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CHAPTER 14

SHIP SERVICE GENERATORS (AC)

INTRODUCTION

All generators change mechanical energy intoelectrical energy. This is the easiest way to transferpower over distances. Fuel is used to operate thediesel or prime mover. The fuel is converted intoenergy to turn the generator. The generator’s move-ment, magnetic field, and associated wiring changethis mechanical energy into electrical energy. Wiresand cables deliver this power to the electrical loads.The motor is designed to change electrical energyback into mechanical energy to do work.

Chapter 13 describes the rectified AC gener-ator which produced a DC output to operate smallDC electrical systems. This chapter describes thethree-phase AC brushless generator which deliversthree-phase AC to the ship’s main electrical distribu-tion system. Most of the large generators used toprovide AC to ships’ electrical systems are of thebrushless type. The brushless generator eliminatesthe weak link (brushes and slip rings) in the generat-ing system and reduces the maintenance required.There are revolving field and revolving armaturebrush and slip ring generators in use today. How-ever, brushless AC generators are used exclusively asthe Army’s ship service power source.

BRUSHLESS ALTERNATING CURRENTGENERATOR CONSTRUCTION

Figure 14-2 illustrates the brushless generator.The main frame or housing (7) is a strong metallicstructure surrounding and retaining the stationarywindings (6). The main frame is, in turn, supportedby mounts. These mounts are not rigid. Rubbercomposition or springs are incorporated into themounts as shock absorbers called resilient mounts.

One end of the generator main frame is boltedto the prime mover’s flywheel housing. The otherend of the generator main frame is bolted to the bellend or end frame (12). The bell end contains abearing (17) that supports the internal rotor shaft.

The other end of the rotor shaft connects to aflexible drive disc (29) and fan (15) assembly. Thedrive disc assembly, in turn, is bolted to the flywheelof the prime mover. When the prime mover turns,the drive disc turns, and the fan pulls cooling air intothe housing to dissipate heat created in the generatorwindings.

The fan can disturb high bilge water and passparticulate of oil over the windings. When oil-covered windings become incapable of transferringsufficient heat to the air stream, the winding insula-tion becomes damaged. It is imperative that lowbilge levels be maintained, and the diesel air boxventilation exhausts away from the fan’s air flow.

Generator Windings

There are four different sets of windings inthe brushless generator (Figure 14-2). Two windings(6 and 10) are connected to the generator mainframe, and two windings (8 and 9) are fitted to therotor shaft. There are no direct mechanical electricalconnections between the rotating and stationarywindings of the generator.

Winding Contamination

Inspect the stationary and rotating windingperiodically for cleanliness. The chief engineer orhis appointed representative will supervise inter-nal inspection of the ship service generator. Neverinspect internal generator components while theprime mover is operating or the generator is con-nected to the switchboard bus. Always secure theprime 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 forremoval of dirt by vacuuming and removal of greaseand oil through wiping with lint-free rags. Thesemethods rarely serve the purpose intended. Con-tamination prevention is the key. Inspect the genera-tor prime mover for gasket and seal leaks. Check

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also adjacent piping and deck plates for liquids andparticles. Once the windings become contaminated,there is no thorough and safe method to clean thegenerator windings on board the vessel. The onlyeffective recourse requires the removal of the genera-tor, its complete disassembly, chemical cleaning, andbaking by the DS/GS maintenance activity.

When contamination is found, use the meggerto check the insulation values. Always disconnect therotating rectifier, voltage regulator, and any othercomponents that house semiconductors. Comparereadings with the appropriate technical manual, withother known good generator readings, or againstmegger historical documentation.

Exciter Field

The exciter field is a stationary DC energizedwinding. This is the winding where the DC magneticfield is initially developed. Even before any voltageregulation takes place, a residual magnetic fieldexists in the poles. During voltage regulation, DCin the exciter field induces an EMF and resultingcurrent flow in the exciter armature. This windingcan be found mounted toward the bell end sectionof the generator.

Exciter Armature

The exciter armature is a three-conductor,three-phase rotating winding. The exciter armatureis located directly inside the exciter stator. A three-phase EMF is induced in the exciter armature as itrotates inside the fixed magnetic field of the exciterfield.

