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Method for In-Field Evaluation of the Stator Winding Connection of Three-Phase Induction Motors to Maximize Efficiency and Power Factor Fernando J. T. E. Ferreira, Member, IEEE, and An´ıbal T. de Almeida, Senior Member, IEEE
Abstract—The performance of the oversized three-phase induction motors can be improved, both in terms of efficiency and power factor, with the proper change of the stator winding connection, which can be delta or star, as a function of their load. A practical method is proposed to quickly and easily evaluate which stator winding connection is more appropriate for the actual motor load profile, in order to increase the motor efficiency and power factor. This new method is suitable for in-field evaluation, because it requires only the use of inexpensive equipment and has enough accuracy to allow a proper decision to be made. The automatic change of the stator winding connection, as a function of the motor line current, is also analyzed. When properly applied, these methods can lead to the improvement of the efficiency and power factor of permanently oversized motors, motors with a load variation between low load and near full load during their duty cycle, and/or motors driving high-inertia, low duty cycle loads. The proposed methods are particularly suitable to industrial plants where typically many electric motor systems are oversized and/or can have a wide load variation. In these conditions, the active and reactive electrical energy bill can be significantly reduced.
Fig. 1. Average load factor by power range for motors, in the industrial and tertiary sectors, in the European Union, 2000 [2].
Index Terms—Energy efficiency, motor oversizing, power factor, savings, stator winding connection change, three-phase induction motor. Fig. 2. Motor stator winding connections. (Left) Star or wye connection. (Right) Delta or triangle connection.
I. INTRODUCTION N industry, more than 90% of the electrical motors are threephase squirrel-cage induction motors, hereafter denominated only by motors [1], [2]. In the European Union, the average load factor for motors, in both industrial and tertiary sectors, is 0.57 (Fig. 1). However, the average load factor per power range in some sectors can be as low as 0.25 [2]. Individual motors in those ranges have even lower load factors. Because the load factor is an average of the motor load during a defined period, the motor load can vary between values lower and higher than the load factor. Motor oversizing is mainly due to the poor motor system design or due to the gross overestimation of the mechanical power required by the load [2]. Additionally, motor oversizing
I
Manuscript received December 10, 2005; revised December 10, 2005. Paper no. TEC-000130-2005. F. J. T. E. Ferreira is with the Department of Electrical Engineering, Engineering Institute of Coimbra (ISEC), Coimbra 3030, Portugal, and also with the 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 Computer Engineering, University of Coimbra, Coimbra 3030, Portugal (e-mail:
[email protected]). Digital Object Identifier 10.1109/TEC.2006.874248
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 very low load and near full load) during their duty cycle can also be found. In these cases, the motor is sized to provide the load peak power, but it can operate during long periods with a very low load. These situations lead to a reduction of both motor efficiency (η) and power factor (λ). For specific conditions, the stator winding connection change from delta (D) to star (Y ) can significantly improve both motor efficiency and power factor. This possibility is only available for motors designed to operate at the nominal power with D connection and with access to the six winding terminals (Fig. 2), which are the vast majority. In this paper, an in-field evaluation method to access the most appropriate motor stator winding connection is proposed and analyzed. The automatic change of the motor stator winding connection, as a function of the motor line current, is also analyzed in the final part of the paper. For both methods, technical and economical considerations associated with motor stator winding connection are presented. The importance of this work is highlighted by the recent concerns on electric motor systems’ optimization in the industrial and tertiary sectors [1]–[3].
