Chapters 18 and 19 provide a comprehensive compilation of nearly 40 years of DC machines and procedures. The DC principles presented here are still valid and provide the means for building the groundwork necessary to understand the DC marine electrical system.
Moreover, the vessels in prepositional fleets, those in storage, and the tugboats and floating cranes currently on station in the marine field require the use of this information. Army marine personnel, active and reserve, need to understand the principles behind the operation of their equipment.
Fundamentally, all electric generators operate on the same principle, regardless of whether they produce AC or DC. Internally, all generators produce AC. If DC is required, a device to rectify, or change, the AC to DC is needed. The DC generators use a device called a commutator just for such a purpose (Figure 18-1). The AC induced into the armature windings is directed to a set of copper segments that, with the aid of the brushes, keeps current moving in a single direction. The commutator and brush assembly is a crude but effective way to rectify the AC to DC (Figure 18-2).
A copper conductor is wound around a metal core called a pole piece or pole shoe. Together, the coil of wire and the pole piece is called the field pole and is bolted directly to the inside of the generator housing or frame. Field poles are always found in pairs. Half of the total number of field poles become electromagnets with the north polarity toward the center of the generator. The other half of the total number of field poles are electromagnets with their south polarity toward the center of the generator. Figure 18-3 shows a four-pole generator.
Direct current is supplied to the field poles to establish a fried magnetic field. This field never changes polarity under normal operating conditions. Other coils of wire are turned by a prime mover in the magnetic field produced by the field poles. These coils of wire are called the armature windings (Figure 18-2).
Armature windings are heavy copper wires wrapped to form coils around a laminated core. The coils of wire are completely insulated from other coils and the laminated core. The coils of wire are also insulated their entire length to prevent turn-to-turn shorts or accidental grounds. Each armature coil is connected to two copper commutator segments. Figure 18-4 shows the armature coils as A, B, C, and so on. Note that each armature coil joins another armature coil at a commutator segment (1, 2, 3, and so on). The brushes are shown inside the commutator segments to show their relative position only (refer to Figure 18-4). The diagram would otherwise become too cluttered if the brushes were shown superimposed over the armature windings.
The commutator is fundamentally a reversing switch synchronized with the action of the armature. Figure 18-5 shows how a commutator performs its work. The simple commutator shown here consists of a cylinder of conducting metal split into two halves called segments. One segment is connected to branch (a) of the armature coil, the other to branch (b). These segments are separated from each other by a space that provides insulation so that the current generated in one branch does not short-circuit directly into the other. Two stationary conductors called brushes make contact with the rotating commutator segments and conduct the generated current from the commutator to the point of application, called the load. Figure 18-5 omits the field pole electromagnet to simplify the illustration. However, it is assumed that the magnet is still in position.
At the instant shown in view A of Figure 18-5, the current in branch (a), which is moving upward through the magnetic field, is flowing toward the commutator. The current in branch (b), which is moving downward through the field, is flowing away from the commutator. When this occurs, the polarity is negative on commutator segment (a) and positive on segment (b). The negative brush is in contact with segment (a), and the positive brush is in contact with segment (b).
As the loop continues to turn, it arrives at the position shown in view B of Figure 18-5. In this position, the branches of the armature coil no longer cut the magnetic field. The current in both conductors drops to zero because a difference in potential no longer exists. In other words, both segments (a) and (b) are at zero potential, and no current flows through the generator or out through the external load. During this period, the two brushes bridge the gap between the segments. As a result, the armature coil is short-circuited on itself. However, since no current is flowing, this condition is harmless.
As the loop continues to turn (view C of Figure 18-5), branch (a) starts to move downward through the magnetic field, and branch (b) starts to move upward. As a result, the polarity of commutator segment (a) changes to positive, and segment (b) changes to negative. However, the continued rotation has also brought segment (a) into contact with the positive brush and segment (b) with the negative brush. As a result, the positive brush develops a positive potential from branch (a), and the negative brush develops a negative potential from branch (b). In other words, at the exact moment when current flow in the conductor loop is reversing, the commutator is counteracting the reversal in the brushes. Current flow is always maintained in the same direction throughout the circuit.
