INTERNATIONAL CONFERENCE ON ENGINEERING UBI2013 - 27-29 Nov 2013 – University of Beira Interior – Covilhã, Portugal Efficiency Analysis of Synchronous Reluctance Motors Jorge O. Estima, A. J. Marques Cardoso CISE – Electromechatronic Systems Research Centre, University of Beira Interior, Covilhã, Portugal [email protected]; [email protected]CT9 – Energy Abstract Regarding the worldwide electrical energy consumption, electric motors are by far the most important electric load, and therefore they are a great focus of interest and research. High efficiency electric motors can lead to significant reductions in the energy consumption and also reduce the environmental impact. Accordingly, this paper presents a performance evaluation of synchronous reluctance motor (SynRM) drives. Electric motors based on this technology were recently introduced in the market, being capable to achieve the IEC 60034-30 Super-Premium Efficiency level class (IE4). A detailed efficiency analysis is presented through the format of efficiency maps, which allow to fully characterize the motor operating range. Results are presented for a wide range of motors starting from 0.55 kW up to 4 kW, and considering two different rated speeds, namely 1500 and 3000 revolutions per minute. Key Words: Synchronous reluctance motors, variable speed AC drives, energy efficiency. Introduction Electric motors are used worldwide in a great variety of applications, consuming a considerable part of the world generated electric energy. As far as energy consumption is concerned, electric motors are responsible for approximately 40% of the electrical energy generated worldwide [1]. In the European Union, electric motors use approximately 70% of the consumed electricity in industry, being therefore the most important load type in this sector. Due to the wide use of electric motors, the improvement of their energy efficiency has become a major research focus. Both energy consumption as well as the environmental impact can be significantly reduced by adopting electric motors with high efficiency levels. Considering the various electric motor types available in the market, the three-phase induction motor is by far the most used machine. During the last decade, there has been a great effort in global harmonization of motor standards in order to promote energy efficiency in electric motors. As a result, the International Electrotechnical Commission (IEC) has introduced new standards dealing with motor testing procedures and efficiency classifications [2]. As an example, the IEC 60034-30 standard proposes four different efficiency classes, namely Standard Efficiency (IE1), High-Efficiency (IE2), Premium Efficiency (IE3), and Super- Premium Efficiency (IE4). A partial representation of the efficiency values for these standards are illustrated in Figure 1. Figure 1 – IEC 60034-30 efficiency standards for 4-pole 50 Hz electric motors [3].
Transcript
INTERNATIONAL CONFERENCE ON ENGINEERING UBI2013 - 27-29 Nov 2013 – University of Beira Interior – Covilhã, Portugal
Efficiency Analysis of Synchronous Reluctance Motors Jorge O. Estima, A. J. Marques Cardoso CISE – Electromechatronic Systems Research Centre, University of Beira Interior, Covilhã, Portugal [email protected]; [email protected]
CT9 – Energy Abstract
Regarding the worldwide electrical energy consumption, electric motors are by far the most important electric load, and therefore they are a great focus of interest and research. High efficiency electric motors can lead to significant reductions in the energy consumption and also reduce the environmental impact. Accordingly, this paper presents a performance evaluation of synchronous reluctance motor (SynRM) drives. Electric motors based on this technology were recently introduced in the market, being capable to achieve the IEC 60034-30 Super-Premium Efficiency level class (IE4). A detailed efficiency analysis is presented through the format of efficiency maps, which allow to fully characterize the motor operating range. Results are presented for a wide range of motors starting from 0.55 kW up to 4 kW, and considering two different rated speeds, namely 1500 and 3000 revolutions per minute.
Key Words: Synchronous reluctance motors, variable speed AC drives, energy efficiency.
Introduction
Electric motors are used worldwide in a great variety of applications, consuming a considerable part of the world generated electric energy. As far as energy consumption is concerned, electric motors are responsible for approximately 40% of the electrical energy generated worldwide [1]. In the European Union, electric motors use approximately 70% of the consumed electricity in industry, being therefore the most important load type in this sector.
Due to the wide use of electric motors, the improvement of their energy efficiency has become a major research focus. Both energy consumption as well as the environmental impact can be significantly reduced by adopting electric motors with high efficiency levels.
