61

1077-2618/06/$20.00©2006 IEEE

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NDUCTION MOTORS ARE OFTEN

the choice for various industrial production

processes. Use of an induction motor, how-

ever, will result in increased lagging power-

factor burdens in the plant. This burden often must be

corrected by adding capacitors. If not, the result is the

penalty of larger kilovoltampere burdens to the intercon-

nected system. For low-speed applications, synchronous

motors may be a better choice

when equipped with accessories

that offer power-factor control.

The synchronous machine, with

the aid of an intelligent excitation controller, can control

power factor to reduce the plant reactive loading to the

connected system.

This article will discuss the application of the synchro-

nous motor at the Lion Oil refinery and the digital excita-

tion system used to control the machine power factor.

Background

Lion Oil is a refinery in El Dorado, Arkansas, that

processes petroleum products. Environmental mandates

that go into effect June 2006 include new clean-air regula-

tions requiring a reduction in sulfur emissions for diesel

fuel from 500 parts to 15 parts per million.

Current refining equipment is not designed to process

the petroleum product at the increased pressures and

higher temperatures needed to meet the U.S. Environ-

mental Protection Agency (EPA) regulations. To meet the

refining requirements, a new, higher-rated motor connect-

ed to a hydrogen-reciprocating compressor would be need-

ed to drive the refining process.

Lion Oil currently utilizes

induction motors ranging in

size from 100–3,500 hp (Figure

1). With such a large number of induction motors,

power factor is an issue at the plant. Capacitors are used

for correction to minimize penalties from the electric

utility. The decision to use a synchronous motor instead

of an induction motor was based upon the need to have

an optimally efficient plant with low operating cost.

Decision for Selection of Synchronous Motor

The induction motor has fixed stator windings that are

electrically connected to the ac power source. Current is

induced in the rotor circuit via transformer action,

I

Plant efficiency benefits resulting from the use of synchronous motors

B Y J I M P A R R I S H , S T E V E M O L L ,

& R I C H A R D C . S C H A E F E R

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resulting in a magnetic field that interacts with the sta-tor field causing rotation.

The rotor must rotate slower than the stator field forthe induction to occur, thus an induction motor operatesat less than synchronous speed using no other powersource to excite the rotor’s field.

Motors, especially those operating at 600 r/min andbelow, have lower operating efficiency and, inherently, alagging power factor that draws exciting current from theconnected source. Since an induction machine draws allexciting current from the ac source, it may take in therange of 0.3 to 0.6 p.u. reactive magnetizing kilovoltam-peres per horsepower of operating load.

The synchronous motor has fixed stator windings electri-cally connected to the ac supply with a separate source ofexcitation connected to a field winding on the rotating shaft.Magnetic flux links the rotor and stator windings, causingthe motor to operate at synchronous speed (see Figure 2).

While induction motors can be started and acceleratedto steady-state running condition simply by applying acpower to the fixed stator windings of the motor, a synchro-nous motor starts as an induction motor. Then, when rotorspeed is near synchronous speed, the rotor is locked in stepwith the stator by the application of dc voltage and cur-rent applied to the rotor of the motor.

Once the synchronous motor is operating at synchro-nous speed, it is possible to alter the power factor of themotor by varying the excitation supplied to the motor’sfield. Since power-factor correction could be provided, asynchronous motor was chosen.

This article will describe the characteristics of the syn-chronous motor, field application, and the excitation sys-tem selected for this application. For Lion Oil, the newsynchronous motor would be rated for 5,000 hp, 13,800Vac, operating at 327 r/min.

Construction of a Synchronous MotorSynchronous motors consist of a fixed stator and a fieldthat rotates concentric with the stator. The stator con-tains armature windings that are electrically connectedto the ac supply system while the rotor contains a fieldwinding that is electrically connected to a source of exci-tation (dc) (see Figure 3).

