Method for In-Field Evaluation of the Stator Winding Connection of Three-Phase Induction Motors to Maximize Efficiency and Power Factor

370 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 21, NO. 2, JUNE 2006

Method for In-Field Evaluation of the Stator WindingConnection of Three-Phase Induction Motors to

Maximize Efficiency and Power FactorFernando J. T. E. Ferreira, Member, IEEE, and Anbal T. de Almeida, Senior Member, IEEE

AbstractThe performance of the oversized three-phase induc-tion motors can be improved, both in terms of efficiency andpower factor, with the proper change of the stator winding con-nection, which can be delta or star, as a function of their load.A practical method is proposed to quickly and easily evaluatewhich stator winding connection is more appropriate for the ac-tual motor load profile, in order to increase the motor efficiencyand power factor. This new method is suitable for in-field evalu-ation, because it requires only the use of inexpensive equipmentand has enough accuracy to allow a proper decision to be made.The automatic change of the stator winding connection, as a func-tion of the motor line current, is also analyzed. When properlyapplied, these methods can lead to the improvement of the effi-ciency and power factor of permanently oversized motors, motorswith a load variation between low load and near full load duringtheir duty cycle, and/or motors driving high-inertia, low duty cycleloads. The proposed methods are particularly suitable to indus-trial plants where typically many electric motor systems are over-sized and/or can have a wide load variation. In these conditions,the active and reactive electrical energy bill can be significantlyreduced.

Index TermsEnergy efficiency, motor oversizing, power factor,savings, stator winding connection change, three-phase inductionmotor.

I. INTRODUCTION

IN industry, more than 90% of the electrical motors are three-phase squirrel-cage induction motors, hereafter denominatedonly by motors [1], [2]. In the European Union, the average loadfactor for motors, in both industrial and tertiary sectors, is 0.57(Fig. 1). However, the average load factor per power range insome sectors can be as low as 0.25 [2]. Individual motors inthose ranges have even lower load factors. Because the loadfactor is an average of the motor load during a defined period,the motor load can vary between values lower and higher thanthe load factor.

Motor oversizing is mainly due to the poor motor systemdesign or due to the gross overestimation of the mechanicalpower required by the load [2]. Additionally, motor oversizing

Manuscript received December 10, 2005; revised December 10, 2005. Paperno. TEC-000130-2005.

F. J. T. E. Ferreira is with the Department of Electrical Engineering, Engi-neering Institute of Coimbra (ISEC), Coimbra 3030, Portugal, and also withthe Institute of Systems and Robotics, University of Coimbra, Coimbra 3030,Portugal (e-mail: [email protected] and [email protected]).

A. T. de Almeida is with the Department of Electrical and Com-puter Engineering, University of Coimbra, Coimbra 3030, Portugal (e-mail:[email protected]).

Digital Object Identifier 10.1109/TEC.2006.874248

Fig. 1. Average load factor by power range for motors, in the industrial andtertiary sectors, in the European Union, 2000 [2].

Fig. 2. Motor stator winding connections. (Left) Star or wye connection.(Right) Delta or triangle connection.

is a widespread practice due to the motor market structure,which is largely dominated by original equipment manufacturers(OEMs). Motors with a wide load variation (e.g., between verylow load and near full load) during their duty cycle can alsobe found. In these cases, the motor is sized to provide the loadpeak power, but it can operate during long periods with a verylow load. These situations lead to a reduction of both motorefficiency () and power factor (). For specific conditions, thestator winding connection change from delta (D) to star (Y ) cansignificantly improve both motor efficiency and power factor.This possibility is only available for motors designed to operateat the nominal power with D connection and with access to thesix winding terminals (Fig. 2), which are the vast majority.

In this paper, an in-field evaluation method to access themost appropriate motor stator winding connection is proposedand analyzed. The automatic change of the motor stator wind-ing connection, as a function of the motor line current, is alsoanalyzed in the final part of the paper. For both methods, techni-cal and economical considerations associated with motor statorwinding connection are presented. The importance of this workis highlighted by the recent concerns on electric motor systemsoptimization in the industrial and tertiary sectors [1][3].

0885-8969/$20.00 2006 IEEE

FERREIRA AND DE ALMEIDA: METHOD FOR IN-FIELD EVALUATION OF THE STATORWINDING CONNECTION 371

Fig. 3. Per-phase equivalent circuit used for the motor simulations.

