ap-5-11.dvi Acta Polytechnica Vol. 51 No. 5/2011 Limited Angle Torque Motors Having High Torque Density, Used in Accurate Drive Systems R. Obreja, I. R. Edu Abstract Atorquemotor is a special electricmotor that is able to develop the highest possible torque in a certain volume. A torque motor usually has a pancake configuration, and is directly jointed to a drive system (without a gear box). A limited angle torque motor is a torque motor that has no rotary electromagnetic field — in certain papers it is referred to as a linear electromagnet. The main intention of the authors for this paper is to present a means for analyzing and designing a limited angle torque motor only through the finite element method. Users nowadays require very high-performance limited angle torque motors with high density torque. It is therefore necessary to develop the highest possible torque in a relatively small volume. A way to design such motors is by using numerical methods based on the finite element method. Keywords: limited angle torque motor, finite element method, high performance, pancake, direct drive. 1 General considerations Our purpose for this paper is to present some as- pects of special torque motors with high torque den- sity. More exactly, our attentionwill focus on limited angle torque motors used in very accurate drive sys- tems. A torque motor is a special electric motor that is usually direct jointed to a drive system (without a gear box). There is generally a pancake structure, which can easily be adapted to the system configura- tion. A limited angle torquemotor is amotor that does not have a rotary electromagnetic field. In certain papers it is referred to as a linear electromagnet. Nowadays users require very special limited angle torque motors with high torque density versus the volume of the motor. The reason is that a perform- ing drive system has to be as small as possible. The authors of this paper have developed very special solutions for motors of this kind. Some as- pects of these solutions will be presented here. 2 Limited angle torque motors with a small air-gap versus limited angle torque motors with a large air-gap A limited torque motor is, usually, a toroidal motor relative to the winding solution. In this way, the to- tal air-gap, relative to the magnet, also includes the winding. The dimension of the air-gap is, generally, more than 1.5 mm. Relative to the overall dimen- sion of the motor, we can consider this to be a large air-gap. Depending on the size of the motor and the internal structure, the value of the air-gap can be several millimeters (4÷5 mm). For motors with the winding assembled in slots, the value of the air-gap may be 0.4 mm to 1.5 mm, depending on the size of the motor and the arrangements for total friction torque. This air-gap is small. We will now present some considerations on the small air-gap and large air-gap solutions. 2.1 A limited angle torque motor with a small air-gap Fig. 1: An example of a limited angle torque motor with a small air-gap 75 Acta Polytechnica Vol. 51 No. 5/2011 Fig. 2: Magnetic field distribution Fig. 3: Torque versus electric angle diagram Fig. 4: Flux density diagram in the air-gap, in the prox- imity of the permanent magnet Analyzing the figures above, it is found that the magnetic field generates 4 main poles, and also four other small poles in the neutral area, because the field lines may close in the neutral area of the poles directly between the magnets and the soft magnetic material of the rotor. A disadvantage is reflected in torque diagram form (the diagram is nonsymmetri- cal, as shown, as the current value is increasing). 2.2 A limited angle torque motor with a large air-gap In the situation of torquemotorswith a largeair-gap, it is found that the magnetic field generates only the 4 main poles very accurately, because the field lines maynot close in the neutral area of the poles directly between the magnets and the soft magnetic material of the rotor, due the dimensions of the air-gap. This advantage is reflected in torque diagram form, which is symmetrical. Fig. 5: An example of a limited angle torque motor with a large air-gap Fig. 6: Magnetic field distribution Fig. 7: Torque versus electric angle diagram 76 Acta Polytechnica Vol. 51 No. 5/2011 Fig. 8: Flux density diagram in the air-gap, in the prox- imity of the permanent magnet 3 Analysis of a limited angle torque motor with a large air-gap Section 2 presented general aspects of the two types of motors. As the motor with a large air-gap is more usual, it will now be analyzed in greater