Since the primary purpose of the field winding is tocreate a rotating magnetic field, the field winding iswound around poles attached to the rotor in a configura-tion that produces magnetic north and south poles that are180 electrical degrees apart (see Figure 4).

During starting of the motor, the field winding is noteffectively coupled with the armature windings in the sta-tor, and no net torque is produced in the field when acpower is connected to the stator winding.

To produce starting torque, a supplementary windingis provided on the rotor that effectively couples electro-magnetically with the armature windings. This winding isa squirrel-cage arrangement of bars placed across eachpoleface that are electrically shorted at each end. Thesquirrel cage winding on the rotor is formally known asthe damper or amortisseur winding.

A synchronous motor.

Stator FrameRotor

AmortisseurWinding

Exciter

2A stator winding.

3

The Lion Oil induction motor.

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When ac power is connected to the stator, current isinduced in the squirrel-cage winding. This results in anet torque that is applied to the rotor, as shown in Figure5(a) and (b).

This squirrel-cage winding also is used in the induc-tion motor to produce motor torque during starting andrunning operation. Torque is produced by the electro-magnetic interaction of stator and rotor only when a slipspeed exists. Thus, the induction motor speed increasesfrom zero until just below synchronous speed as torquedecreases with increasing rotor speed.

As noted, the synchronous motor is started like aninduction motor, with the torque on the rotor dependenton the difference between the rotor speed and the fre-quency of the power being applied to the stator winding.The torque supplied by the squirrel cage is at a maxi-mum when ac power is first supplied to the stator wind-ing and decreases as the rotor accelerates until it reacheszero torque when the rotor approaches synchronousspeed. The absolute value of the accelerating torque is afunction of the resistance of the bars in the squirrel cage.Higher resistance bars produce higher starting torque(and hotter squirrel-cage winding), while lower resis-tance bars produce lower starting torque with less heatgeneration.

If the starting torque produced by the squirrel-cagewinding is not adequate to roll the rotor, the rotor issaid to be locked, and ac power must be quicklyremoved from the stator windings to avoid overheatingboth the armature and the squirrel cage windings. Asshown in Figure 6, the stator winding is connected tothe ac supply system at startup, and the bars of thesquirrel-cage winding on the rotor produces an acceler-ating torque on the rotor. The field winding is connect-ed to a field discharge resistor during startup, while noexternal excitation is applied to the field (see Figure 6).

As the rotor accelerates, the field winding is coupledto the stator field via the armature winding. AC currentis induced into the field based upon the differencebetween the frequency of the applied ac voltage to themotor and the frequency associated with the instanta-neous speed of the rotor. If the field winding were opencircuited during startup, dangerously high voltages couldbe induced in the field winding, hence, a discharge resis-tor is used to limit the voltage seen by the field windingduring startup while dissipating the energy induced inthe field. The resistance of the discharge resistor alsoaffects the synchronizing torque available as the rotorapproaches rated speed. Synchronizing torque increaseswith the resistance of the field discharge resistor.

A low resistance produces lower synchronizingtorque, as shown in Figure 7. Insulation limits of thefield winding limit the size of the resistor, becauseinduced field voltage increases with increasing values ofthe discharge resistor. Sizing the discharge resistor is anart that balances starting torque and allowable fieldvoltage during startup.

Starting the Synchronous MotorOver the years, several methods have been used to startsynchronous motors. The most common approach is

starting across the line, with full ac voltage to the arma-ture windings.

The squirrel-cage windings begin the task of accelerat-ing the motor from zero speed. As the motor speedincreases, the discharge resistor provides the torquerequired for the motor to reach synchronous speed. Whenthe synchronous speed is reached, the discharge resistor isswitched out of the field circuit, and excitation can beapplied to lock the stator and field poles into synchronism.It is important to properly time the application of

A rotor for a synchronous motor.

4

During start, the rotor poles will rotate at a slower speed than

the stator frequency, inducing a slip.