The commutator is the basis of all DC machines (generators and motors). In practice, many armature coils are used. Individual commutator segments are insulated by mica, and a commutator segment is provided for each armature coil lead. There is no difference in the basic principles of the generator or the motor. For this reason, the term "machine" is often used to identify both components when dealing in generalities.
The DC generator may supply electrical ship service loads or just charge batteries. The generator is designed to incorporate its own field poles as part of the electrical load circuit. In this manner, the generator can provide for its own field current in the development of its magnetic field.
Magnetic lines of force exist between two magnets. These magnets represent the field poles. Circular magnetic lines of force exist around any current-carrying conductor. These current-carrying conductors are representative of the armature coils.
Separately, each of these magnetic fields has its own neutral plane. The neutral plane is the area outside the influence of the magnetic fields. The magnetic field of the field. poles alone show the neutral plane perpendicular to the lines of flux (Figure 18-6 view A). Current flow in the armature conductors (view B) without the field pole flux present produces a neutral plane parallel to the lines of flux. In each instance, the neutral plane is located in the same place and outside of the magnetic fields.
The brushes are designed to short-circuit an armature coil when it is located outside the influence of the field poles' magnetic field (in the neutral plane). In this manner, the commutator will not be damaged by excessive sparking because the armature coils are not undergoing induction. When brushes short-circuit two segments that have their armature coils undergoing induction, excessive sparking results, and there is a proportional reduction in EMF (voltage). In Figure 18-6 view C, AB illustrates the original (mechanical) neutral plane. If the brushes were left in this position and the neutral plane shifted, several armature windings would be short-circuited while they were having an EMF induced into them. There would be a great deal of arcing and sparking. Provided the distribution current demands remained constant, the brushes could be moved to the A'B' position where the neutral plane has shifted. This would reduce the amount of sparking at the commutator and sliding brush connections.
However, constantly changing current is the rule, rather than the exception for DC machines.
The effects of armature reaction are observed in both the DC generator and motor. To reduce the effects of armature reaction, DC machines use high flux density in the pole tips, compensating windings, and commutating poles.
Pole Tip Reduced Cross-Sectional Area
The cross-sectional area of the pole tip is reduced by building the field poles with laminations having only one tip (Figure 18-7). These laminations are alternately reversed when the pole core is stacked so that a space is left between alternate laminations at the pole tips. The reduced cross section of iron at the pole tips increases the flux density so that they become saturated. The cross magnetizing and demagnetizing forces of the armature will not affect the flux distribution in the pole face to as great an extent as they would at reduced flux densities.
The compensating winding consists of conductors imbedded in the pole faces parallel to the armature conductors (Figure 18-8). The winding is connected in series with the armature and is arranged so that the magnetizing forces are equal in magnitude and opposite in direction to those of the armature's magnetizing force. The magnetomotive force of the compensating winding therefore neutralizes the armature magnetomotive force, and armature reaction is practically eliminated. Because of the relatively high cost, compensating windings are ordinarily used only on high-speed and high-voltage DC machines of large capacity.
Commutating poles, or interpoles, provide the required amount of neutralizing flux without shifting the brushes from their original position. Figure 18-9 shows the commutating or interposes located midway between the main field poles. The smaller interposes establish a flux in the proper direction and of sufficient magnitude to produce satisfactory commutation. They do not contribute to the generated EMF of the armature as a whole because the voltages generated by their fields cancel each other between brushes of opposite polarity.
The commutating poles are also connected in series with the armature (Figure 18-9 view A). As current increases in the armature, with a resulting increase in armature reaction, the current through the commutating poles also increases. Because these two fields counteract each other, the variable armature reaction is counteracted proportionally. Small DC machines may have only one of these poles.
COMMUTATION
Commutation is the process of reversing the current in the individual armature coils and conducting current to the external circuit during the brief interval of time required for each commutator segment to pass current under a brush. In Figure 18-10, commutation occurs simultaneously in the two coils that are undergoing momentary short circuit by the brush coil B by the negative brush and coil J by the positive brush. As mentioned previously, the brushes are placed on the commutator in a position that short-circuits the coils that are moving through the electrical neutral plane. There is no voltage generated in the coils at that time, and no sparking occurs between commutator and brush.
