current, and the generator is called a DC generator. 30 AVIA-1030S3 31  General 12-128 Construction Features of DC Generators  Generators used on aircraft may differ somewhat in design, since various manufacturers make them.  All, however, are of the same general construction and operate similarly.  The major parts, or assemblies, of a DC generator are a field frame (or yoke), a rotating armature, and a brush assembly.  The parts of a typical aircraft generator are shown in General Figure 12-277. 32  General 12-128 Construction Features of DC Generators  General Figure 12-277. ( or Airframe Figure 9-45) 33  General 12-131 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 generator.  In View A of General Figure 12-278A, the frame for a two-pole generator is shown in a cross- sectional view.  A four-pole generator frame is shown in View B of General Figure 12-278B.  In small generators, the frame is made of one piece of iron, but in larger generators, it is usually made up of two parts bolted together. 34  General 12-132 Field Frame  General Figure 12-278 35  General 12-131 Field Frame  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 General Figure 12-278, are bolted to the inside of the frame and form a core on which the field coil windings are mounted.  The poles are usually laminated to reduce eddy 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.  https://www.youtube.com/watch?v=zJ23gmS3KHY eddy current 2 min  The entire frame including field poles, is made from high quality magnetic iron or sheet steel. 36  Airframe Vol 1. 9-31 Field Frame  Airframe Figure 9-46 37  General 12-131 Field Frame  A practical DC 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 that fits over the iron core of the pole to which it is securely fastened. [General Figure 12- 279]  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 DC of the machine. 38  General 12-132 Field Frame  General Figure 12-279 39  General 12-131 Armature  The armature assembly of a generator consists of two primary elements: the wire coils (called windings) wound around an iron core and the commutator assembly.  The armature windings are evenly spaced around the armature and mounted on a steel shaft.  The armature rotates inside 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.  A typical armature assembly is shown in Airframe Figure 9-47. 40  Airframe Vol 1. 9-31 Armature  Airframe Figure 9-47. 41  General 12-133 Drum-Type Armature  General Figure 12-281 42  General 12-132 Drum-Type Armature  The wave winding is used in generators that are designed for high voltage outputs.  The two ends of each coil are connected to commutator segments separated by the distance between poles.  This results in a series arrangement of the coils and is additive of all the induced voltages. 43  Airframe Vol 1. 9-31 Commutators  Airframe Figure 9-48 shows a cross-sectional view of a typical commutator.  The commutator is located at the end of an armature and consists of copper segments divided by a thin insulator.  The insulator is often made from the mineral mica.  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. 44  Airframe Vol 1. 9-31 Commutators  Airframe Figure 9-48 45  Airframe Vol 1. 9-31 Commutators  The brushes are free to slide up and down in their holders in order to follow any irregularities in the surface of the commutator.  The constant making and breaking of electrical connections between the brushes and the commutator segments, along with the friction between the commutator and the brush, causes brushes to wear out and need regular attention or replacement.  For these reasons, the material commonly used for brushes is high-grade carbon. 46  Airframe Vol 1. 9-31 Commutators  The carbon 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 current path for the armature windings.  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. 47  Airframe Vol 1. 9-31 Armature  Airframe Figure 9-47. 48  General 12-134 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 field is called armature reaction.  [General Figure 12-283]  Armature windings of a generator are spaced so 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. 49  General 12-134 Armature Reaction  General Figure 12-283 50  General 12-134 Armature Reaction  When the magnetic field is not distorted, there is usually 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. 51  General 12-134 Armature Reaction  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. 52  General 12-134 Armature Reaction  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. 53  General 12-134 Compensating Windings  Compensating windings or 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.  The compensating windings consist of a series of coils embedded in slots in the pole faces.  These coils are also connected in series with the armature.  Consequently, this series connection with the armature produces a magnetic field in the compensating windings that varies directly with the armature current.  The compensating windings are wound in such a manner that the magnetic field produced by them will counteract the magnetic field produced by the armature. 54  General 12-134 Compensating Windings  As a result, the neutral plane will remain stationary with any magnitude of armature current.  With this design, once the brushes are set correctly, they do not need to be moved again.  General Figure 12-284A illustrates how the windings are set into the pole faces. 