The development of a high-efficiency aquaculture water pump using self-adhesive electrical steel

Traditional water pump use AC motor combines with gearbox to achieve high torque and low speed to protect fish form injury by blades, which will shorten the life of waterwheel. Besides, the efficiency of the gearbox is about 40 ∼ 50%, resulting in low efficiency of whole system, and translated into poor reliability. The waterwheel in this study uses surface permanent magnet (SPM) motor instead of AC motor. However, the design of conventional waterwheel PM motor has the following drawbacks: (a) the slot fill factor cannot be too low; (b) the spacing between the rotor magnet grooves is too large, which affect the efficiency of the motor. To improve the motor efficiency without changing the shape of the stator core, this study developed an assembled stator core to improve the slot fill rate of motor winding. However, if the stack length of the stator core is too high, there will be a problem of core breakage. Therefore, this study introduced the self-adhesive steel to develop a self-adhesive assembled stator core to improve the breakage situation, and also to take advantage of the higher slot fill rate of the assembled core. Furthermore, in order to solve the problem of large spacing between the rotor magnet grooves, we optimize the magnetic circuit design by using simulation to improve the motor performance, such as magnet size, slot opening size, etc, to make the back electromotive force waveform as a sine wave. In addition, this design also reduces the thickness of magnets to reduce the cogging torque of the motor, and effectively reduces magnet usage to reduce the cost. As a result, this study developed a high-efficiency waterwheel SPM motor about 86.32% at 123 rpm, by using self-adhesive assembled stator core to increase the slot fill factor. The overall efficiency of system reaches about 78.72%.


Introduction
Traditional aquaculture aeration systems commonly utilize AC induction motors to power waterwheels (figure 1). However, the low torque of the AC motor requires a reduction gearbox to decrease rotation speed and increase torque [1]. It is important to note that the efficiency of the traditional AC motor is only around 60%, and the addition of the reduction gear box decreases the overall efficiency to approximately 40% [1][2][3][4]. Furthermore, gear boxes are easily corroded by seawater, leading to frequent replacements and increased maintenance costs [5]. Therefore, this research aims to develop a permanent magnet waterwheel motor for an aquaculture water pump system. The objective is to achieve an overall system efficiency of greater than 75% and a motor efficiency exceeding 85%. Additionally, the permanent magnet motor developed in this study should maintain high torque at low speeds, ensuring motor efficiency exceeding 75%. This approach can save more than 35% of power consumption, eliminate the need for the gear reduction box, and prevent damage to the gear box due to seawater erosion. In this study, we proposed the concept of assembled self-adhesive stator cores, which can be applied to waterwheel motors to improve the traditional waterwheel motor stator which mostly Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. uses one-piece cores or assembled cores made by riveting and stacking. Typically, assembled stators use several single-tooth stators connected in series to form a complete stator core, which reduces the tedious and difficult winding process to achieve a higher slot fill factor. Compared to one-piece stator cores, assembled cores have higher motor efficiency due to higher slot occupancy. However, if the stack length of the assembled single-tooth core is too high, there will be a problem of core breakage, so the traditional stacking method uses riveting or welding to increase the structural rigidity of the core, but it will lead to processing degradation of the magnetic properties of the core. Therefore, in this study, the development of self-adhesive stator cores through selfadhesive electrical steel not only improves the fracture of the cores, but also maintains the magnetic properties of the electrical steel. The goal of this work is that the overall system efficiency is greater than 75% and the motor efficiency exceeds 85%. Besides, the permanent magnet motor developed in this research needs to be able to maintain high torque at low speeds, at which time the motor efficiency can exceed 75%. The high-efficiency surface permanent magnet (SPM) motor developed in this study can be widely applied to the aeration pump used in aquaculture in Taiwan, which can reduce the electricity consumption of the aquaculture water pump system and the maintenance cost of the motor. The aeration pump runs 24 h a day and consumes considerable power. It can be said that electricity bills are one of the biggest costs in aquaculture. If the application of the new high-efficiency SPM motor could be widely used in Southeast Asia where aquaculture is thriving, it will have the opportunity to reduce a lot of energy consumption for the earth in the near future. As well as saving more than 35% of the power consumption, the expense of the gear reduction box can also be saved, and it can totally avoid the situation that the reduction gearbox is easily damaged due to the erosion of sea water.

