Reluctance motor
A reluctance motor is a type of electric motor in which the torque in the rotor is generated exclusively by the reluctance force and not to a significant extent by the Lorentz force , as is the case with magnetically excited machines. This means that the machine is not equipped with permanent magnets , nor are there any electrical windings on the rotor . As a result, there is no need for any kind of wear- prone slip rings and brushes . The rotor has pronounced poles and consists of a highly permeable , soft magnetic material such as electrical steel .
Compared to machines with permanent magnet excitation, such as the synchronous motor , the reluctance motor has a low torque density . This means that the nominal torque , based on the volume of the machine, is lower. Since the motor type in the synchronous operating mode by means of a frequency converter has a higher degree of efficiency than other asynchronous and synchronous motors, it is nevertheless economical.
Basic principle
The movement comes about because the system strives for minimal magnetic resistance (reluctance). As a model you can imagine a toroidal core coil with one leg loose ( see picture ). Due to the reluctance force , the loose leg will strive for the position in which the reluctance reaches its minimum (i.e. the inductance reaches its maximum). In the reluctance motor, two poles of the stator and two poles of the rotor always form a "ring" in that the rotor positions itself so that the reluctance is as low as possible (i.e. parallel to the field lines).
An alternative (somewhat more mathematical) approach is that the magnetic field vector that penetrates the rotor is broken down into several components. Since the rotor goes into saturation, the partial vectors will not reach their full "length", but only a maximum, which is higher in the direction through the poles (because of lower reluctance) than in other directions. If the “shortened” components are added vectorially, their vectorial sum points in a different direction than the causal field vector of the stator. The rotor now turns until the vectors point in the same direction.
Because the stator poles are magnetized with a time offset, the force always acts in a different direction and a rotation occurs.
advantages
The advantage of a reluctance motor is the fact that losses practically only occur in the stationary stator, which can therefore be easily cooled from the outside. Correspondingly built reluctance motors are therefore tolerant of brief overloads. Due to the comparatively simple structure of the rotor without coils or special materials (no permanent magnets and no materials such as rare earths are required), the rotor can be made robust and with an appropriate design tolerant of overspeed.
disadvantage
The main disadvantage of the reluctance motor is the pulsating torque, which is particularly important when the number of stator poles is small. Further disadvantages are pulsating radial forces between rotor and stator, which load the bearings and are responsible for a comparatively high level of noise. In addition, as in the asynchronous machine , a reactive current is required to build up the rotating field . This reactive current increases the apparent power of the electronic converters.
Types
There are essentially four types of reluctance motors:
- Synchronous reluctance motor
- The synchronous reluctance motor has a wound multi-phase stator (stator) like an asynchronous machine . The rotor (rotor) is not round, but has pronounced poles. A direct start-up of this motor type on the network is i. d. Usually not possible, so it is usually controlled by a frequency converter.
- Asynchronous motor with reluctance torque
- If the frequency converter is to be dispensed with, the motor can be equipped with a squirrel cage like an asynchronous machine and is then called an asynchronous motor with reluctance torque . It then starts up like an asynchronous motor up to the vicinity of the asynchronous equilibrium speed. Then the reluctance effect prevails and the rotor rotates synchronously with the rotating field. A three-phase motor running synchronously with the stator frequency can be built in a simple and relatively inexpensive way , but this design has disadvantages compared to operation with a frequency converter as compared to synchronous motors and asynchronous motors, a lower efficiency , a lower power density and above all a high reactive power requirement.
For this reason, it makes sense to use a frequency converter-fed synchronous reluctance motor whenever possible.
- Switched reluctance machine
- (SRM for short, from English: switched reluctance motor , also SR-drive ): Such reluctance motors, like the other types, have a different number of pronounced teeth on the rotor and stator. The stator teeth are wound with coils that are switched on and off alternately. The teeth with the energized windings each attract the closest teeth of the rotor like an electromagnet and are switched off when (or shortly before) the teeth of the rotor are opposite the attracting stator teeth. In this position the next phase is switched on on other stator teeth, which attracts other rotor teeth. Generally, a switched reluctance motor has three or more phases. But there are also special designs with only two or one phase. In order to switch over at the right time, the machine is usually provided with a rotor position encoder . There are also encoderless control methods based on the stator current or the torque. Reluctance motors of this type are characterized by high robustness and low construction costs. Like asynchronous machines, they do not generate any torque when they are de-energized. A residual magnetization often leads to a small detent torque in the de-energized state. At low speeds they are superior to asynchronous machines in terms of torque density due to the high number of pole pairs that can be produced, and at higher speeds they are clearly inferior. In this regard, they are definitely inferior to permanent magnet synchronous machines.