55  General 12-135 Compensating Windings  General Figure 12-284A 56  General 12-134 Interpoles  An interpole is a pole placed between the main poles of a generator.  An example of interpole placement is shown in General Figure 12-284B.  This is a simple two-pole generator with two interpoles.  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. 57  General 12-134 Interpoles  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 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. 58 AVIA-1030S4 59  Airframe Vol 1. 9-32 Types of DC Generators  There are three types of DC generator: series wound, parallel (shunt) wound, and series- parallel (or compound wound).  The appropriate generator is determined by the connections to the armature and field circuits with respect to the external circuit.  The external circuit is the electrical load powered by the generator.  In general, the external circuit is used for charging the aircraft battery and supplying power to all electrical equipment being used by the aircraft.  As their names imply, windings in series have characteristics different from windings in parallel. 60  Airframe Vol 1. 9-32 Series Wound DC Generators  The series generator contains a field winding connected in series with the external circuit. [Airframe Figure 9-49]  Series generators have very poor voltage regulation under changing load, since the greater the current is through the field coils to the external circuit, the greater the induced EMF’s and the greater the output voltage is.  When the aircraft electrical load is increased, the voltage increases; when the load is decreased, the voltage decreases.  Since the series wound generator has such poor voltage and current regulation, it is never employed as an airplane generator.  Generators in airplanes have field windings, that are connected either in shunt or in compound formats. 61  Airframe Vol 1. 9-32 Series Wound DC Generators  Airframe Figure 9-49 62  Airframe Vol 1. 9-32 Parallel (Shunt) Wound DC Generators  A generator having a field winding connected in parallel with the external circuit is called a shunt generator.  [Airframe Figure 9-50] It should be noted that, in electrical terms, shunt means parallel.  Therefore, this type of generator could be called either a shunt generator or a parallel generator.  In a shunt generator, any increase in load causes a decrease in the output voltage, and any decrease in load causes an increase output voltage. 63  Airframe Vol 1. 9-32 Parallel (Shunt) Wound DC Generators  Airframe Figure 9-50 64  Airframe Vol 1. 9-32 Parallel (Shunt) Wound DC Generators  This occurs since the field winding is connected in parallel to the load and armature, and all the current flowing in the external circuit passes only through the armature winding (not the field).  As shown in Airframe Figure 9-50A, the output voltage of a shunt generator can be controlled by means of a rheostat inserted in series with the field windings.  As the resistance of the field circuit is increased, the field current is reduced; consequently, the generated voltage is also reduced.  As the field resistance is decreased, the field current increases and the generator output increases.  In the actual aircraft, the field rheostat would be replaced with an automatic control device, such as a voltage regulator. 65  Airframe Vol 1. 9-32 Compound Wound DC Generators  A compound wound generator employs two field windings one in series and another in parallel with the load.  [Airframe Figure 9-51]  This arrangement takes advantage of both the series and parallel characteristics described earlier.  The output of a compound wound generator is relatively constant, even with changes in the load. 66  Airframe Vol 1. 9-33 Compound Wound DC Generators  Airframe Figure 9-51 67  General 12-137 Compound Wound DC Generators  General Figure 12-288. 68  Airframe Vol 1. 9-33 Generator Ratings  A DC generator is typically rated for its voltage and power output.  Each generator is designed to operate at a specified voltage, approximately 14 or 28 volts.  It should be noted that aircraft electrical systems are designed to operate at one of these two voltage values.  The aircraft’s voltage depends on which battery is selected for that aircraft.  Batteries are either 12 or 24 volts when fully charged. 69  Airframe Vol 1. 9-33 Generator Ratings  The generator selected must have a voltage output slightly higher than the battery voltage.  Hence, the 14-or 28-volt rating is required for aircraft DC generators.  The power output of any generator is given as the maximum number of amperes the generator can safely supply.  Generator rating and performance data are stamped on the nameplate attached to the generator.  When replacing a generator, it is important to choose one of the proper ratings. 70  Airframe Vol 1. 9-33 Generator Ratings  The rotation of generators is termed either clockwise or counterclockwise, as viewed from the driven end.  The direction of rotation may also be stamped on the data plate.  It is important that a generator with the correct rotation be used; otherwise, the polarity of the output voltage is reversed.  The speed of an aircraft engine varies from idle rpm to takeoff rpm; however, during the major portion of a flight, it is at a constant cruising speed.  The generator drive is usually geared to turn the generator between 1⅛ and 1½ times the engine crankshaft speed.  Most aircraft generators have a speed at which they begin to produce their normal voltage.  