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IV. EXPERIMENTAL RESULTS
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, ω is the motor speed, Pelec is the input active power, and Pmech is the output mechanical power (useful power): η=
Pmech T ·ω = . Pelec Pelec
(1)
In the absence of voltage unbalance and motor electromechanical asymmetries, the active power Pelec absorbed by the motor is given by (2), where VLL is the line-to-line voltage (rms), IL is the line current (rms), and λ is the power factor: √ Pelec = 3 · VLL · IL · λ. (2) The motor load ψ is defined by (3), where PN is the motor nominal power: ψ=
Pmech . PN
(3)
The motor slip s is given by (4), where ω sync is the synchronous speed: s=
ωsync − ω . ωsync
(4)
III. SIMULATED RESULTS The efficiency–load curves for three motors were simulated in the MATLAB–Simulink software, using the motor per-phase √ equivalent circuit (Fig. 3). For the Y connection, a voltage 3 times lower than the voltage considered in the D connection was considered in the simulation. For the 3-kW motor (Brand A), the per-phase equivalent circuit parameters were experimentally obtained. For the 11- and 300-kW motors, the per-phase equivalent circuit 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 simulated motor efficiency–load curves for both D and Y connections for the three motors can be seen, as well as the motor parameters. The intersection point between the efficiency–load curves, hereafter denominated by point α, for the efficiency–load simulated curves for the D and Y connections corresponds to a motor load of 0.36, 0.42, and 0.47, for the 3-, 11-, and 300-kW motors, respectively. Note that, according to the Fig. 4, the load corresponding to the crossover point α increases with the motor nominal power, because the efficiency curves become flatter (due to the relative lower core losses).
The motor testing facility used in the experimental tests fulfils the IEEE 112 Standard requirements [6]. To measure the electrical and mechanical variables, a high-accuracy power analyzer is used (Yokogawa WT1030M). A dynamometer (Magtrol HD815-8NA) is used as a variable load, which includes an encoder to measure speed, and a load cell to measure the torque. The power analyzer acquires the values of both sensors and directly measures the motor efficiency according to (1). Thirteen totally enclosed fan-cooled motors of five different brands (denominated in this paper by A, B, C, D, and E), with nominal powers between 185 W and 7.5 kW, were tested. In Table I, the nameplate values of the motors, considering the D connection, can be seen. Eleven motors have four poles, one has two poles, and the remaining one has six poles. In all the tests, the motor temperature stability was guaranteed, for the same room temperature. The temperature correction of the motor parameters was not considered, in order to allow a real evaluation of the motor performance for both D and Y connections 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 line current, as a function of the load, for Y and D connections (for the 3-kW (Brand A) and 5-kW (Brand B) motors, both with four poles) are presented. For a motor load lower than point α, the motor efficiency and power factor for the Y connection are higher than for the D connection [Fig. 5(a) and (b)]. For any motor 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 narrow low power range), the point α has no regular relation with brand, nominal power, and number of poles, and it is between ψ = 0.27 and ψ = 0.50 (average ψ = 0.37, see Table I). However, as it can be seen in Section III, for motors with significant higher power, the point α moves to a higher load. The experimental and simulated point α for the 3-kW motor (Brand A) are approximately in accordance. Note that the difference of the motor operating temperature for both D and Y connections and different load points is not considered in the simulation. From Fig. 5, it can be concluded that the user should evaluate several factors before changing the motor stator winding connection. The most important factor should be the motor efficiency. For a specific load below point α, the increase in the power factor and in the slip after the D to Y connection change is well known. V. METHODS FOR DIFFERENT LOAD PROFILES The motor stator winding connection change can be made either by a manual method (permanent change) or by an automatic method (dynamic change). Each method should be chosen according to the motor-load profile. If the load profile is similar to the load shape of the Fig. 6(a) or (b), the stator winding should be permanently connected, after starting, in Y or D, respectively. In both cases, if the motor load slightly crosses the point α load level, during short periods, the respective connection can still be used (this issue is addressed in the Section VIII). If the load
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Fig. 4.
