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PLANT EFFICIENCIES BENEFIT BY SELECTION OFSYNCHRONOUS MOTOR

Authors:Jim Parrish Steve Moll Richard C. SchaeferLion Oil Electric Machinery Basler ElectricEldorado, AR Minneapolis, MN Highland, IL

KEY WORDS AND PHRASES:Synchronous motors, powerfactor, reactive loading, SCR, field forcing, underexcitationlimiting, overexcitation limiting, digital excitation.

ABSTRACT: Induction motors are often the choice forvarious industrial production processes. Use of an inductionmotor, however, will result in increased lagging power factorburdens in the plant. This burden often must be corrected byadding capacitors. If not, the result is the penalty of largerkVA burdens to the interconnected system. For low speedapplications, synchronous motors may be a better choicewhen equipped with accessories that offer power factorcontrol. The synchronous machine, with the aid of anintelligent excitation controller, can control power factor to

reduce the plant reactive loading to the connected system.

This paper will discuss the application of the synchronousmotor at the Lion Oil Refinery and the digital excitationsystem used to control the machine power factor.

I. BACKGROUND

Lion Oil is a refinery in El Dorado, Arkansas, that processespetroleum products. Environmental mandates that go intoeffect June 2006 include new clean air regulations requiring areduction in sulfur emissions for diesel fuel from 500 parts to15 parts per million.

Current refining equipment is not designed to process thepetroleum product at the increased pressures and highertemperatures needed to meet the EPA regulations. To meetthe refining requirements, a new higher-rated motorconnected to a hydrogen-reciprocating compressor would beneeded to drive the refining process.

Lion Oil currently utilizes induction motors, ranging in sizefrom 100 to 3500 horsepower. With such a large number ofinduction motors, power factor is an issue at the plant.Capacitors are used for correction to minimize penalties fromthe electric utility. The decision to use a synchronous motorinstead of an induction motor was based upon the need tohave an optimally efficient plant with low operating cost.

Figure 1: Picture of Lion Oil Motor

II. DECISION FOR SELECTION OF SYNCHRONOUSMOTOR

The induction motorhas fixed stator windings that areelectrically connected to the ac power source. Current isinduced in the rotor circuit via transformer action, resulting ina magnetic field that interacts with the stator field causingrotation.

The rotor must rotate slower than the stator field for theinduction to occur, thus an induction motor operates at lessthan synchronous speed using no other power source toexcite the rotors field.

Motors, especially those operating at 600 RPM and below,have lower operating efficiency and, inherently, a laggingpower factor that draws exciting current from the connectedsource. Since an induction machine draws all exciting currentfrom the ac source, it may take in the range of 0.3 to .6 p.u.reactive magnetizing kVA per HP of operating load.

The synchronous motor has fixed stator windings electricallyconnected to the ac supply with a separate source of

excitation connected to a field winding on the rotating shaft.Magnetic flux links the rotor and stator windings causing themotor to operate at synchronous speed. See Figure 2.

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Figure 2: Synchronous Motor

While induction motors can be started and accelerated tosteady state running condition simply by applying ac power tothe fixed stator windings of the motor, a synchronous motorstarts as an induction motor. Then, when rotor speed is nearsynchronous speed, the rotor is locked in step with the statorby application of dc voltage and current applied to the rotor ofthe motor.

Once the synchronous motor is operating at synchronousspeed, it is possible to alter the power factor of the motor byvarying the excitation supplied to the motors field. Sincepower factor correction could be provided, a synchronousmotor was chosen.

This paper will describe the characteristics of thesynchronous motor, field application and the excitationsystem selected for this application. For Lion Oil, the newsynchronous motor would be rated for 5000 HP, 13,800 Vac,operating at 327 RPM.

III. CONSTRUCTION OF ASYNCHRONOUS MOTOR

Synchronous motors consist of a fixed stator and a field thatrotates concentric with the stator. The stator containsarmature windings that are electrically connected to the acsupply system while the rotor contains a field winding that iselectrically connected to a source of excitation (dc). SeeFigure 3.

Figure 3: Stator Winding

Since the primary purpose of the field winding is to create arotating magnetic field, the field winding is wound around

poles attached to the rotor in a configuration that producesmagnetic north and south poles that are 180 electricaldegrees apart. See Figure 4

Figure 4: A Rotor for a Synchronous Motor

During starting of the motor, the field winding is noteffectively coupled with the armature windings in the statorand no net torque is produced in the field when ac power isconnected to the stator winding.