Rotor

AC StatorInput

Pole ofSquirrel Cage

Stator

DC RotorInput

Field DischargeResistor in

Circuit

++ ++++

+ + + + +

−−−−−

−−

−−

−

−Field Removed

on Start6

(a) Relation of the squirrel-cage conductors to the magnetic

field. (b) A rotor field pole with a squirrel cage.

(a) (b)

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excitation to the main field. The purpose of the dc excita-tion system is to apply current to the field winding, creat-ing a rotating electromagnet field that couples the rotorfield to the rotating ac field in the armature winding whenthe motor is operating at synchronous speed. When dcexcitation is applied to the motor field, the position of therotor with respect to the stator magnetic field determinesthe reaction of the rotor.

If the north and south rotor and stator poles arealigned, such that the magnetic flux lines flow easily fromthe rotor through the airgap, the rotor flux will lock instep with the stator flux, and the motor will become syn-chronous. If the rotor poles are 180 electrical degrees outof phase with the stator poles, but motor acceleration isdecreasing the angle of displacement, it is likely thataccelerating torque plus magnetic attraction will combineto draw the rotor rapidly into pole alignment with thestator. Synchronizing additionally depends on the slip fre-quency between rotor and stator. Synchronizing torquefrom the magnetic linkage of rotor and stator must besufficient to accelerate the rotor to keep it locked in step.

The most straightforward approach to synchronousmotor starting is to monitor the frequency of the voltageacross the field discharge resistor. As the motor nears syn-chronous speed, the slip frequency approaches zero acrossthe resistor (see Figure 8).

At a specific slip frequency and rotor angle, dc isapplied to the main field, and the discharge resistor isswitched out to provide a very smooth transition fromstarting to synchronous operation. Modern application ofthe field for synchronous motors utilizes solid-state devicesthat control the removal of the discharge resistor and applythe excitation at the proper time in lieu of the breakers orcontactors that have been used historically in the past.

Synchronous OperationTo understand normal operation of the motor, it isimportant to understand why the field rotates at syn-chronous speed. Each set of field poles is matched withan equivalent set of stator poles. Since like poles, forexample north and north, repel, and unlike poles attract,the poles of the rotating field tend to align themselveswith opposite poles produced in the stator by the acpower applied to the armature windings of the motor, asshown in Figure 9.

When this condition is achieved, the motor is said tobe in step. When the motor is lightly loaded, the center-line of the rotor poles will nearly align across the air gapbetween the pole face and the stator with the center line ofan opposite polarity stator pole.

As the motor is loaded, the angular displacement of thecenter lines will increase. As the angular displacementapproaches approximately 45 mechanical degrees (90 elec-trical degrees), as shown in Figure 10, the motor isapproaching pullout torque. Beyond this point, the rotorpoles will no longer be in step with the stator poles, andthe rotor poles will slip, producing a sudden increase incurrent draw from the ac system.

Once the synchronous motor’s field poles are in stepwith the stator frequency, two factors determine the syn-chronous speed of the motor: the first is the frequency ofOpposite polarity poles are matched.

Ro

ta

ti o

n

N N

S

S

9

A synchronous motor after dc voltage is applied to the

revolving rotor field and the field is in step with the stator

frequency.

Rotor

AC StatorInput

Pole ofSquirrel Cage

S

S

N N

StatorField Discharge

Resistor OutField Applied Near

Synchronism

DC RotorInput

++

+++++

− −−−−

−

−−−−

++++−

8

The squirrel cage at different starting resistance values.

LowResistance

Total Torque

HighResistance

Cage Contribution

Per

cent

Tor

que

20

Percent Speed

40

160

200

80 1000

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the applied voltage, and the second is the number of polesin the motor.

speed = frequency × 120

poles.