Called the “coming in” speed, it is usually about 1,500 rpm. 71  General 12-137 Generator Terminals  On most large 24-volt generators, electrical connections are made to terminals marked B, A, and E.  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. 72  General 12-137 Generator Terminals  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. 73 AVIA-1030S5 74  General 12-137 Inspection  The following information about the inspection and maintenance of DC generator systems is general in nature because of the large number of differing aircraft generator systems.  These procedures are for familiarization only.  Always follow the applicable manufacturer’s instructions for a given generator system. 75  General 12-138 Inspection  In general, the inspection of the generator installed in the aircraft should include the following items:  1. Security of generator mounting.  2. Condition of electrical connections.  3. Dirt and oil in the generator. If oil is present, check engine oil seal. Blow out dirt with compressed air.  4. Condition of generator brushes.  5. Generator operation.  6. Voltage regulator operation. 76  Airframe Vol 1. 9-33 DC Generator Maintenance  Airframe Figure 9-52 77  General 12-138 Condition of Generator Brushes  Sparking of brushes quickly reduces the effective brush area in contact with the commutator bars.  The degree of such sparking should be determined.  Excessive wear warrants a detailed inspection.  The following information pertains to brush seating, brush pressure, high mica condition, and brush wear.  Manufacturers usually recommend the following procedures to seat brushes that do not make good contact with slip rings or commutators. 78  General 12-138 Condition of Generator Brushes  Lift the brush sufficiently to permit the insertion of a strip of No. 000, or finer, sandpaper under the brush, rough side out. [General Figure 12-289]  Pull the sandpaper in the direction of armature rotation, being careful to keep the ends of the sandpaper as close to the slip ring or commutator surface as possible in order to avoid rounding the edges of the brush.  When pulling the sandpaper back to the starting point, raise the brush so it does not ride on the sandpaper.  Sand the brush only in the direction of rotation. 79  General 12-138 Condition of Generator Brushes  General Figure 12-289 80  General 12-138 Condition of Generator Brushes  After the generator has run for a short period, brushes should be inspected to make sure that pieces of sand have not become embedded in the brush and are collecting copper.  Under no circumstances should emery cloth or similar abrasives be used for seating brushes (or smoothing commutators), since they contain conductive materials that will cause arcing between brushes and commutator bars.  Excessive pressure will cause rapid wear of brushes.  Too little pressure, however, will allow “bouncing” of the brushes, resulting in burned and pitted surfaces. 81  General 12-138 Condition of Generator Brushes  The pressure recommended by the manufacturer should be checked by the use of a spring scale graduated in ounces.  Brush spring tension is usually adjusted between 32 to 36 ounces; however, the tension may differ slightly for each specific generator.  When a spring scale is used, the measurement of the pressure that a brush exerts on the commutator is read directly on the scale.  The scale is applied at the point of contact between the spring arm and the top of the brush, with the brush installed in the guide. 82  General 12-138 Condition of Generator Brushes  The scale is drawn up until the arm just lifts off the brush surface.  At this instant, the force on the scale should be read.  Flexible low resistance pigtails are provided on most heavy current carrying brushes, and their connections should be securely made and checked at frequent intervals.  The pigtails should never be permitted to alter or restrict the free motion of the brush.  The purpose of the pigtail is to conduct the current, rather than subjecting the brush spring to currents that would alter its spring action by overheating. 83  General 12-138 Condition of Generator Brushes  The pigtails also eliminate any possible sparking to the brush guides caused by the movement of the brushes within the holder, thus minimizing side wear of the brush.  Carbon dust resulting from brush sanding should be thoroughly cleaned from all parts of the generators after a sanding operation.  Such carbon dust has been the cause of several serious fires as well as costly damage to the generator.  Operation over extended periods of time often results in the mica insulation between commutator bars protruding above the surface of the bars.  This condition is called “high mica” and interferes with the contact of the brushes to the commutator. 84 DC Generator Maintenance  Airframe Figure 9-52 85  General 12-139 Condition of Generator Brushes  Whenever this condition exists, or if the armature has been turned on a lathe, carefully undercut the mica insulation to a depth equal to the width of the mica, or approximately 0.020 inch.  Each brush should be a specified length to work properly.  If a brush is too short, the contact it makes with the commutator will be faulty, which can also reduce the spring force holding the brush in place.  Most manufacturers specify the amount of wear permissible from a new brush length. 