There are two paths for current through the armature winding. One current flow moves in the opposite direction of the armature rotation, starting at segment 9 and moving to segment 2 through coils I to C. The other current flow moves in the direction the armature rotates, from segment 10 to segment 1 through coils K to A. In this example, the armature maintains two parallel paths for current flow. Current in the coils will reverse directions between the right side and the left side of the armature.
If the load current is 100 amperes, each path will contain 50 amperes. Thus, each coil on the left side of the armature carries 50 amperes in a given direction, and each coil on the right side of the armature carries 50 amperes in the opposite direction. The reversal of the current in a given coil occurs during the time that particular coil is being short-circuited by a brush. For example, as coil A approaches the negative brush, it is carrying the full value of 50 amperes which flows through commutator segment 1 and the left half of the negative brush where it joins 50 amperes from coil C.
At the instant shown, the negative brush spans half of segment 1 and half of segment 2. Coil B is on short circuit and is moving parallel to the field so that its generated voltage is zero, and no current flows through it. As rotation continues in a clockwise direction, the negative brush spans more of segment 1 and less of segment 2. When segment 2 leaves the brush, no current flows from segment 2 to the brush, and commutation is complete.
As coil A continues into the position of coil B, the induced EMF becomes negligible, and the current in A decreases to zero. Thus, the current in the coils approaching the brush is reduced to zero during the brief interval of time it takes for coil A to move to the position of coil B. During this time, the flux collapses around the coil and induces an EMF of self-induction which opposes the decrease of current. Thus, if the EMF of self-induction is not neutralized, the current will not decrease in coil A, and the current in the coil lead to segment 1 will not be zero when segment 1 leaves the brush. This delay causes a spark to form between the toe of the brush and the trailing edge of the segment. As the segment breaks contact with the brush, this action burns and pits the commutator.
The reversal of current in a coil takes place very rapidly. For example, in an ordinary four-pole generator, each coil passes through the process of commutation several thousand times per minute. It is important that commutation be done with as little sparking as possible.
The IEEE Recommended Practice for Electric Installations on Shipboard defines successful commutation in the following manner: "Successful commutation is attained if neither the brushes nor the commutator is burned or injured in an acceptance test; or in normal service to the extent that abnormal maintenance is required. The presence of some visible sparking is not necessarily evidence of unsuccessful commutation."
A commutator with a brown film is an indication of successful commutation. This film should be allowed to remain. To help finely adjust commutation, a small incremental brush adjustment is provided on the brush rigging. When dealing with a generator, the brush rigging may be moved to show the highest voltage reading with limited sparking. This is not a normal maintenance adjustment. Extreme care must be exercised. This adjustment should be done only by a qualified individual.
Generators may have more than one pair of field poles used in combination. This construction is especially advisable on large generators because it permits the production of a given voltage at a much lower speed. For example, to produce a given voltage, a two-pole machine must be driven twice as fast as a four-pole machine and three times as fast as a six-pole machine, assuming equal pole strength in all cases.
DC generators are classified according to their field excitation methods. There are four common types of DC generators:
Series Wound Generator
Figure 18-11 shows the elements of a series wound generator, semipictorially in view A and schematically in view B. The field winding of any generator supplies the magnetic field necessary to induce a voltage in the armature. In most generators, this field winding is supplied with electrical energy by the generator itself. If the generator is connected as shown in Figure 18-11, it is called series wound. One commutator brush is connected to the external load through a switch, the other through a field winding.
Figure 18-12 shows the principle of the shunt wound generator, semipictorially in view A and schematically in view B. In this type of generator, the field coil is connected directly across the commutator brushes. The armature and shunt field are connected in parallel.
In practical machines, the shunt winding is usually provided with a series-connected variable resistance or field rheostat as shown in view A. This permits the strength of the field to be varied to compensate for changes in load.
Inherent Regulation of the Shunt Generator. Internal changes, both electrical and mechanical, that occur in a generator automatically with load change give the generator certain typical characteristics by which it may be identified. These internal changes are referred to as the inherent regulation of the generator.