To produce starting torque, a supplementary winding is

provided on the rotor that effectively coupleselectromagnetically with the armature windings. This windingis a squirrel cage arrangement of bars placed across eachpoleface that are electrically shorted at each end. Thesquirrel cage winding on the rotor is formally known as thedamper or amortisseur winding.

When ac power is connected to the stator, current is inducedin the squirrel cage winding. This results in a net torque thatis applied to the rotor, as shown in Figure 5a and b.

Figure 5a: Relation of the Squirrel Cage Conductors to

Magnetic Field

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Figure 5b: A Rotor Field Pole with Squirrel Cage

This squirrel cage winding also is used in the induction motorto produce motor torque during starting and runningoperation. Torque is produced by the electromagneticinteraction of stator and rotor only when a slip speed exists.Thus, the induction motor speed increases from zero until

just below synchronous speed as torque decreases withincreasing rotor speed.

As noted, the synchronous motor is started like an inductionmotor with the torque on the rotor dependent on thedifference between the rotor speed and the frequency of thepower being applied to the stator winding. The torquesupplied by the squirrel cage is at a maximum when acpower is first supplied to the stator winding and decreases asthe rotor accelerates, until it reaches zero torque when therotor approaches synchronous speed. The absolute value ofthe accelerating torque is a function of the resistance of thebars in the squirrel cage. Higher resistance bars producehigher starting torque (and hotter squirrel cage winding) whilelower resistance bars produce lower starting torque with lessheat generation.

If the starting torque produced by the squirrel cage winding isnot adequate to roll the rotor, the rotor is said to be lockedand ac power must be quickly removed from the statorwindings to avoid overheating both the armature and thesquirrel cage windings. As shown in Figure 6, the statorwinding is connected to the ac supply system at startup, andthe bars of the squirrel cage winding on the rotor producesan accelerating torque on the rotor. The field winding isconnected to a field discharge resistor during startup, whileno external excitation is applied to the field See Figure 6.

Figure 6: During Start, the Rotor Poles will Rotate at a SlowerSpeed than the Stator Frequency, Inducing a Slip.

As the rotor accelerates, the field winding is coupled to thestator field via the armature winding. AC current is inducedinto the field based upon the difference between the

frequency of the applied ac voltage to the motor and thefrequency associated with the instantaneous speed of therotor. If the field winding were open circuited during startup,dangerously high voltages could be induced in the fieldwinding, hence a discharge resistor is used to limit thevoltage seen by the field winding during startup whiledissipating the energy induced in the field. The resistance ofthe discharge resistor also affects the synchronizing torqueavailable as the rotor approaches rated speed. Synchronizingtorque increases with the resistance of the field discharge

resistor.

A low resistance produces lower synchronizing torque, asshown in Figure 7.

Insulation limits of the field winding limit the size of theresistor, because induced field voltage increases withincreasing values of the discharge resistor. Sizing thedischarge resistor is an art that balances starting torque andallowable field voltage during startup.

Figure 7: Squirrel Cage at Different Starting ResistanceValues

IV. STARTING THE SYNCHRONOUS MOTOR

Over the years several methods have been used to startsynchronous motors, the most common approach is startingacross the line with full ac voltage to the armature windings.

The squirrel cage windings begin the task of accelerating themotor from zero speed. As the motor speed increases, thedischarge resistor provides the torque required for the motorto reach synchronous speed. When the synchronous speedis reached, the discharge resistor is switched out of the fieldcircuit and excitation can be applied to lock the stator andfield poles into synchronism. It is important to properly timethe application of excitation to the main field. The purpose ofthe dc excitation system is to apply current to the fieldwinding, creating a rotating electromagnet field that couplesthe rotor field to the rotating ac field in the armature windingwhen the 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 determines thereaction of the rotor.

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Figure 11: Field Application Circuit Drawing

VII. SOLID STATEFIELD APPLICATION SYSTEMS

The efficiencies of solid state field application control hasprovided an economical and reliable means of applyingexcitation to the motors field at the precise moment thatinsures optimum synchronization. See Figure 12

The function of the Field Application System provides threeimportant elements:

# Provide a discharge path for the current induced inthe field of the motor during starting, and open thiscircuit when excitation is applied.

# Apply field excitation positively when the motorreaches an adequate speed. The excitation shouldbe applied with such polarity that maximum torquewill be obtained at the time of pull-in.