86  General 12-139 Condition of Generator Brushes  When a brush has worn to the minimum length permissible, it must be replaced.  Some special generator brushes should not be replaced because of a slight grooving on the face of the brush.  These grooves are normal and will appear in AC and DC generator brushes which are installed in some models of aircraft generators.  These brushes have two cores made of a harder material with a higher expansion rate than the material used in the main body of the brush.  Usually, the main body of the brush face rides on the commutator.  However, at certain temperatures, the cores extend and wear through any film on the commutator. 87 AVIA-1030Z44 88  Airframe Vol 1. 9-34 Theory of Generator Control  All aircraft are designed to operate within a specific voltage range (for example 13.5–14.5 volts).  And since aircraft operate at a variety of engine speeds (remember, the engine drives the generator) and with a variety of electrical demands, all generators must be regulated by some control system.  The generator control system is designed to keep the generator output within limits for all flight variables.  Generator control systems are often referred to as voltage regulators or generator control units (GCU).  Aircraft generator output can easily be adjusted through control of the generator’s magnetic field strength. 89  Airframe Vol 1. 9-34 Theory of Generator Control  Remember, the strength of the magnetic field has a direct effect on generator output.  More field current means more generator output and vice versa.  Airframe Figure 9-54 shows a simple generator control used to adjust field current.  When field current is controlled, generator output is controlled.  Keep in mind, this system is manually adjusted and would not be suitable for aircraft.  Aircraft systems must be automatic and are therefore a bit more complex. 90  Airframe Vol 1. 9-34 Theory of Generator Control  Airframe Figure 9-54 91  Airframe Vol 1. 9-34 There are two basic types of generator controls:  electro-mechanical and solid-state (transistorized).  The electromechanical type controls are found on older aircraft and tend to require regular inspection and maintenance.  Solid-state systems are more modern and typically considered to have better reliability and more accurate generator output control. 92  Airframe Vol 1. 9-35 Functions of Generator Control Systems  Most generator control systems perform a number of functions related to the regulation, sensing, and protection of the DC generation system.  Light aircraft typically require a less complex generator control system than larger multiengine aircraft.  Some of the functions listed below are not found on light aircraft. 93  Airframe Vol 1. 9-35 Functions of Generator Control Systems  Voltage Regulation  The most basic of the GCU functions is that of voltage regulation.  Regulation of any kind requires the regulation unit to take a sample of a generator output and compare that sample to a known reference.  If the generator’s output voltage falls outside of the set limits, then the regulation unit must provide an adjustment to the generator field current.  Adjusting field current controls generator output. 94  Airframe Vol 1. 9-35 Functions of Generator Control Systems  Overvoltage Protection  The overvoltage protection system compares the sampled voltage to a reference voltage.  The overvoltage protection circuit is used to open the relay that controls the field excitation current.  It is typically found on more complex generator control systems. 95  Airframe Vol 1. 9-35 Functions of Generator Control Systems  Parallel Generator Operations  On multiengine aircraft, a paralleling feature must be employed to ensure all generators operate within limits.  In general, paralleling systems compare the voltages between two or more generators and adjust the voltage regulation circuit accordingly. 96  Airframe Vol 1. 9-35 Functions of Generator Control Systems  Overexcitation Protection  When one generator in a paralleled system fails, one of the generators can become overexcited and tends to carry more than its share of the load, if not all of the loads.  Basically, this condition causes the generator to produce too much current.  If this condition is sensed, the overexcited generator must be brought back within limits, or damage occurs.  The overexcitation circuit often works in conjunction with the overvoltage circuit to control the generator. 97  Airframe Vol 1. 9-35 Functions of Generator Control Systems  Differential Voltage  This function of a control system is designed to ensure all generator voltage values are within a close tolerance before being connected to the load bus.  If the output is not within the specified tolerance, then the generator contactor is not allowed to connect the generator to the load bus. 98  Airframe Vol 1. 9-35 Functions of Generator Control Systems  Reverse Current Sensing  If the generator cannot maintain the required voltage level, it eventually begins to draw current instead of providing it.  This situation occurs, for example, if a generator fails.  When a generator fails, it becomes a load to the other operating generators or the battery.  The defective generator must be removed from the bus.  The reverse current sensing function monitors the system for a reverse current.  Reverse current indicates that current is flowing to the generator not from the generator.  If this occurs, the system opens the generator relay and disconnects the generator from the bus. 99  Airframe Vol 1. 