Together, the exciter field and exciter armaturedevelop a three-phase AC. In effect, this is a rotatingarmature generator. This portion of the generator isused to provide the excitation necessary for the mainfield portion. The exciter field and armature operatein the same manner described in Chapter 13. Theexciter portion is the generator that develops thepower necessary to develop the magnetic field in themain generator portion. Since current is inducedinto the armature without the aid of wires, brushesand slip rings are eliminated.

Rotating Rectifier

The output developed from the exciter portionof the generator is AC. To produce the enhancedthree-phase output from the main armature of thegenerator (necessary for the large power require-ments of the distribution system), the main field mustbe provided a direct current source. To change (orrectify) the exciter portion output from AC to DC,the rotating rectifier is used. The rotating rectifierprovides the same conversion of AC to DC as thediode combination discussed in Chapter 13 for thebelt-driven alternator.

Main Rotating Field

The main rotating field (8 in Figure 14-2) canconsist of four to eight individual coils or pole pieceskeyed to the rotor shaft. The coils are connected inseries and consist of only one wire. The direction thatthe wire is wound around the pole piece determinesthe magnitude polarity of each individual field coil.Rectified DC, from the rotating rectifier (11),develops the revolving magnetic field inside the main

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field generator portion providing alternate fixed field short-circuited at each end to allow currents topolarities. circulate so that a magnetic field can be produced to

oppose any change in prime mover motion.Amortisseur or Damper Winding

Main ArmatureEmbedded in the face of each main field pole

piece is the Amortisseur or damper windings. Theseare necessary for generators that operate in parallel.These become very important when dealing withfrequency. The frequency of an AC generator mustnot change. These damper windings prevent hunt-ing during parallel operation. Damper windingsare copper or aluminum conductors embeddedjust below the surface of the rotor. They are

The main armature (6 in Figure 14-2 view B) isbolted to the inside of the main frame. There arethree windings, each of which are spaced 120mechanical and electrical degrees apart. They maybe connected in either wye or delta configurations asrequired for the application. The main armaturewindings are connected directly to the electrical sys-tem through the switchboard.

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BRUSHLESS ALTERNATING CURRENTGENERATOR OPERATION

The brushes and slip rings used by many smallgenerators become intense maintenance problems.This area is extremely prone to contamination. Asthe brush slides over the slip ring, a certain amountof arcing may take place. To eliminate brushes, twogenerators are coupled together in a single housing.A rectified rotating armature generator, similar tothe one discussed in Chapter 13, provides a directcurrent source for the rotating field of the maingenerator. Putting these two generators togethereliminates the need for any physical mechanical con-nection between the moving and the stationary parts

of the generator. Figure 14-4 is a pictorial diagramof the electrical circuits in the generic brushless ACgenerator.

Generator Residual Magnetism

Residual magnetism exists in all ferrous metalthat has had a current carried around it. In manygenerators, there is not enough material to provide asubstantial residual magnetic field to use in creatingan EMF. The ship service generator has a lot ofmetal. The material mass maintains enough of aresidual magnetic field in the exciter field to inducean EMF in the exciter armature when there is motion.

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Outline of Operation

The following is a basic outline of the brushlessgenerator operation:

The prime mover starts. The primemover crankshaft revolves, and the gen-erator shaft is moving. This turns theexciter armature, the main field, and therotating rectifier.

The exciter initiates an EMF. The rotatingexciter armature cuts the residual mag-netic field left over in the exciter field polepieces. A small EMF is induced in thewye-wound rotating exciter armaturewindings. The exciter portion of themachine operates as a revolving armaturegenerator.

Exciter AC is rectified to DC. The smallexciter three-phase AC is directed to therotating rectifier. The diodes rectify theAC to a pulsating DC. Five wires areconnected to the rotating rectifier.Three wires are from the three-phaseexciter armature, and two wires direct theDC output to the main field winding.

The main field induces an EMF into themain armature. Direct current enters therotating main field. As the rotor shaftturns the main field, the alternatingpolarities induce an EMF of alternatingpotentials in the main armature windings.

Three-phase AC is produced from themain armature. The main armature hasthree windings producing three-phase AC.The main portion of the generator isoperating as a revolving field generator.Initially, only a small three-phase EMF isproduced.

Voltage control takes over. The voltageregulator senses an undervoltage condi-tion and diverts the current flow back tothe stationary exciter field. In this case, theCF exciter field winding is used for theinitial voltage buildup and some short-circuited conditions. The current flowthrough the exciter field winding increases

its magnetic field. The exciter armatureconductors now cut through a greatermagnetic field, and the induced EMF inthe exciter armature is increased.

