All generators change mechanical energy into electrical energy. This is the easiest way to transfer power over distances. Fuel is used to operate the diesel or prime mover. The fuel is converted into energy to turn the generator. The generator's movement, magnetic field, and associated wiring change this mechanical energy into electrical energy. Wires and cables deliver this power to the electrical loads. The motor is designed to change electrical energy back into mechanical energy to do work.
Chapter 13 describes the rectified AC generator which produced a DC output to operate small DC electrical systems. This chapter describes the three-phase AC brushless generator which delivers three-phase AC to the ship's main electrical distribution system. Most of the large generators used to provide AC to ships' electrical systems are of the brushless type. The brushless generator eliminates the weak link (brushes and slip rings) in the generating system and reduces the maintenance required. There are revolving field and revolving armature brush and slip ring generators in use today. However, brushless AC generators are used exclusively as the Army's ship service power source.
Figure 14-2 illustrates the brushless generator. The main frame or housing (7) is a strong metallic structure surrounding and retaining the stationary windings (6). The main frame is, in turn, supported by mounts. These mounts are not rigid. Rubber composition or springs are incorporated into the mounts as shock absorbers called resilient mounts.
One end of the generator main frame is bolted to the prime mover's flywheel housing. The other end of the generator main frame is bolted to the bell end or end frame (12). The bell end contains a bearing (17) that supports the internal rotor shaft.
The other end of the rotor shaft connects to a flexible drive disc (29) and fan (15) assembly. The drive disc assembly, in turn, is bolted to the flywheel of the prime mover. When the prime mover turns, the drive disc turns, and the fan pulls cooling air into the housing to dissipate heat created in the generator windings.
The fan can disturb high bilge water and pass particulate of oil over the windings. When oil-covered windings become incapable of transferring sufficient heat to the air stream, the winding insulation becomes damaged. It is imperative that low bilge levels be maintained, and the diesel air box ventilation exhausts away from the fan's air flow.
Generator Windings
There are four different sets of windings in the brushless generator (Figure 14-2). Two windings (6 and 10) are connected to the generator main frame, and two windings (8 and 9) are fitted to the rotor shaft. There are no direct mechanical electrical connections between the rotating and stationary windings of the generator.
Winding Contamination
Inspect the stationary and rotating winding periodically for cleanliness. The chief engineer or his appointed representative will supervise internal inspection of the ship service generator. Never inspect internal generator components while the prime mover is operating or the generator is connected to the switchboard bus. Always secure the prime mover fully and ensure other power sources, such as the emergency generator or shore power, cannot feed the generator being serviced.
Textbook maintenance practices call for removal of dirt by vacuuming and removal of grease and oil through wiping with lint-free rags. These methods rarely serve the purpose intended. Contamination prevention is the key. Inspect the generator prime mover for gasket and seal leaks. Check also adjacent piping and deck plates for liquids and particles. Once the windings become contaminated, there is no thorough and safe method to clean the generator windings on board the vessel. The only effective recourse requires the removal of the generator, its complete disassembly, chemical cleaning, and baking by the DS/GS maintenance activity.
When contamination is found, use the megger to check the insulation values. Always disconnect the rotating rectifier, voltage regulator, and any other components that house semiconductors. Compare readings with the appropriate technical manual, with other known good generator readings, or against megger historical documentation.
Exciter Field
The exciter field is a stationary DC energized winding. This is the winding where the DC magnetic field is initially developed. Even before any voltage regulation takes place, a residual magnetic field exists in the poles. During voltage regulation, DC in the exciter field induces an EMF and resulting current flow in the exciter armature. This winding can be found mounted toward the bell end section of the generator.
Exciter Armature
The exciter armature is a three-conductor, three-phase rotating winding. The exciter armature is located directly inside the exciter stator. A three-phase EMF is induced in the exciter armature as it rotates inside the fixed magnetic field of the exciter field.
