Performance Comparison of Three Different AC ...

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[1] Paul WaIde, and Conrad U. Brunner, Energy-efficiency policy opportunities for electric motor- driven systems, p. 39, 2011. [2] A. T. de Almeida, F. Ferreira and ...
Performance Comparison of Three Different AC Variable Speed Motor Drives Fernando Bento, Jorge O. Estima, Antonio J. Marques Cardoso CISE – Electromechatronic Systems Research Centre, Universidade da Beira Interior, Covilhã, Portugal Abstract Electric motors consume a substantial part of the electric energy generated worldwide. However, the concerns related to energy resources shortage are leading to the adoption of measures to improve energy efficiency and, hence, reduce the share of energy consumed by electric drives. The use of Variable Frequency Drives (VFDs) provides an effective way to reach such efficiency targets. Besides allowing the speed control of electric motors, enabling relevant energy savings in certain applications, VFDs are becoming increasingly popular due to their new capabilities and improved efficiency. To assess the performance of the latest state-of-the-art electric drives, this paper presents a comparison study of the most relevant AC electric drives based on three different motor types: squirrel cage induction motor (IM), permanent magnet synchronous motor (PMSM) and synchronous reluctance motor (SynRM). A detailed performance analysis is performed for each kind of drive by evaluating important parameters such as drive global efficiency, motor efficiency, power factor, input current and operating temperature. Then, a comparison analysis is made in order to highlight the most relevant advantages and disadvantages of each drive system.

1. Introduction Electric motors play an important role in a wide number of applications. It is estimated that 44% to 46% of the electrical energy generated worldwide is consumed by this type of load. Currently, industry is the most important activity sector contributing to these numbers. Nearly 64% of the worldwide electricity consumption related to this sector is made by electric motors [1]. Implementation of measures that increment energy efficiency and cut the share of energy consumed by electric drives are underway. The development of motor standards aimed to promoting the adoption of highly energy efficient electric drives is one of those measures. The International Electrotechnical Commission (IEC), one of the most important organizations involved in the development of standards related to electric and electronic technologies, has developed standards concerning energy efficiency classifications for electric motors. The IEC 60034-30 standard created four levels of energy efficiency: Standard Efficiency (IE1), High-Efficiency (IE2), Premium Efficiency (IE3), and Super-Premium Efficiency (IE4) [2]. Many manufacturers have already introduced, in their portfolios, electric drives designed to comply with the IE4 efficiency class, allowing for lower operating costs and, consequently, higher economical revenue. However, the development and proliferation of highly efficiency motors does not ensure, by itself, that the prominent energy efficiency targets are achieved in the industry sector. Other solutions should be implemented as well. The main motor applications running in the industry include conveyors, pumps, fans and compressors. Some of those applications have potential to attain significant energy savings, as it is the case of fans and pumps, by simply controlling the motor speed according to the application requirements. Currently, this is not achieved in most cases, as the three-phase squirrel cage induction machine directly connected to the grid is, by far, the most prevalent electric drive used in the industry [3]. Variable Frequency Drives (VFDs) can effectively solve this problem. Recent developments in the power electronics field are boosting the diffusion of VFDs that are now more efficient and reliable than ever. Power semiconductors are the key elements of a power converter, which means that efficiency, reliability and cost-effectiveness of a power converter depend mainly on the features of these devices.

The development of the Silicon IGBT, for instance, allowed the development of lighter and cheaper power converters, with switching frequencies in the order of kHz, capable of handling medium to high power levels [4]. Other advantages of these kind of semiconductors include the low on-state resistance, high-voltage capability and insulated gate. However, with the development of new materials in the semiconductor production, namely wide bandgap materials, it is expected that VFDs will become even more efficient, reliable, and costeffective. The main advantages of these materials lie on their high electrical and thermal conductivity properties, and high blocking voltage capability. Several advancements in the power conversion, such as reduction of the thermal stress, lower switching and conduction losses, higher switching frequencies, and power converters with smaller passive components, will be possible as soon as wide bandgap semiconductors penetrate in the market in a large scale [5]. To evaluate the performance of the most common electric drives available in the industry, the next sections present a comparison study of the most relevant AC electric drives, based on three different motor types: squirrel cage induction motor (IM), permanent magnet synchronous motor (PMSM) and synchronous reluctance motor (SynRM). The comparison makes a general evaluation of each drive, supported by experimental results, considering parameters such as the motor and global drive efficiency, supply current and voltage, power factor and stator temperature.

2. Experimental Test Bench To evaluate the performance of the three AC electric drives, the test bench shown in Figure 1 was built. It consists of a power converter, compatible with the three electric motors under analysis, supplying the AC electric motor, a digital power analyser, a hysteresis dynamometer and its controller.