The process is repeated until satisfactoryvoltage is achieved. The increased exciterarmature current is rectified by the rotat-ing rectifier and directed again to therotating main field. The increased mag-netic field, of the rotating main field,sweeps past the conductors in the station-ary main armature. This produces agreater three-phase EMF. Normal volt-age control is maintained by the regulatorcontrolling current to the exciter field.

Permanent Magnet Generator (PMG)

Newer Army generators employ six separatewindings. The additional two windings are identi-cal in operation to any pair of field and armaturewindings described above. These extra windingsprovide external excitation for the generator in thesame way the four winding generator provided forits own self-excitation.

On Cummins generators and some Caterpillargenerators, a permanent magnet generator has beenadded to the generator assembly. The magnet ismounted on the rotor and is located inside the per-manent magnet armature. When the generator isrunning, the PMG magnet generates an EMF in thePMG armature, providing current directly to theautomatic voltage regulator for control of the exciterfield. The permanent magnet provides definite volt-age output on start-up and greater voltage controlunder extreme load conditions.

GENERATOR VOLTAGE CONTROL

The voltage regulator (Figure 14-5) controlsthe output of the generator by controlling the mag-netic field in the stationary exciter field winding. Thevoltage regulator senses the generator’s output volt-age directly from the generator’s main armaturewindings or indirectly through generator cable con-nections within the switchboard. The voltageregulator may monitor only a portion of the singlephase (Figure 14-6) from the main armature’s three-phase or each phase directly from the switchboard.

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The generator output voltage is controlled bycontrolling the current in the exciter field windings.Self-excited generators redirect part of their mainarmature AC output to the voltage regulator. Insidethe voltage regulator, the armature AC is rectified toDC. The voltage regulator applies the DC to theexciter field to increase or decrease the magneticfield. When the exciter field magnetic strength isgreat, the generator’s output is improved. With adecrease in the exciter field strength, the output ofthe generator is reduced.

Separately excited generators sense the out-put voltage in the same manner as self-excitedgenerators but derive the current for exciter fieldcontrol directly from a permanent magnet genera-tor designed exclusively for that purpose. In eachcase, changes in the magnetic exciter field strengthis derived from DC supplied from the voltageregulator.

FLASHING THE FIELD

Initially, self-excited ship service generatorsmay need to have the fields flashed to establish the

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residual magnetism necessary to start the exciterinduction process.

NOTE: Read the manufacturer’srecommendations carefully. Damageto the generator or voltage regulatorwill result if proper procedures arenot correctly followed.

Generally, flashing the field is done by connect-ing a battery to the exciter field terminals. By allow-ing directing current to flow through the exciter fieldwindings, a residual magnetic field is established.The direct current source must be connected cor-rectly. Failure to do so can damage the voltageregulator semiconductors. Observe the field andbattery polarity. Ensure the connections are positiveto positive and negative to negative.

MAIN ARMATURE THREE-PHASECONNECTIONS

Brushless generators are usually 240- or 450-volt three-phase AC machines. They are rated inkVA at a specified power factor. Many generatormanufacturers produce a basic unit with a variety ofvoltage and current possibilities.

The main armature consists of six individualwindings. Two windings, as a pair, are connected toeach other in series or parallel. Each armature wind-ing pair is then connected to the other two armaturewinding pairs to form the common wye or deltacombination. The actual connection between eachwinding is completed outside of the generator’sframe in the attached terminal connector box. In thismanner, the user can connect the individual armaturewinding pairs in series or parallel and the pairs in thedelta or wye configuration. The configuration isselected for the type of voltage and current require-ments that best fit the application.

PHASES

Only a single-phase EMF (voltage) can beinduced (produced) in a single pair of the armaturewindings. Since there are three such pairs of wind-ings, three separate single-phase EMF values areinduced. It is the development of each of the threesingle-phase values that together produce thethree-phase output from the armature windings(Figure 14-7).

Phases are often referred to in the followingmanners:

Three-phase: phase to phase to phase,A-B-C.

Single-phase high voltage: phase to phase,A-B, B-C, A-C.

Single-phase low voltage: phase toneutral, A-N, B-N, or C-N.