Research background
In response to the catastrophe of global marine resources, since 2014, the amount of direct food supplied to humans by aquaculture fisheries has exceeded that of capture fisheries, and they have continued to grow rapidly [6][7][8]. The production of global aquaculture has been continuing to rise significantly to meet the huge demand of the world, being estimated to witness a growth of around 4% in 2019 [9]. According to historical statistics, the aquaculture area in Taiwan in 2017 was approximately 43,877 hectares, most of which are located in the southern part of the west bank. Taiwan used to be world-renowned in the field of aquaculture, but as the number of countries invested in aquaculture has increased significantly and the cost of aquaculture has increased year by year, the international competitiveness of Taiwan's aquaculture industry has declined rapidly in recent years. In the traditional high-density aquaculture fishery, in order to increase the amount of dissolved oxygen in the pond [10], an aerator must be used to draw water to improve the survival rate of fish and shrimp. According to reports, Taiwan's aquatic motors use 8.5 billion kWh of electricity a year. Electricity is one of the three major costs of the aquaculture industry. Therefore, if energy-saving waterwheel motors can be developed, it can not only alleviate the power shortage crisis in Taiwan, but also increase the competitiveness of the aquaculture industry. In the future, if the application of the new high-efficiency SPM motor could be widely used in Asia such as China, India, Vietnam, Indonesia, Thailand, Philippines, Bangladesh, and Malaysia where aquaculture is booming, it would contribute to the energy-saving and carbon-reduction of the Earth.
Since the waterwheel motor is usually directly exposed to the high temperature of the Sun, it is constantly in contact with the seawater, causing many corrosion conditions and adhesion of moss, as shown in figure 2. It is understood that there are three main factors for the failure of the waterwheel: (1) The temperature of the coil rises due to excessively high temperature of copper wire in summer, which will eventually cause the insulation of the winding coil to melt and cause the motor to fail; (2) Sea water corrodes the motor shell, causing a short circuit and damage in the driver or motor; (3) Severe adhesion of moss leads to increased motor load, resulting in slower speed and even motor burnout.
In response to the above three conditions, the following five points must be paid attention to in the future development of waterwheel motors [10][11][12][13]: (1) The current density of the rated point of the motor must be adjusted after considering the ambient temperature. Although the waterwheel has seawater to assist in heat dissipation, it must be adjusted down to the air-cooled current density range to avoid coil burnout and damage due to harsh environments; (2) The shell of the waterwheel must be made of corrosion-resistant material, and the surface must be coated with waterproof paint or marine paint; (3) The driver should not be placed above the waterwheel motor, and should be centralized in the distribution box on the shore. As well as avoiding the driver from being corroded by seawater, it can also be easily repaired; (4) Considering that the adhesion of moss will increase the load of the waterwheel, the margin of the motor output specification must be increased. For example, for a 1 HP motor, the specification must reach 1.2 HP, and for a 2 HP motor, the specification needs to reach 2.2 HP; (5) The general height specification of the fan blade shaft core of the aeration system and the waterwheel system is 110 mm. Therefore, the outer diameter of the motor housing can be designed to be within 220 mm, and the outer diameter of the stator can be designed to be less than 200 mm.
