Pole wheel
The pole wheel is the part of a rotating single-phase or three-phase synchronous machine on which the DC- fed excitation winding is accommodated, i.e. on the rotor . In the case of linear motors , the magnets that ensure the excitation and generate the load-bearing forces are also called the pole wheel. This article only deals with the pole wheel of the synchronous machine.
Layout and function
The pole wheel carries permanent magnets or an excitation winding that generate a magnetic field. The latter are coils that are fed with direct current . Today, neodymium-iron-boron (NdFeB) or, in cheaper machines, ferrites are mostly used as permanent magnets . Because of the high costs, rare earth magnets such as samarium cobalt (SmCo) are only used for special small series.
Rotor angle
The so-called pole wheel angle (or also load angle) is the angle at which the pole wheel of a synchronous machine leads ( generator operation ) or lags ( motor operation ) the synchronous rotating field . In the spring model of a synchronous machine in generator mode, with the pole wheel voltage U P and the stator voltage U S , the spring force F spring represents the load from the network, which is counteracted by the torque M supplied by a drive machine and thus the force F M. In engine operation, the spring force would correspond to the engine torque, which counteracts the load torque. The rotor angle must not be greater than 90 ° for stable operation; rotor angles in the range of 20 ° to 30 ° at nominal power are common . At 90 ° the machine delivers the greatest torque in motor operation, at a larger angle the machine “tilts”, which is why the greatest torque is also called the overturning moment. This behavior is shown graphically in the current locus of the synchronous machine. Metaphorically speaking, the imaginary spring breaks at the overturning moment, and the motor gets “out of step”. If the synchronous machine is operated in an unstable manner in generator mode, it can no longer convert the mechanical power into electrical power, it “goes through”, accelerates above its rated speed and can be damaged or destroyed without being switched off.
With a higher number of pole pairs , the (electrical) pole wheel angle and the mechanical pole wheel angle differ from one another. The conversion takes place via the number of pole pairs.
Pole wheel in measurement technology
In measurement technology , the pole wheel consists of many small permanent magnets. Around the circumference, the north and south poles alternate approximately every 0.5 to 5 mm. The magnetic poles serve as a measuring standard and are scanned with coils or magnetic field sensors ( list ). The pole wheel is used like an incremental encoder as a speed or angle encoder . Compared to optical graduated disks, the pole wheel is less sensitive to dirt and moisture (e.g. condensation) and, depending on the version, also more cost-effective, but not as precise due to the coarser division due to the limited number of magnets. In addition to a single incremental track, several magnetic tracks can also be implemented on one pole wheel.
literature
- Gregor D. Häberle, Heinz O. Häberle: Transformers and electrical machines in power engineering systems. 2nd edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 1990, ISBN 3-8085-5002-3 .
- Gerd Fehmel, Horst Flachmann, Otto Mai: The master's examination in electrical machines. 12th edition, Vogel Buchverlag, Oldenburg / Würzburg, 2000, ISBN 3-8023-1795-5 .
Individual evidence
- ↑ Terms and definitions specific to maglev trains . PDF (323 kB).
- ↑ Trial synchronous machine ( memento of the original from November 25, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 882 kB) . ETH Zurich