Single-phase connections using the neutral arethe least common combination on commercial orArmy watercraft. The neutral connection iseliminated on all Army watercraft ship service gen-erators but retained on some three-phase delta-wye-connected transformers.

Figure 14-8 illustrates the three-phase windingcombination of the wye-connected armature.

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The three-phase condition is the culminationof all the windings, A-B-C. This produces the highestvoltage and current for a given period of time in theelectrical system. The three-phase condition takesadvantage of three independent electrical circuitsalmost simultaneously. Figure 14-9 shows how onecircuit (between the generator armature and themotor stator windings) at a time is completed. Arma-ture windings A to B and the motor’s equivalent A(T1) to B (T2) windings complete one circuit (viewA). After 120 degrees of generator rotor rotation,the B to C circuit starts (view B) in the samemanner as the A to B circuit; 240 rotor degreesafter the A to B circuit started, the C to A circuitis starting (view C). In effect, three single-phasecurrents are delivered to three motor windings invarious amplitudes over the same period of time.

NOTE: For clarity, three differentperiods of time are used to reflect acurrent at a maximum amplitude inone specific direction per completedcircuit. In reality, current is movingin various amplitudes and directionsat any point in time as illustrated bythe three-phase sine wave.

A phase is the reoccurring electrical event, orvalue, found between any combination of the genera-tor’s armature terminals. In other words, a phaseis 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, orterminals A, B, and C (Figure 14-10).

The electrical value found between any twoterminals is a single-phase event. The electrical valuefound between three terminals is a three-phase event.There cannot be a two-phase value derived fromthese terminals. The single-phase circuit has anelectrical load between two of the generator ter-minals. This is the only way to provide for the com-plete circuit required for current flow. The armaturewinding has the difference in potential required toattract the electron back. For example, terminals Aand B complete one phase.

The three-phase circuit uses three such com-binations in varying amplitudes at the same time(Figure 14-11). Although each sine wave is usuallyidentified as A, B, or C, each sine wave is a combina-tion of a completed circuit. A better representationof the three-phase sine waves would identify eachwave as the circuit it completes, such as A-B, B-C, orC-A. In this manner, it is easy to see the three singlephases, operating out of phase by 120 electrical andmechanical degrees. It also becomes apparent thatwith the loss of anyone winding (A for example), onlyone complete circuit phase is left. Without phase A,there cannot be a completed circuit between A-B orC-A. This leaves only B-C and a single-phase event.This electrical three-phase malfunction is calledsingle phasing.

The following terms describe the operation ofthe generator and the transformer:

Phase to phase to phase. This is a three-phase event using all the available voltageand current values in the entire generatorarmature over a period of time.

Phase to phase. This is a single-phaseevent, providing the total available voltageand current value from two individualphases. This is a high voltage single phase.There is no difference in voltage betweenphase-phase and phase-phase-phasevalues in the same machine.

Phase to neutral. This is a single-phaseevent using any voltage and current thatcan be induced into one armature windingalone. This is a low voltage single-phasevalue.

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WINDING COMBINATIONS

The generator main armature has six individualwindings (Figure 14-12). Two windings are for use ineach phase-to-neutral combination. Each of the twoleads from each winding may be brought out of thearmature to a connection box for connecting exter-nally. How these windings are wired together willdetermine the current and voltage characteristics ofthe generator output terminals. Although this chap-ter deals primarily with ship service operations, thecombinations of windings remain pertinent to trans-formers and motors alike.

The neutral is used as a reference point whendealing with the Army’s AC generators. Currently,the neutral is isolated within the connector box andleft unconnected. However, it is necessary to under-stand the neutral conductor and its effects on theelectrical system. The LSV uses the neutral leadfrom delta-wye transformers. The transformer’sprimary side receives three conductors from the gen-erator armature. The secondary side of the LSVthree-phase transformer uses the neutral terminal

and provides four leads and an extra low voltagecapability. Whether windings are connected in anarmature, in a single three-phase transformer, or inthree single-phase transformers, the wye and deltacombinations all apply equally.

WYE-CONNECTED ARMATURE

Marine generators are almost exclusively wye-connected. The wye is a series winding because anycompleted circuit, A-B, B-C, or C-A, has only onepath for current to flow. The neutral (N) representsthe common connection point where one end of eachof the individual phase windings are connected. Theother end of the phase windings deliver power to theelectrical system as terminals: A, B, or C.