Together, the exciter field and exciter armature develop a three-phase AC. In effect, this is a rotating armature generator. This portion of the generator is used to provide the excitation necessary for the main field portion. The exciter field and armature operate in the same manner described in Chapter 13. The exciter portion is the generator that develops the power necessary to develop the magnetic field in the main generator portion. Since current is induced into the armature without the aid of wires, brushes and slip rings are eliminated.
Rotating Rectifier
The output developed from the exciter portion of the generator is AC. To produce the enhanced three-phase output from the main armature of the generator (necessary for the large power requirements of the distribution system), the main field must be provided a direct current source. To change (or rectify) the exciter portion output from AC to DC, the rotating rectifier is used. The rotating rectifier provides the same conversion of AC to DC as the diode combination discussed in Chapter 13 for the belt-driven alternator.
Main Rotating Field
The main rotating field (8 in Figure 14-2) can consist of four to eight individual coils or pole pieces keyed to the rotor shaft. The coils are connected in series and consist of only one wire. The direction that the wire is wound around the pole piece determines the magnitude polarity of each individual field coil. Rectified DC, from the rotating rectifier (11), develops the revolving magnetic field inside the main field generator portion providing alternate fixed field polarities.
The main armature (6 in Figure 14-2 view B) is bolted to the inside of the main frame. There are three windings, each of which are spaced 120 mechanical and electrical degrees apart. They may be connected in either wye or delta configurations as required for the application. The main armature windings are connected directly to the electrical system through the switchboard.
The brushes and slip rings used by many small generators become intense maintenance problems. This area is extremely prone to contamination. As the brush slides over the slip ring, a certain amount of arcing may take place. To eliminate brushes, two generators are coupled together in a single housing. A rectified rotating armature generator, similar to the one discussed in Chapter 13, provides a direct current source for the rotating field of the main generator. Putting these two generators together eliminates the need for any physical mechanical connection between the moving and the stationary parts of the generator. Figure 14-4 is a pictorial diagram of the electrical circuits in the generic brushless AC generator.
The following is a basic outline of the brushless generator operation:
Permanent Magnet Generator (PMG)
The voltage regulator (Figure 14-5) controls the output of the generator by controlling the magnetic field in the stationary exciter field winding. The voltage regulator senses the generator's output voltage directly from the generator's main armature windings or indirectly through generator cable connections within the switchboard. The voltage regulator may monitor only a portion of the single phase (Figure 14-6) from the main armature's three-phase or each phase directly from the switchboard.
Only a single-phase EMF (voltage) can be induced (produced) in a single pair of the armature windings. Since there are three such pairs of windings, three separate single-phase EMF values are induced. It is the development of each of the three single-phase values that together produce the three-phase output from the armature windings (Figure 14-7).
Phases are often referred to in the following manners:
Figure 14-8 illustrates the three-phase winding combination of the wye-connected armature.
The three-phase condition is the culmination of all the windings, A-B-C. This produces the highest voltage and current for a given period of time in the electrical system. The three-phase condition takes advantage of three independent electrical circuits almost simultaneously. Figure 14-9 shows how one circuit (between the generator armature and the motor stator windings) at a time is completed. Armature windings A to B and the motor's equivalent A (T1) to B (T2) windings complete one circuit (view A). After 120 degrees of generator rotor rotation, the B to C circuit starts (view B) in the same manner as the A to B circuit; 240 rotor degrees after the A to B circuit started, the C to A circuit is starting (view C). In effect, three single-phase currents are delivered to three motor windings in various amplitudes over the same period of time.
A phase is the reoccurring electrical event, or value, found between any combination of the generator's armature terminals. In other words, a phase is the voltage and current found between terminals A to B, B to C, C to A, A to N, B to N, C to N, or terminals A, B, and C (Figure 14-10).