Benefits of the BrushlessExcited Synchronous MotorThe brushless excited synchronous motor is the mostcommon type of exciter supplied today for use withsynchronous motors, requiring no brushes or collector-ring maintenance. The exciter is physically direct-con-nected to the motor shaft. The brushless exciter has athree-phase ac armature winding (see Figure 11). Thestationary field winding is on poles on the stator and isconnected to an excitation supply source. The generat-ed ac current is directly connected along the shaft to arotating three-phase diode wheel, where the ac is recti-fied to dc before going to the motor’s main field. Themagnitude of the motor field current is adjusted bychanging the current to the stationary exciter field bya dc source.

Solid-State Field Application SystemsThe efficiencies of solid-state field application controlhas provided an economical and reliable means of apply-ing excitation to the motor’s field at the precise momentthat insures optimum synchronization (see Figure 12).

The function of the field-application system providesthree important elements:

■ It provides a discharge path for the current inducedin the field of the motor during starting and opensthis circuit when excitation is applied.

■ It applies field excitation positively when the motorreaches an adequate speed. The excitation should beapplied with such polarity that maximum torquewill be obtained at the time of pull-in.

■ It removes excitation and reapplies the field dischargeresistor immediately if the motor pulls out of step.

Referring to Figure 11, the field-discharge resistor pro-tects the motor field winding from the high voltageinduced during starting and provides the voltage source for

the control circuit. The ac output of the exciter is convertedto dc by the rotating rectifier diodes. This output is switchedon or off to the motor field winding by a silicon controlledrectifier, SCR-1, which is gated by the control circuit.

The solid-state field application system shown in Figure 11 has a control circuit that keeps SCR-1 from firinguntil the induced field-current frequency is very low, repre-senting a close approach to synchronous speed, and then firesthe rectifier SCR-1 at the proper time and applies excitation tothe synchronous motor field. At the same time, the field dis-charge resistor is removed from the circuit. This is done by theinherent operating characteristics of silicon controlled rectifierSCR-2. This frequency-sensitive part of the control circuitensures that field excitation is applied at the proper pull-inspeed for successful synchronizing and at the proper polarity togive maximum pull-in torque with minimum line disturbance.

A field application circuit drawing.

Exciter

Field Armature

DC toExciter Field

Rectifier

Rotating Equipment

Field Application Circuit Field

SynchronousMotor

Armature

Three-PhaseAC

SCR-1

D10

SCR-2

FDR21 3

6

54

Rotating HeatSink Assembly

ControlUnit

11

Center lines of opposite polarity poles with angular

displacement approaching 45◦.

+ + + + +

+ + + + +

−−−−

−−−−−−

Center LineRotor Lags

Angular Displacementof Approximately

90 Electrical Degrees

45°

Center LineStator

N

N

S

S

10

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The control circuit operates to remove excitationshould the motor pull out of step due to a voltage step orexcessive mechanical load. On the first half cycle afterpull-out, the induced field voltage causes the net fieldcurrent to pass through zero, turning SCR-1 off, automat-ically removing excitation. SCR-2 operates to connect thefield discharge resistor back in to the circuit. During thistime, the motor operates as an induction motor.

When conditions permit, the field is then reapplied asduring starting. In Figure 11, the voltage from the exciterrectifier is blocked by SCR-1 until the point of synchro-nization. The field has an alternating voltage causing cur-rent to flow first through SCR-2 and the dischargeresistor. On the next half cycle, current flows through thediode and discharge resistor. The control circuit waitsuntil the frequency drops to the preset value, indicatingthe rotor is at an adequate speed. Then, after a north poleon the stator is in the right position to be attracted towhat will be a south pole on the rotor, it triggers SCR-1 toapply excitation.

If the rotor does not synchronize, it will slip a pole,and the induced field voltage will oppose the excitervoltage, causing the current to go to zero, turning SCR-1off. SCR-2 is Figure 11 turned on only at a voltage high-er than the exciter voltage, so it will not be on whenSCR-1 is on.