In the wye armature (Figure 14-13), the seriesconnection is the key to the voltage and currentoutput. If each phase winding can develop a specificnumber of amperes and volts, then the generator’stotal output characteristics can be calculated. Forexample, the phase winding A-N, B-N, or C-N candevelop an induced EMF at 260 volts with a maxi-mum resulting current of 100 amperes.

Basic series circuit rules apply. In a seriescircuit, amperage remains constant. Therefore, cur-rent available to the electrical system from any phase-to-phase combination, A-B, B-C, or C-A, is 100amperes.

Line current = phase current = 100 amperes

Phase-to-phase (or line) voltage, asdescribed in the series circuit rules, is the sum ofthe individual phase winding voltages. In this case,

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260 volts from A-N, for example, cannot be added to260 volts from B-N because the same magnetic fluxof the generator’s main field does not affect themequally. The phase windings are displaced in timeand space by 120 degrees. Instead, a constant hasbeen developed through vector mathematics as1.732. This figure will always hold true for basicelectrical needs. Figure 14-14 shows the connec-tion between voltage in a series circuit and the1.732 multiplication factor.

The total voltage in a series circuit equals thesum of the voltages. However, Figure 14-14 showsthat the north magnetic polarity is influencing onearmature winding in its entirety. The second over-lapping armature winding is being affected to a greatextent, but not fully. The third overlapping armaturewinding is not affected at all by the north field polepolarity. The magnetic influence of the single polecannot affect each physically displaced windingequally. Think of the 1.732 multiplication factor asthe following:

Voltage total = one complete armature winding + NOTE: This is an oversimplification73 percent of the other armature winding of the entire electrical process. These

Et = 260 volts + (.73)(260 volts)armature winding effects happen toall three windings by two different

Et = 260 volts + 190 volts (approximate)polarities constantly in various degreesat any given time.

Et = 450 volts

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Multiplying the voltage by 1.732 gives the volt-age 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 voltagevalue be desired, the wye can be tapped at theneutral connection. This provides the phase-to-neutral voltage. In this case, A-N, B-N, or C-Nwould provide another voltage value possibility of260 volts (Figure 14-15).

DELTA-CONNECTED ARMATURE

The delta connects the windings in parallel(Figure 14-16). The positive end of each winding isconnected to the negative end of another winding.The terminals are labeled A, B, and C.

The delta connection in Figure 14-17 showsthat a complete circuit between any phase providestwo paths for current to flow. For example, currentmay leave armature terminal A, go through the motorstator, and return to armature terminal B to completethe A-B circuit). The current in the C-A and the B-Cphases are also affected by the rotor field in variousdegrees. Notice that there is no single common con-nection. All phases in the armature are connectedtogether in parallel during any single-phase completecircuit. The parallel circuit rules, therefore, are thekey to understanding the delta connection.

The same example values that were used withthe wye-connected windings are used for the delta.Each phase winding can develop an induced EMF of260 volts with a maximum current value of100 amperes (Figure 14-18). The basic parallel rulestates that voltage remains constant. Therefore—

Line voltage = phase voltage = 260 volts

Line current is the sum of the individual cur-rents developed in the generator. Since the magneticfield of the rotor cannot affect all the phase windingsequally, the constant 1.732 must be used for thecalculations.

Line current = (phase current) x (1.732)

Line current = (100 amps) x (1.732)

Line current = 173 amperes

POWER CONSIDERATIONS

Both the wye- and the delta-wound generatorhave approximately the same power capability. Thewye generator is the most favorable generator for theapplication. By using the higher voltage and lowercurrent (450 volts and 100 amperes), smallerdiameter conductors, contacts, and circuit breakerscan be used in the electrical system. Dual voltagemotors will run cooler and last longer with the lowercurrent value.

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The delta generator uses 260 volts at 173amperes to provide the same power needs to theelectrical distribution system. Increased currentmeans increased heat. Heat is the element ofdestruction to electrical components. The increasein current requires increased cable, contactor, andmotor protective size. Any increase in size is anincrease in cost.

Figure 14-19 details some of the possible con-figurations for wiring ship service generators. Themany options let the manufacturer keep costs low byreducing the number of different generators thatmust be built and stocked. This lets the consumerdetermine what application of the generator bestserves him.

WYE-DELTA VOLTAGES

When using identical windings, the high deltais the same voltage as the low wye and half the voltageof the high wye (Table 14-1). The parallel delta is halfthe voltage of the high delta and low wye and one-fourth the voltage of the high wye.