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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 I EXPERIMENTAL VALUES FOR THE INDICATORS IN THE POINT α
efficiency, the mechanical power, and the active electrical power values, for both D and Y connections, are equal [see (1) and (3)]. Therefore, in the point α, the relation (5) is true α IL(D λαY ) = . (5) α IL(Y λαD ) To identify point α, four indicators based on the motor in-field measurements and motor nameplate values (nominal values) are analyzed: • two line current-based indicators (KI 1 and KI 2 ); • two slip-based indicators (Ks1 and Ks2 ). The proposed indicators are based in values easily obtained in the field, using common measurement devices (voltmeter, clamp ammeter, and stroboscopic tachometer), namely, the rms lineto-line voltage, the rms line current, and the motor speed. The measurement of the power factor is avoided because it requires the use of a power factor measurement device, a wattmeter or a power analyzer, which, to have sufficient accuracy, are expensive devices. The indicators KI 1 , KI 2 , Ks1 , and Ks2 are defined by (6)– (9), where IN is the motor nominal line current, VN the motor nominal line-to-line voltage, Vmeas is the actual motor line-toline voltage, sN is the motor nominal slip, and smeas is the actual motor slip: KI 1 =
IL(D ) IN
IL(D ) IL(Y ) 2 ωsync − ωmeas(D ) VN = · ωsync − ωN Vmeas
KI 2 = profile is similar to the load shape of Fig. 6(c), the stator winding connection should be automatically managed by a suitable control device. VI. PERMANENT CHANGE OF THE WINDING CONNECTION When the stator winding connection √ is changed from D to Y , the winding voltage decreases 3 times. In point α, the
Ks1
= Ks2 =
(6) (7)
Smeas(D ) · VN2 2 SN · Vmeas
(8)
Smeas(D ) . Smeas(Y )
(9)
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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 KI 1 and Ks1 are obtained without disconnecting the motor and the indicators KI 2 and Ks2 require the motor stator winding connection change. In Table I, a summary of the indicator values, their average values, standard deviation, and variation with load, in relation to the point α, for the tested motors is presented. In Table II, a summary of the obtained indicator values, in relation to the point α, for the simulated motors is presented.
The standard deviation σ of a generic variable x is given by (10), where n is the number of samples: 2 n ni=1 x2i − ( ni=1 xi ) . (10) σ= n2 − n It is also important to evaluate the variation of each indicator, when the motor load is moving away from the point α. In
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TABLE II SIMULATED VALUES FOR THE INDICATORS IN THE INDICATORS IN THE POINT α
Table I, the average variation of the indicators in the neighborhood of point α (±10% variation) is presented. The indicator Ks1 is easy to obtain (it requires a stroboscopic tachometer and a voltmeter) but has errors related to the speed measurement device errors (typically ±1 r/min) and to the nameplate speed errors due to the numerical rounding process (the speed is rounded to 5-r/min multiples) [7]. The indicator Ks1 includes a voltage correction related to the fact that, for a constant torque, the motor slip is approximately inversely proportional to the voltage square. Therefore, if there is a difference between the motor actual voltage and its nominal voltage, it is necessary to compensate the slip, considering the relation between both voltages. The variation between Ks1 for the tested motors, in the point α, is reduced (σ = 0.06 for an average equal to 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 operating in the zone where energy consumption reduction can be obtained after the stator winding connection change from D to Y . In the simulated data, it can be concluded that Ks1 can slightly increase with the motor rated power. The indicator Ks2 is also easy to obtain (it also requires a stroboscopic tachometer and a voltmeter) and it is more reliable than Ks1 , but requires the motor stator winding to be changed. The variation between Ks2 for the tested motors, in the point α, is reduced (σ = 0.03 for an average equal to 0.27). It can be concluded that if a motor has a Ks2 ≥ 0.30, there is a high possibility (100% of the tested and simulated motors verify that condition) of being operating in the zone where energy consumption reduction can be obtained after the stator winding connection change from D to Y . The KI 1 is not a good indicator because, when the motor load is moving away from point α, for the tested motors, it has a very low average variation (±2%), tending to 0% for motors with PN ≤ 1 kW. The KI 2 average is 1.67 (σ = 0.11), which is also equal to the ratio between the Y and D power factors, in point α, as it was demonstrated in (5). All indicators present low standard deviation, but those with higher variation, when the motor load is moving away from point α, are more appropriate for the selection of the best connection mode. In general, the slip-based indicators are more suitable to in-field purposes because they have both lower standard deviation and higher average variation as a function of the motor load. Additionally, the measurement of the motor slip is normally easier and faster than the measurement of the motor line current.