# Remove excitation and reapply the field dischargeresistor immediately if the motor pulls out of step.

Referring to Figure 11, the field discharge resistor protectsthe motor field winding from the high voltage induced duringstarting and provides the voltage source for the controlcircuit. The ac output of the exciter is converted to dc by therotating rectifier diodes. This output is switched on or off tothe motor field winding by a silicon controlled rectifier SCR-1,which is gated by the control circuit.

Figure 12: Brushless Exciter

The solid state field application system shown inFigure 11 has a control circuit that keeps the SCR-1 fromfiring until the induced field current frequency is very low,representing a close approach to synchronous speed, andthen fires the rectifier SCR-1 at the proper time and appliesexcitation to the synchronous motor field. At the same time,the field discharge resistor is removed from the circuit. This isdone by the inherent operating characteristics of siliconcontrolled rectifier, SCR-2. This frequency sensitive part of

the control circuit ensures that field excitation is applied atthe proper pull-in speed for successful synchronizing and atthe proper polarity to give maximum pull-in torque withminimum line disturbance.

The control circuit operates to remove excitation should themotor pull out of step due to a voltage step or excessivemechanical load. On the first half cycle after pull-out, theinduced field voltage causes the net field current to passthrough zero, turning SCR-1 off, automatically removing

excitation. SCR-2 operates to connect the field dischargeresistor back in to the circuit. During this time the motoroperates as an induction motor.

When conditions permit, field is then re-applied as duringstarting. In Figure 11, the voltage from the exciter rectifier isblocked by SCR-1 until the point of synchronization. The fieldhas an alternating voltage causing current to flow firstthrough SCR-2 and the discharge resistor. On the next halfcycle, current flows through the diode and discharge resistor.The control circuit waits until the frequency drops to thepreset value, indicating the rotor is at an adequate speed.Then, after a North pole on the stator is in the right position tobe attracted to what will be a South pole on the rotor, ittriggers SCR-1 to apply excitation.

If the rotor does not synchronize, it will slip a pole, theinduced field voltage will oppose the exciter voltage causingthe current to go to zero, turning SCR-1 off. SCR-2 is Figure11turned on only at a voltage higher than the exciter voltageso it will not be on when SCR-1 is on.

VIII. SYNCHRONOUS MOTOR CONTROLS POWERFACTOR

Power factor is the factor by which apparent power, or kVA,is multiplied to obtain actual power, or kW, in an ac system. Itis the ratio of the in-phase component of current to totalcurrent. It also is the cosine of the angle by which the current

lags (or leads) the voltage. The conversion of electricalenergy to mechanical energy in a motor is accomplished bymagnetic fields. The poles rotate around the stator. Whenvoltage is applied to a motor, an armature current flows toprovide the necessary magnetic push (mmf or ampere turns)to produce a flux that in turn, produces a voltage (back emf)that opposes the applied voltage. This mmf is a magnetizingcurrent. It is loss-less, except for the I

2R losses in the winding

and any core loss due to the changing flux in the iron. Themagnetic energy is transferred from the line to the motor andback again each half cycle. The net power is zero, and thepower 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, the field poleson the rotor are in line with the rotating magnetic poles of thestator. If dc is applied to the rotor pole windings, the rotor cansupply the necessary ampere turns to generate the flux thatproduces the internal motor voltage. Thus, the field currentcan replace part or all of the magnetizing current. In fact, ifmore dc field current is supplied, the increased flux will try toincrease the line voltage. To increase the line voltage, themotor will supply ac magnetizing current to all magnets onthe system to increase their magnetic flux. This is leadingpower factor. The synchronous torque developed is roughly

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proportional to the angle of lag (load angle) of the motor rotorwith respect to the terminal voltage, which, at full load, is inthe area of 20 to 30 electrical degrees. If a restraining force isapplied to the motor shaft, it will momentarily slow down untila torque is developed equal to the applied restraint. Themotor will then continue to operate at synchronous speed.

Figure 13 illustrates how the change in the excitation causesthe ac line current to either lead the ac source voltage, be inphase with the source voltage or lag the source voltage

depending upon if the synchronous motor is over or underexcited.

Figure 13: Leading and Lagging Current as Affected by Field

Excitation on theSynchronous Motor

V CURVESIt is generally assumed that the line voltage will besubstantially 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 known fromtheir shape as V curves. These are represented in Figure14 that for each curve the power is constant and theexcitation is varied to give different magnetizing currents. Theminimum value of line amperes for each load condition is at1.0 or unity power factor. As excitation is decreased, the linecurrent will increase and the motor will operate at lagging

power factor.