9-35 Generator Controls for High Output Generators  Most modern high output generators are found on turbine powered corporate-type aircraft.  These small business jets and turboprop aircraft employ a generator and starter combined into one unit.  This unit is referred to as a starter generator.  A starter-generator has the advantage of combining two units into one housing, saving space and weight.  Since the starter-generator performs two tasks, engine starting and generation of electrical power, the control system for this unit is relatively complex. 100  Airframe Vol 1. 9-35 Generator Controls for High Output Generators  A simple explanation of a starter-generator shows that the unit contains two sets of field windings.  One field is used to start the engine and one used for the generation of electrical power.  [Airframe Figure 9-55]  During the start function, the GCU must energize the series field and the armature causes the unit to act like a motor.  During the generating mode, the GCU must disconnect the series field, energize the parallel field, and control the current produced by the armature. 101  Airframe Vol 1. 9-35 Generator Controls for High Output Generators  Airframe Figure 9-55 102  Airframe Vol 1. 9-35 Generator Controls for High Output Generators  At this time, the starter generator acts like a typical generator.  Of course, the GCU must perform all the functions described earlier to control voltage and protect the system.  These functions include voltage regulation, reverse current sensing, differential voltage, overexcitation protection, overvoltage protection, and parallel generator operations.  A typical GCU is shown in Airframe Figure 9-56.  In general, modern GCUs for high-output generators employ solid-state electronic circuits to sense the operations of the generator or starter-generator. 103  Airframe Vol 1. 9-36 Generator Controls for High Output Generators  Airframe Figure 9-56. 104  Airframe Vol 1. 9-36 Generator Controls for High Output Generators  The circuitry then controls a series of relays and/or solenoids to connect and disconnect the unit to various distribution busses.  One unit found in almost all voltage regulation circuitry is the zener diode.  The zener diode is a voltage sensitive device that is used to monitor system voltage.  The zener diode, connected in conjunction to the GCU circuitry, then controls the field current, which in turn controls the generator output. 105  Airframe Vol 1. 9-36 Generator Controls for Low-Output Generators  A typical generator control circuit for low-output generators modifies current flow to the generator field to control generator output power.  As flight variables and electrical loads change, the GCU must monitor the electrical system and make the appropriate adjustments to ensure proper system voltage and current.  The typical generator control is referred to as a voltage regulator or a GCU.  Since most low-output generators are found on older aircraft, the control systems for these systems are electromechanical devices. 106  Airframe Vol 1. 9-36 Generator Controls for Low-Output Generators  (Solid-state units are found on more modern aircraft that employ DC alternators and not DC generators.)  The two most common types of voltage regulator are the carbon pile regulator and the three-unit regulator.  Each of these units controls field current using a type of variable resistor.  Controlling field current then controls generator output.  A simplified generator control circuit is shown in Airframe Figure 9-57. 107  Airframe Vol 1. 9-36 Generator Controls for Low-Output Generators  Airframe Figure 9-57. 108  Airframe Vol 1. 9-36 Generator Controls for Low-Output Generators  Carbon Pile Regulators  The carbon pile regulator controls DC generator output by sending the field current through a stack of carbon disks (the carbon pile).  The carbon disks are in series with the generator field.  If the resistance of the disks increases, the field current decreases and the generator output goes down.  If the resistance of the disks decreases, the field current increases and generator output goes up.  As seen in Airframe Figure 9-58, a voltage coil is installed in parallel with the generator output leads. 109  Airframe Vol 1. 9-36 Generator Controls for Low-Output Generators  Airframe Figure 9-58 110  Airframe Vol 1. 9-36 Generator Controls for Low-Output Generators  The voltage coil acts like an electromagnet that increases or decrease strength as generator output voltage changes.  The magnetism of the voltage coil controls the pressure on the carbon stack.  The pressure on the carbon stack controls the resistance of the carbon; the resistance of the carbon controls field current and the field current controls generator output.  Carbon pile regulators require regular maintenance to ensure accurate voltage regulation; therefore, most have been replaced on aircraft with more modern systems. 111  Airframe Vol 1. 9-37 Generator Controls for Low-Output Generators  Three-Unit Regulators  The three-unit regulator used with DC generator systems is made of three distinct units.  Each of these units performs a specific function vital to correct electrical system operation.  A typical three-unit regulator consists of three relays mounted in a single housing.  Each of the three relays monitors generator outputs and opens or closes the relay contact points according to system needs.  