The application of self-adhesive electrical steel to motors is a hot topic these days. As shown in figure 3, the 80 kW traction motor developed by our team with both self-adhesive and riveted core designs using 50CS470 electrical steel is fabricated. The CS31 self-adhesive coating was developed by China Steel Corporation [14]. A water-cooling housing is used and both of them are measured on the dynamometer platform as shown in figure 4. Figure 5 refers to the torque-speed curve and motor efficiency maps of the two motors. It is obvious that the efficiency of the self-adhesive-core motor is higher, and the motor's high-efficiency area (>96%) is about 14.65% higher than the riveted core version [14]. Figure 6 shows the original design of the motor demonstrated by our previous research, which was made of electrical steel (grade 35CS250) and NdFeB magnets (grade N35SH). The electrical steel and NdFeB magnets used by this motor are both manufactured by China Steel Corporation (CSC). Its rated power is 1 HP, and the structure is inner rotor type and Surface-Mounted Permanent-Magnet Motor (SPM) type. The surface of the rotor fixes the magnets in the form of grooves, and the outer diameter of the stator is 170 mm (O.D. = 170).

Original design of 1 HP waterwheel motor
From the relevant analysis of the following motor design, it can be seen that this motor has several shortcomings [15][16][17][18]: • The stator winding for the inner rotor type motor adopts the concentrated winding method. The winding machine will be limited by the slot opening size. The winding speed must be slowed down, otherwise there will be a risk of striking the pin, and the slot fill factor cannot be too high.
• The rotor area is too large, causing the motor to be too heavy.
• The waterproof protection level of the motor case is too low, and sea water easily penetrates into the motor from the shaft center.
• The distance between the magnet grooves in the rotor iron core is too large, which makes the magnetic circuit discontinuous and easy to leak magnetic flux. This design leads to an incomplete sine wave of magnetic field intensity distribution of the rotor, which will affect the efficiency of the motor.  • The magnet is embedded and tightly mounted with the rotor iron core, which makes the magnetizing process more difficult. If magnetizing first, it is difficult to insert the magnet. Therefore, the magnet must be inserted by percussion, which will easily destroy the rotor balancing.

Optimal design of 1 HP waterwheel motor
In order to improve the efficiency of the motor without altering the shape of the stator and rotor cores, we attempt to develop an assembled stator to increase the slot fill factor of the motor. The assembled stator uses several individual single-tooth stators connected in series to form a complete stator core. Its advantage is that the single-tooth stator can be individually wound, which can achieve a higher slot fill factor [15]. The disadvantage is that the winding process of multiple windings is longer and more difficult [16][17][18]. Each set of the single-tooth stator can be bonded together, or riveted by mechanical characteristics (such as punching cuts). However, if the stack length of the assembled stator core is too large, there will be a problem with the iron core breaking from it [15].
In response to the above problems, this research realized a self-adhesive coating film with electrical steel (50CS470) to develop a self-adhesive assembled stator core. Its preparation method is to first use round rivets to fix the iron core with low-damage cross sections, and then use pressure baking to make the iron core selfadhesive. Figures 7(a) and (b) are the side views of the self-adhesive single-tooth stator, and the completed selfadhesive assembled stator core is shown in figure 7(c). Using a self-adhesive 50CS470 electrical steel to  demonstrate the motor with the assembled stator, as well as improving the fracture of the assembled stator core, it also takes advantage of the better slot full rate of the assembled stator. Under the condition of the same driver, the efficiency of the motor in the overall operating range is increased to 68.7 ∼ 75.3%, but it still cannot fulfill the performance of the motor developed with NdFeB magnet [18]. Therefore, we will carry out a new design for the rotor to improve the motor performance in the following sections.