At no load, when the generator is disconnected from the distribution system, the armature current equals the field current. This is because the shunt field is the only electrical load in the generator's circuit. In Figure 18-12 view B, the armature current flows through the shunt field winding. The winding is actually in series with the armature winding at this time. The shunt field winding has a relatively high resistance, and armature current is kept low. The voltage dropped in the armature is kept low as well because of the small current in the armature and field circuit. Voltage drop in the armature equals the current through the armature multiplied by the internal resistance of the armature (E = IR). With low armature resistance and low field current, there is little armature voltage (IR) drop, and the generated voltage equals the terminal voltage.
With a load applied, the armature IR drop increases but is relatively small compared with the generated voltage. The terminal voltage decreases only slightly provided the speed is maintained at the rated RPM.
Loading is added to generators by increasing the number of parallel paths across the generator terminals. This action reduces the total load circuit resistance. When electrical loads are connected to the generator, the shunt field stops operating like a series load to the armature. Now that all the loads are connected in parallel with the armature, the voltage across each load will remain relatively constant. If the voltage can remain relatively constant in a generator's field, then there is sufficient force to maintain a constant current flow through the field windings. As long as the field is constant, then the armature-induced EMF can be constant.
Since the terminal voltage is approximately constant with the shunt field generator, armature current increases directly with the load. Since the shunt field acts as a separate parallel branch circuit, it receives only a slightly reduced voltage, and its field current does not change to any great extent. Thus, with low armature resistance and a relatively strong field, there is only a small variation in terminal voltage between no load and full load.
External Voltage Characteristics. Curve A of Figure 18-13 shows a graph of the variation in terminal voltage with load on a shunt generator. This curve shows that the terminal voltage of a shunt generator falls slightly with increase in load from no-load to full-load condition. It also shows that with heavy overload the terminal voltage falls more rapidly. The shunt field current is reduced, and the magnetizing effect of the field falls to a low value. The dotted curve A indicates the way terminal voltage falls beyond the breakdown point. In large generators, the breakdown point occurs at several times the rated load current. Generators are not designed to be operated at these large values of current.
The compound wound generator uses both the series and shunt fields. The series field coils are made of a relatively small number of turns of large diameter copper conductor, either circular or rectangular in cross-section area, 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 magnetic field that influences the total magnetic field of the generator. Figure 18-14 schematically shows a compound wound generator of the type known as a long shunt, semipictorially in view A and schematically in view B.
In extreme cases, flashing the field of Army marine DC generators may be done in this manner:
Short and Long Shunt. In the short shunt generator shown in Figure 18-14, the shunt field is connected directly across the commutator and does not receive its current through the series field. The long shunt generator (Figure 18-15) has a shunt field connected to one commutator and what might be called the far end of the series field winding. Long shunt machines are usually used on shipboard.
When it becomes necessary to identify these two windings, an ohmmeter can be used. The large diameter series winding should have a very low resistance. The small diameter shunt winding should have a much higher resistance. Figure 18-16 shows how these windings are marked in the line diagram.
The following are descriptions of under-, flat-, and over-compounding:
Figure 18-18 shows the diverter rheostat in shunt with the series field. View A shows the series field operating at maximum current because the shunt rheostat is adjusted for full resistance. This means that minimum current goes through the rheostat, and maximum current goes through the series field. View A illustrates a compound generator adjusted for an over-compounded condition. In this situation, the generator is designed for a greater voltage at full load than at no load. The maximum resistance position compensates for extreme changes in current demands. A drop in voltage, under extremely high current demands, is prevented.
View C illustrates the diverter adjusted for the under-compounded condition. The rheostat is adjusted for minimum resistance. Most of the current bypasses the series field, and the generator operates with the characteristics of a shunt generator.
The preceding two examples are the extreme conditions. It is the intent of the operator to adjust the diverter for the most stable voltage condition under the immediate electrical load demands of the distribution system. Adjusting the diverter between these two extremes provides the voltage regulation characteristics necessary for operating the generator at or near full-load conditions (View B).
Applications. The compound wound generator is commonly used for shipboard DC power. It is versatile and will stand a wide variety of loads. This is particularly important on cargo ships as the loading from a single winch, for example, may vary from half the capacity of the generator when the winch is hoisting to what might be considered less than zero when the winch is lowering a load.