Synchronous Motor Controls Power FactorPower factor is the factor by which apparent power, orkilovoltamperage, is multiplied to obtain actual power,or kilowatts, in an ac system. It is the ratio of the in-phase component of current to total current. It also is thecosine of the angle by which the current lags (or leads)the voltage. The conversion of electrical energy tomechanical energy in a motor is accomplished by mag-netic fields. The poles rotate around the stator. Whenvoltage is applied to a motor, an armature current flowsto provide the necessary magnetic push [magnetomotiveforce (mmf) or ampere turns] to produce a flux that inturn, produces a voltage [back electromotive force (emf)]that opposes the applied voltage. This mmf is a magne-tizing current. It is lossless, except for the I2R losses inthe winding and any core loss due to the changing fluxin the iron. The magnetic energy is transferred from theline to the motor and back again each half cycle. The netpower is zero, and the power factor is zero.

The power factor of a synchronous motor is controllablewithin its design and load limits. It may operate at unity,leading, or in rare cases, lagging power factor.

Once the synchronous motor is synchronized, thefield poles on the rotor are in line with the rotatingmagnetic poles of the stator. If dc is applied to therotor pole windings, the rotor can supply the necessaryampere turns to generate the flux that produces theinternal motor voltage. Thus, the field current canreplace part or all of the magnetizing current. In fact, ifmore dc field current is supplied, the increased fluxwill try to increase the line voltage. To increase the linevoltage, the motor will supply ac magnetizing currentto all “magnets” on the system to increase their mag-netic flux. This is leading power factor. The synchro-nous torque developed is roughly proportional to theangle of lag (load angle) of the motor rotor with respectto the terminal voltage, which, at full load, is in thearea of 20–30 electrical degrees. If a restraining force isapplied to the motor shaft, it will momentarily slowdown until a torque is developed equal to the appliedrestraint. The motor will then continue to operate atsynchronous speed.

Figure 13 illustrates how the change in the excitationcauses the ac line current to either lead the ac source volt-age, be in phase with the source voltage, or lag the sourcevoltage, depending upon whether the synchronous motoris over or underexcited.

V CurvesIt is generally assumed that the line voltage will be sub-stantially constant, and it is apparent from the precedingdiscussion that load, excitation, line amperes, and powerfactor are closely related. This relationship is readilyexpressed by a family of characteristic curves knownfrom their shape as “V” curves. These are represented inFigure 14; for each curve, the power is constant and theexcitation is varied to give different magnetizing cur-rents. The minimum value of line amperes for each loadcondition is at 1.0 or unity power factor. As excitation isdecreased, the line current will increase, and the motorwill operate at lagging power factor.

Leading and lagging current as affected by field excitation

on the synchronous motor.

I Lead Bus Voltagefor Leading P. F.

OverexcitedSynchronous Machines

System Bus Voltage

I Lag Bus Voltagefor Lagging P. F.

Synchronous Machines

13

A brushless exciter.

Field Discharge ResistorsField Discharge Resistors

SCR-2 and D10SCR-2 and D10

Exciter ArmatureExciter Armature

Control ModuleControl Module

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System Power Factor CorrectionBy operating synchronous motors with a leading powerfactor, the overall system power factor can be shiftedtoward unity. The power factor of a synchronous motorcan be controlled by varying the amount of excitationcurrent delivered to the motor field during operation. Asthe dc field excitation is increased, the power factor of themotor load, as measured at the motor terminal, becomesmore leading as the overexcited synchronous motor pro-duces vars. If excitation is decreased, the power factor ofthe motor shifts toward lagging, and the motor willimport vars from the system.

The decision for Lion Oil to go with a synchronousmotor was based on the need to min-imize economic penalties due to poorpower factor at the utility connec-tion. An example of how penaltiescan occur is illustrated in Figures15–17.

Figures 15–17 show one-line dia-grams of a synchronous motor oper-ating at leading, unity, and laggingpower factors. When operating atleading power factor as shown in Fig-ure 15, the field excitation isincreased above the level required toproduce unity power factor.

