SMC materials, which have been known for decades and extensively studied for several years, are made of small ferromagnetic pieces, enclosed by a thin layer of an insulating material . The insulation is made in various technological processes, ranging from oxide isolation, polymer isolation and resin insulation. The relatively high content of the dielectric (insulation) in the composite improves its resistivity and deteriorate its magnetic properties such as lower relative magnetic permeability and flat B-H magnetization curve. Unfortunately, the mechanical properties are lower than those for laminated magnetic materials . Therefore, it is essential for the appropriate selection of technological process parameters to produce the SMC material. It concerns both the step of preparing a magnetic powder (grinding time), as well as the process of molding (pressure and temperature of the pressing). Production of large-sized components introduces additional technological problems tied to uniform pressure and material density of the component. Currently, many researchers are working in the selection of the composition and sintering conditions, so as to achieve optimum material properties [3, 4, 5, 6, 7, 8, 9, 10]. This is the first problem investigated in this work. The second problem considered and analyzed here is the use of SMC material in the construction of electric motors, especially those small and with fractional power. These motors have relatively small geometrical dimensions and use classical solutions (core made of a laminations), then operating parameters reduction (resulting from e.g. the damage of part of the material, during forming the shape of the core, by performing the punching process) arises. In the literature there are a number of research results related to the use of SMC materials for the motor core’s construction [5, 11, 12, 13, 14]. Available works can be divided into the following groups: operating parameters of high speed motors ; optimization of the motor core ; design of motors having complex 3-D cores or core manufactured by separate elements [13, 14]. This paper describes the results of research, both measurement, technology and simulation, carried out on the mass produced LSSR (Line-Started Synchronous Reluctance) motor of a nominal power of 120 W.
2 Experimental activity
For this work a mass-produced, 4-pole line-started 120 W synchronous reluctance motor was chosen as the object of research. The motor is dedicated for use with a three-phase 400 V/50 Hz sinusoidal supply. Self-start of the motor is guaranteed by a rotor cage existence. The experimental study was conducted by energizing the motor windings with sinusoidal voltage with a frequency that varied in the range 20-50 Hz. The authors wanted to avoid Joule loss (in the rotor cage) to be taken into account in the calculation process. Therefore the motor version without the rotor cage was studied. Lack of the rotor cage was the reason causing of the some difficulties at motor starting . However, it was possible to reach complete synchronization of the rotor. The test stand was equipped with the following devices: a programmable three-phase power supply (18kVA, 360AMXT Pacific Power Source), a high accuracy digital wattmeter, a digital oscilloscope (used to record the phase current and phase voltage waveforms). The stator winding temperature was on-line monitored by means of the embedded thermal sensors and the sensor readings are used to compute the stator Joule losses. The executed experiments recorded the phase current and phase voltage waveforms, as well as determined the iron loss in the motor core. These values and waveforms was then used as a reference basis for assessing the adequacy and accuracy of the 3D FEM model, adopted in the prosecution of the research. A very good convergence was achieved in simulation results, both in the form of a waveform of the input current and phase voltage (having characteristic deformation resulting from the lack of rotor cage and core saturation) as well as iron loss in the stator core – see Fig. 1. These results were the basis to consider that the built FEM model reconstructs very well the characteristics and electromagnetic phenomena in the test motor.
3 The research on innovative SMC material
The advantages that SMCs can offer with respect to the traditional lamination steels (mainly a 3D ferromagnetic behavior) must be evaluated considering both magnetic and mechanical aspects. Taking as a reference an Insulated Iron Powder Compound (I.I.P.C.) available on the market, novel SMC materials can be obtained by mixing common iron powders together with organic resins. In Fig.2 we can see the comparison of B-H curves for commonly used magnetic laminations (M600-50A for the tested motor), a SMC material available on the market and the innovative SMC material proposed by the Authors. The research, carried out with the support of a polymeric research group, has focused the attention on epoxy resins as innovative binders for SMC realization. The main goal was the improvement of the mechanical properties, maintaining the magnetic characteristics of the I.I.P.C. taken as reference (that is in practice the only material adopted for these kind of applications). In a past activity the Authors adopted such I.I.P.C. to realize parts of a motor, but the results highlighted insufficient mechanical properties; it seemed necessary to find a way to significantly increase the mechanical resistance of at least 100% or more.
The base of the SMCs preparation is a common ferromagnetic powder, without any insulating layer on the grains; the addition of the binder keeps together the grain structure, and provides electrical insulation. At first the most suitable binder has been selected, then the impact of different binder percentages and different compacting pressures has been observed. This activity has been made possible due to the capability of self-producing the samples in our own laboratories. The realization process is totally under control: from the powder mixing, to the samples realization in the mold, up to the wounded toroid structure for the magnetic measurements and for mechanical tests.
The experimental results showed similar magnetic characteristics and increased mechanical performance with respect to the basic I.I.P.C. product.
In Fig. 3 the hysteresis loop is reported as an example of how the introduction of the epoxy resin impacts on the magnetic behavior of the pure ferromagnetic powder, while in Fig. 4 the magnetic characteristics of samples obtained with different bonder percentages are shown.
The mechanical characteristics of the SMC have been verified with bending tests to evaluate the strength of the samples, which can be expressed through the so called “Transverse Rupture Strength” (or TRS), with a three-points bending test, typically adopted for brittle materials.
The sensitivity of the mechanical resistance with respect to the binder percentage and to the adopted mold pressures has been investigated, and the results are shown in Fig. 5 where the TRS value concerning the I.I.P.C. is reported as a reference horizontal line. From the mechanical point of view low binder percentage and high molding pressures improve mechanical performances.