II. MOTOR EFFICIENCY AND MOTOR LOAD

The motor efficiency can be measured by the direct method,according to (1), where T is the torque, is the motor speed,Pelec is the input active power, and Pmech is the output mechan-ical power (useful power):

=PmechPelec

=T Pelec

. (1)

In the absence of voltage unbalance and motor electrome-chanical asymmetries, the active power Pelec absorbed by themotor is given by (2), where VLL is the line-to-line voltage (rms),IL is the line current (rms), and is the power factor:

Pelec =

3 VLL IL . (2)The motor load is defined by (3), where PN is the motor

nominal power:

=PmechPN

. (3)

The motor slip s is given by (4), where sync is the syn-chronous speed:

s =sync

sync. (4)

III. SIMULATED RESULTSThe efficiencyload curves for three motors were simulated

in the MATLABSimulink software, using the motor per-phaseequivalent circuit (Fig. 3). For the Y connection, a voltage 3times lower than the voltage considered in the D connection wasconsidered in the simulation. For the 3-kW motor (Brand A), theper-phase equivalent circuit parameters were experimentally ob-tained. For the 11- and 300-kW motors, the per-phase equivalentcircuit parameters were obtained from book data in [4] and [5].The mechanical losses component, as a function of motor speed,was also considered in the simulations. In Fig. 4, the simulatedmotor efficiencyload curves for both D and Y connections forthe three motors can be seen, as well as the motor parameters.

The intersection point between the efficiencyload curves,hereafter denominated by point , for the efficiencyload sim-ulated curves for the D and Y connections corresponds to amotor load of 0.36, 0.42, and 0.47, for the 3-, 11-, and 300-kWmotors, respectively.

Note that, according to the Fig. 4, the load corresponding tothe crossover point increases with the motor nominal power,because the efficiency curves become flatter (due to the relativelower core losses).

IV. EXPERIMENTAL RESULTS

The motor testing facility used in the experimental tests fulfilsthe IEEE 112 Standard requirements [6]. To measure the elec-trical and mechanical variables, a high-accuracy power analyzeris used (Yokogawa WT1030M). A dynamometer (Magtrol HD-815-8NA) is used as a variable load, which includes an encoderto measure speed, and a load cell to measure the torque. Thepower analyzer acquires the values of both sensors and directlymeasures the motor efficiency according to (1).

Thirteen totally enclosed fan-cooled motors of five differentbrands (denominated in this paper by A, B, C, D, and E), withnominal powers between 185 W and 7.5 kW, were tested. InTable I, the nameplate values of the motors, considering the Dconnection, can be seen. Eleven motors have four poles, one hastwo poles, and the remaining one has six poles.

In all the tests, the motor temperature stability was guaran-teed, for the same room temperature. The temperature correctionof the motor parameters was not considered, in order to allowa real evaluation of the motor performance for both D and Yconnections and different load points.

A summary of the experimental results is presented in Table I.In Fig. 5, the motor efficiency, power factor, speed, and linecurrent, as a function of the load, for Y and D connections (forthe 3-kW (Brand A) and 5-kW (Brand B) motors, both withfour poles) are presented. For a motor load lower than point ,the motor efficiency and power factor for the Y connection arehigher than for the D connection [Fig. 5(a) and (b)]. For anymotor load, the D to Y change also leads to a speed decrease[Fig. 5(c)]. For the tested motors (which are all in a very narrowlow power range), the point has no regular relation with brand,nominal power, and number of poles, and it is between = 0.27and = 0.50 (average = 0.37, see Table I). However, as itcan be seen in Section III, for motors with significant higherpower, the point moves to a higher load.

The experimental and simulated point for the 3-kW motor(Brand A) are approximately in accordance. Note that the dif-ference of the motor operating temperature for both D and Yconnections and different load points is not considered in thesimulation.

From Fig. 5, it can be concluded that the user should eval-uate several factors before changing the motor stator windingconnection. The most important factor should be the motor ef-ficiency. For a specific load below point , the increase in thepower factor and in the slip after the D to Y connection changeis well known.

V. METHODS FOR DIFFERENT LOAD PROFILES

The motor stator winding connection change can be made ei-ther by a manual method (permanent change) or by an automaticmethod (dynamic change). Each method should be chosen ac-cording to the motor-load profile. If the load profile is similar tothe load shape of the Fig. 6(a) or (b), the stator winding should bepermanently connected, after starting, in Y or D, respectively.In both cases, if the motor load slightly crosses the point loadlevel, during short periods, the respective connection can stillbe used (this issue is addressed in the Section VIII). If the load


Fig. 4. Simulated motor efficiency, as a function of the load, for motors with different power rating: (a) 3 kW, (b) 11 kW, and (c) 300 kW.