The three-phase circuit uses three such combinations in varying amplitudes at the same time (Figure 14-11). Although each sine wave is usually identified as A, B, or C, each sine wave is a combination of a completed circuit. A better representation of the three-phase sine waves would identify each wave as the circuit it completes, such as A-B, B-C, or C-A. In this manner, it is easy to see the three single phases, operating out of phase by 120 electrical and mechanical degrees. It also becomes apparent that with the loss of anyone winding (A for example), only one complete circuit phase is left. Without phase A, there cannot be a completed circuit between A-B or C-A. This leaves only B-C and a single-phase event. This electrical three-phase malfunction is called single phasing.
The following terms describe the operation of the generator and the transformer:
The generator main armature has six individual windings (Figure 14-12). Two windings are for use in each phase-to-neutral combination. Each of the two leads from each winding may be brought out of the armature to a connection box for connecting externally. How these windings are wired together will determine the current and voltage characteristics of the generator output terminals. Although this chapter deals primarily with ship service operations, the combinations of windings remain pertinent to transformers and motors alike.
In the wye armature (Figure 14-13), the series connection is the key to the voltage and current output. If each phase winding can develop a specific number of amperes and volts, then the generator's total output characteristics can be calculated. For example, the phase winding A-N, B-N, or C-N can develop an induced EMF at 260 volts with a maximum resulting current of 100 amperes.
Line current = phase current = 100 amperes
Phase-to-phase (or line) voltage, as described in the series circuit rules, is the sum of the individual phase winding voltages. In this case, 260 volts from A-N, for example, cannot be added to 260 volts from B-N because the same magnetic flux of the generator's main field does not affect them equally. The phase windings are displaced in time and space by 120 degrees. Instead, a constant has been developed through vector mathematics as 1.732. This figure will always hold true for basic electrical needs. Figure 14-14 shows the connection between voltage in a series circuit and the 1.732 multiplication factor.
The total voltage in a series circuit equals the sum of the voltages. However, Figure 14-14 shows that the north magnetic polarity is influencing one armature winding in its entirety. The second overlapping armature winding is being affected to a great extent, but not fully. The third overlapping armature winding is not affected at all by the north field pole polarity. The magnetic influence of the single pole cannot affect each physically displaced winding equally. Think of the 1.732 multiplication factor as the following:
Voltage total = one complete armature winding + 73 percent of the other armature winding
Et = 260 volts + (.73)(260 volts)
Et = 260 volts + 190 volts (approximate)
Et = 450 volts
NOTE: This is an oversimplification of the entire electrical process. These armature winding effects happen to all three windings by two different polarities constantly in various degrees at any given time.
Multiplying the voltage by 1.732 gives the voltage supplied to the electrical system by phase A-B (or B-C or C-A):
(Phase-to-neutral voltage) x (1.732) = line voltage
(260 volts) x (1.732) = 450 volts
Should an additional single-phase voltage value be desired, the wye can be tapped at the neutral connection. This provides the phase-to-neutral voltage. In this case, A-N, B-N, or C-N would provide another voltage value possibility of 260 volts (Figure 14-15).
The delta connects the windings in parallel (Figure 14-16). The positive end of each winding is connected to the negative end of another winding. The terminals are labeled A, B, and C.
The delta connection in Figure 14-17 shows that a complete circuit between any phase provides two paths for current to flow. For example, current may leave armature terminal A, go through the motor stator, and return to armature terminal B to complete the A-B circuit). The current in the C-A and the B-C phases are also affected by the rotor field in various degrees. Notice that there is no single common connection. All phases in the armature are connected together in parallel during any single-phase complete circuit. The parallel circuit rules, therefore, are the key to understanding the delta connection.
The same example values that were used with the wye-connected windings are used for the delta. Each phase winding can develop an induced EMF of 260 volts with a maximum current value of 100 amperes (Figure 14-18). The basic parallel rule states that voltage remains constant. Therefore--
Line voltage = phase voltage = 260 volts
Line current = (phase current) x (1.732)
Line current = (100 amps) x (1.732)
Figure 14-19 details some of the possible configurations for wiring ship service generators. The many options let the manufacturer keep costs low by reducing the number of different generators that must be built and stocked. This lets the consumer determine what application of the generator best serves him.