Fig. 7.
In-field method to evaluate the motor stator winding connection.
Therefore, it can be concluded that the Ks1 is the most appropriate indicator for a preliminary evaluation of the motor efficiency improvement possibility, before the stator winding connection change. After changing the stator winding connection, Ks2 can be used to check with more accuracy the motorefficiency improvement. On the basis of the previous conclusions, a simple in-field method to evaluate which connection is more appropriated for the motor stator winding, as a function of the motor slip, can be defined based only on the Ks1 and Ks2 indicators (see Fig. 7). In this evaluation, the higher loads of the motors during their duty cycle should be considered. Firstly, the possibility of motor efficiency improvement after the stator winding connection change from D to Y should be determined based on the nameplate and actual motor speed and voltage, using Ks1 . The D to Y change should only be made if Ks1 ≤ 0.25, with a fair possibility of efficiency improvement. After the D to Y change, a slip based re-evaluation should be made using Ks2 . If Ks2 ≥ 0.30 the Y connection should be maintained, otherwise the winding should be reconnected to D. Note that, even if there are no significant efficiency improvements due to the proximity between the motor load and the point α, the power factor still significantly improves. Although the proposed method was only experimentally validated for the 185 W–7.5 kW motor power range, in principle, it can be applied to all the motors, because Ks2 has a very low dependency on the motor rated power and Ks1 can slightly increase with the motor rated power, as was demonstrated by the simulated results (see Table II). The permanent stator winding connection should be reevaluated periodically if the load characteristics change. The proposed method is suitable for grossly oversized motors and/or motors driving loads with low duty cycles and high inertia (e.g.,
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VIII. TECHNICAL CONSIDERATIONS A. Motor Load and Speed
Fig. 8. Basic topology of an electronic device for the automatic change of the motor stator winding connection [8].
press machines and high-inertia saws 1 ). Because it requires only low-cost and easy-to-use equipment (a stroboscopic tachometer and a voltmeter), the proposed method can be integrated in the group of the low-cost measures with a significant energy savings potential. VII. AUTOMATIC CHANGE OF THE WINDING CONNECTION The automatic change of the stator winding connection is particularly suitable for motors with significant load variation during their duty cycle, including relatively long, low load operating periods (below point α). The automatic connection change in such motors can lead to significant energy savings and improvement of the motor power factor in the low load operating periods, largely compensating the additional modest investment. The experimental results, using a microcontroller based electronic device (described in detail in [8] and shown in Fig. 8), for the automatic change of the motor stator winding connection, as a function of the motor line current, are presented. The device controls the D/Y and the line contactors. The connection control is based on the current measurement because it is the variable most suitable to be acquired and processed by an electronic 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 (Brand A), 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 in the point α for the D and Y connections, the stator winding connection is automatically and properly changed, as a function of the motor line current, leading to an improvement of the motor efficiency and power factor, for loads lower than point α. The duration of each different operating period of the motor duty cycle should be long enough to avoid an excessive number of stator winding connection changes, in order to avoid a significant decrease of the contactors and motor lifetime. Examples of loads in which the automatic change method can be potentially applied with possible energy savings include industrial mixers, elevating conveyors, and high-inertia saws. 1 In these load types, if the motor stator winding is Y connected, and the time between the maximum load periods is sufficient to allow the acceleration and speed stabilization of the inertia wheel, there are no operating problems. For high-inertia loads, D connection starting can be used, in order to reduce the starting period.