TABLE IEXPERIMENTAL VALUES FOR THE INDICATORS IN THE POINT

profile is similar to the load shape of Fig. 6(c), the stator wind-ing connection should be automatically managed by a suitablecontrol device.

VI. PERMANENT CHANGE OF THE WINDING CONNECTIONWhen the stator winding connection is changed from D to

Y , the winding voltage decreases

3 times. In point , the

efficiency, the mechanical power, and the active electrical powervalues, for both D and Y connections, are equal [see (1) and(3)]. Therefore, in the point , the relation (5) is true

IL(D )

IL(Y )=

YD

. (5)

To identify point , four indicators based on the motor in-fieldmeasurements and motor nameplate values (nominal values) areanalyzed:

two line current-based indicators (KI1 and KI2); two slip-based indicators (Ks1 and Ks2).The proposed indicators are based in values easily obtained in

the field, using common measurement devices (voltmeter, clampammeter, and stroboscopic tachometer), namely, the rms line-to-line voltage, the rms line current, and the motor speed. Themeasurement of the power factor is avoided because it requiresthe use of a power factor measurement device, a wattmeter or apower analyzer, which, to have sufficient accuracy, are expensivedevices.

The indicators KI1,KI2,Ks1, and Ks2 are defined by (6)(9), where IN is the motor nominal line current, VN the motornominal line-to-line voltage, Vmeas is the actual motor line-to-line voltage, sN is the motor nominal slip, and smeas is the actualmotor slip:

KI1 =IL(D )

IN(6)

KI2 =IL(D )

IL(Y )(7)

Ks1 =(

sync meas(D )sync N

)(

VNVmeas

)2

=Smeas(D ) V 2NSN V 2meas

(8)

Ks2 =Smeas(D )

Smeas(Y ). (9)


Fig. 5. Experimental results for the 3-kW four-pole motor (Brand A) and for the 5.5-kW four-pole motor (Brand B): (a) motor efficiency, (b) motor power factor,(c) motor speed, and (d) motor line current, as a function of the load.

Fig. 6. Motor load profiles for (a) permanent Y connection, (b) permanent D connection, and (c) automatic management of the connection.

The indicators KI1 and Ks1 are obtained without disconnect-ing the motor and the indicators KI2 and Ks2 require the motorstator winding connection change.

In Table I, a summary of the indicator values, their averagevalues, standard deviation, and variation with load, in relationto the point , for the tested motors is presented. In Table II,a summary of the obtained indicator values, in relation to thepoint , for the simulated motors is presented.

The standard deviation of a generic variable x is given by(10), where n is the number of samples:

=

nn

i=1 x2i (

ni=1 xi)

2

n2 n . (10)

It is also important to evaluate the variation of each indica-tor, when the motor load is moving away from the point . In


TABLE IISIMULATED VALUES FOR THE INDICATORS IN THE INDICATORS IN THE POINT

Table I, the average variation of the indicators in the neighbor-hood of point (10% variation) is presented.

The indicator Ks1 is easy to obtain (it requires a strobo-scopic tachometer and a voltmeter) but has errors related to thespeed measurement device errors (typically1 r/min) and to thenameplate speed errors due to the numerical rounding process(the speed is rounded to 5-r/min multiples) [7]. The indicatorKs1 includes a voltage correction related to the fact that, for aconstant torque, the motor slip is approximately inversely pro-portional to the voltage square. Therefore, if there is a differencebetween the motor actual voltage and its nominal voltage, it isnecessary to compensate the slip, considering the relation be-tween both voltages. The variation between Ks1 for the testedmotors, in the point , is reduced ( = 0.06 for an average equalto 0.30). It can be concluded that if a motor has a Ks1 0.25,there is a fair possibility (93% of the tested motors and 100%of the simulated motors verify that condition) of being oper-ating in the zone where energy consumption reduction can beobtained after the stator winding connection change from Dto Y . In the simulated data, it can be concluded that Ks1 canslightly increase with the motor rated power.