- Reluctance stepper motor
- A reluctance stepper motor can in principle be constructed in the same way as a switched reluctance motor. In contrast to this, it is switched without knowledge of the rotor position, making it simpler but less reliable ( step losses ). In contrast to other stepper motors, the reluctance stepper motor is not able to hold its position in the de-energized state.
Areas of application
Reluctance motors are well suited for medium-sized drives (diameter from 100 to 300 mm) with short switch-on times. Due to their simple and robust construction (for example no rotor windings or magnets) they are very well suited for operation in harsh environments. For small motors they are ruled out because of insufficient power density and insufficient efficiency, and for large ones because of insufficient energy efficiency and too high reactive power requirement. Versions up to 52 kW are currently known.
Another area of application for synchronously running reluctance motors is in the textile industry for the synchronous unwinding of yarn.
Switched reluctance motors were used in hybrid electric vehicles as a parallel hybrid drive because, unlike permanent magnet motors, they run without losses when driven by the internal combustion engine and, above all, have a higher torque than asynchronous motors when starting . As a fully electrically powered car that has model 3 of Tesla a reluctance motor.
One advantage of synchronously running and switched reluctance motors is the cost-effective manufacture of the motor. With switched reluctance motors, the control electronics are somewhat more expensive than with other motor technologies due to the high reactive power requirement. As a result of lower prices for electronic components, they are now also attractive for use in larger household appliances ( washing machines , cleaning pumps).
literature
- Ernst Hörnemann, Heinrich Huebscher, Dieter Jagla: Electrical engineering, industrial electronics . Westermann, Braunschweig 2001, ISBN 3-14-221730-4 .
- Hans-Günter Boy, Horst Flachmann, Otto Mai: Electrical machines and control technology . In: The master's examination . 4th edition. Vogel Verlag und Druck, Würzburg 1983, ISBN 3-8023-0725-9 .
- Electrical engineering expertise . In: European reference book series: For electrotechnical professions . 18th edition. Verl. Europa-Lehrmittel, 1989, ISBN 3-8085-3018-9 .
- Peter Friedrich Brosch : The new energy saver - IE4 with reluctance motor . In Konstruktions , No. 7/8, 2011, pp. 14-17
Web links
Individual evidence
- ^ Dieter Gerling: Lecture Electrical Machines and Drives . University of the Federal Armed Forces Munich, S. 150-159 ( online ). Online ( Memento from December 13, 2013 in the Internet Archive )
- ↑ Comparison of the efficiency of reluctance motors (IE4) and IE3 motors . KSB Aktiengesellschaft . Archived from the original on December 13, 2013. Retrieved December 4, 2013.
- ↑ Reinhard Kluger: IE4 drive package with synchronous reluctance motor . Electrical engineering. February 4, 2013. Retrieved December 8, 2013.
- ↑ Peter F. Brosch, University of Hanover: Synchronous reluctance motors achieve efficiency class IE4 . Energie 2.0 Kompendium 2013. Archived from the original on September 24, 2015. Retrieved on December 8, 2013.
- ↑ Peter F. Brosch, University of Hanover: Synchronous reluctance motor versus asynchronous motor. publish-industry Verlag GmbH, June 2, 2014, accessed October 5, 2018 .
- ↑ http://www.energie.ch/themen/industrie/antriebe/#Reluktanzmotor Comparison of reluctance motor, synchronous motor, asynchronous motor
- ↑ Siemens Aktiengesellschaft: SIMOTICS SD-VSD4000 MOTOR TYPE: 1TV4222B IEC LV motor, synchronous reluctance motor. Siemens Aktiengesellschaft, July 22, 2019, accessed on August 19, 2020 .
- ↑ Tesla Model 3 dual motor performance version features both an AC induction and a permanent magnet motor (February 26, 2019)
- ↑ Belgians develop electric motor without rare earths (February 25, 2013)
