Synchronous reluctance motor

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A synchronous reluctance motor is a three-phase synchronous machine with a rotor that either has a so-called flux barrier section or pronounced poles. The stator (stator) of the synchronous reluctance motor has, as in other three-phase machines , three coils spatially offset by 120 °, which are fed by three-phase alternating voltage. In this rotating field generated by the stator , the rotor has preferred directions due to the different magnetic conductivity along the circumference . In this motor, the torque is generated due to the reluctance force in the preferred directions and not, as in other electrical machines, due to the Lorentz force .

As with all synchronous motors, the rotor rotates synchronously with the rotating field of the supply voltage network. The speed is linked to the frequency of the alternating voltage via the number of pole pairs . In practice, synchronous reluctance motors are usually 4-pole.

history

The principle of the reluctance motor with the anisotropy of the magnetic conductivity in the rotor has long been known. Jaroslaw K. Kostko published a corresponding article as early as 1923. Various manufacturers offer reluctance motors for operation directly on the grid. The rotor usually has a short-circuit starting winding for asynchronous self -starting . This type of engine is only found in niches, e.g. B. in the textile industry , application. In 1998 Vagati succeeded in optimizing the rotor geometry in such a way that torque ripple and noise emissions are systematically and significantly reduced.

Structure and way of working

The stator (stator) of a synchronous reluctance motor usually has the same structure as that of a commercially available asynchronous motor with distributed windings. The rotor (rotor) is to avoid eddy currents designed as a laminated core of electric sheets. This has a special sheet metal cut geometry with flow guide and flow barrier sections (e.g. Fig. 1). The winding distributed in the stator slots generates a rotating field in the air gap of the motor when supplied with three-phase current . When powered by a frequency converter , the speed can be increased from zero to the operating speed and adjusted during operation. A suitable rotor position control in the frequency converter ensures that the rotor does not fall out of step, especially when the load changes. In synchronous reluctance motors that are to be operated on a rigid network, the rotor is often designed with a rotor cage (similar to those of squirrel cage asynchronous machines). This enables an asynchronous start-up on the network. As soon as the speed of the rotor approaches the synchronous speed, the reluctance torque predominates, so that the rotor can synchronize (“step into step”) and follow the rotating field.

Sheet metal section of a 4-pole synchronous reluctance motor according to the US patent by Vagati

In the preferred magnetic direction (d-direction) there is a low magnetic resistance and the magnetic flux is well guided in the iron. At a 45 ° angle to this (q-direction), the air barriers obstruct the magnetic flux. If the stator winding is energized, poles and gaps are created in the rotor. If the rotor follows the stator field synchronously, its poles are connected to the poles of the rotating field via a spring (cf. spring model in the article pole wheel ). When the load is applied, the runner remains a little behind and a rotor angle is created which disappears again when the load is removed . Equivalently, the rotor leads the stator field in generator mode. The d and q directions of the rotor correspond to the respective axes of the coordinate system defined by the D / q transformation . The stator currents can be used to determine the d and q components in the rotor via the D / q transformation.

Torque generation

The picture on the right shows the principle of torque generation in more detail. If a strip of sheet iron is in a magnetic field , it tries to turn in the direction of the field lines and to take up an energetically favorable position parallel to the field lines. If you turn it out of this position, a torque M is created. The same thing happens in the motor. When the motor is loaded, the pole wheel briefly lags behind the rotating field and the load-dependent pole wheel angle δ (delta) is created. If the load is too great, the engine falls "out of step" and stops.

Torque

The torque M of the synchronous reluctance motor can be calculated from the motor data. You get it to:

with the motor constant k , the inductances in the q and d directions L q and L d , the magnetic flux Ψ and the rotor angle δ . The formula shows that the ratio L d / L q possible must be large, a large torque M to obtain.

The better the flux guides work in the d-axis and the more the flux is hindered in the q-axis, the greater the motor torque.