In order to improve the performance of this motor and further reduce the cost, we only change the design of the rotor under the prerequisites of using the same mold as the current stator. According to previous studies, the disadvantage of the initial motor design is that the distance between the magnet grooves in the rotor iron core is too large, which makes the magnetic circuit discontinuous and easy to leak magnetic flux. This leads to problems such as the incomplete sine wave of magnetic field intensity distribution of the rotor, which causes the motor efficiency to decrease. The improvement method in this research is to optimize the design of the magnetic circuit to make the waveform of the back electromotive force (back EMF) a sine wave. Figure 8 depicts the parameters of 1 HP waterwheel motor. Figure 9 shows the difference before and after the improvement. The conventional waterwheel permanent magnet motor design has a large spacing between the rotor magnet grooves (i.e., figure 9(a)), and the magnetic circuit is not continuous and the rotor magnetic wave plane is not a complete sine wave, resulting in a decrease in motor efficiency [2][3][4]. In this study, we optimize the magnetic circuit design by using electromagnetic analysis simulation software (JMAG-Express) to improve the motor performance, such as magnet size, slot opening size, stator tooth width, etc, to make the back EMF waveform as a sine wave. In addition, this design also reduces the thickness of the magnets to reduce the cogging torque of the motor, and effectively reduces the magnet usage to reduce the cost. Figure 10 shows the rotor core for preliminary proofing of the motor prototype. This rotor core uses 50CS470 electrical steel with self-adhesive coating film (CS31), which is first bonded and then wire-cut to produce a rotor core. The rotor uses NdFeB magnets of the same grade as the original motor design (N42SH).
According to calculations, the original design of the motor has a magnet consumption of 122461.5 mm 3 , and the amount of magnet used in the optimized design is 88215.92 mm 3 , which can save about 28% of the amount of magnets and reduce the cost by about 7%. In addition, this optimized design is conducive to the process of mounting the magnet to the surface of the rotor core. Compared with the previous process of tightly fitting and inserting the magnet, it not only reduces the difficulty of the process to increase the feasibility of motor mass production, but also shortens the manufacturing process. The back EMF constant (K b ), one of the performance indexes of the motor, is also larger than the initial design (about 5.6% increase), which shows that the motor efficiency is higher, the input current is smaller and the rise of coil temperature is lower under the same output    torque and the same winding specifications. Table 1 is the characteristic table of the optimized design rotor. It can be seen that the optimized design of the motor has a higher efficiency (73.7 ∼ 93.2%).
In this work, the temperature rise test of this 1 HP optimized design waterwheel motor is carried out. The actual measured coil temperature rise is about 5°C lower than the original design (The operating temperatures of the original design and optimized design at 600 W, 700 W, and 800 W input are 52.8°C, 69.4°C, 86.7°C, and 47.3°C, 64.3°C, 85.3°C, respectively. However, it can be seen from the coil temperature rise test data that the optimized design rotor does not meet the temperature tolerance of electrical insulators Class B (80°C) when the input is 800 W (about 1 HP). In consideration of cost and without changing the grade of electrical steel and NdFeB magnet, it is necessary to increase the volume of the motor to increase the output torque. Therefore, we will further carry out the design of the 2 HP waterwheel motor. According to the previous analysis, the diameter of the motor casing cannot exceed 220 mm, and the maximum stator outer diameter cannot be larger than 200 mm. Considering that a thicker cast iron casing may be used in the future to increase the corrosion resistance of the motor, the stator outer diameter is set to 190 mm. Next, we will further develop a waterwheel motor with an overall efficiency (including PFC, variable speed drives, and motor) exceeding 75% at the rated point. Under the preset PFC (power-factor-correction) efficiency of 95%, the efficiency of the single motor is expected to be 85%, and the efficiency of the matched driver is 95%, as shown in table 2. Other specifications of the motor are: rated speed = 111 rpm, rated torque = 80 Nm, rated voltage = 380 V, outer diameter of stator = 190 mm, and stack length of the stator core = 150 mm.