If the power supply system isbeing operated to maintain a fixedvoltage at the terminals of themotor, var production by the motorreduces the vars required from thesystem to maintain the preset termi-nal voltage.

As the motor excitation is reduceduntil the motor operates at unitypower factor (Figure 16), the supplycurrent and voltage shift in phase.The kilovoltampere burden suppliedto the utility increases from 3,914 to4,217 kVA. If excitation is furtherreduced, the motor will operate atlagging power factor, and the systempower factor will shift to lagging aswell. Notice the kilovoltampere bur-den from the utility is now 4,470kVA (see Figure 17).

Under these conditions, the sys-tem must carry a larger kilovoltam-pere load than when the motor isoperating at unity or leading powerfactor. At 0.8 lagging power factor,the system must provide the local varsupport. Thus, operation of the syn-chronous motor can substantiallyaffect the power factor of the load asseen from the supply system, and thekilovoltamperage of the load as seenfrom the utility system resulting inpenalties because of low incomingpower factor.

Excitation System Controls the Power FactorTo take full advantage of the synchronous motor, it isnecessary to have an excitation system that will maintainconstant power factor regardless of load and ac supplyvariations to the excitation controller. Today’s excitationsystems are designed with features to help improve thequality of machine control. For this system, a digitalcontroller was specified by the engineering consultantwith the following features:

■ power-factor control■ underexcitation limiting■ overexcitation limiting■ manual control.

A synchronous motor at unity power factor.

Utility Supply

IE Adjusted for 1.0 PF

Load Demand

2,700 KW2,025 KVARS Lagging

3,375 KVA0.8 PF Lagging

3,700 KW2,025 KVARS Lagging

4,217 KVA0.87 PF Lagging Sync

Motor

1,000 KW1,000 KVA0 KVARS

1.0 PF

16

A synchronous motor in leading PF.

Utility Supply

I

E Overexcited

Load Demand

2,700 KW2,025 KVARS Lagging

3,375 KVA0.8 PF Lagging

3,700 KW1275 KVARS Lagging

3914 KVA0.94 PF Lagging

Sync Motor

750 KVARSLeading

1,000 KW1,250 KVA

0.80 PFLeading

15

Characteristic V curves.

160

FullLoad

Full Load1.0 P.F.

Full Load.8 P.F.

HalfLoad

NoLoad

120

80

40

00 25 50 75

C

0

D

B

A

% of Full Load .8 P. F. Excitation

% o

f Ful

l Loa

d .8

P. F

. Lin

e A

mpe

res

100

LeadLag

125 150

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The digital control, however, also includes functionsto provide added value to the plant and the mainte-nance personnel who are responsible for the system.These features include:

■ loss of voltage sensing and automatic transfer tomanual control to prevent a machine trip caused bya loss of PT fuse at the controller input

■ autotracking between power-factor mode and manu-al control to avoid bumps and a system disturbanceduring a unit transfer

■ oscillography, a diagnostic tool to evaluate plantissues such as a machine trip to help determine andlocate the problem

■ sequence of events that tabulates all events thatoccur from the time the motor is started to the timethe machine is shutdown.

Automatic Power-Factor Control The digital controller is designed to maintain a specificcosine angle by measuring the real power into the motor

and adjusting the excitation intothe field to provide the correctamount of kilovars to maintain thecosine angle. Figure 18 illustratesthe constant angle as a function ofkilowatts into the machine versuskilovars out of the machine. Anyvoltage variation to the system orload change that would affect thepower factor is immediately modi-fied via the excitation system torestore the cosine angle and powerfactor of the machine.

The automatic control elimi-nates the concern with an ac supply

variation to the excitation system that could otherwiseresult in pole slip due to too little excitation for themotor field. Additionally, digital systems are equippedwith safeguards to prevent pole slip from occurring.These include:

■ field forcing margins■ underexcitation limiting■ overexcitation limiting.

