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.
In general, the inspection of the generator installed in the aircraft should include the following items:- Security of generator mounting
- Condition of electrical connections
- Dirt and oil in the generator—if oil is present, check engine oil seal. Blow out dirt with compressed air.
- Condition of generator brushes
- Generator operation
- Voltage regulator operation
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.
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| Figure 1. Seating brushes with sandpaper. |
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.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.
Excessive pressure causes rapid wear of brushes. Too little pressure, however, allows “bouncing” of the brushes, resulting in burned and pitted surfaces.
A carbon, graphite, or light metalized brush should exert a pressure of 11⁄2 to 21⁄2 psi on the commutator. 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. 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. 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. 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 1.20 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. 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.
DC Motors
Most devices in an airplane, from the starter [Figure 2] to the automatic pilot, depend upon mechanical energy furnished by DC motors. A DC motor is a rotating machine, which transforms DC energy into mechanical energy. It consists of two principal parts—a field assembly and an armature assembly. The armature is the rotating part in which current carrying wires are acted upon by the magnetic field.
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| Figure 2. DC series starter motor |
Whenever a current carrying wire is placed in the field of a magnet, a force acts on the wire. The force is not one of attraction or repulsion; however, it is at right angles to the wire and also at right angles to the magnetic field set up by the magnet. The action of the force upon a current carrying wire placed in a magnetic field is shown in Figure 3. A wire is located between two permanent magnets. The lines of force in the magnetic field are from the North pole to the South pole. When no current flows, no force is exerted on the wire, but when current flows through the wire, a magnetic field is set up about it. [Figure 3] The direction of the field depends on the direction of current flow. Current in one direction creates a clockwise field about the wire, and current in the other direction, a counterclockwise field.
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| Figure 3. Force on a current carrying wire |
Since the current carrying wire produces a magnetic field, a reaction occurs between the field about the wire and the magnetic field between the magnets. When the current flows in a direction to create a counterclockwise magnetic field about the wire, this field and the field between the magnets add or reinforce at the bottom of the wire because the lines of force are in the same direction. At the top of the wire, they subtract or neutralize, since the lines of force in the two fields are opposite in direction. Thus, the resulting field at the bottom is strong and the one at the top is weak. Consequently, the wire is pushed upward. [Figure 3C] The wire is always pushed away from the side where the field is strongest.
If current flow through the wire were reversed in direction, the two fields would add at the top and subtract at the bottom. Since a wire is always pushed away from the strong field, the wire would be pushed down.Force Between Parallel Conductors
Two wires carrying current in the vicinity of one another exert a force on each other because of their magnetic fields. An end view of two conductors is shown in Figure 4. |
| Figure 4. Fields surrounding parallel conductors |
In Figure 4A, electron flow in both conductors is toward the reader, and the magnetic fields are clockwise around the conductors. Between the wires, the fields cancel because the directions of the two fields oppose each other. The wires are forced in the direction of the weaker field, toward each other. This force is one of attraction. In Figure 4B, the electron flow in the two wires is in opposite directions.
The magnetic fields are, therefore, clockwise in one and counterclockwise in the other, as shown. The fields reinforce each other between the wires, and the wires are forced in the direction of the weaker field, away from each other. This force is one of repulsion.To summarize: conductors carrying current in the same direction tend to be drawn together; conductors carrying current in opposite directions tend to be repelled from each other.
Developing Torque
If a coil in which current is flowing is placed in a magnetic field, a force is produced which causes the coil to rotate. In the coil shown in Figure 5, current flows inward on side A and outward on side B. The magnetic field about B is clockwise and that about A, counterclockwise. As previously explained, a force develops which pushes side B downward. At the same time, the field of the magnets and the field about A, in which the current is inward, adds at the bottom and subtracts at the top. Therefore, A moves upward. The coil rotates until its plane is perpendicular to the magnetic lines between the North and South poles of the magnet, as indicated in Figure 5 by the white coil at right angles to the black coil.
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| Figure 5. Developing a torque |
The tendency of a force to produce rotation is called torque. When the steering wheel of a car is turned, torque is applied. The engine of an airplane gives torque to the propeller. Torque is developed also by the reacting magnetic fields about the current carrying coil just described. This is the torque, which turns the coil.