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GENERATOR PHASE BALANCE

The ship’s distribution system has beenspecifically designed with certain conditions in mind.Your specific involvement with the actual design isunlikely. However, the marine engineer’s direct in-fluence on the electrical system is evident on eachvessel. The Army requires that all the military inven-tory in its possession be maintained by configurationcontrol standards. This means that all equipmentwill not be modified, added to, or altered in anymanner without the proper approval of appropriatecommands.

Phase-to-neutral, phase-to-phase, and three-phase relationships were discussed earlier. A vesselis designed to accommodate electrical growth ofapproximately 10 percent. When three-phaseequipment is added to the distribution system,within the system’s capabilities, few problems areencountered. The three-phase motor current drawis distributed over all three generator armaturephases equally. There is no unbalanced condition.

However, if a single-phase device is added tothe distribution system, an additional electrical loadwill be imposed on one phase of the generator’sarmature only. The lowered resistance, fromanother resistance in parallel, means that more cur-rent will be delivered by the generator’s armaturephase that is involved. This also creates a voltagedrop due to the internal resistance within that genera-tor phase and increases the inductive reactancewithin that phase. This unbalances the generator. Iftoo many single-phase loads are added to the samegenerator armature phase, overheating will occur.Moreover, if three-phase equipment is improperlyreplaced with a single-phase component, the entire

generator overall three-phase current demand isreduced and replaced by an equal current demandon one phase alone. This is extremely detrimental tothe generator.

Generators function adequately within certainparameters of electrical imbalance. However, thisis not for the marine engineer to determine. Anychanges must be submitted in writing through thevessel maintenance office. Changes and additions tothe electrical system, both approved and expedient,must be logged in the engineering logbook indicatingthe circuit, the phases involved, and the starting kVAif the component is a motor.

Every time you turn on a blender in the galley,turn your bunk lights off, or start the coffee pot,a single-phase circuit has been individually changed.These changes are considered and taken intoaccount when the distribution system is designed.Improper component substitutions and un-authorized additions and modifications have notbeen considered in the design of the electrical system.

FREQUENCY

The frequency of a generator must neverchange. If the frequency of the generator changes,the speed of the motors and the contactor operationwill be adversely affected. Frequency is measured incycles per second or hertz. The equipment on boardArmy vessels operates at a frequency of 60 hertz.Frequency is concerned with how frequently therotor’s magnetic field sweeps past the armaturewinding and induces a usable EMF. Each time anorth and south pair of poles rotates one completerevolution, one cycle of AC is developed. If a rotor

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with one north and one south pole, called a two-polemachine, is revolved 60 times a second, it willproduce a frequency of 60 hertz (cycles per second).Rotor revolutions, however, are not measured incycles per second They are measured in revolutionsper minute (RPM).

Prime mover speed is an important factor infrequency. The other factor is generator construc-tion. Generators are classified by the number ofpoles. For example, a two-pole generator has onepair of main field poles. A north and south poleconstitute a pair. They are connected to the rotorshaft. A four-pole generator has two pairs of poleson the rotor and two pairs of poles in each phase inthe armature.

The number of poles determines the speed theprime mover must turn the rotor to maintain a fre-quency of 60 hertz. A four-pole generator must havea prime mover turn the rotor at 1,800 RPM to attaina frequency of 60 hertz (cycles per second).

Frequency = (number of poles) x (RPM)(2 poles per pair*) x (6O seconds per minute**)

Frequency = (4 poles) x (1,800 RPM)(2 poles per pair*) x (60 seconds per minute**)

Frequency = 7,200120

Frequency = 60 revolutions (or cycles) per second

*2 poles per pair is a constant used to account for therequirement of two poles of one north and one southpolarity for each individual cycle of events.

**6O seconds per minute is a constant used to convertevents per minute to cycles per second.

Poles and Frequency Relationships

Table 14-2 lists some of the more commonspeed and rotor pole relationships of the AC genera-tor for 60 hertz operation.

Factors Affecting Frequency

When a large conductor motor is first started(connected to the line), the resistance of the distribu-tion system is reduced. This reduction in resistanceis because the loads in the distribution system areconnected in parallel (Rt is always less than thesmallest resistance in a parallel circuit). As distribu-tion system resistance decreases, current from thegenerator increases through the main armaturewindings. This results in the following braking actionWithin the generator:

Large main armature currents react withan increased main field magnetic flux(initiated by the voltage regulator to main-tain voltage as the high current movesthrough the main armature windings to the load).