The proposed SMC material, adopting an epoxy binder, presents a very significant increment of the mechanical resistance with respect to the commercial I.I.P.C., and this result is obtained without penalties for the magnetic prerogatives; these last ones are better when small binder percentages are adopted.
4 3D FEM simulation results
Research studies of the mass produced 120 W and 550 W motors, having core made of a standard M600-50A material, have been published in prestigious journals and conference proceedings [15, 16, 17]. These studies, supported by experimental activities have allowed for calibration of the 3D FEM model used in the present study. This model was then used to determinate the operating parameters of the motor having a core made of the SMC material. The test motor has a rotor made by the punching die used for the production of induction motor having the same dimensions. Part of the rotor teeth has been removed, thereby forming a geometric anisotropy of the rotor – Fig. 6 and Fig. 7.
During this research the authors adopted three variants of the SMC material application: only for the stator core (variant 1), only for the rotor core (variant 2), both for the stator and the rotor cores (variant 3). In addition, they decided to leave unchanged the winding parameters such as the winding type, the number of series turns per phase and the diameter of the wire. The authors assumed that from an economic point of view, apart from changing the material core, it is not possible to complete reconstruction of the motor. Only a small change in the length of the core package is possible.
The simulations were performed by energizing the stator winding by three-phase voltage of 400 V/50 Hz (Y connected windings). The starting point of the analysis was assumption that the core length of the motors, having a core made of SMC, Somaloy and M600-50A materials, is the same. The efficiency values of the investigated motor, for such assumption, calculated by 3D FEM models are reported in Table 1, where it is possible to appreciate the performance detriment moving from the laminations to SMC materials. However, the efficiency reduction was lower for the variant 2, where the SMC material was used for the rotor only.
The motor efficiency calculated by FEM model (at 50 Hz, rated torque, core length of 35 mm)
|Variant 1||Variant 2||Variant 3|
Computer simulations also allowed determination of consumed current as well as loss in the stator core. In the case of the test motor, the dominant loss is the Joule component in the stator windings, and possibly loss enlargement in the stator core. The application of SMC material does not significantly affect the motor efficiency.
The main reason for reduction of the motor efficiency, is the consumed current increase, resulting from worsening B-H magnetization curve of the SMC material. This leads to increase in the Joule losses in the stator winding. Calculated input phase currents and stator iron loss are presented in Table 2.
FEM-computed phase current and stator iron losses (at 50 Hz, rated torque, core length of 35 mm)
|Material||Phase current [A]|
|Variant 1||Variant 2||Variant 3|
|Stator iron loss [W]|
|M600-50A||3.7 (base line)|
Further studies were focused on:
- The changes in the core length from 35 mm to 40 mm. The proposed variation range does not require the change of the master necessary to form the stator coil winding; in this way the same stator winding production costs are maintained.
- The changes in the outer shape of the rotor teeth - Fig. 8.
The change of the core length (for reference tooth shape) causes:
- –reduction in consumed current by 10% (variant 1 and 2); by 20% (variant 3);
- –increase in motor efficiency by 3% (variant 1 and 2); by 5% (variant 3) – for Somaloy; by 1.5% (variant 1 and 2); by 4% (variant 3) – proposed SMC.
Leaving the reference core length (35 mm) and using proposed SMC material, the change in the shape of the outer rotor tooth causes:
- –for wider outer tooth – slightly larger consumed current (0.45 A and 0.62 A) and reduction in efficiency (64.5% and 50.4%) – for variant 2 and 3 respectively; slight differences in consumed current (0.55 A) and efficiency (57.4%) – for variant 1;
- –for narrower outer tooth - slightly larger consumed current (0.48 A and 0.63 A) and reduction in efficiency (63.3% and 50.2%) – for variant 2 and 3 respectively; slight differences in consumed current (0.49 A) and efficiency (60.3%) – for variant 1.
The change of the outer teeth shape in the rotor has an effect on the flux density change in these teeth (lower flux density in whole teeth - see the variant in Fig. 8b) or the flux density change in the teeth area located near the gap (higher flux density only in this part of the teeth - see the variant in Fig. 8c).
The authors present the results of research conducted with an innovative SMC material, characterized by a significantly improved magnetization curve compared to those that have commercial SMC materials available on the market (e.g. Somaloy material). Moreover, the proposed novel material has improved mechanical characteristics but unfortunately it has a greater specific loss with respect to competitive SMC materials. This is a disadvantage of this material, and therefore its use (for the current parameters) must be well thought out. Therefore, further studies are needed on the influence of processing parameters. The effect of the use of two types of SMC materials (commercial material available on the market and novel one proposed by the authors) in the reluctance motor of fractional power, is described in the second part of the paper. The authors accepted the assumption that from economic viewpoint, a re-design of the core geometry is not rational. For this reason, the stator geometry remains unchanged, with the possibility to small extend a package, so there was no need to re-design and initiate a new manufacturing of the punching die and the stator winding former. That is why the authors only tested the effects of the use of the SMC material instead of the currently used laminations type M600-50A. It is difficult to directly compare the results achievable in the available literature, because the authors did not find such a comparable solution. Known works, with similar research, concern motors in which the stator core is made of SMC material or motors in which the rotor shape was optimized, using other than our optimization criteria. As was mentioned before, for the examined problem, minimizing processing cost was particularly important. That’s why the rotor geometry was not changed, except minor corrections of the outer teeth. General conclusions regarding the second part of the conducted research are as follows: the most efficient solution (due to the motor efficiency) is the usage of the SMC material in the rotor only; it is possible to achieve a similar efficiency as the reference motor (made of the M600-50A material) after packet extension by 10% and use of the novel SMC material. Analyzed small geometry corrections of the outer rotor teeth does not have a positive impact on the motor efficiency, and therefore the authors suggest to leave the rotor geometry the same as in the reference motor.
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