The indicator Ks2 is also easy to obtain (it also requires astroboscopic tachometer and a voltmeter) and it is more reliablethan Ks1, but requires the motor stator winding to be changed.The variation between Ks2 for the tested motors, in the point, is reduced ( = 0.03 for an average equal to 0.27). It canbe concluded that if a motor has a Ks2 0.30, there is a highpossibility (100% of the tested and simulated motors verifythat condition) of being operating in the zone where energyconsumption reduction can be obtained after the stator windingconnection change from D to Y .

The KI1 is not a good indicator because, when the motorload is moving away from point , for the tested motors, it hasa very low average variation (2%), tending to 0% for motorswith PN 1 kW.

The KI2 average is 1.67 ( = 0.11), which is also equal tothe ratio between the Y and D power factors, in point , as itwas demonstrated in (5).

All indicators present low standard deviation, but those withhigher variation, when the motor load is moving away frompoint , are more appropriate for the selection of the best con-nection mode. In general, the slip-based indicators are moresuitable to in-field purposes because they have both lower stan-dard deviation and higher average variation as a function of themotor load. Additionally, the measurement of the motor slip isnormally easier and faster than the measurement of the motorline current.

Fig. 7. In-field method to evaluate the motor stator winding connection.

Therefore, it can be concluded that the Ks1 is the most ap-propriate indicator for a preliminary evaluation of the motorefficiency improvement possibility, before the stator windingconnection change. After changing the stator winding connec-tion, Ks2 can be used to check with more accuracy the motor-efficiency improvement.

On the basis of the previous conclusions, a simple in-fieldmethod to evaluate which connection is more appropriated forthe motor stator winding, as a function of the motor slip, canbe defined based only on the Ks1 and Ks2 indicators (seeFig. 7). In this evaluation, the higher loads of the motors duringtheir duty cycle should be considered. Firstly, the possibilityof motor efficiency improvement after the stator winding con-nection change from D to Y should be determined based onthe nameplate and actual motor speed and voltage, using Ks1.The D to Y change should only be made if Ks1 0.25, witha fair possibility of efficiency improvement. After the D to Ychange, a slip based re-evaluation should be made using Ks2. IfKs2 0.30 the Y connection should be maintained, otherwisethe winding should be reconnected to D.

Note that, even if there are no significant efficiency improve-ments due to the proximity between the motor load and the point, the power factor still significantly improves.

Although the proposed method was only experimentally val-idated for the 185 W7.5 kW motor power range, in principle,it can be applied to all the motors, because Ks2 has a very lowdependency on the motor rated power and Ks1 can slightly in-crease with the motor rated power, as was demonstrated by thesimulated results (see Table II).

The permanent stator winding connection should be re-evaluated periodically if the load characteristics change. Theproposed method is suitable for grossly oversized motors and/ormotors driving loads with low duty cycles and high inertia (e.g.,


Fig. 8. Basic topology of an electronic device for the automatic change of themotor stator winding connection [8].

press machines and high-inertia saws1). Because it requires onlylow-cost and easy-to-use equipment (a stroboscopic tachometerand a voltmeter), the proposed method can be integrated in thegroup of the low-cost measures with a significant energy savingspotential.

VII. AUTOMATIC CHANGE OF THE WINDING CONNECTIONThe automatic change of the stator winding connection is

particularly suitable for motors with significant load variationduring their duty cycle, including relatively long, low load oper-ating periods (below point ). The automatic connection changein such motors can lead to significant energy savings and im-provement of the motor power factor in the low load operatingperiods, largely compensating the additional modest investment.

The experimental results, using a microcontroller based elec-tronic device (described in detail in [8] and shown in Fig. 8),for the automatic change of the motor stator winding connec-tion, as a function of the motor line current, are presented. Thedevice controls the D/Y and the line contactors. The connec-tion control is based on the current measurement because it isthe variable most suitable to be acquired and processed by anelectronic device for industrial purposes.

In Fig. 9(a), the motor efficiency, power factor, current ratios,and speed ratios are shown for the 3 kW four-pole motor (BrandA), as a function of the load, for both Y and D connections.After proper calibration of the setpoints 1 and 2 [see Fig. 9(b)],which correspond to the two levels of the motor line current inthe point for the D and Y connections, the stator windingconnection is automatically and properly changed, as a functionof the motor line current, leading to an improvement of themotor efficiency and power factor, for loads lower than point .

The duration of each different operating period of the motorduty cycle should be long enough to avoid an excessive numberof stator winding connection changes, in order to avoid a signifi-cant decrease of the contactors and motor lifetime. Examples ofloads in which the automatic change method can be potentiallyapplied with possible energy savings include industrial mixers,elevating conveyors, and high-inertia saws.