A typical three unit regulator is shown in Airframe Figure 9-59. 112  Airframe Vol 1. 9-37 Generator Controls for Low-Output Generators  Airframe Figure 9-59. 113  Airframe Vol 1. 9-37 Generator Controls for Low-Output Generators  Voltage Regulator  The voltage regulator section of the three-unit regulator is used to control generator output voltage.  The voltage regulator monitors generator output and controls the generator field current as needed.  If the regulator senses that system voltage is too high, the relay points open and the current in the field circuit must travel through a resistor.  This resistor lowers field current and therefore lowers generator output. 114  Airframe Vol 1. 9-37 Generator Controls for Low-Output Generators  Remember, generator output goes down whenever generator field current goes down.  As seen in Airframe Figure 9-60, the voltage coil is connected in parallel with the generator output, and it therefore measures the voltage of the system.  If voltage gets beyond a predetermined limit, the voltage coil becomes a strong magnet and opens the contact points.  If the contact points are open, field current must travel through a resistor and therefore field current goes down. 115  Airframe Vol 1. 9-37 Generator Controls for Low-Output Generators  Airframe Figure 9-60 116  Airframe Vol 1. 9-37 Generator Controls for Low-Output Generators  The dotted arrow shows the current flow through the voltage regulator when the relay points are open.  Since this voltage regulator has only two positions (points open and points closed), the unit must constantly be in adjustment to maintain accurate voltage control.  During normal system operation, the points are opening and closing at regular intervals.  The points are in effect vibrating.  This type of regulator is sometimes referred to as a vibrating type regulator. 117  Airframe Vol 1. 9-37 Generator Controls for Low-Output Generators  As the points vibrate, the field current raises and lowers and the field magnetism averages to a level that maintains the correct generator output voltage.  If the system requires more generator output, the points remain closed longer and vice versa.  Current Limiter  The current limiter section of the three-unit regulator is designed to limit generator output current.  This unit contains a relay with a coil wired in series with respect to the generator output. 118  Airframe Vol 1. 9-37 Generator Controls for Low-Output Generators  As seen in Airframe Figure 9-61, all the generator output current must travel through the current coil of the relay.  This creates a relay that is sensitive to the current output of the generator.  That is, if generator output current increases, the relay points open and vice versa.  The dotted line shows the current flow to the generator field when the current limiter points are open.  It should be noted that, unlike the voltage regulator relay, the current limiter is typically closed during normal flight.  Only during extreme current loads must the current limiter points open; at that time, field current is lowered and generator output is kept within limits. 119  Airframe Vol 1. 9-37 Generator Controls for Low-Output Generators  Airframe Figure 9-61 120  Airframe Vol 1. 9-38 Generator Controls for Low-Output Generators  Reverse-Current Relay  The third unit of a three-unit regulator is used to prevent current from leaving the battery and feeding the generator.  This type of current flow would discharge the battery and is opposite of normal operation.  It can be thought of as a reverse current situation and is known as reverse current relay.  The simple reverse current relay shown in Airframe Figure 9-62 contains both a voltage coil and a current coil. 121  Airframe Vol 1. 9-38 Generator Controls for Low-Output Generators  Airframe Figure 9-62 122  Airframe Vol 1. 9-38 Generator Controls for Low-Output Generators  The voltage coil is wired in parallel to the generator output and is energized any time the generator output reaches its operational voltage.  As the voltage coil is energized, the contact points close and the current is then allowed to flow to the aircraft electrical loads, as shown by the dotted lines.  The diagram shows the reverse current relay in its normal operating position; the points are closed and current is flowing from the generator to the aircraft electrical loads.  As current flows to the loads, the current coil is energized and the points remain closed.  If there is no generator output due to a system failure, the contact points open because magnetism in the relay is lost.  With the contact points open, the generator is automatically disconnected from the aircraft electrical system, which prevents reverse flow from the load bus to the generator.  A typical three-unit regulator for aircraft generators is shown in Airframe Figure 9-63 123  Airframe Vol 1. 9-38 Generator Controls for Low-Output Generators  Airframe Figure 9-63. 124  Airframe Vol 1. 9-38 Generator Controls for Low-Output Generators  As seen in Airframe Figure 9-63, all three units of the regulator work together to control generator output.  The regulator monitors generator output and controls power to the aircraft loads as needed for flight variables.  Note that the vibrating regulator just described was simplified for explanation purposes.  A typical vibrating regulator found on an aircraft would probably be more complex. 125 Review  https://www.youtube.com/watch?v=MXQzUhORgKM motors and gens 30min 126