Field ForcingField forcing provides a means to maintain constantvoltage into the field, even when the ac supply voltagedrops as much 30–40%. Hence, if the field voltagerequired by the motor were 100 Vdc at 0.9 power factorlead, and the digital controller were selected to provide150 Vdc maximum ceiling voltage, the digital con-troller would be able to provide 100 Vdc to the fieldeven if the supply voltage into the controller were todrop 50%. The additional margin could mean the dif-ference between continue process control or a machinetrip and plant outage.

Underexcitation LimitingDigital controllers also are equipped with underexcita-tion limiters. These devices have always been popularfor generators but also very practical for synchronousmotors using digital controllers. The underexcitationlimiter monitors the kilowatts into the synchronousmachine as compared to the kilovars being supplied.Should the kilovars drop below acceptable levels neededto maintain synchronism, the underexcitation limiterwill cause an increase in excitation to prevent a machinetrip. Figure 19 illustrates an apparent drop in machinevars. Notice how the kilovars drop to a level but arelimited to any further reduction because of the underex-citation limiter.The motor capability curve.

1010Q− KVAR

10Q+ JVAR

9

9

8

8

7

7

6

6

5

5

4

4

3

3

2

2

1

1

0Per Unit

9 8 7 6 5

Incoming Real Power

4 3 2 1 0

Stability Limitof Motor

Increasing Load

37° - .8 P. F.

Power Factor ControlMaintains a

Constant Angle

RatedKVA

18

A synchronous motor at 0.9 power factor lagging.

Utility Supply

I

EAdjusted for 0.9 PF Lagging

Load Demand

2,700 KW2,025 KVARS Lagging

3,375 KVA0.8 PF Lagging

3,700 KW2,509 KVARS

4,470 KVA0.83 PF

Sync Motor

484 KVARS

1,000 KW1,111 KVA

0.90 PFLagging

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Overexcitation LimitingTo ensure that too much excitation is not applied intothe rotor for extended periods of time, the excitationsystem is equipped with an overexcitation limiter. Theoverexcitation limiter monitors the field current, andif it should remain too high for extended periods, theexcitation limiter will react by restricting any furtherincreased field current and restore the level within asafe operating region for the machine. Figure 20 illus-trates how a bump in the system has occurred, andkilovars are limited because of the overexcitation lim-iter limiting any further increases. Observe that theoverexcitation limiter limits at three levels to allowshort time var boosting that could benefit the system.

Other FeaturesToday’s digital excitation controllers, such as the typeprovided for Lion Oil, also includefield protection that might other-wise have been neglected or pro-vided by other discreet devices.These additional features includefield overvoltage and field over-current as well as over and underterminal voltage monitoring.Since brushless motors use diodesto rectifiy the ac to dc from theexciter output, failure of thesediodes can lead to a faulted exciterphase and subsequent overloadand excessive vibration of theexciter (see Figure 21). New digi-tal controllers are equipped withdiode-failure detectors for thebrushless exciter that will alarmshould failure occur and allow fora timely shutdown to repair fault-ed devices. The overexcitation limiter operation on a synchronous motor.

#1:Ib [0.35 pu]

0.35

0.350.15

0.150.51

0.25

#1:Kvar [0.15 pu]

#1:Ifd [0.51 pu]

0 5 10

Seconds

15 20 25 30

20

The system one-line interconnect.

Exciter

Field Armature

DigitalController

Rectifier

Rotating Equipment

Field Application Circuit Field

SynchronousMotor

Armature

3 PhaseAC

SCR-1

D10

PT

CT

SCR-2

FDRControl

Unit

21 3

6

54

Rotating HeatSink Assembly

ACSupplyPower

21

Underexcitation limiter operation for a synchronous motor.