When using identical windings, the high delta is the same voltage as the low wye and half the voltage of the high wye (Table 14-1). The parallel delta is half the voltage of the high delta and low wye and one-fourth the voltage of the high wye.
Frequency = (number of poles) x (RPM)
(2 poles per pair*) x (60 seconds per minute**)
Frequency = (4 poles) x (1,800 RPM)
(2 poles per pair*)
x (60 seconds per minute**)
Frequency = 7,200
120
Frequency = 60 revolutions (or cycles) per second
*2 poles per pair is a constant used to account for the requirement of two poles of one north and one south polarity for each individual cycle of events.
** 60 seconds per minute is a constant used to convert events per minute to cycles per second.
Poles and Frequency Relationships
Table 14-2 lists some of the more common speed and rotor pole relationships of the AC generator for 60 hertz operation.
-- The diesel flywheel has stored energy which tends to keep the diesel moving at the same speed.
Figure 14-20 view A shows the damper windings. These windings are nothing more than metal bars parallel to the rotor shaft (view B). The forward ends of these bars are connected together by shorting rings. The aft ends of these bars are connected in a like manner. When the magnetic fields from the main field and the main armature change in respect to each other, an EMF is induced in the damper bars. A resulting current flows, and a magnetic field is established because the bars are short-circuited at each end. The magnetic field that develops in the damper windings is a result of any change in the magnetic flux between the field and the armature windings. The magnetic field in the damper windings opposes the effects that created it.
Resistance of the Armature Conductors
The DC generator output can be measured easily in watts. To calculate DC power, multiply the current by the voltage (review Power in Chapter 2). Unlike DC, AC does not maintain a constant amplitude. Further, the current and voltage are influenced by the very nature of the reversing current flow characterized by AC (Figure 14-23). To understand how these circumstances affect the actual generator output, the actual values available at the generator terminals must be understood.
Inductance was discussed in Chapter 6. Every motor, generator, and transformer has a coil of wire called an inductor. By the very nature of AC and its effects on the inductors in the circuit, current often lags voltage. The average current lag is represented by a decimal known as the power factor (PF). Unity power factor indicates that current and voltage arrive together and are in phase with each other. Unity power factor has a decimal representation of 1.0. Unity is the best use of electrical power. This condition results when all the power is consumed in the circuit. The ratio of unity to current lag is approximately 80 percent or .8 PF.
The inductors in the motors actually become their own miniature generators, inducing an EMF that opposes a change in current.
The current from the generator must overcome the resistance in the wires of the motor as well as overcome this counter EMF. The extra current developed by the generator, necessary to overcome the CEMF, is not consumed by the motor. It is considered to be a shuttle power, existing in the electrical system moving between the generator and the motor. This condition also results between two generators when they are improperly paralleled.
When AC is applied only to resistors, lights, and heaters, all the power is consumed in the circuit. The power consumed in the resistive AC circuit can be computed the same as in the DC system. This is the true power (or active power) that has been consumed and is expressed in kilowatts (KW).
Figure 14-24 view A shows that the current and voltage rise and fall together. Only when the peak current and voltage are in phase is the product of voltage and amperage the same as the power consumption of the load in watts.
This situation is inconceivable on board a vessel. The current held momentarily suspended in time by the CEMF generated by the action of the inductive coils cannot be successfully multiplied together to determine the power consumption of the electrical system. In view B, the current is delayed behind the voltage. The apparent power (power that the generator apparently sees that must be added to compensate) is represented by kVA.
Power factor is the percentage of the true power to apparent power or --
The electrical system must be designed to operate on the apparent power of the system, not the true power of the system. Apparent power is always greater than true power when there is a power factor less than 1.0 (unity).
The following are some additional formulas that may be useful:
(Three-phase applications)