Figure 1: Experimental Test Bench The motor speed is controlled by the power converter PumpDrive R (KSB202). The power converter uses a Voltage Vector Control (VVC+) strategy for the three drives. The load torque is imposed by the hysteresis dynamometer Magtrol HD-815, which is controlled by its programmable controller Magtrol DSP7001. Speed and load torque signals are sent from the programmable controller to the power analyser. Besides speed and load torque, other relevant signals are acquired/computed using the power analyser Yokogawa WT1800. The power analyser, connected in series with the power circuit in two distinctive points - input of the power converter KSB202 and input of the AC electric motor measures the voltage, current and active power on those points, and computes other important parameters used for the drive performance analysis, such as power factor, mechanical power and efficiency. Figure 2 depicts an overview of the experimental setup. The mechanical power at the motor shaft Pmec is determined using the load torque Tmec and the angular speed of the motor shaft ωmec (1):

Pmec = Tmec × ω mec

(1)

The converter and motor efficiency levels are important metrics in the assessment of the electric motor drive system performance. They are determined using the active power values, computed at the different points of the circuit. 2

Figure 2: Experimental setup schematic view. The converter efficiency ηconv is computed as the ratio between the active power at the converter input Pin and the active power at the motor input Pmotor:

η conv =

Pin × 100% Pmotor

(2)

In the case of the motor, the ratio between the active power at the motor input Pmotor and the mechanical power at the shaft Pmec gives the motor efficiency ηmotor:

η motor =

Pmotor × 100% Pmec

(3)

In turn, the global drive efficiency ηdrive is computed as the ratio between the input power and the mechanical power developed by the electric motor:

η drive =

Pin × 100% Pmec

(4)

Another relevant metric of the drive performance is the power factor of the motor supply. It is calculated using the motor input power Pmotor, the motor supply voltage Vph-ph and its line current Il :

cos(θ ) =

Pmotor Pmotor = S motor 3 × V ph − ph × I l

(5)

Detailed information regarding the technical parameters of the three electric motors used in the experiment can be found in Table I of the Appendix.

3. Experimental Results With the aim of fully understanding the capabilities and advantages of each electric drive, contour maps with the most relevant variables are plotted. A brief analysis of those maps is also presented. All three electric drives were operated over a wide range of load torque values, comprised between 1.4 Nm and 14 Nm (from 10% to 100% of rated torque), as well as a wide range of mechanical speeds, comprised in the range of 375 to 1500 rpm (from 25% to 100% of rated speed). The operation below 375 rpm was not considered in this work, as some electric drives subjected to the experimental tests were unable to operate under rated load torque condition for such low mechanical 3

speed values (below 375 rpm), precluding a fair comparison between all three electric drives for such operating conditions. 3.1. Efficiency Figure 3 shows the efficiency maps of each electric drive under analysis.

(a)

(b)

(c)

Figure 3: Efficiency maps of the electric drives under analysis: (a) IM; (b) PMSM; (c) SynRM. 4

Efficiency maps on the left side are related to the AC electric motors, while the efficiency maps on the right side are related to the global electric drives, i.e., the set power-converter + motor. Considering the AC electric motors efficiency maps (left side of Figure 3), it can be observed that, as expected, all the motors share the feature of high efficiency levels while operating at high rotational speed and load torque. At rated operating conditions, the PMSM shows the higher efficiency (91.7%), followed by the SynRM (86.6%), and the IM (86.3%). Nevertheless, a low variation of the PMSM efficiency with the speed is verified while the PMSM operates at low load conditions. The decay in the motor efficiency with the reduction of the motor speed is significantly higher in the case of the IM and the SynRM, if compared to the PMSM, demonstrating the high dependence of the IM and SynRM efficiency with the mechanical speed. Furthermore, the PMSM efficiency map shows a somehow significant decrease in efficiency, proportional to the load torque, when the load torque decreases below 4 Nm. The IM efficiency range is [66.1 - 86.3] %; in turn, the SynRM efficiency range is [34.2 86.9] %; and the PMSM efficiency range is [72.9 - 91.7] %. Considering now the efficiency maps for the global drives shown on the right side of Figure 3, it is visible that no significant changes are introduced in their appearance when compared to the corresponding motor efficiency values. The efficiency increases with the motor rotational speed, reaching its peak for rated operating conditions. A more in-depth comparison between the motor efficiency and global drive efficiency also allow to testify a small reduction of the global drive efficiency as the load torque increases, as a result of higher conduction losses in the converter semiconductors. This trend is common to all drives, as the power converter used in the drive is a common element in all of them. At rated operating conditions, the IM and SynRM drives global efficiency reach 84.32%, while the PMSM drive reaches a global efficiency of 89.4%. Small deviations between the experimental data and the expected values are verified in some of the efficiency maps presented in the paper. Such deviations might be a consequence of the power converter auto-tuning function, which determines some of the motor parameters autonomously. A thinner and more in-depth tuning of the motor parameters in the power converter would banish these small deviations. 3.2. Motor Supply Voltage and Current Figure 4 presents the motor supply voltage (left side of Figure 4) and current (right side of Figure 4) related to each of the electric drives under study. It can be stated that the supply voltage maps differ significantly between each other. In the IM drive (Figure 4a), the supply voltage is proportional to the mechanical speed and load level, especially while the drive operates at low load torque. However, the supply voltage tends to depend exclusively on the mechanical speed when the load torque is higher than 7 Nm (half rated torque). For the PMSM (Figure 4b) and SynRM drives (Figure 4c), the supply voltage shows a strong correlation with the mechanical speed, but meaningless correlation with the load torque. Considering now the supply current (right side of Figure 4), it is easily stated that the supply current of the electric drives under analysis depend, most of all, on the load torque. Nevertheless, a small deviation from this behavior is stated in the IM drive (Figure 4a). Under rated load operation and low mechanical speed, a relatively significant increment in the supply current is verified. Also, the SynRM drive supply current map has a deviation from its typical variation at low speed and torque, where the supply current increases.