Considering the various electric motor types available in the market, the three-phase induction motor is by far the most used machine. During the last decade, there has been a great effort in global harmonization of motor standards in order to promote energy efficiency in electric motors. As a result, the International Electrotechnical Commission (IEC) has introduced new standards dealing with motor testing procedures and efficiency classifications [2]. As an example, the IEC 60034-30 standard proposes four different efficiency classes, namely Standard Efficiency (IE1), High-Efficiency (IE2), Premium Efficiency (IE3), and Super-Premium Efficiency (IE4). A partial representation of the efficiency values for these standards are illustrated in Figure 1.
Figure 1 – IEC 60034-30 efficiency standards for 4-pole 50 Hz electric motors [3].
INTERNATIONAL CONFERENCE ON ENGINEERING UBI2013 - 27-29 Nov 2013 – University of Beira Interior – Covilhã, Portugal
With the aim to improve efficiency of induction motors, more copper and lamination materials can be used. Further efficiency gains can be achieved by casting copper cages in squirrel-cage induction motors. As main disadvantages, the motor frame size is typically increased as well as the manufacturing costs. Moreover, there are problems associated to the cast copper cage.
Due to the copper high melting point, the die casting process is more demanding and complex. On the other side, the too high die casting temperature will lower the life expectancy of moulds, increasing even more the cost of producing cast copper rotors [4].
In the last years, line-start permanent magnet synchronous motors (PMSMs) were introduced in the market, allowing to achieve the Super-Premium Efficiency class (IE4). Comparing with induction machines, these motors can also be directly connected to the grid and present other advantages such as synchronous speed operation, higher efficiency (mainly due to the enormous decrease of the rotor losses), higher power density and higher power factor. However, this technology is not very mature yet since this motor type still presents some limitations. Due to the starting torque characteristics, these motors present some restrictions when the load presents high inertia and requires high starting torque. On the other hand, the generated cogging torque may cause relatively high levels of noise and vibration during the motor starting [5]-[6]. Line-start PMSMs can also be used with a converter for variable speed operation, eliminating these problems. Probably the biggest drawback of PMSMs is their price since the rare earth materials used in the magnets are still relatively expensive.
More recently, a different AC drive was release in the market based on synchronous reluctance motors (SynRM). Comparing with induction motors and PMSMs, the major difference is the rotor structure that is built taking into account the magnetic reluctance principle. Despite this concept dates back to 1923, only with the more recent improvements of the rotor design and the advance of power converters, this drive technology become suitable for industrial use.
The rotor design of synchronous reluctance motors present a magnetically anisotropic structure and it is only built using punched electric steel plates stacked together to form the rotor package. Therefore, no cage or magnets are used, making this motor cheaper to produce, more robust and avoiding the negative impact of rare-earth materials mining, used in the magnets production.
Accordingly, this paper presents a performance evaluation of eight SynRMs, divided into two groups of four motors (0.55 kW, 1.1 kW, 2.2 kW and 4 kW) according to their rated speed (1500 rpm and 3000 rpm). A detailed efficiency analysis is presented through the format of efficiency maps, which allow to fully characterize each motor operating range.
Synchronous Reluctance Motors Comparing with the standard AC machines such as induction motors and PMSMs (sinusoidal back-EMF), the stator construction of SynRMs is also very similar. However, the rotor design of these machines is unique, as depicted in Figure 2.
(a)
(b)
Figure 2 – SynRM rotor: (a) cross-section schematic representation; (b) detail of a built rotor.
INTERNATIONAL CONFERENCE ON ENGINEERING UBI2013 - 27-29 Nov 2013 – University of Beira Interior – Covilhã, Portugal
The rotor is built in such a way in order to take advantage of the magnetic reluctance principle. Considering a four pole rotor as shown in Figure 1a, it presents four high- and four low-reluctance axes. In practical terms, reluctance is equivalent to the magnetic resistance. The axes with low reluctance can be referred to as the direct or d-axis, while the axes with high reluctance can be referred to as the quadrature or q-axis.
When the motor is supplied by applying exciting currents to the stator windings, a rotating magnetic field is produced in the air gap. Then, the rotor attempts to align its most magnetically conductive axis, the d-axis, with the applied field, in order to minimize the reluctance in the magnetic circuit. As result, torque is produced in the air gap between the stator and rotor whenever the air gap rotating field and the d-axis of the rotor are not aligned. The generated torque amplitude is directly proportional to the difference between the inductances on the d- and q-axes. Consequently, the greater this difference, the greater is the torque production [7].