Speed Control of Generator Output
Since for a given load the output of a DC generator is approximately proportional to the speed at which it is driven (assuming constant field strength), it is possible to control the output by varying the speed. However, most diesel generators are designed to be run at a certain constant speed most suitable for their construction. Therefore, speed control of generator output is seldom satisfactory except in specialized applications, such as propulsion generators.
Field Strength Control of Generator Output
For a given load, the voltage output of any generator is proportional to the field strength of its field poles. The most practical way to regulate generator voltage is to control the field strength. This may be done by placing resistances in series with the shunt field winding, by placing resistances across the series field winding, or by tapping a winding so that any part or all of it may be included in the circuit as desired.
The most practical method of varying field strength is by inserting a variable resistor or rheostat in series with the shunt winding of a compound generator or in the only winding of the simple shunt generators. Figure 18-19 view A shows the circuit of a simple shunt generator. View B shows the circuit of a compound generator. Since the shunt generator, when lightly loaded, tends to deliver a higher voltage than it does as the load increases, it is ordinarily started with a large value of resistance in series with the shunt winding. This keeps the voltage down to a normal value. As loading is increased, the operator cuts more and more of the resistance out of the circuit. At maximum load, the remaining shunt field resistance is very low. This method of control is also used with the compound wound generator, as shown in view B. It is used not so much to compensate for wide voltage variations with loading, but ordinarily to bring the voltage of the compound generator up to a value suitable to connect it across the switchboard bus when it is to be paralleled with another generator.
When a generator is started, the voltage is adjusted by a rheostat in series with the shunt field. When the resistance is increased in the rheostat, current in the shunt field is reduced. With a reduction in shunt field current, a decrease in EMF results. The generator now produces less voltage. If the shunt field resistance is reduced, the generated voltage increases. Figure 18-20 shows the rheostat in series with the shunt field. The shunt field adjusts the no-load voltage of the generator. All Army marine generators have this control. As the electrical distribution system requires the generator to produce more and more current, the generated voltage drops lower and lower. This voltage drop must be manually compensated for by adjusting the shunt field rheostat.
Figure 18-21 illustrates the simple circuit required for operating compound generators in parallel. It is necessary to watch the voltage and amperage much more closely when generators are operating in parallel to prevent troubles that might occur if one generator were to take more than its share of the load. Paralleled generators need to divide the current equally between them. If they do not, the dominant generator will pick up more and more current from the other generator. Eventually, and without any protective devices, the dominant generator will try to motorize the unloaded generator. Because of the like internal resistances of the generators (the maximum power transfer theory), current flow will become excessive, and damage will occur. The reverse current relay is designed to prevent one generator from trying to motorize the other generator.
Suppose that generator 1 in Figure 18-21 is already online and is delivering to the bus its normal electromotive force of 120 volts and its full-rated current of 100 amperes. If the load is increased, it will be necessary to start generator 2 to prevent generator 1 from becoming disconnected from the distribution system by its own circuit breaker. Figure 18-21 shows generator 1 as connected in the circuit and delivering power to the line. If the load is to be increased, it will be necessary to bring generator 2 up to speed so that its voltage will be correct before connecting it into the line with generator 1. For this reason, switch 2 is not closed until generator 2 has been brought up to speed. When constant speed has been reached and generator 2 is at operating temperature, generator 2 must have its shunt field rheostat adjusted so that its voltage is about 1 to 5 volts higher than generator 1.
If the voltage of generator 2 were adjusted to the same voltage as generator 1 and then they both were connected to the bus, generator 2 voltage would decrease. The reduction in generator 2 voltage would result because of the addition of an electrical load. Generator 1 would have an increase in terminal voltage because of the reduction in its electrical load. Generator 1 would start to take more and more of the electrical load from generator 2. Generator 2 could eventually become a load itself, and generator 1 may even try to drive it as a motor. Basically DC generator paralleling is quite simple.
To place one generator on line --
To parallel generators --
The generators are paralleled. To secure a generator, follow the sequence below:
NOTE: Just as when dealing with any other component, always check the manufacturer's manual or technical references for specific information. The above procedure has been provided in lieu of the information lost to antiquity.
NOTE: Maintenance and repair procedures of the DC motor and generator can be found in TM 5-764, Electric Motor and Generator Repair, dated September 1964.