detail. Fig. 9: Torque motor with a large air-gap in no load con- ditions — magnetic field distribution Fig. 10: Tangential fluxdensity diagram in the rotor hub, through the magnet axis Fig. 11: Normal flux density diagram in the rotor hub, in the neutral area Fig. 12: Tangential flux density diagram, on the magnet, through the magnet axis Fig. 13: Normal flux density diagram, transversal on the magnet, in the base of the magnet area Fig. 14: Normal flux density diagram, transversal on the magnet, in the base of the middle area 77 Acta Polytechnica Vol. 51 No. 5/2011 Fig. 15: Normal flux density diagram, transversal on the magnet, at the limit to the air-gap area Fig. 16: Tangential flux density diagram in the air-gap area, through the magnet axis Fig. 17: Normal flux density diagram in the air-gap area, on polar pitch Fig. 18: Tangential flux density diagram in winding area, through the magnet axis Fig. 19: Normal fluxdensity diagram in thewinding area, on polar pitch Fig. 20: Tangential flux density diagram in the stator core area, through the magnet axis Fig. 21: Normal flux density diagram in the stator core area Fig. 22: Torque motor with a large air-gap in load condi- tions at 1A – magnetic field distribution 78 Acta Polytechnica Vol. 51 No. 5/2011 Fig. 23: Tangential fluxdensity diagram in the rotor hub, through the magnet axis Fig. 24: Normal flux density diagram in the rotor hub, in the neutral area Fig. 25: Tangential flux density diagram, on the magnet, through the magnet axis Fig. 26: Normal flux density diagram, transversal on the magnet, in the base of the magnet area Fig. 27: Normal flux density diagram, transversal on the magnet, in the middle area Fig. 28: Normal flux density diagram, transversal on the magnet, at the limit to the air-gap area Fig. 29: Tangential flux density diagram in the air-gap area, through the magnet axis Fig. 30: Normal flux density diagram in the air-gap area, on polar pitch 79 Acta Polytechnica Vol. 51 No. 5/2011 Fig. 31: Tangential flux density diagram in the winding area, through the magnet axis Fig. 32: Normal fluxdensitydiagram in thewinding area, on polar pitch Fig. 33: Tangential flux density diagram in the stator core area, through the magnet axis Fig. 34: Normal flux density diagram in the stator core area, through the magnet axis Fig. 35: Normal flux density diagram in the stator core area Fig. 36: Torque motor with a large air-gap in load condi- tions at 5A — distribution of the magnetic field Fig. 37: Tangential flux density diagram in a rotor hub, through the magnet axis Fig. 38: Normal flux density diagram in the rotor hub, in the neutral area 80 Acta Polytechnica Vol. 51 No. 5/2011 Fig. 39: Tangential flux density diagram, on the magnet, through the magnet axis Fig. 40: Normal flux density diagram, transversal on the magnet, in the base of the magnet area Fig. 41: Normal flux density diagram, transversal on the magnet, in the middle area Fig. 42: Normal flux density diagram, transversal on the magnet, at the limit to the air-gap area Fig. 43: Tangential flux density diagram in the air-gap area, through the magnet axis Fig. 44: Normal flux density diagram in the air-gap area, on polar pitch Fig. 45: Tangential flux density diagram in the winding area, through the magnet axis Fig. 46: Normal fluxdensity diagram in thewinding area, on polar pitch 81 Acta Polytechnica Vol. 51 No. 5/2011 Fig. 47: Tangential flux density diagram in the stator core area, through the magnet axis Fig. 48: Normal flux density diagram in the stator core area, through the magnet axis Fig. 49: Normal flux density diagram in the stator core area The diagrams above present three different situ- ations of a torque motor, limited angle, that can be met in applications: no-load conditions, or a very low load, below 1A, which is the most usual situa- tion, and below a 5A load, which is considered to be a peak torque situation. It is extremely important to consider the mag- netic field in any of the above situations, because the goal of the motor design is to put maximum power into a certain volume (maximum torque), while as- suring that themotor operates properly in anywork- ing regime of interest. There are many areas where a designer controls the evolution of themagnetic field in variousworking regimes, but thosementioned above are themost im- portant: to control the status of the dispersion field between the magnets; to control the level of the field and the influence of the winding field in the perma- nent magnet area; to control the level of saturation in the rotor hub area, as well as in