Figure 14: Characteristic V Curves

IX. SYSTEM POWER FACTOR CORRECTION

By operating synchronous motors with a leading powerfactor, the overall system power factor can be shifted towardunity. The power factor of a synchronous motor can becontrolled by varying the amount of excitation currentdelivered to the motor field during operation. As the dc fieldexcitation is increased, the power factor of the motor load, asmeasured at the motor terminal, becomes more leading asthe overexcited synchronous motor produces vars. If

excitation is decreased, the power factor of the motor shiftstoward lagging, and the motor will import vars from thesystem.

The decision for Lion Oil to go with a synchronous motor wasbased upon the need to minimize economic penalties due topoor power factor at the utility connection. An example isillustrated in Figures 15-17 of how penalties can occur.

Figures 15-17 show one-line diagrams of a synchronous

motor operating at leading, unity, and lagging power factors.When operating at leading power factor as shown in Figure15, the field excitation is increased above the level requiredto produce unity power factor.

Figure 15: Synchronous Motor in Leading PF

If the power supply system is being operated to maintain afixed voltage at the terminals of the motor, var production bythe motor reduces the vars required from the system tomaintain the preset terminal voltage.

Figure 16: Synchronous Motor at Unity Power Factor

As the motor excitation is reduced until the motor operates atunity power factor (Figure 16), the supply current and voltageshift in phase. The kVA burden supplied to the utilityincreases from 3914 kVA to 4217 kVA; if excitation is furtherreduced, the motor will operate at lagging power factor andthe system power factor will shift to lagging as well. Noticethe kVA burden from the utility is now 4470 kVA. See Figure17.

Figure 17: Synchronous Motor at 0.9 Power Factor Lagging

Under these conditions, the system must carry a larger kVAload than when the motor is operating at unity or leadingpower factor. At .8 lagging power factor, the system mustprovide the local var support. Thus, operation of the

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synchronous motor can substantially affect the power factorof the load as seen from the supply system, and the kVA ofthe load as seen from the utility system resulting in penaltiesbecause of low incoming power factor.

X. Excitation System Controls thePower Factor

To take full advantage of the synchronous motor, it isnecessary to have an excitation system that will maintain

constant power factor regardless of load and ac supplyvariations to the excitation controller. Todays excitationsystems are designed with features to help improve thequality of machine control. For this system, a digital controllerwas specified by the engineering consultant with thefollowing features:

# Power Factor Control

# Under Excitation Limiting

# Over Excitation Limiting

# Manual Control

The digital control, however, also includes functions toprovide added value to the plant and the maintenancepersonnel who are responsible for the system. Thesefeatures 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 andManual Control to avoid bumps and a systemdisturbance during 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 specific cosineangle by measuring the real power into the motor andadjusting the excitation into the field to provide the correctamount of kvars to maintain the cosine angle. Figure 18illustrates the constant angle as a function of kW into themachine versus kvars out of the machine. Any voltagevariation to the system or load change that would affect thepower factor is immediately modified via the excitationsystem to restore the cosine angle and power factor of themachine.

Figure 18: Motor Capability Curve

The automatic control eliminates the concern with an ac

supply variation to the excitation system that could otherwiseresult in pole slip due to too little excitation for the motor field.

Additionally, digital systems are equipped with safeguards toprevent pole slip from occurring. These include:

# Field Forcing Margins

# UnderExcitation Limiting

# OverExcitation Limiting

Field ForcingField forcing provides a means to maintain constant voltageinto the field even when the ac supply voltage drops as much30-40%. Hence if the field voltage required by the motorwere 100 Vdc at .9 power factor lead and the digital controllerwere selected to provide 150 Vdc maximum ceiling voltage,

The digital controller would be able to provide 100 Vdc to thefield even if the supply voltage into the controller were to drop50%. The additional margin could mean the differencebetween continue process control or a machine trip and plantoutage.

UnderExcitation LimitingDigital controllers also are equipped with underexcitationlimiters. These devices have always been popular forgenerators, but also very practical for synchronous motorsusing digital controllers. The underexcitation limiter monitorsthe kW into the synchronous machine as compared to thekvars being supplied. Should the kvars drop belowacceptable levels needed to maintain synchronism, theunderexcitation limiter will cause an increase in excitation to

prevent a machine trip. Figure 19 illustrates an apparent dropin machine vars. Notice how the kvar drops to a level but islimited to any further reduction because of theunderexcitation limiter.