In order to properly control the motor, a frequency converter must be used since the performance is dependent on the information about the position of the rotor (easily obtained using sensorless control due to the high saliency ratio). Therefore, it will not operate correctly connected to the grid. With the converter, SynRMs can run smoothly due to the sinusoidal air gap field distribution and operate with sinusoidal current.
All this makes the rotor construction of SynRMs less complex than for induction motors or PMSMs. Without a cage or magnets, the rotor has a plain structure since it only consists of punched laminated electrical steel sheets to form flux barriers that are fitted in the shaft. The synchronous reluctance motor is therefore designed with magnetically conductive material, iron, in the d-axis and magnetically insulating material, air, in the q-axis.
Hence, the rotor construction is more robust than either induction motors or PMSMs. On the other side, the absence of rotor cage means that SynRMs operate at lower temperatures than induction motors, increasing their life-time since the cool running of the rotor also means lower bearing temperatures, which in turn increase the reliability of the bearing system.
Comparing with PMSMs, no magnets are used, resulting in lower production costs (due to the expensive rare earth materials), avoiding simultaneously the problem of demagnetization due to possible overheating problems. Moreover, since no back-EMF voltage is induced, SynRMs are inherently safe and eliminate the need for converter over-voltage protection. The maintenance is also much easier since if a bearing eventually needs to be replaced, having no magnetic forces, unlike a PMSM, the bearing change of a SynRM is as easy as for an induction motor.
Due to the lack of a cage and magnets, the rotor inertia is also smaller. This feature becomes very advantageous in high dynamic applications where low inertia enables faster operating cycles and brings further benefits in energy efficiency.
As main disadvantage, it can be pointed out the low motor power factor due to the need of rotor magnetization. This means that, comparing with induction motors, the power converter must be designed to a higher current rating. However, since there is always the converter between the motor and the grid, the lower SynRM power factor is not visible on the grid side and consequently does not have an impact on the grid supply dimensioning.
Electric Motor Test Bench
With the aim to perform a detailed evaluation of the SynRM drive, a dedicated electric motor test bench was built. Basically, the experimental setup comprises a converter that supplies the SynRM, a hysteresis dynamometer and its corresponding controller, and a digital power analyzer (Figure 3).
INTERNATIONAL CONFERENCE ON ENGINEERING UBI2013 - 27-29 Nov 2013 – University of Beira Interior – Covilhã, Portugal
(a)
(b)
Figure 3 – Electric motor test bench details: (a) power analyzer and dynamometer controller; (b) SynRM and hysteresis dynamometer.
The SynRM load torque is imposed by a Magtrol HD-815 hysteresis dynamometer and it is precisely adjusted using a Magtrol DSP6001high speed programmable controller. This device also provides the speed and torque signals to the power analyzer, which allows to calculate the motor mechanical power (Figure 4).
Figure 4 – Schematic representation of the experimental setup.
The used digital power analyzer is a state-of-the-art Yokogawa WT3000, which
provides all the required measurements with the highest accuracy available. This device is connected in series with the power circuit in order to measure all the converter input and output quantities. As a result, the converter and SynRM efficiency values are calculated by
the direct measurement of the converter input power (in
P ), the converter output power,
corresponding to the motor input power (motor
P ), and the mechanical power available at the
motor shaft (mec
P ). Accordingly, the efficiency values are given by:
100%motor
conv
in
P
Pη = × (1)
100%mec
motor
motor
P
Pη = × (2)
Other important quantities such as voltages, currents and power factor, are also directly calculated and obtained by the power analyzer.
INTERNATIONAL CONFERENCE ON ENGINEERING UBI2013 - 27-29 Nov 2013 – University of Beira Interior – Covilhã, Portugal
Experimental Results
For the experimental results, a total of 8 SynRMs were evaluated in the laboratory, four of them with a rated speed of 1500 rpm and the remaining ones with a rated speed of 3000 rpm. For both rated speeds, the power of each tested motor corresponds to 0.55 kW, 1.1 kW, 2.2 kW and 4 kW (Figure 5).
Figure 5 – Global view of the eight SynRMs used for the experimental tests.
With the aim to perform a detailed analysis of each SynRM, a great number of data
samples were acquired. Then, using a dedicated interpolation algorithm, all results are presented through the format of contour maps, allowing to fully characterize the motor operating range.