#1:Vave [1.0 pu]

#1:Ib [0.35 pu]

#1:Kvar [0.21 pu]

#1:Vid [0.66 pu]

0 0.5 1

Seconds

1.5 2

19

IEE

E I

ND

US

TRY

AP

PLI

CA

TIO

NS

MA

GA

ZIN

E •

MA

R|

AP

R 2

00

6 •

WW

W.I

EE

E.O

RG

/IA

S

70

Excitation System InterconnectionUnlike constant voltage supplies that require only an acvoltage source to provide a fixed dc into the field, theautomatic control system requires inputs from instrumentPTs for measuring motor line voltage and a current trans-former signal input for measuring the reactive current outof the motor (see Figure 21).

Inputs for starting the excitation systems are inter-faced with the motor control, so when the motor breakercloses, the excitation system will apply excitation intothe exciter field. The excitation system initially isenabled in manual control with a fixed dc current levelto the field. Once the motor field is synchronized to theline, the system switches to power-factor control forautomatic operation.

Outputs are available to monitor any operation of theexcitation limiters and general annunciation of any protec-tion alarms either by hard contact or by a Modbus RS 485serial port.

Testing and EvaluatingDuring testing, V-curve checks are performed to evaluatemotor characteristics. Historically, the V curve wasobtained by installing chart recorders and appropriatetransducers to measure watts and vars. In the digital exci-tation system, curves can be obtained using the oscillogra-phy that can speed commissioning time and providepermanent records of the motor’s history. Figure 22 illus-trates kvars and ac line current when field excitation ischanged, forming a V curve of the synchronous machine.Notice how the kvars increase with increasing excitation.As the motor line current valleys between the two ac linecurrent peaks, the kvars are zero. A further reduction infield current now causes the kvars to further decrease,causing a lagging ac line current and lagging power factorin the system.

ConclusionFor low speed applications and power factor concerns, thebenefits offered by the synchronous motor can providelong-term improvements that can represent cost savingsto the plant. The brushless exciter favors a more stream-lined control via a solid-state device to apply excitation tothe field with increased reliability and economic benefits.Accurate control of the synchronous motor power factoras implemented by a digital excitation system (Figure 23)offers more efficient control of the machine to help avoidpotential machine trips due to low ac power supply varia-tions to the excitation system. Features included in theexcitation system also provide more complete monitoringof the synchronous machine to safeguard the unit underabnormal operating conditions. Special attention shouldbe provided for long time storage of the motor prior toinstallation to be sure the motor is adequately protectedfrom moisture.

References[1] A.E. Fitzgerald, C. Kingsley, Jr., and A. Kusko, Electric Machinery: The

Processes, Devices, and Systems of Electromechanical Energy Conversion, 3rd.ed. New York: McGraw-Hill, 1971.

[2] T.C. Lloyd, Electric Motors and their Applications. New York: Wiley-Interscience, 1969.

[3] R.C. Schaefer, “Synchronous motor controller prevents pole slip andsaves cost when operating brushless synchronous motors,” ApplicationNote 125, Basler Electric, Highland, IL, 1997.

[4] R.C. Schaefer, “Synchronous motor control,” presented at the BaslerElectric Power Control and Protection Conference, Oct. 1993.

[5] G. Oscarson, J. Impertson, B. Impertson, and S. Moll, “The ABCs ofSynchronous Motors,” Electric Machinery Company, Minneapolis,MN, 1987.

Jim Parrish is with Lion Oil in Eldorado, Arkansas. Steve Mollis with Electric Machinery in Minneapolis, Minnesota. RichardC. Schaefer (Richschaefer@basler.com) is with Basler Electric inHighland, Illinois. This article first appeared in its originalform at the 2005 IEEE Pulp and Paper Industry TechnicalComittee Conference.

Testing the synchronous motor operation using V curves.

#1:Ib [0.67 pu]

#1:Kvar [0.62 pu]

#1:Vfd [0.36 pu]

0 5 10

Seconds

22

A picture of the new excitation system.

23