The right-hand motor rule can be used to determine the direction a current carrying wire moves in a magnetic field. As illustrated in Figure 6, if the index finger of the right hand is pointed in the direction of the magnetic field and the second finger in the direction of current flow, the thumb indicates the direction the current carrying wire moves. |
| Figure 6. Right-hand motor rule |
The amount of torque developed in a coil depends upon several factors: the strength of the magnetic field, the number of turns in the coil, and the position of the coil in the field. Magnets are made of special steel that produces a strong field. Since there is torque acting on each turn, the greater the number of turns on the coil, the greater the torque. In a coil carrying a steady current located in a uniform magnetic field, the torque varies at successive positions of rotation. [Figure 7] When the plane of the coil is parallel to the lines of force, the torque is zero. When its plane cuts the lines of force at right angles, the torque is 100 percent. At intermediate positions, the torque ranges between 0 and 100 percent.
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| Figure 7. Torque on a coil at various angles of rotation |
Basic DC Motor
A coil of wire through which the current flows rotates when placed in a magnetic field. This is the technical basis governing the construction of a DC motor. [Figure 8] However, if the connecting wires from the battery were permanently fastened to the terminals of the coil and there was a flow of current, the coil would rotate only until it lined itself up with the magnetic field. Then, it would stop, because the torque at that point would be 0.
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| Figure 8. Basic DC motor operation |
A motor, of course, must continue rotating. It is therefore necessary to design a device that reverses the current in the coil just at the time the coil becomes parallel to the lines of force. This creates torque again and causes the coil to rotate. If the current reversing device is set up to reverse the current each time the coil is about to stop, the coil can be made to continue rotating as long as desired.
One method of doing this is to connect the circuit so that, as the coil rotates, each contact slides off the terminal to which it connects and slides onto the terminal of opposite polarity. In other words, the coil contacts switch terminals continuously as the coil rotates, preserving the torque and keeping the coil rotating. In Figure 8, the coil terminal segments are labeled A and B. As the coil rotates, the segments slide onto and past the fixed terminals or brushes. With this arrangement, the direction of current in the side of the coil next to the North-seeking pole flows toward the reader, and the force acting on that side of the coil turns it downward. The part of the motor that changes the current from one wire to another is called the commutator.Position A
When the coil is positioned as shown in Figure 8A, current flows from the negative terminal of the battery to the negative (−) brush, to segment B of the commutator, through the loop to segment A of the commutator, to the positive (+) brush, and then back to the positive terminal of the battery. By using the right-hand motor rule, it is seen that the coil rotates counterclockwise. The torque at this position of the coil is maximum, since the greatest number of lines of force is being cut by the coil.
Position B
When the coil has rotated 90° to the position shown in Figure 8B, segments A and B of the commutator no longer make contact with the battery circuit and no current can flow through the coil. At this position, the torque has reached a minimum value, since a minimum number of lines of force are being cut. However, the momentum of the coil carries it beyond this position until the segments again make contact with the brushes, and current again enters the coil; this time, though, it enters through segment A and leaves through segment B. However, since the positions of segments A and B have also been reversed, the effect of the current is as before, the torque acts in the same direction, and the coil continues its counterclockwise rotation.
Position C
On passing through the position shown in Figure 8C, the torque again reaches maximum.Position D
Continued rotation carries the coil again to a position of minimum torque as in Figure 8D. At this position, the brushes no longer carry current, but once more the momentum rotates the coil to the point where current enters through segment B and leaves through A. Further rotation brings the coil to the starting point and, thus, one revolution is completed.
The switching of the coil terminals from the positive to the negative brushes occurs twice per revolution of the coil.The torque in a motor containing only a single coil is neither continuous nor very effective, for there are two positions where there is actually no torque at all. To overcome this, a practical DC motor contains a large number of coils wound on the armature. These coils are so spaced that, for any position of the armature, there are coils near the poles of the magnet. This makes the torque both continuous and strong. The commutator, likewise, contains a large number of segments instead of only two. The armature in a practical motor is not placed between the poles of a permanent magnet but between those of an electromagnet, since a much stronger magnetic field can be furnished. The core is usually made of a mild or annealed steel, which can be magnetized strongly by induction. The current magnetizing the electromagnet is from the same source that supplies the current to the armature.