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(a)

(b)

(c)

Figure 4: Motor supply voltage (left) and current (right) for: (a) IM; (b) PMSM; (c) SynRM.

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3.3. Motor Power Factor Figure 5 shows the power factor maps for the three electric drives used in the experiment. The power factor was calculated using the RMS phase-phase voltage Vph-ph, RMS line current Il, and motor input power Pin values, acquired with the power analyser. Equation (5) was used to compute the power factor for each operation point.

(a)

(b)

(c)

Figure 5: Motor power factor: (a) IM; (b) PMSM; (c) SynRM. As depicted in Figure 5, the IM (Figure 5a) and SynRM (Figure 5c) drives power factor is proportional to the mechanical speed and load torque level. However, a small deviation occurs in the IM drive, when a high load torque and low mechanical speed is imposed. The power factor decrement results from the higher supply current of the drive on this operating condition, as visible in Figure 4a. For the PMSM drive, the power factor (Figure 5b) depends mainly on the drive mechanical speed, especially when the load torque is higher than 4 Nm. Below this torque level, the power factor has a proportional relation with the mechanical speed and load torque. As expected, the PSMM drive shows the highest power factor at rated operating conditions (0.80), followed by the IM (0.70) and the SynRM (0.62). 3.4. Efficiency at different operating points Figure 6 and Figure 7 depict the efficiency curves for both motor and global drive, considering changes in a single parameter: load torque (Figure 6) or mechanical speed (Figure 7).

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(a)

(b)

Figure 6: Motor and global drive efficiency, considering different mechanical speed levels: (a) 750 rpm; (b) 1500 rpm (rated speed). A detailed analysis of Figure 6 allows to conclude that the motor and global drive efficiency is low when the motor is subjected to low load torque values. Additionally, the difference between the motor and global drive efficiency shortens as the load torque increases. This behavior is common to all drives, but it is more evident for the PMSM and IM drives. Comparing Figure 6a and Figure 6b, it is seen that the speed variation does not introduce significant changes on the evolution of the efficiency curves of the PMSM and SynRM drives; only a small increment of the efficiency occurs when the speed increases. However, the IM drive shows a degradation of the performance while it operates at lower speed and higher torque (Figure 6a), that does not occur at rated mechanical speed (Figure 6b). At rated speed, the IM drive efficiency curves surpass the efficiency curves of the PMSM drive when the load torque decreases below 2 Nm and, at the same time, tend to superimpose with the SynRM ones when the load torque approaches to the nominal value.

(a)

(b)

Figure 7: Motor and global drive efficiency, considering different load torque levels: (a) 7 Nm; (b) 14 Nm (rated load torque). The results of Figure 7 show that the efficiency of both motor and global drive increases with the mechanical speed. Comparing Figure 7a and Figure 7b, and considering the operation at lower mechanical speed, it is also stated that there is a decrease of the PMSM efficiency (from 84.8% to 79.6%) and IM efficiency (from 67.4% to 62.6%), but an increase in the SynRM efficiency (from 64.1% to 65.0%). At rated mechanical speed, there is a general increment on the drives efficiency as the 8

load increases. Furthermore, the difference between the motor and global drive efficiency shortens as the mechanical speed increases. This behavior is common to all three drives. 3.5. Thermal Performance To assess the thermal performance of the three drives under analysis, the drives were operated at half-load condition (rated speed and 50% of the rated load torque). The temperature values were acquired using a PT100 sensor probe placed inside the motor frame, next to the motor front cover. Figure 6 depicts the temperature sensor arrangement inside the motor frame.