1In these load types, if the motor stator winding is Y connected, and the timebetween the maximum load periods is sufficient to allow the acceleration andspeed stabilization of the inertia wheel, there are no operating problems. Forhigh-inertia loads, D connection starting can be used, in order to reduce thestarting period.

VIII. TECHNICAL CONSIDERATIONS

A. Motor Load and SpeedThe motor speed and load variation after the stator winding

connection change also deserve to be analyzed. After the statorwinding connection change from D to Y , the motor line currentsignificantly decreases and the motor speed slightly decreases(in the point , the motor slip increases 34 times). After the Yto D change, the motor line current significantly increases andthe motor speed slightly increases. The decrease of the motorspeed after the D to Y change is related to the stator windingvoltage decrease (decreases 3 times) and the consequent re-shape of the motor torque-speed curve.2 The slight increase ordecrease of the motor speed after the stator winding connec-tion change generally leads to an increase or decrease of themotor load, respectively. This fact can lead to significant powerreductions in constant, linear, or quadratic torque loads, partic-ularly for the last ones (e.g., centrifugal pumps and fans). For aspeed variation of = (D Y )/D several outcomes arepossible depending on the type of load, namely

loads with constant horsepower, Y D , loads with constant torque, Y D (1), loads with linear torque, Y D (1)2, and loads with quadratic torque, Y D (1)3.Care must be taken to ensure that the motor speed after sta-

tor winding change from D to Y is still appropriate to thedriven load operation. For example, in a centrifugal pump, itis necessary to guaranty that the speed reduction does not leadto insufficient fluid flow (the pump flow is proportional to thespeed) and lifting incapacity3 (the pump head is proportionalthe speed square).

However, the lower the motor load is, and the higher the motorrated power is, the lower the motor speed variation will be, afterstator winding connection change. If the D to Y change is madenear the point , the motor slip never exceeds the motor nominalslip.

B. Motor Start-Up PrecautionsWhen the motor stator winding is connected in the Y mode,

the starting torque is reduced approximately to 1/3 of the nom-inal value (for D connection), which can lead to a significantincrease of the starting period or even to the lack of startingcapabilities. If the Y starting mode is adopted, the user shouldevaluate the increase of the starting timeframe and the increaseof the temperature that can result from such situation, poten-tially leading to a decrease in the motor lifetime. Therefore, theuser has to evaluate if the motor torque is able to acceleratethe motor in a suitable timeframe, particularly for high-inertialoads and/or loads with high demanding torque requirements(e.g., constant horsepower or constant torque loads).

In the starting instant, the winding current in Y mode is

3times lower than for the D mode. Therefore, the Joules losses

2The torque is approximately proportional to the voltage square.3If there is a system head associated with providing a lift to the fluid in a

pumping system, the pump must overcome the corresponding static pressure.


Fig. 9. Motor efficiency, power factor, current (p.u.), and speed (p.u.) as a function of the load, for the 3-kW four-pole motor (Brand A): (a) without automaticchange and (b) with automatic change.

Fig. 10. Motor efficiency, winding current (p.u.), and line current (p.u.) as afunction of the load for the 3-kW four-pole motor (Brand A).

for the starting period in Y mode are, approximately, 1/3 ofthose for the D mode. Thus, the motor starting timeframe canincrease, approximately, three times without an increase in themotor thermal stress.

C. Motor Losses and TemperatureConsidering the steady state, when the motor operates in Y

mode with a load below point , the overall losses are lowerthan those for the D mode, leading to a lower motor operatingtemperature and longer motor lifetime. For a motor load belowpoint , the stator winding connection change form D to Y leadsto a decrease in the core losses, and can lead to the decrease of thestator winding current for low-power motors (Fig. 10), but may

Fig. 11. Simulated motor efficiency, winding current (p.u.), and line current(p.u.) as a function of the load for a 300-kW six-pole motor.

not lead to a stator winding current decrease for mediumhighpower motors (Fig. 11). This is related to the balance betweencore (or magnetic) and electrical Joule losses.

Note that, for the Y connection, the stator winding currentand the line current are equal, but for the connection, D thestator winding current is

3 times lower than the line current.