The motor speed and load variation after the stator winding connection change also deserve to be analyzed. After the stator winding connection change from D to Y , the motor line current significantly decreases and the motor speed slightly decreases (in the point α, the motor slip increases 3–4 times). After the Y to D change, the motor line current significantly increases and the motor speed slightly increases. The decrease of the motor speed after the D to Y change √ is related to the stator winding voltage decrease (decreases 3 times) and the consequent reshape of the motor torque-speed curve. 2 The slight increase or decrease of the motor speed after the stator winding connection change generally leads to an increase or decrease of the motor load, respectively. This fact can lead to significant power reductions in constant, linear, or quadratic torque loads, particularly for the last ones (e.g., centrifugal pumps and fans). For a speed variation of ∆ω = (ωD − ωY )/ωD several outcomes are possible depending on the type of load, namely • loads with constant horsepower, ψY ≈ ψD , • loads with constant torque, ψY ≈ ψD (1 − ∆ω), • loads with linear torque, ψY ≈ ψD (1 − ∆ω)2 , and • loads with quadratic torque, ψY ≈ ψD (1 − ∆ω)3 . Care must be taken to ensure that the motor speed after stator winding change from D to Y is still appropriate to the driven load operation. For example, in a centrifugal pump, it is necessary to guaranty that the speed reduction does not lead to insufficient fluid flow (the pump flow is proportional to the speed) and lifting incapacity 3 (the pump head is proportional the speed square). However, the lower the motor load is, and the higher the motor rated power is, the lower the motor speed variation will be, after stator winding connection change. If the D to Y change is made near the point α, the motor slip never exceeds the motor nominal slip. B. Motor Start-Up Precautions When the motor stator winding is connected in the Y mode, the starting torque is reduced approximately to 1/3 of the nominal value (for D connection), which can lead to a significant increase of the starting period or even to the lack of starting capabilities. If the Y starting mode is adopted, the user should evaluate the increase of the starting timeframe and the increase of the temperature that can result from such situation, potentially leading to a decrease in the motor lifetime. Therefore, the user has to evaluate if the motor torque is able to accelerate the motor in a suitable timeframe, particularly for high-inertia loads 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 3 times lower than for the D mode. Therefore, the Joules losses 2 The
torque is approximately proportional to the voltage square. 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. 3 If
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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 automatic change and (b) with automatic change.
Fig. 10. Motor efficiency, winding current (p.u.), and line current (p.u.) as a function of the load for the 3-kW four-pole motor (Brand A).
for the starting period in Y mode are, approximately, 1/3 of those for the D mode. Thus, the motor starting timeframe can increase, approximately, three times without an increase in the motor thermal stress. C. Motor Losses and Temperature Considering the steady state, when the motor operates in Y mode with a load below point α, the overall losses are lower than those for the D mode, leading to a lower motor operating temperature and longer motor lifetime. For a motor load below point α, the stator winding connection change form D to Y leads to a decrease in the core losses, and can lead to the decrease of the stator 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 medium–high power motors (Fig. 11). This is related to the balance between core (or magnetic) and electrical Joule losses. Note that, for the Y connection, the stator winding current and the line current are √ equal, but for the connection, D the stator winding current is 3 times lower than the line current. For motors operating with a load below point α (as well as for loads higher than point α), the D to Y change leads to an increase of the motor rotor losses (as a result of the increase of the rotor current), as it can be seen in the Fig. 12 (for a 3-kW motor), which depends on the motor parameters and load. For the motors operating with a load below point α, after the D to Y change, the motor stator winding and rotor currents are
FERREIRA AND DE ALMEIDA: METHOD FOR IN-FIELD EVALUATION OF THE STATOR–WINDING CONNECTION
Fig. 12.