Figure 6: PT100 sensor position. Environment temperature was also measured and recorded, in order to adjust the stator temperature according to the environment temperature evolution during the experiment. The data was acquired and sent to a computer using a National Instruments NI cDAQ-9174 data acquisition board and a NI 9217 module. Temperatures are recorded until the motor thermal steady-state condition is met. This condition, defined by standard IEC 60034-1:2004 [6], is reached when the gradient of the straight line between corresponding points of successive duty cycles on a temperature plot is lower than 2 K/h (Kelvin per hour).

Figure 7: Stator temperature evolution. 9

The results show that the SynRM temperature increment during the first hour of the experiment is higher than in the IM and PMSM. It is also observed that the SynRM stator temperature stabilizes much faster than in the IM and PMSM. At the end of the experiment, the temperature of both the IM and SynRM is quite similar, despite the differences in the behavior of the IM and SynRM. During the first 20 minutes, the PMSM and IM temperature values are quite similar. Then, the PMSM temperature increases at a lower rate, leading to a lower temperature at the end of the experiment. The PMSM stator temperature stabilizes at 33.9 ºC; the IM stator temperature reaches 36.8 ºC, while the SynRM stator temperature increases until 37.1 ºC. As stated in [3], such evolution can be influenced by several factors. However, the main reason for this behavior lies on the SynRM frame materials and their physical properties. The quick increment of the SynRM temperature verified in the first half of the experiment is a result of the higher current used to supply the SynRM. At the same time, the temperature evolution of the IM and PMSM during this period is a result of the similar physical properties of both motor frames. Then, the SynRM thermal stability is achieved in less time due to the high thermal conductivity of the SynRM stator/frame, which allows a more effective heat transfer to the environment and a counterbalance of the higher Joule losses of this motor. On the other hand, the low temperature of the PMSM stator at the end of the experiment is a consequence of the lower Joule losses and higher efficiency of this motor, when compared to the IM and SynRM.

4. Conclusions This paper has presented a performance analysis of the most common electric drives available in the industry. To obtain a high resemblance degree between all drives, the same power converter is used to supply the motors, using a vector control strategy. A detailed results analysis shows that all drives reach their efficiency peak at rated operating conditions, or close to that point. It is relevant to refer the good indicators of the SynRM drive performance for low speed operating conditions, which are very similar to those of the PMSM. This feature makes this type of drive desirable for applications where speed regulation is important. The results also demonstrate the good performance of the power converter used to control each motor. The converter shows high efficiency levels, without relevant changes, for all the operating range, in all drives. Despite that, the power converter performance on the SynRM drive should be highlighted, as the converter efficiency remains almost unchanged over the entire operating range of this drive. It is stated that the SynRM drive has a good thermal performance, comparable to the IM and PMSM drives, despite the higher supply current related to the SynRM.

Acknowledgement The authors gratefully acknowledge Reel/KSB company for the collaboration and for providing the power converter and the PMSM used in this paper. The authors also acknowledge the support of the Portuguese Foundation for Science and Technology under Project No. UID/EEA/004131/2013 and Project No. SFRH/BSAB/118741/2016.

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Appendix Table I – Parameters of the electric motors IM

PMSM

SynRM

Power (kW)

2.2

2.2

2.2

Speed (rpm)

1435

1500

1500

Frequency (Hz)

50

75

50

Torque (Nm)

14.6

14

14

Voltage (V)

400

400

400

Current (A)

4.56

4.4

5.7

Efficiency (%)

87

91.6

89.5

Frame Size

100L

90L

100L

Frame Material

Cast Iron

Cast Iron

Aluminium

Weight (kg)

33

17

25

Efficiency Class

IE3

IE4

IE4

References [1]

Paul WaIde, and Conrad U. Brunner, Energy-efficiency policy opportunities for electric motordriven systems, p. 39, 2011.

[2]

A. T. de Almeida, F. Ferreira and A. Quintino, “Economical considerations of super highefficiency three-phase motors”, 48th IEEE Industrial & Commercial Power Systems Conference, Louisville, KY, 2012, pp. 1-13.

[3]

J. O. Estima and A. J. M. Cardoso, “Super Premium Synchronous Reluctance Motor Evaluation”, International Conference on Energy Efficiency in Motor Driven Systems, 28-30 October, 2013.

[4]

Bimal K. Bose, Power electronics and motor drives: advances and trends, Academic press, 2010.

[5]

Peter Friedrichs and Marc Buschkühle, “The Future of Power Semiconductors | PowerGuru Power Electronics Information Portal”, 2016.

[6]

IEC 60034-1, Rotating electrical machines – Part 1: Rating and performance, 2004.

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