Both strong magnetic fields, produced byincreased current flow through the wind-ings, slow the rotation of the generatorshaft just as effectively as if a friction brakeis applied. A form of this, known aselectric braking, is commonly used in largemotors to bring rotation to a rapid halt.

The magnetic braking effect makes genera-tor rotation by the prime mover most dif-ficult. To help the prime mover overcomethis difficulty, several components areused:

The diesel governor maintains preciseengine speed control.

The diesel flywheel has stored energywhich tends to keep the diesel movingat the same speed.

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The damper windings are required forall generators that operate in parallel.These damper windings are short-circuited bars embedded in the mainfield (rotor) windings.

Damper Winding Construction

Figure 14-20 view A shows the damper wind-ings. These windings are nothing more than metalbars parallel to the rotor shaft (view B). The forwardends of these bars are connected together by shortingrings. The aft ends of these bars are connected in alike manner. When the magnetic fields from themain field and the main armature change in respectto each other, an EMF is induced in the damper bars.A resulting current flows, and a magnetic field isestablished because the bars are short-circuited ateach end. The magnetic field that develops in thedamper windings is a result of any change in themagnetic flux between the field and the armaturewindings. The magnetic field in the damper windingsopposes the effects that created it.

Damper Winding Operation

When the rotor is turning at the required speedand there is no change in the current demands on thegenerator armature, then the rotor field, inducedarmature field, and the damper windings all movetogether. The two fields and the damper windingsare magnetically linked together. However, there isno relative movement between them and no inducedEMF in the damper windings.

If the prime mover changes speed, resulting ina speed change of the rotor, there is a change in themagnetic field link between the rotor field, theinduced armature field, and the short-circuiteddamper windings. This provides a relative motionbetween the magnetic fields and the damper wind-ings. A voltage is induced with a resulting currentflow in the damper windings. This current flow setsup its own magnetic field. This magnetic field isgoverned by the property of induction and Lenz’sLaw, which states, "An induced effect is always suchas to oppose the cause that produced it."

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This means that the speed decrease of the rotoris opposed magnetically by the damper windings.The magnetic field of the damper windings tries topush the rotor along and maintain speed. If the rotorwould speed up, the induced EMF in the damperwindings would develop a magnetic field that wouldoppose this speed increase, slowing the rotor to itsoriginal speed.

VOLTAGE VARIATIONS UNDER LOAD

When the generator is disconnected from theelectrical system, no load is applied. Then the ter-minal voltage is exactly the same as the generatedvoltage. There is no current flow without a completecircuit.

When the electrical circuit is completed andcurrent flow is established in the armature, thencertain phenomena take place. This is called theinternal impedance of the generator. These factorsaffect the terminal voltage of the generator.

Resistance of the Armature Conductors

This is the IR drop so often mentioned. Ascurrent flows through the wire in the armature, itencounters resistance. The resistance of copperincreases as its temperature increases. Any timecurrent encounters resistance, a certain amount offorce is necessary to overcome this resistance. Theforce that is used is voltage. Voltage is lost drivingcurrent through the armature windings of the genera-tor. Although this voltage drop is small because ofthe small resistance encountered, the terminal volt-age decreases as the current increases in response tothe electrical system demands. The higher the cur-rent through the armature winding resistance, thegreater the voltage consumed.

Armature Reaction

This is the combined effects of the rotor’s mag-netic field and the field developed in the armature ascurrent is delivered to the electrical system. As longas the generator is not supplying current to theelectrical system, the rotor’s field is distributedevenly across the air gap to the main armature.

When the generator supplies inductive loads,the current developed in the armature opposes therotor’s field and weakens it. This develops the

lagging power factor and decreases terminal voltage.This is extremely common to Army watercraft.

When current leads voltage out of the armatureconductors (because of capacitor banks andsynchronous motors), then a leading power factordevelops. The armature current flow actuallystrengthens the magnetic field across the air gap andcombines with the rotor field. This increases ter-minal voltage.