For motors operating with a load below point (as well asfor loads higher than point ), the D to Y change leads to anincrease of the motor rotor losses (as a result of the increase ofthe rotor current), as it can be seen in the Fig. 12 (for a 3-kWmotor), which depends on the motor parameters and load.

For the motors operating with a load below point , after theD to Y change, the motor stator winding and rotor currents are


Fig. 12. (a) Motor stator winding and rotor currents and (b) motor per-phase losses, as a function of the load, for the 3-kW four-pole motor (Brand A).

lower than the nominal values, for steady-state. Below point ,the motor operating temperature is lower in the Y connectiondue to lower overall losses.

A potential benefit of the Y connection is that it eliminatesthe circulating currents, which can exist in the D-connectedwindings, and are related to operation with unbalanced systems.The circulating currents are responsible for additional windinglosses.

IX. ECONOMICAL CONSIDERATIONSThe increase of the motor efficiency and power factor leads

to a reduction in the motor operating costs. Oversized motorsare by far the most important cause of poor power factor inpower systems networks, additionally leading to large voltagefluctuations. This problem is particularly serious in develop-ing countries, which already face an undercapacity problem. Inpractical terms, the power factor increase leads to a decrease ofreactive energy bill and to a better exploitation of the electric in-stallations, including lower losses. The efficiency improvementhas direct impact on the electricity bill.

Considering the D to Y change in the operating periods withloads under point , the value of the annual savings S(/year)is given by (11), where i is the motor operating period with aduration hi (h/year), in which the mechanical power is P imech(kW) and an electrical energy cost Ci (/kWh).

Except for constant power loads, after the D to Y change,the motor input active power decreases not only due to themotor efficiency increase, but also due to the slight decreaseof the motor speed, which leads to a decrease of the requiredmechanical power:

S =

i

[(P imech(D )

iD

P imech(Y )

iY

) hi Ci

]. (11)

For the automatic change, the longer the motor operatingperiods with a load below the point are, the higher the energysavings potential is.

A. Permanent Winding Connection ChangeFor the economical analysis of the stator winding permanent

change, only one example is considered. Assuming that the 7.5-kW motor (Brand A) with D-connected windings drives a cen-trifugal fan at 25% of full load (Pmech = 1871 W), the efficiencyis 74%, the power factor is 0.35, the speed is 1489 r/min, and thetorque is 12 Nm. The D to Y change results in the speed reduc-tion to 1463 r/min, the torque reduction to 11.6 Nm and, con-sequently, the motor load reduction to 24% (Pmech = 1777 W),with an efficiency of 82% and a power factor of 0.76. Becausethe Y -connection speed is 1.7% lower (26 r/min) than Dspeed, there is a 5.0% reduction in the required fan power. Con-sidering 8000 h/year and 0.05 /kWh, the D to Y change leadsto annual savings of 144 /year. Additionally, there is a powerfactor increase of approximately 0.41 (from 0.44 to 0.85).

B. Automatic Winding Connection ChangeFor the economical analysis of the automatic change, some

examples are considered. To simplify the estimation of the en-ergy savings, the impact of the slight variation of the motorspeed after stator winding connection change is not considered.Two types of loads are considered in the following economicalanalysiselevating conveyors and mixers. It is also consideredthat the described loads operate 16 h/day and 360 days/year,and that the average electrical energy cost is 0.05 /kWh. It isconsidered that the elevating conveyor operates 12 h/day at 25%of full load and 4 h/day at 95% of full load (Fig. 13). The mixeroperates 7 h/day at 25% of full load, 5 h/day at 15% of full load,and 4 h/day at 95% of full load (Fig. 14). The estimated cost forthe electronic device presented in the Fig. 8 is 50 [8].

Considering the 3-kW motor (Brand A) with the automaticchange, the energy savings are 419 kWh/year and 444 kWh/yearfor the conveyor and mixer, respectively. This can be translatedinto 21 /year and 22 /year, respectively. For both cases, thepayback time for the automatic change device can be less than2.4 years. For motors with the same operating conditions anda rated power 3.5 times higher than the previously considered,


Fig. 13. Elevating conveyor with different load levels. (a) Motor load = 25%. (b) Motor load= 95%.

Fig. 14. Mixer with different load levels. (a) Motor load= 25%. (b) Motorload= 95%. (c) Motor load= 15%.

the energy savings can increase about 2.7 times, reducing thepayback time to less than ten months. The average daily powerfactor of the 3-kW motor improves by 0.31 (increases from 0.47to 0.78) and 0.31 (increases from 0.44 to 0.75) for the conveyorand mixer, respectively. The motor power factor improvementfor 25% and 15% of full load is 0.41 (from 0.37 to 0.78) and0.39 (from 0.28 to 0.67), respectively.