Figure 6 presents the efficiency maps calculated for all four 1500 rpm SynRMs. Analyzing the obtained data, it can be concluded that in general, high motor efficiency values are achieved for high load torque and speed operation. Additionally, it can be seen that the 2.2 kW and 4 kW SynRMs can maintain relatively high efficiency values for partial load torque.
With the aim to better analyze this, Figure 7 presents the efficiency results for all four motors, considering the operation at rated speed (1500 rpm).
The efficiency values shown in Figure 7 allow to conclude that the 2.2 kW and 4 kW SynRMs present a remarkable performance for partial load, being able to operate with an efficiency level relatively near to the one obtained at full load. The two less powerful motors also present a good performance for partial load values above 50%.
Further than this, the results presented in Figure 7 also allow to verify that at full load operation and taking into account each motor power class, all four motors present a very high efficiency. In general, comparing the obtained values with the efficiency standards presented in Figure 1, it can be concluded that these motors comply with the highest standard, corresponding to the Super-Premium efficiency class (IE4).
INTERNATIONAL CONFERENCE ON ENGINEERING UBI2013 - 27-29 Nov 2013 – University of Beira Interior – Covilhã, Portugal
300 450 600 750 900 1050 1200 1350 15000.5
1
1.5
2
2.5
3
3.5
56.0
57.8
59.7
59.7
61.5
61.5
63.3
63.3
65.2
65.2
67.0
67.0
68.9
68.9
70.7
70.7
72.5
72.5
74.4
74.4
76.2
76.2
78.1
78.1
79.979.9
81.8
Mechanical Speed (rpm)
Load
Torq
ue
(Nm
)
0.55 kW SynRM Efficiency (%)
300 450 600 750 900 1050 1200 1350 15001
2
3
4
5
6
7
65.7
67.0
67.0
68.4
68.4
69.7
69.7
71.1
71.1
72.5
72.5
73.8
73.8
75.2
75.2
76.6
76.6
77.9
77.9
79.3
79.3
80.6
80.6
82.0
82.0
83.4
84.7
Mechanical Speed (rpm)
Load
Torq
ue
(Nm
)
1.1 kW SynRM Efficiency (%)
300 450 600 750 900 1050 1200 1350 15002
4
6
8
10
12
14
72.673.875.0
75.0
76.2
76.2
77.4
77.4
78.7
78.7
79.9
79.9
81.1
81.1
82.3
82.3
83.5
83.5
84.7
84.7
86.0
86.0
87.2
87.2
88.4
Mechanical Speed (rpm)
Load
Torq
ue
(Nm
)
2.2 kW SynRM Efficiency (%)
600 900 1200 1500
4
6
8
10
12
14
16
18
20
22
24
76.7
77.7
78.7
79.7
80.7
81.782.8
83.8
83.8
84.8
84.8
85.8
85.8
86.8
87.8 88.8
88.8
89.8
89.8
90.8
90.8
Mechanical Speed (rpm)
Load
Torq
ue
(Nm
)
4 kW SynRM Efficiency (%)
Figure 6 – Efficiency maps for all four 1500 rpm SynRMs.
10 20 30 40 50 60 70 80 90 10070
75
80
85
90
95
Load Torque (%)
SynR
M E
ffic
iency
(%
)
0.55 kW
1.1 kW
2.2 kW
4 kW
Figure 7 – Efficiency results for the 1500 rpm motors at rated speed operation.
Regarding the analysis for the 3000 rpm SynRMs, Figure 8 presents the efficiency maps
calculated for the four motors.
