Article by Marco Villani, Dept. of Industrial and Information Engineering and Economics, University of L’Aquila
In this paper the influence of semi-processed steels on the single-phase motor performance has been evaluated. A right choice of electrical steel among the commercial ones can increase the efficiency without affecting the manufacturing process. Further improvement can be achieved if this choice is combined with a design optimization of the motor. An example is shown for a split-phase capacitor motor, commonly used in compressors for household appliances
Introduction
The single-phase induction motors are widely used in several applications like as compressors, pumps, farm and homework ship tools and conveyors. These motors are generally available in ratings from as small as 1/8 hp and larger, although special applications may require less than 1/8 hp. The design of these motors is based on universally accepted physical and mathematical principles which have been verified by the experimental methods. However, with fast changing technological developments, the knowledge of these principles is often insufficient to produce the “optimal” design. The improvement of energy efficiency in the domestic appliances and the establishment of minimum efficiency standards for refrigerators and freezers has stimulated the manufacturers towards an accurate motors design; they are continuously involved in design optimization with a view to improving efficiency whilst remaining competitive in the marketplace. The improvement of motor performance mainly depends on the semi-processed electrical steel quality, that is on the right choice among materials available on the market; obviously the highest efficiency can be achieved by combining a “suitable” sheet with an optimized motor design, so that the properties of the electrical steels are well exploited. In this paper a specific software, developed by the author, has been used in order to evaluate the influence of different electrical steels on the motor performance and to optimize the design. The proposed procedure has been applied to optimize the design of a split-phase capacitor motor, commonly used in compressors for household appliances. The optimized designs have been compared with the standard one, in order to evaluate the effective performance improvement and energy saving.
Analysis of single-phase induction motors
Gains in efficiency through modified design and manufacturing processes relies on the ability of the designer to predict the motor performance to a high degree of accuracy. This in turns leads to a demand for improved analytical tools to examine the motor design in ever-increasing detail. The analysis of single-phase induction motor has been carried out by specific software that allows to evaluate the motor performance for different operating conditions. The physical description of the motor is reduced to equivalent parameters such as resistance and inductances: the adopted model takes into account the influence of saturation on stator and rotor reactances and the influence of the skin effect on rotor parameters. It also includes thermal analysis through a thermal network. All performance of the motor (steady-state, no-load, starting operation) are determined through multiloop iterative calculations. The code has been refined on the basis of design and test data of several single-phase induction motors that have been selected by inquiring the main national electrical Manufacturers. The program is very flexible and allows to compute the performance characteristics of any type of single-phase induction motors (split-phase, capacitor-start, permanent split-capacitor motor); it is suitable for rated powers in the range between 50W and 300 W. The validity of the mathematical model has been verified by means of experimental tests on split-phase capacitor motor (for refrigerator) with the following name plate data: 150 W rated power, 220 V, 50 Hz, 2 pole, 4 mF capacitance. Fig. 1 shows a detail of stator core and windings. Fig. 2 and 3 show the comparisons between the experimental and simulation curves with reference to the torque and efficiency. These results point out a good agreement between calculations and measurements and confirm the goodness of the analytical tool.
Semi-processed steels
The lamination material is subjected to a variety of processes, during the manufacture of an electrical machine, that influence the iron losses and permeability. Mechanical and thermal stresses are well-known to have a direct if unquantifiable effect upon the losses. Clearly certain manufacturing processes are unavoidable and will inevitably subject to material to mechanical forces. Punching for example creates a high stress zone close to the punched edge above all for stator and rotor figures of small sizes (e.g. single-phase induction motors), where the teeth are very thin. In order to release these residual stresses, the annealing treatment by customers, so called either “Customer Annealing” or “Stress Relief Annealing” (SRA), is needed to develop an exaggerated grain growth and, thereby, to achieve low core loss and high permeability required to improve the efficiency of electric motors.This treatment allows to modify crystalline texture in order to have a more suitable magnetically crystal orientation; crystalline texture can affect the magnetic material permeability. It is important to underline that the effect of the annealing process is very sensitive to the operating conditions, such as the soaking temperature, the annealing time and the annealing atmosphere temperature. Therefore, controlling this condition to an optimum state is considered very important for customers to fully take the advantage of the annealing. However, the customers employ their own SRA conditions which are all different even for the same material and don’t seem to be willing to share the technical information each other. In this analysis four different steels (labelled with S1, S2, S3 and S4) have been considered whose main characteristics, after customer annealing process, are summarized in Tab.1.