Inductive Reactance

This is encountered when an EMF is inducedin a conductor. Induction produces a counter EMF.The current moving through the conductor and themagnetic fields cutting the adjacent turns of the con-ductor are all that is necessary to produce an EMFin the opposite direction. Inductive reactance inthe armature may be as great as 30 to 50 timesthat of armature resistance. Two effects can beencountered with inductive reactance:

When the load current is in phase with theterminal voltage or when the load currentlags the terminal voltage (due to inductivereactance from motor loads), then addi-tional voltage must be generated to over-come the armature’s inductive reactance(Figure 14-21).

When the current leads the terminal volt-age, as is the case with capacitors orsynchronous motors, then the inductivereactance in the armature aids thegenerated voltage. Less voltage must begenerated than when resistance was theonly consideration (Figure 14-22).

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VOLTAGE REGULATION

The voltage regulation of an AC generator isthe change of voltage from full load to no load,expressed in percentage of full-load volts. This infor-mation is necessary to determine how much changeto expect in the terminal voltage between no load andfull load.

Percent regulation =(no-load volts) - (full-load volts) x 100

(full-load volts)

Percent regulation

Percent regulation

= 465 volts - 450 volts x 100450 volts

= 3.33 percent

EFFECTIVE ALTERNATING CURRENTVALUES

Voltage and current are often expressed inspecific values. Alternating current changes not onlyin direction, but in constantly changing amplitudes.The value most commonly expressed is the genera-tor’s effective value.

For instance, the 450-volt AC generatorproduces a 450-volt output with the same effect as the450-volt DC generator. The effective value of thevoltage or current is computed as follows:

Square the instantaneous values.

Determine the mean average.

Extract the square root.

14-20

The effective value is also called the root meansquared (RMS) value. Mathematically, it is deter-mined to be a factor of .707 of the peak value.

POWER FACTOR

The DC generator output can be measuredeasily in watts. To calculate DC power, multiply thecurrent by the voltage (review Power in Chapter 2).Unlike DC, AC does not maintain a constantamplitude. Further, the current and voltage areinfluenced by the very nature of the reversing currentflow characterized by AC (Figure 14-23). To under-stand how these circumstances affect the actual gen-erator output, the actual values available at thegenerator terminals must be understood.

Inductance was discussed in Chapter 6.Every motor, generator, and transformer has a coilof wire called an inductor. By the very nature ofAC and its effects on the inductors in the circuit,current often lags voltage. The average current lagis represented by a decimal known as the powerfactor (PF). Unity power factor indicates that cur-rent and voltage arrive together and are in phasewith each other. Unity power factor has a decimalrepresentation of 1.0. Unity is the best use ofelectrical power. This condition results when allthe power is consumed in the circuit. The ratio ofunity to current lag is approximately 80 percent or.8 PF.

The inductors in the motors actually becometheir own miniature generators, inducing an EMFthat opposes a change in current.

The current from the generator must overcomethe resistance in the wires of the motor as well as

FM 55-509-1

overcome this counter EMF. The extra currentdeveloped by the generator, necessary to overcomethe CEMF, is not consumed by the motor. It isconsidered to be a shuttle power, existing in theelectrical system moving between the generator andthe motor. This condition also results between twogenerators 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 canbe computed the same as in the DC system. This isthe true power (or active power) that has been con-sumed and is expressed in kilowatts (KW).

Figure 14-24 view A shows that the current and

This situation is inconceivable on board a ves-sel. The current held momentarily suspended in timeby the CEMF generated by the action of the inductivecoils cannot be successfully multiplied together todetermine the power consumption of the electricalsystem. In view B, the current is delayed behind thevoltage. The apparent power (power that the genera-tor apparently sees that must be added to compen-sate) is represented by kVA.

Power factor is the percentage of the truepower to apparent power or –

PF = KW (true power)kVA (apparent power)

voltage rise and fall together. Only when the peakcurrent and voltage are in phase is the product of PF = 125 KWvoltage and amperage the same as the power con- 156 kVAsumption of the load in watts.

PF = .80

The electrical system must be designed tooperate on the apparent power of the system, not thetrue power of the system. Apparent power is alwaysgreater than true power when there is a power factorless than 1.0 (unity).

The following are some additional formulasthat may be useful:

(Three-phase applications)

KW= (1.732) x E x I x PF1,000

kVA = (1.732) x E x I1,000

HP= (1.732 x E x I x 100 x PF746 x efficiency

RPM = 2 x 60 x frequencypoles

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