the stator core area; to control the level and distribution of the field in the winding area. 4 Conclusion Limited angle torque motors are important compo- nents used in various applications: control systems, automatic systems, drive systems, etc. Most limited angle torquemotors have a pancake configuration,whichmeans two separate parts: a ro- tor and a stator. In this way, such motors can easily be adapted and assembled in the above systems. Present day applications require limited angle torque motors with: the smallest possible relative size and dimensions; the highest possible torque den- sity, which means the highest possible torque con- stant relative to a certain volume; a high insulation class,whichmeanshaving theability towork inharsh ambient conditions, at high current density. General classical formulas can be used to design a limited angle torque motor, but it is difficult to achieve the best results in this way. It is only by us- ing numerical methods, based on finite element ana- lysis, that themagnetic field can be controlled in any area of interest ensuring that: any area is properly dimensioned; the total volume is used optimally; and the maximum torque constant is obtained. This way was used to analyze, design and pro- duce many types of very well performing limited angle torque motors, with different sizes and elec- tric parameters, with outer diameters from 15 mm to 300 mm, and also with a torque constant from 5 mNm/A to 5 Nm/A. One of the most special limited angle torque mo- tors has two poles and an angular excursion of ±40 degrees. When there was an outer diameter of 40 mm and a length of 9 mm, it was necessary to have 20 mNm/A as the torque constant, but 75 mNm at 4.5A.Earliermotors hadworkedproperly only up to 2.5 A. At a higher current, saturation caused faster degradation of the torque constant. Only using nu- mericalmethodswas it possible to design amagnetic circuit and a winding distribution that would meet the application requirements. Acknowledgement This work was supported by CNCSIS-UEFISCDI, projectPNII-RU,No. 1/28.07.2010,“High-precision strap-down inertial navigators, based on the connec- tion and adaptive integration of the nano and micro 82 Acta Polytechnica Vol. 51 No. 5/2011 inertial sensors in low cost networks, with a high de- gree of redundance”, code TE-102/2010. References [1] Hoole, S.R.: Computer-aided analysis and design of electromagnetic device. Elsevier, 1989. [2] Haberman,R.: Elementary applied partial differ- ential equations. Prentice-Hall, 1987. [3] Ierusalimschy,R., deFigueiredo,L.H., Celes,W.: ReferenceManual of the Programming, Language Lua 4.0. http://www.lua.org/manual/4.0/ [4] Allaire, P. E.: Basics of the finite element method. 1985. [5] Silvester, P. P.: Finite elements for electrical en- gineers. Cambridge University Press, 1990. [6] Plonus, M.: Applied electromagnetic. McGraw- Hill, 1978. [7] Chari,M.V.K., Salon, S. J.: Numerical Methods in Electromagnetism. Academic Press in Electro- magnetism, 2000. [8] Boulassel, Z. B., Mekideche, M. R.: Modeling and Design of a Synchronous Permanent Mag- nets Machine, Studies in Applied Electromagnet- ics and Mechanics, Vol. 34 – Computer Field Models of Electromagnetic Devices, IOS Press 2010, pp. 389–397. [9] Bernat, J., Kajda, M., Kolota, J., Stepien, S.: Brushless DC Motor Optimization Using FEM Parallel Simulation Technique with Look Up Ta- bles, Studies in Applied Electromagnetics and Mechanics, Vol. 34 – Computer Field Mod- els of Electromagnetic Devices, IOS Press 2010, pp. 435–439. About the authors Radu Obreja was born on December 22, 1983. He received his bachelor’s degree and master’s degree in electrical engineering from the Faculty of Electri- cal Engineering,PolitehnicaUniversity of Bucharest, Romania. At present he is a PhD student at the Military Technical Academy. His research interests are in special electric machines, inertial navigation systems, low-cost navigation sensors and integration technologies. Ioana Raluca Edu was born on December 4, 1984. She received her bachelor’s degree and mas- ter’s degree in electrical engineering from the Fac- ulty of Electrical Engineering, PolitehnicaUniversity of Bucharest, Romania. At present she is a PhD stu- dent at the Faculty of Electrical Engineering. Her research interests are in inertial navigation systems, low-cost navigation sensors and integration technolo- gies. Radu Obreja E-mail: radu@sistemeuroteh.ro Military Technical Academy Bucharest, Romania Ioana Raluca Edu E-mail: edu ioana raluca@yahoo.com Politehnica University of Bucharest Romania 83