S1 is a typical commercial steel used by manufacturers in their standard motors and it was assumed like “reference material”. S2 is an alternative commercial steel with lower loss respect to S1, while S3 and S4 represent alternative “premium steels” with significant low loss and high permeability. The software has allowed to evaluate the influence of magnetic material on the performance of the motor tested in the previous section. Particularly, the electrical steel only has been changed without modifying the stator winding, stack length and stator and rotor figures. The results are presented in Tab.2 and concern the input power, the line current, the main winding current, the auxiliary winding current and the losses. Fig. 4 shows the trend of efficiency as function of the motor speed and steel quality.
These curves point out how the use of S3 and S4 gives rise to higher efficiencies not only at rated speed (about 2910 rpm) but also at low speed; the efficiency improvement is mainly due to a significant iron loss reduction rather than copper losses because the input currents are quite similar. The percentage difference on iron losses between the design with S1 and S4 one is about of 22%. These results demonstrate how a right choice of electrical steel among the commercial ones can increase the motor efficiency and it represent a convenient design strategy that does not heavily affect the cost of manufacturing process.
High efficiency designs
Further improvement in motor efficiency can be achieved if a right choice of material is combined with a design optimization of the motor. The optimization has been formulated as a constrained minimization of an Objective Function and solved by an optimization procedure. This procedure has been used to optimize motors to be employed in applications where high efficiency is required. In this way the final design will be an optimized motor rather than a derated one. The chosen independent variables of the optimization concern the inner and outer stator diameters Di, Do the stack length Ls (Fig.1), the stator and rotor slots dimensions, the number of turns and wire size of stator winding and the capacitance value. These variables have been varied according to the motor Manufacturer suggestions and internal room of compressor. The proposed design procedure has been applied for the optimization of the considered split-phase capacitor motor, by using the commercial steel S1 and the “premium steel” S4. The performance of the standard and optimized designs are shown in Tab.3.
The optimization with the “reference material” (S1) gives rise to an efficiency improvement of about 3% that is always higher than the best results previously found by substitution of the lamination material (standard design with S4); in this case the increase of copper weight is about 16% and 7% for the iron one. The optimized design with S4 presents, with respect to the standard one, an increase of efficiency of about 7%, a total loss reduction of 37% and an increase on copper and iron weights of 18% and 8% respectively. Moreover, both optimized designs present a new capacitance of 5 mF.
The optimization results allow to evaluate the effective energy saving due to a substitution of the standard motors with high efficiency ones. Let us suppose, as a specific application, a common refrigerator (with a capacity of 180 liters) that employs the considered split-phase capacitor motor, and daily operating hours (15 minutes in 1 hour). Under these hypotheses the daily and annual energy consumed by two motors can be evaluated (Tab.4); the cost of 0.18 euro/kWh has been fixed (in the domestic field).
The saving of money makes up for the high efficiency motor over-cost due to the increase of active material weight and new capacitor. The “national” energy saving can be calculated with reference to the total numbers of refrigerators. Let us suppose 30 million of the above-mentioned refrigerators (the estimated number of families in Italy). In this case the difference of the energy consumed is about 6%; from economic point of view this difference corresponds to an annual saving of about 100 million euro.
Conclusions
The influence of the semi-processed electrical steels on single-phase motor performance has been evaluated. Four electrical steels have been chosen, among the commercial ones, whose performance take into account the customer annealing process. Variations on efficiency and iron losses have been found showing that the choice of electrical steel’s an important factor in motor design. The highest efficiency increase has been achieved by combining a “suitable” steel with design optimization. This solution requires high cost of investment for the Manufacturers, due to the stamping renewal, but gives rise to an improvement of motor performance that can produce significant energy savings.