Advantages and disadvantages

Due to the inertia of the rotor and the unconditional synchronous mode of operation, the synchronous reluctance motor (without damper cage) can only be operated with a frequency converter. With modern control technology, speed control is now also possible without a resolver (rotor position encoder). A disadvantage is therefore the increased investment costs by the cost of the converter. Due to the robust design, the probability of failure is not increased compared to a regulated asynchronous machine. With synchronous reluctance motors, there are almost no losses in the rotor. As a result, it is more efficient than conventional asynchronous motors and, for this reason, it generates less heat. The winding losses in the stator are slightly higher because of the higher proportion of magnetization in the stator. Due to the overall lower losses, the international efficiency class IE4 can be achieved with the motor, the exact curve of which over the nominal power in the CD (Committee Draft) of IEC 60034-30 Ed. 2 is set. Thanks to this property, synchronous reluctance motors have again gained importance since around 2009 for use in so-called rotating equipment (pumps, fans, compressors and turbines). In comparison to permanent magnet excited synchronous motors, the advantage of manufacturing is that no rare earth- based magnetic materials are used that have been classified as particularly “critical metals” by the European Commission 14 as part of its “raw materials initiative”.

literature

  • Peter F. Brosch: In: Zeitschrift Elektrotechnik - Edition June 2011, p. 36ff.
  • TA Lipo: Synchronous Reluctance Machines-A Viable Alternative for AC Drives? In: Electric Machines & Power Systems . tape 19 , no. 6 , 1991, pp. 659-671 , doi : 10.1080 / 07313569108909556 .
  • TJE Miller, A. Hutton, C. Cossar, DA Staton: Design of a synchronous reluctance motor drive . In: IEEE Transactions on Industry Applications . tape 27 , no. 4 , August 1991, pp. 741-749 , doi : 10.1109 / 28.85491 .
  • I. Boldea, L. Tutelea, CI Pitic: PM-assisted reluctance synchronous motor / generator (PM-RSM) for mild hybrid vehicles: electromagnetic design . In: IEEE Transactions on Industry Applications . tape 40 , no. 2 , April 2004, p. 492–498 , doi : 10.1109 / TIA.2004.824434 .
  • M.-I. Lamghari-Jamal, J. Fouladgar, E.-H. Zaim, D. Trichet: A magneto-thermal study of a high-speed synchronous reluctance machine . In: IEEE Transactions on Magnetics . tape 42 , no. 4 , April 2006, p. 1271 -1274 , doi : 10.1109 / TMAG.2006.871956 .
  • TA Lipo, PC Krause, “Stability analysis of a reluctance synchronous machine,” IEEE Trans. Power Appl. Syst., Vol. PAS-86, No. 7, pp. 825-834, Jul. 1967
  • AJO Cruickshank, RW Menzies, AF Anderson: Axially laminated anisotropic rotors for reluctance motors . In: Proceedings of the Institution of Electrical Engineers . tape 113 , no. December 12 , 1966, p. 2058-2060 , doi : 10.1049 / piee.1966.0358 .
  • D. Platt: Reluctance motor with strong rotor anisotropy . In: IEEE Transactions on Industry Applications . tape 28 , no. 3 , June 1992, pp. 652-658 , doi : 10.1109 / 28.137453 .
  • HH. Hofmann, SR Sanders: High-speed synchronous reluctance machine with minimized rotor losses . In: IEEE Transactions on Industry Applications . tape 36 , no. 2 , April 2000, p. 531-539 , doi : 10.1109 / 28.833771 .
  • Patent FI118940B1 : Rotor for electric machine. Published on May 15, 2008 , inventor: J. Kolehmainen. ‌
  • J. Kolehmainen, J. Ikaheimo: Motors With Buried Magnets for Medium-Speed ​​Applications . In: IEEE Transactions on Energy Conversion . tape 23 , no. 1 , March 2008, p. 86-91 , doi : 10.1109 / TEC.2007.914331 .
  • J. Kolehmainen: Machine with a rotor structure supported only by buried magnets . In: Andrzej Krawczyk, Sławomir Wiak, Ivo Doležel (Eds.): Advanced Computer Techniques in Applied Electromagnetics . IOS Press, 2008, ISBN 978-1-58603-895-3 , pp. 240-246 , doi : 10.3233 / 978-1-58603-895-3-240 (presentation at the Int. Symp. Electromag. Fields Elect. Eng. (ISEF), Prague, Czech Republic, Sept. 2007).
  • XB Bomela, MJ Kamper: Effect of stator chording and rotor skewing on average torque and torque ripple of reluctance synchronous machine . In: 1999 IEEE Africon . tape 2 , 1999, p. 687-690 , doi : 10.1109 / AFRCON.1999.821849 .

Individual evidence

  1. ↑ JK Kostko: polyphase reaction synchronous motor . In: Journal AIEE . tape 42 , 1923, pp. 1162-1168 .
  2. ↑ Patent US5818140 : Synchronous reluctance electrical motor having a low torque ripple design. Published October 6, 1998 , inventor: A. Vagati. ‌
  3. ↑ Background paper rare earths, status January 2011, Öko-Institut eV, Berlin office, Schicklerstrasse 5-7, 10179 Berlin