Considering the simulated 300-kW motor with automaticchange, the energy savings are 10887 kWh/year and 12099kWh/year for the conveyor and mixer, respectively. This canbe translated into 544 /year and 605 /year, respectively. Forthis case, the payback time for the automatic change device canbe 1 month. The daily average of the 300-kW motor power fac-tor improves by 0.15 (from 0.76 to 0.91) and 0.19 (from 0.70 to0.89) for the conveyor and mixer, respectively. The motor powerfactor improvement for 25% and 15% of full load is 0.20 (from0.71 to 0.91) and 0.33 (from 0.52 to 0.85), respectively.

X. CONCLUSIONGrossly oversized three-phase induction motors operate with

lower efficiency and power factor, which is by far the mostimportant cause of poor power factor in industrial installations.In some situations, motor performance can be improved bothin terms of efficiency and power factor through stator windingconnection change from delta to star. However, for variableload motors, permanent connection change is not an acceptablesolution.

With grossly oversized motors, there are substantial benefitsin terms of efficiency and power factor, by operating the motorin the Y -connection mode instead of the D-connection mode.This paper provides a technique, based on simple measurements,which can be used to select the most appropriate operation mode.

For variable-load motors, with long, low load periods andsome near full load periods during their duty cycle, an automaticstator winding change system can be implemented, particularlyfor those motors already started by stardelta method. This paperprovides the basics for the design of such system. This methodcan be implemented with a modest investment.

The replacement of an oversized standard efficiency motorby a properly sized high efficiency motor is, in most cases, aneconomical advantageous option, but requires additional invest-ment. Of course, for the variable-load motors, with near full loadoperating periods, a smaller motor cannot be used. Therefore, ifthe user applies the described methods to three-phase inductionmotors meeting the criteria described in the paper, the active andreactive electrical energy bill can significantly be reduced. Ad-ditionally, if the motor average efficiency increases, the motoroverall losses decrease and, therefore, the motor lifetime in-creases. However, for large motors, the authors recommend theusers to first consult the motor manufacturer before changingthe motor stator winding connection.

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[6] IEEE Test Procedure for Polyphase Induction Motors and Generators,IEEE Standard 112, 2004.

[7] Determining Electric Motor Load and Efficiency, U.S. Dept. Energy, FactSheet, Motor Challenge, 1997.

[8] F. Ferreira, A. de Almeida, G. Baoming, S. Faria, and J. Marques, Auto-matic change of the stator-winding connection of variable-load three-phaseinduction motors to improve the efficiency and power factor, in Proc.IEEE Int. Conf. Ind. Technol., Hong Kong, Dec. 1417, 2005, pp. 13311336.


Fernando J. T. E. Ferreira (M06) received thelincentiate degree in electrical engineering and theM.Sc. degree in automation and systems from theUniversity of Coimbra, Coimbra, Portugal.

He is currently teaching in the Department of Elec-trical Engineering, Engineering Institute of Coimbra(ISEC), Coimbra. Since 1998, he has been a Re-searcher in the Institute of Systems and Robotics,University of Coimbra, in the area of energy-efficientmotor technologies.

Dr. Ferreira was a recipient of the Best PaperAward at the 2001 IEEE/IAS Industrial and Commercial Power Systems Tech-nical Conference.

Anbal T. de Almeida (SM03) received the Ph.D.degree in electrical engineering from Imperial Col-lege, University of London, London, U.K.

He is currently a Professor in the Department ofElectrical Engineering and Computers, Universityof Coimbra, Coimbra, Portugal. He is the coauthor of5 books on energy efficiency and more than 100 pa-pers in international journals, meetings, and confer-ences. He has coordinated four European projectsdealing with energy-efficient motor technologies, in-cluding electronic variable-speed drives. He is also a

Consultant of the European Commission 4th and 5th Framework Programmes.He has also participated as a Consultant on several international projects topromote energy efficiency in developing countries.

Dr. de Almeida was a recipient of the Best Paper Award at the 2001 IEEE/IASIndustrial and Commercial Power Systems Technical Conference.

Documents

Method for In-Field Evaluation of the Stator Winding Connection of Three-Phase Induction Motors to Maximize Efficiency and Power Factor