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(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 α, the motor operating temperature is lower in the Y connection due to lower overall losses. A potential benefit of the Y connection is that it eliminates the circulating currents, which can exist in the D-connected windings, and are related to operation with unbalanced systems. The circulating currents are responsible for additional winding losses. IX. ECONOMICAL CONSIDERATIONS The increase of the motor efficiency and power factor leads to a reduction in the motor operating costs. Oversized motors are by far the most important cause of poor power factor in power systems networks, additionally leading to large voltage fluctuations. This problem is particularly serious in developing countries, which already face an undercapacity problem. In practical terms, the power factor increase leads to a decrease of reactive energy bill and to a better exploitation of the electric installations, including lower losses. The efficiency improvement has direct impact on the electricity bill. Considering the D to Y change in the operating periods with loads under point α, the value of the annual savings S(€/year) is given by (11), where i is the motor operating period with a i duration hi (h/year), in which the mechanical power is Pmech i (kW) and an electrical energy cost C (€/kWh). Except for constant power loads, after the D to Y change, the motor input active power decreases not only due to the motor efficiency increase, but also due to the slight decrease of the motor speed, which leads to a decrease of the required mechanical power:
S=
i
i Pmech(D ) i ηD
−
i Pmech(Y )
ηYi
·h ·C i
i
.
(11)
For the automatic change, the longer the motor operating periods with a load below the point α are, the higher the energy savings potential is.
A. Permanent Winding Connection Change For the economical analysis of the stator winding permanent change, only one example is considered. Assuming that the 7.5kW motor (Brand A) with D-connected windings drives a centrifugal fan at 25% of full load (Pmech = 1871 W), the efficiency is 74%, the power factor is 0.35, the speed is 1489 r/min, and the torque is 12 N·m. The D to Y change results in the speed reduction to 1463 r/min, the torque reduction to 11.6 N·m and, consequently, the motor load reduction to 24% (Pmech = 1777 W), with an efficiency of 82% and a power factor of 0.76. Because the Y -connection speed is 1.7% lower (−26 r/min) than D speed, there is a 5.0% reduction in the required fan power. Considering 8000 h/year and 0.05 €/kWh, the D to Y change leads to annual savings of 144 €/year. Additionally, there is a power factor increase of approximately 0.41 (from 0.44 to 0.85).
B. Automatic Winding Connection Change For the economical analysis of the automatic change, some examples are considered. To simplify the estimation of the energy savings, the impact of the slight variation of the motor speed after stator winding connection change is not considered. Two types of loads are considered in the following economical analysis—elevating conveyors and mixers. It is also considered that the described loads operate 16 h/day and 360 days/year, and that the average electrical energy cost is 0.05 €/kWh. It is considered 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 mixer operates 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 for the electronic device presented in the Fig. 8 is 50 € [8]. Considering the 3-kW motor (Brand A) with the automatic change, the energy savings are 419 kWh/year and 444 kWh/year for the conveyor and mixer, respectively. This can be translated into 21 €/year and 22 €/year, respectively. For both cases, the payback time for the automatic change device can be less than 2.4 years. For motors with the same operating conditions and a rated power 3.5 times higher than the previously considered,
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Fig. 13.
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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) Motor load = 95%. (c) Motor load = 15%.
the energy savings can increase about 2.7 times, reducing the payback time to less than ten months. The average daily power factor of the 3-kW motor improves by 0.31 (increases from 0.47 to 0.78) and 0.31 (increases from 0.44 to 0.75) for the conveyor and mixer, respectively. The motor power factor improvement for 25% and 15% of full load is 0.41 (from 0.37 to 0.78) and 0.39 (from 0.28 to 0.67), respectively. Considering the simulated 300-kW motor with automatic change, the energy savings are 10887 kWh/year and 12099 kWh/year for the conveyor and mixer, respectively. This can be translated into 544 €/year and 605 €/year, respectively. For this case, the payback time for the automatic change device can be 1 month. The daily average of the 300-kW motor power factor improves by 0.15 (from 0.76 to 0.91) and 0.19 (from 0.70 to 0.89) for the conveyor and mixer, respectively. The motor power factor improvement for 25% and 15% of full load is 0.20 (from 0.71 to 0.91) and 0.33 (from 0.52 to 0.85), respectively. X. CONCLUSION Grossly oversized three-phase induction motors operate with lower efficiency and power factor, which is by far the most important cause of poor power factor in industrial installations. In some situations, motor performance can be improved both in terms of efficiency and power factor through stator winding connection change from delta to star. However, for variable load motors, permanent connection change is not an acceptable solution.
