Rotating field
In electrical engineering, a rotating field is a magnetic field that continuously rotates around an axis of rotation .
Basics
Rotating fields are generated to drive the shafts of three-phase motors and independently starting AC motors. The rotating field magnetically pulls the rotor, which is fastened coaxially on the shaft of the motor .
Due to the time offset of the three alternating currents, three-phase alternating currents offer the possibility of generating a rotating field through the circular arrangement of coils. In the simplest case, six coils are arranged in a circle and the opposing coils are connected together to form a pole pair that forms the two poles of an electromagnet. Interchanging any two external conductors of the three-phase alternating current causes the direction of rotation to be reversed.
Temporal sequence of the individual phase-shifted voltages as vectors
A rotating field can also be generated with two-phase alternating current. Due to the time offset of two alternating currents by 90 °, it forms a rotating field. Two-phase alternating current does not play an essential role in energy transmission, since the conductor cross-sections are poorly utilized, but it is used in some machines such as the two-phase synchronous motor to generate a rotating field.
Mathematical approach
The pole pairs are magnetized in turn by the alternating current, so that, ideally, a rotating magnetic field results whose speed corresponds to the frequency of the alternating current:
By multiplying the number of pole pairs per outer conductor , the speed is reduced:
The maximum speed at the usual frequency of the power grid of 50 Hz is:
This results in a rotating field speed per minute of 3000 min −1 .
Rotating field as the sum of three alternating fields
As in the animation (above), the rotating magnetic field of a single-pole pair three-phase machine can be generated by summing three alternating fields that are phase-shifted by 120 degrees in space and time . The following applies accordingly in the space vector representation :
After mathematical transformation, the sum of the three alternating fields results in the total field:
The magnetic induction of the three-phase rotating field thus has a temporally constant 1.5 times the amount of the induction of the individual alternating fields and rotates with the angular frequency .
Special applications
AC motors that start up independently generate a rotating field through a phase shift . A phase shift of the electrical current is achieved by a capacitor , a phase shift of the magnetic flux by short-circuit rings on the iron core (see capacitor motor , shaded pole motor ).
Rotating field measuring device
Rotary field measuring devices are used to identify the orientation of the field . Early models were equipped with a small electric motor , current devices work on an electronic basis and can also indicate the failure of a phase.
Individual evidence
- ^ Thomas Keve / Helmut Roeloffzen: Electrical machine building block Berliner Union & Kohlhammer 1978; ISBN 3-408-53529-9 , p. 188
literature
- Horst Stöcker: Pocket book of physics. 4th edition, Verlag Harry Deutsch, Frankfurt am Main, 2000, ISBN 3-8171-1628-4
- 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
- Günter Springer: Electrical engineering arithmetic book. 11th improved edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 1992, ISBN 3-8085-3371-4
Web links
- Do-it-yourself rotating field meter
- Video: Magnetic rotating field . Institute for Scientific Film (IWF) 2004, made available by the Technical Information Library (TIB), doi : 10.3203 / IWF / C-14889 .