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Figure 19: Underexcitation Limiter Operation forSynchronous Motor

OverExcitation LimitingTo ensure that too much excitation is not applied into therotor for extended periods of time, the excitation system isequipped with an overexcitation limiter. The overexcitation

limiter monitors the field current and if it should remain toohigh for extended periods, the excitation limiter will react byrestricting any further increased field current and restore thelevel within a safe operating region for the machine. Figure20 illustrates how a bump in the system has occurred andkvars are limited because of the overexcitation limiter limitingany further increases. Observe that the overexcitation limiterlimits at three levels to allow short time var boosting thatcould benefit the system.

Figure 20: Overexcitation Limiter Operation on SynchronousMotor

Other FeaturesTodays digital excitation controllers such as the typeprovided for Lion Oil also include field protection that mightotherwise have been neglected or provided by other discreetdevices. These additional features include field overvoltageand field overcurrent, as well as, over and under terminalvoltage monitoring. Since Brushless motors use diodes torectifiy the ac to dc from the exciter output, failure of thesediodes can lead to a faulted exciter phase, and subsequentoverload and excessive vibration of the exciter. See Figure21. New digital controllers are equipped with diode failuredetectors for the Brushless exciter that will alarm shouldfailure occur and allow for a timely shutdown to repair faulteddevices.

XI. Excitation System Interconnection

Unlike constant voltage supplies that require only an acvoltage source to provide a fixed dc into the field, theautomatic control system requires inputs from instrument PTsfor measuring motor line voltage and a current transformersignal input for measuring the reactive current out of themotor. See Figure 21.

Figure 21 System One Line Interconnect

Inputs for starting the excitation systems are interfaced withthe motor control, so when the motor breaker closes, theexcitation system will apply excitation into the exciter field.The excitation system initially is enabled in manual controlwith a fixed dc current level to the field. Once the motor fieldis synchronized to the line, the system switches to powerfactor control for automatic operation.

Outputs are available to monitor any operation of theexcitation limiters and general annunciation of any protectionalarms either by hard contact or by a Modbus RS 485 serialport.

XII. TESTING AND EVALUATING

During testing, V curves checks are performed to evaluatemotor characteristics. Historically, the V curve was obtained

by installing chart recorders and appropriate transducers tomeasure watts and vars. In the digital excitation system,curves can be obtained using the oscillography that canspeed commissioning time and provide permanent records ofthe motors history. Figure 22 illustrates kvars and ac linecurrent when field excitation is changed forming a V curve ofthe synchronous machine. Notice how the kvars increasewith increasing excitation. As the motor line current valleysbetween the two ac line current peaks, the kvars are zero. Afurther reduction in field current now causes the kvars tofurther decrease, causing a lagging ac line current andlagging power factor in the system.

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Figure 22: Testing the Synchronous Motor Operation using VCurve

Figure 23: Picture of new excitation system.

XIII. CONCLUSION

For low speed applications and power factor concerns, thebenefits offered by the synchronous motor can provide longterm improvements that can represent cost savings to theplant. The brushless exciter favors a more streamlinedcontrol via a solid-state device to apply excitation to the fieldwith increased reliability and economic benefits. Accuratecontrol of the synchronous motor power factor asimplemented by a digital excitation system offers moreefficient control of the machine to help avoid potentialmachine trips due to low ac power supply variations to theexcitation system. Features included in the excitation systemalso provide more complete monitoring of the synchronousmachine to safeguard the unit under abnormal operatingconditions. Special attention should be provided for long time

storage of the motor prior to installation to be sure the motoris adequately protected from moisture.

REFERENCES1. Fitzgerald, Kingsley, Kusko, Electric Machinery,

19712. T. Cox Lloyd, Electric Motors and their Applications,

19693. R. C. Schaefer, Synchronous Motor Controller

Prevents Pole Slip and Saves Cost when Operating

Brushless Synchronous Motors.,Application Note125, Basler Electric, 1997

4. R. C. Schaefer, Synchronous Motor Control, BaslerElectric Power Control and Protection, Conference,October 1993

5. Gerry Oscarson, Jack Impertson, Ben Impertson,Steve Moll., The ABCs of Synchronous Motors,1987

For More InformationFor information on Basler Electrics

complete range of exciter solutions, visitthe download section at

http://www.basler.comto access productdocumentation, Application Notes, andTechnical Papers.

To discuss your application, consultBasler at the factory at (618) 654-2341 oremail us at info@basler.com