With grossly oversized motors, there are substantial benefits in terms of efficiency and power factor, by operating the motor in 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 and some near full load periods during their duty cycle, an automatic stator winding change system can be implemented, particularly for those motors already started by star–delta method. This paper provides the basics for the design of such system. This method can be implemented with a modest investment. The replacement of an oversized standard efficiency motor by a properly sized high efficiency motor is, in most cases, an economical advantageous option, but requires additional investment. Of course, for the variable-load motors, with near full load operating periods, a smaller motor cannot be used. Therefore, if the user applies the described methods to three-phase induction motors meeting the criteria described in the paper, the active and reactive electrical energy bill can significantly be reduced. Additionally, if the motor average efficiency increases, the motor overall losses decrease and, therefore, the motor lifetime increases. However, for large motors, the authors recommend the users to first consult the motor manufacturer before changing the motor stator winding connection. REFERENCES [1] A. de Almeida, P. Bertoldi, and H. Falkner, Energy Efficiency Improvements in Electric Motors and Drives. Berlin, Germany: Springer-Verlag, 2000. [2] A. de Almeida, F. E. Ferreira, and P. Fonseca, “Improving the penetration of energy-efficient motors and drives,” ISR-University of Coimbra, European Commission, Directorate-General for Transport and Energy, SAVE II Programme, Mar. 2000. [3] A. de Almeida et al., “VSDs for electric motor systems,” ISR-University of Coimbra, European Commission, Directorate-General for Transport and Energy, SAVE II Programme, May 2001. [4] P. Alger, Induction Machines—Their Behavior and Uses, 2nd ed. New York: Gordon and Breach, 1969. [5] H. Beaty and J. Kirtley, Electric Motor Handbook. New York: McGrawHill, 1998. [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, Fact Sheet, Motor Challenge, 1997. [8] F. Ferreira, A. de Almeida, G. Baoming, S. Faria, and J. Marques, “Automatic change of the stator-winding connection of variable-load three-phase induction motors to improve the efficiency and power factor,” in Proc. IEEE Int. Conf. Ind. Technol., Hong Kong, Dec. 14–17, 2005, pp. 1331– 1336.
FERREIRA AND DE ALMEIDA: METHOD FOR IN-FIELD EVALUATION OF THE STATOR–WINDING CONNECTION
Fernando J. T. E. Ferreira (M’06) received the lincentiate degree in electrical engineering and the M.Sc. degree in automation and systems from the University of Coimbra, Coimbra, Portugal. He is currently teaching in the Department of Electrical Engineering, Engineering Institute of Coimbra (ISEC), Coimbra. Since 1998, he has been a Researcher in the Institute of Systems and Robotics, University of Coimbra, in the area of energy-efficient motor technologies. Dr. Ferreira was a recipient of the Best Paper Award at the 2001 IEEE/IAS Industrial and Commercial Power Systems Technical Conference.
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An´ıbal T. de Almeida (SM’03) received the Ph.D. degree in electrical engineering from Imperial College, University of London, London, U.K. He is currently a Professor in the Department of Electrical Engineering and Computers, University of Coimbra, Coimbra, Portugal. He is the coauthor of 5 books on energy efficiency and more than 100 papers in international journals, meetings, and conferences. He has coordinated four European projects dealing with energy-efficient motor technologies, including 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 to promote energy efficiency in developing countries. Dr. de Almeida was a recipient of the Best Paper Award at the 2001 IEEE/IAS Industrial and Commercial Power Systems Technical Conference.
