summary
Based on the analysis of the five major losses affecting motor efficiency, different solutions are proposed for each type of loss, and the effectiveness of these solutions is verified through targeted experiments. A comprehensive comparison shows that double-layer concentric unequal-turn low-harmonic windings have significant advantages in reducing losses, saving costs, and improving process feasibility, making them a promising candidate for widespread application in the design and production of high-efficiency motors.
introduction
Since electric motor systems account for a significant proportion of electricity consumption in all countries worldwide, and with the increasingly tense energy situation, countries around the world have launched energy-saving initiatives for electric motor systems. my country has 1.7 billion kilowatts of electric motors, consuming 60% to 70% of the country's total electricity consumption. The 12th Five-Year Plan of China set specific requirements for energy conservation and emission reduction targets.
This paper systematically analyzes the five major losses affecting motor efficiency, proposes specific measures to improve motor efficiency based on the characteristics of different losses, conducts relevant experimental research, and summarizes the methods for developing high-efficiency motors.
1. Performance Analysis of High-Efficiency Motors
1.1 Measures and methods to reduce motor losses
(1) Measures to reduce the copper loss of motor
Stator copper losses are related to stator current and winding resistance.
A common method is to increase the winding cross-sectional area to reduce the winding current.
To reduce resistance and electrical density, this method is subject to constraints from factors such as slot fill factor. Therefore, in the development of high-efficiency and energy-saving motors, some specifications have enlarged the outer diameter of the laminations to increase the slot area and allow for the placement of...
Add more copper wire. Additionally, the overall winding resistance can be reduced by improving the end design of the winding and decreasing the length of the ends.
Currently, most electric motors use cast aluminum rotors. Copper has a higher electrical conductivity than aluminum, and theoretically, motors with cast copper rotors can reduce rotor losses by 38%. Using cast copper rotors can significantly improve motor efficiency.
(2) Measures to reduce iron loss
The most common way to reduce iron loss is to use high-quality silicon steel sheets with high magnetic permeability and low loss. In addition, the thickness of the silicon steel sheet is one of the important factors affecting the iron loss of the motor. The mainstream silicon steel sheets in China are 0.5mm thick, while many foreign products are starting to use silicon steel sheets that are 0.35mm or even 0.27mm thick. As the thickness of the silicon steel sheet decreases, the magnetic performance also improves.
In addition to designing the motor's magnetic circuit appropriately, reducing the iron loss of a motor also requires considering the impact of the motor core manufacturing process. Generally, iron loss increases significantly after silicon steel sheets are processed into cores and pressed into the frame. The commonly used lamination annealing process involves annealing the laminations after stamping and shearing to eliminate stress and restore the good properties of the cold-rolled silicon steel sheets, which plays a significant role in reducing iron loss.
(3) Measures to reduce mechanical wear
Mechanical losses are divided into ventilation losses and friction losses. Ventilation losses can be effectively reduced by optimizing the airflow structure and improving the design of the fan and shroud; friction losses can be reduced by using high-quality low-friction bearings and low-friction lubricating greases.
(4) Measures to reduce stray losses
Stray losses can be reduced through design and manufacturing process optimization. Design improvements include optimizing the air gap, using low-harmonic windings, and optimizing slot fit. Manufacturing processes include insulating the rotor slots to reduce high-frequency lateral current losses and using punched air gaps instead of machined air gaps to reduce high-frequency losses on the rotor surface. Additionally, solid parts such as motor end covers can be made of non-magnetic materials to limit eddy current losses caused by leakage magnetic fields.
1.2 Research on Low Harmonic Winding Technology
The form of the stator winding determines the fundamental frequency distribution coefficient and harmonic content, significantly impacting motor performance. Commonly used stator winding forms include single-layer and double-layer windings. Double-layer windings can reduce harmonic content through measures such as long and short pitches. Low-harmonic windings can generate near-sinusoidal air gap flux using unequal winding turns and "Δ-Y" series connection methods. For high-efficiency motors, it is necessary to use low-harmonic windings to improve the motor's magnetomotive force waveform, thereby reducing stray losses.
In low-harmonic windings, double-layer concentric unequal-turn windings are easy to install, requiring no change in the lead-out configuration, and offer good manufacturing feasibility. Furthermore, they save copper wire, achieving both material savings and increased efficiency. The essence of an unequal-turn low-harmonic winding is to appropriately distribute the number of conductors within the slots, causing the slot current to be distributed sinusoidally along the core surface, thus obtaining a magnetomotive force curve close to a sinusoidal shape. The following example, a short-span unit motor with 18 slots per pole pair and q=3, illustrates the calculation method for the turns ratio of a low-harmonic winding coil.
When the A-phase current reaches its maximum, the amplitude of the fundamental magnetomotive force will be equal to 2.
Solving this system of equations yields the turns ratio of each coil. Based on this, we developed motor design software based on double-layer concentric unequal-turn windings, specifically for the design of low-harmonic winding motors.