3. Development of OD190 waterwheel motor for aquaculture water pump system 3.1. Motor design According to the analysis in section 2.3, it is known that the design limit for the development of this project is that the maximum outer diameter of the stator is 190 mm, and the rated point is 80 ∼ 90 Nm. After considering the winding factor and other factors, the number of slot/pole is determined to be 15S/14P, and enter the parameters such as motor power, voltage, pole number, slot number, speed, stack length of the stator core, and the selection of magnet material in the JMAG Express program input. Then, optimize the design according to the motor size parameters in table 3, and finally, the optimized parameters can be obtained. In addition, the computer-aided design drawing program software AutoCAD is also used to calculate the maximum fullness (slot fill factor) of the motor winding, as shown in figure 11. Figure 12 shows the simulation results using JMAG Express. According to the I-T curve and I-Eff. curve of the results, when the current value is about 3.6 A, the torque value can reach the rated point of 80 Nm, and the efficiency is about 87%, which meets the goal of this research (85%). On the other hand, when the current value is 6.5 A, the torque value is 138 Nm, the output power can reach 1.5 kW (2 HP), and the efficiency is about 78.3%.
In this study, the grade of the electrical steel utilized in the motor is 50CS470 (manufactured by China Steel Corporation, CSC), and its B 50 (the magnetic flux density at 5000 A m −1 ) is 1.72 T, as shown in figure 13. Then confirm whether the waveform of the EMF is a sine wave by simulating the no-load voltage of the motor at 111 rpm, as demonstrated in figure 14. It can be seen from figure 14 that the waveform has no obvious distortion, and the voltage is 321.58V, and the back EMF constant (K b = V peak /ω no-load ) can be calculated as K b = 321.58V/ (111 × 2π/60) = 27.67 V·s/rad. After optimizing the motor size parameters, the JMAG-Designer software is used to confirm that the magnetic flux of the tooth is smaller than the B 50 of the electrical steel 50CS470. Figure 15 (a) shows the magnetic flux distribution of the optimized motor. It can be seen that the magnetic flux of the tooth is about 1.69 T, which has not been magnetically saturated. Figure 15(b) is the magnetic field lines distribution of this motor, and there is no issue of excessive magnetic field concentration. Figure 15 can be used as a reference for future rotor weight reduction designs. The area with less magnetic flux distribution or without magnetic field line distribution can be hollowed out to further reduce the weight of the rotor.

Preparation of O.D.190 waterwheel motor prototype
In this study, single-slot stator cores are used for assembled stator winding to increase the slot fullness rate of the winding. Compared with the integrated stator cores, the slot fullness rate of the assembled stator winding can be increased by about 5 ∼ 10%. In addition, the difficulty of single-slot winding is relatively simple. Figure 16 shows the finished single-slot stator winding and 15-slots assembled stator core after combination. Figure 17 illustrates the finished O.D.190 waterwheel motor made of self-adhesive stator cores using selfadhesive electrical steel. Due to the high torque requirement of the waterwheel motor, a larger outer diameter and a higher stack length core are generally used. Therefore, during the winding process, it is easy to cause the core to break due to the tension of the winding wire. In order to solve the problem of interruption of the stator core caused by high stack length, the process of multiple riveting or welding stacking is generally used. However, riveting destroys the stator flux path, reduces motor efficiency, and increases motor iron losses; welding destroys the interlayer impedance of the stator core, increases copper losses in the eddy current, and also causes deterioration of the core magnetism due to the high temperature of welding. In view of the above-mentioned drawbacks of the known techniques, the concept of a self-adhesive stator core is proposed in this study. Figure 18 demonstrates a single-tooth self-adhesive assembled stator core, which can be divided into figure 18(a) without rivet fixing and figure 18(b) with circular rivet fixing. Compared to traditional square riveted (V-shaped riveted) and welded assembled cores, single-tooth self-adhesive assembled stator cores without rivet fixing can avoid process degradation from riveting and welding, and round rivets are less likely to damage the core than square rivets. The self-adhesive assembled stator cores in this study improve the problem that the rivet-assembled cores used in conventional waterwheels are prone to break due to excessive winding tension during the winding Figure 11. The maximum fullness (slot fill factor) of the motor winding calculated by AutoCAD. process because of their high stack length. By increasing the winding tension, the quality and neatness of the winding can be improved, and the occupancy rate can be increased. Figure 19 shows the finished prototype of the assembled stator core winding demonstrated in this study, which indicates that the winding is very neat, and thus it can also take advantage of the better fill rate of the assembled stator core. To further understand the effect of riveting points on the iron loss of the assembled stator core of the waterwheel motor, we also prepared an assembled core of the same size with square riveting points for comparison, as shown in figure 20. And the iron loss measurement is carried out by the iron loss measuring instrument (BROCKHAUS MPG-200D), and the measurement setup is shown in figure 21. Figures 22(a)-(d) show the difference in iron loss measured for riveted cores and self-adhesive stator cores under different AC magnetic fields. The experimental results show that the iron loss of the self-adhesive core is lower than that of the riveted core at 50 Hz, 60 Hz, 200 Hz and 400 Hz by about 28.74%, 29.68%, 30.86% and 27.68%. As a result, selfadhesive assembled stator cores have obvious advantages over traditional riveted assembled cores.