INTERNATIONAL CONFERENCE ON ENGINEERING UBI2013 - 27-29 Nov 2013 – University of Beira Interior – Covilhã, Portugal
600 900 1200 1500 1800 2100 2400 2700 3000
0.4
0.6
0.8
1
1.2
1.4
1.6
63.1
64.6
66.2
67.7
69.2
69.2
70.8
70.8
70.8
72.3
72.3
73.8
73.8
75.4
75.4
76.9
76.9
78.5
78.5
80.0
80.0
81.5
83.1
Mechanical Speed (rpm)
Load
Torq
ue
(Nm
)
0.55 kW SynRM Efficiency (%)
600 900 1200 1500 1800 2100 2400 2700 30000.5
1
1.5
2
2.5
3
3.5
65.967.4
68.9
70.4
70.4
71.9
71.9
73.4
73.4
74.8
74.8
76.3
76.3
77.8
77.8
79.379.3
80.8
80.8
82.3
82.3
83.8
83.8
85.3
85.3
86.8
Mechanical Speed (rpm)
Load
Torq
ue
(Nm
)
1.1 kW SynRM Efficiency (%)
600 900 1200 1500 1800 2100 2400 2700 30001
2
3
4
5
6
7
71.2
72.5
73.8
75.2
76.5
77.8
77.8
77.8
79.1
79.180.5
80.5
81.8
81.8
83.1
83.1
84.4
84.4
85.7
85.7
87.1
87.1
88.4
Mechanical Speed (rpm)
Load
Torq
ue
(Nm
)
2.2 kW SynRM Efficiency (%)
600 900 1200 1500 1800 2100 2400 2700 3000
2
3
4
5
6
7
8
9
10
11
12
72.4
73.775.0
76.3
77.6
78.8
78.8
80.1
80.1
81.4
81.482.7
82.7
84.0
84.0
85.3
85.3
86.6
86.6
87.9
87.9
89.2
Mechanical Speed (rpm)
Load
Torq
ue
(Nm
)
4 kW SynRM Efficiency (%)
Figure 8 – Efficiency maps for all four 3000 rpm SynRMs.
In a similar way to the efficiency values measured for the 1500 rpm motors, the
results obtained in Figure 8 also allow to verify that high motor efficiency values are achieved for high load torque and speed operation. The exception is the 4 kW motor since it achieves the highest efficiency value at 2400 rpm.
With the aim to evaluate the partial load operation, Figure 9 presents the efficiency results as a function of load torque for all four motors, considering the operation at rated speed (3000 rpm).
10 20 30 40 50 60 70 80 90 10065
70
75
80
85
90
Load Torque (%)
SynR
M E
ffic
iency
(%
)
0.55 kW
1.1 kW
2.2 kW
4 kW
Figure 9 – Efficiency results for the 3000 rpm motors at rated speed operation.
By analyzing Figure 9, it can be observed that the 3000 rpm SynRMs do not have a
partial load operation as good as the one seen for 1500 rpm motors and shown in Figure 7.
INTERNATIONAL CONFERENCE ON ENGINEERING UBI2013 - 27-29 Nov 2013 – University of Beira Interior – Covilhã, Portugal
Additionally, it can be verified that the efficiency values obtained for the 4 kW motor are very similar to the ones obtained for the 2.2 kW, and therefore lower than what should be expected. This is justified by the fact that this motor presents higher friction losses due to bad lubrication or bearing quality, contributing to the decrease of the motor efficiency.
Finally, and with the exception of the 4 kW SynRM, it can be also concluded that according to each power class, all motors achieve the IE4 Super-Premium IEC standard.
Conclusions This paper has presented an efficiency evaluation of eight synchronous reluctance
motors divided into two groups according to their rated speed. With the aim to perform a detailed analysis, a great number of data samples were acquired, allowing to generate efficiency contour maps that fully characterize each SynRM operating range.
The obtained results allow to conclude that, in general, motor efficiency values are proportional to the load torque and speed operation.
Analyzing the two motor groups of 1500 rpm and 3000 rpm, the performed measurements have shown that the first group presents a better behavior for partial load operation, since a relatively high efficiency value is maintained for a wide variation of the load torque level.
As far as the efficiency standards is concerned, it can be concluded that in general the tested motors comply with the highest efficiency standard, namely the IEC IE4, corresponding to the Super-Premium efficiency level. As a result, and comparing with the majority of AC motor drives in variable speed applications, the use of these high efficiency motors can benefit from lower electric energy consumption and reduced operating costs.
Concluding, despite the magnetic reluctance principle applied in electric motors is not a new concept, only until very recently this technology became sufficiently mature, allowing a few manufacturers to market release this new type of motor drive. Thanks to their characteristics, it is expected that SynRM drives become a serious competitor against induction motors drives, achieving simultaneously high efficiency levels as PMSMs but at a lower cost.
Acknowledgement The authors gratefully acknowledge KSB company for their collaboration and for providing the SynRMs drive used in this paper. The authors also acknowledge the financial support from the Portuguese Foundation for Science and Technology (FCT) under project nº SFRH/BPD/87135/2012.
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INTERNATIONAL CONFERENCE ON ENGINEERING UBI2013 - 27-29 Nov 2013 – University of Beira Interior – Covilhã, Portugal
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