2. Verification of the effectiveness of the high-efficiency motor design
2.1 Increasing effective materials to improve motor efficiency
There are two aspects to increasing the effective materials: one is to increase the amount of copper in the motor,
The efficiency improvement is attributed to two main factors: firstly, the use of aluminum reduces stator and rotor resistance; secondly, the use of higher-performance materials, such as high-performance silicon steel sheets to reduce iron losses and cast copper rotors to reduce rotor copper losses. Table 1 shows the losses and effective material usage of four 45kW-2 motors that meet Level 3 and Level 2 energy efficiency standards, respectively. The data in Table 1 shows that the efficiency improvement mainly stems from the reduction in copper and iron losses on the stator and rotor sides, with stator-side copper losses, rotor-side copper losses, and iron losses decreasing by an average of 20%–30%. Based on the data in Table 1 and the design and manufacturing process of these motors, the following measures can be summarized to improve motor efficiency.
(1) Increase the core length and increase the winding cross-sectional area.
The core length of the 45kW motor, which is upgraded from Level 3 to Level 2 energy efficiency, has increased from 200mm to 230mm, a 15% increase. Under the same magnetic load requirements, the increased core length reduces the number of turns in the motor, thereby increasing the cross-sectional area of a single coil turn and reducing stator resistance.
(2) Higher performance silicon steel sheets were adopted. Silicon steel sheets with lower unit iron loss and better performance were adopted. With the increase of core length, the total iron loss was reduced.
(3) Enlarging the slot size and increasing the amount of copper wire and cast aluminum: The amount of copper used in Level 2 energy efficiency is 22.4% higher than that in Level 3 energy efficiency, and the amount of aluminum used is 6.9% higher. This directly leads to a reduction in the resistance of the stator and rotor.
2.2 Improve the process to increase motor efficiency
A prototype of the punched air gap motor was fabricated and compared with a conventional motor. The comparison results are shown in Table 2. The four 22kW-4 prototypes in Table 2 were designed according to Level 2 energy efficiency (i.e., IE3, 93%). Prototypes 1 and 2 used the punched air gap method, while prototypes 3 and 4 used the machined air gap method. As can be seen from Table 2, the stray losses of prototypes 1 and 2 are significantly lower than those of prototypes 3 and 4, averaging about 30% lower. This indicates that the direct punched air gap method is significantly effective in reducing stray losses.
After punching and shearing, the plastic deformation along the edge of the shear separation line of cold-rolled silicon steel sheets causes the accumulation of internal stress and changes in physical properties, resulting in a decrease in magnetic permeability and an increase in iron loss. This poses a disadvantage to fully utilizing the excellent magnetic permeability of cold-rolled silicon steel sheets. Selecting a suitable annealing process can eliminate punching and shearing stress and restore the performance of cold-rolled silicon steel sheets. A specific experimental study was conducted on the lamination annealing process of several motor specifications, and the performance comparison is shown in Table 3. The comparison shows that the iron loss of the annealed motor is lower than that of the unannealed motor, and the power factor of the motor is also improved due to the restoration of the magnetic properties of the silicon steel sheets.
2.3 Improving motor efficiency by using low-harmonic windings
Using our self-developed electromagnetic calculation software, we modified the windings of several motor specifications. After adopting a double-layer concentric unequal-turn winding, the harmonic content was significantly reduced. Taking a 110kW-4 motor as an example, the changes in harmonic coefficients before and after the winding change are shown in Table 4. As can be seen from Table 4, after adopting the double-layer concentric unequal-turn winding, the fundamental frequency coefficient remained at 98.8% of its original value, showing little change, while the harmonic coefficients of other orders were significantly reduced. The reduction in higher-order harmonic content can reduce stray losses in the motor.
Using concentric windings allows for a shorter end, saving copper wire usage. Table 5 shows the changes in copper usage before and after the winding modification. The 110kW-4 motor did not have its end intentionally shortened, saving 6.3% of copper wire. The 15kW-6 and 55kW-2 motors, however, had their ends shortened and the wire gauge slightly adjusted, saving approximately 10% of copper wire.
Table 6 shows a comparison of test data between the prototype using low-harmonic windings and the prototype using ordinary windings. The comparison data in Table 6 shows that using low-harmonic windings reduces stray losses by 40%–50% and improves efficiency by 0.4–0.7 percentage points. By rationally designing and replacing ordinary windings with low-harmonic windings, stray losses can be reduced and motor efficiency improved while saving motor costs, resulting in significant energy savings and efficiency gains.
2.4 Comparison of the effects of different measures
Through the above theoretical analysis and actual prototype testing, it can be seen that different measures have varying effects and costs in improving motor efficiency. For a long time, most domestic motor manufacturers have primarily focused on increasing the utilization of effective materials when designing and producing high-efficiency motors. However, with the continuous improvement of motor energy efficiency levels, this approach has become increasingly limited. Directly punching out the air gap can significantly reduce stray losses, but it requires a high level of manufacturing expertise for the rotor core. Annealing can reduce iron losses in motors, but it requires specialized processes and equipment. Using double-layer concentric low-harmonic windings, however, only requires changing the winding's winding method, making the process highly feasible. It not only improves efficiency but also saves copper wire, demonstrating significant effectiveness in the trial production of high-efficiency motors and showing broad application prospects.
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