In this study, the motor performance is tested by the dynamometer (HA-ADCM-10 HP-100Nm/20Nm) developed by HSY ATSystem Corporation, the measured back EMF constant K b is 24.6V·s/rad, as shown in

Comparison of measured and simulated values of motor performance difference
In order to compare the motor performance difference between the measured value and the simulated value, the characteristic of the magnet (B r value) is often used as the result under the condition of tolerating all process variation in the motor design [16]. From the simulation results in section 3.1, it is known that the simulated  value of back EMF constant K b is 27.67 V·s/rad, and the measured value is 24.6 V·s/rad. According to the difference between the measured value and the simulated value, the characteristics of the magnet are modified and the winding specifications are adjusted. Finally, the simulation results are quite close to the actual measurement results, as shown in figure 23.
According to the calculations, the magnet volume used in the original motor design was 122461.5 mm 3 , while the improved design reduced the magnet volume to 88215.92 mm 3 , saving approximately 28% of magnet usage and reducing costs by approximately 7%. Additionally, the new rotor design achieved a magnetic flux even higher than the initial design, by approximately 10%, with a magnet usage reduction of 28%. A higher magnetic flux indicates a higher magnetic loading output, which results in higher motor efficiency. Furthermore, the back EMF constant Kb, one of the motor performance indicators, was also greater in the new design than in the initial design, increasing by about 5.6%. At a working point of 100 rpm and an output torque of 30 Nm, the motor efficiency of the new design was 93.2%, surpassing the previous design's efficiency of 74.1%.

Conclusion
For aquaculture water pump system application, a high-efficiency SPM motor with a self-adhesive assembled stator core using a self-adhesive 50CS470 electrical steel produced by CSC is developed in this work. JMAG-Express and JMAG-Designer motor simulation software are used to analyze the influence of the characteristics of electrical steel on the iron loss, copper loss, efficiency and torque of the waterwheel motor. Finally, a prototype of the waterwheel motor suitable for mass production is realized. When the waterwheel motor is operated with a torque close to the operating point of 80 Nm, the overall efficiency of the system is 78.72% (open loop) and the motor efficiency is 86.32%. The relevant research results of this study have been practically applied to the aeration pump used in aquaculture in Taiwan, which can reduce considerable electricity consumption of the aquaculture water pump system and the maintenance cost of the motor. The relevant results of this work are also suitable for the development of other high-torque and low-speed waterwheel motors or industrial motor      products in the future. For the current related products, low-speed and high-torque DC motors have a wide range of applications. If induction motors are to achieve this characteristic, their relative volume must be very large, and they need to be equipped with a deceleration device to reduce the speed. In the future, this work can be further applied to other low-speed and high-torque DC motors, such as negative pressure fan motors, blower motors, large ceiling fan motors, direct drive washing machine motors, mixer motors, and applications for hanging motors of heavy objects, etc.