Rotating magnetic field
As a rotating field, a magnetic field is called in electrical engineering, which continuously rotates about an axis of rotation.
Basics
Rotating fields are generated to drive the waves of AC motors and independently -starting AC motors. The rotating field pulls the coaxially on the shaft of the motor rotor with fixed magnetically.
Three-phase AC offers the possibility to produce by the circular arrangement of a rotating field coil by the time offset of the three alternating currents. In the simplest case, to six coils are arranged in a circle and the respective opposite coils interconnected to form a pole pair, which form the two poles of an electromagnet. The interchange of any two phase conductors of three-phase alternating current causes a reversal of the direction of rotation.
- Visualization of the rotating field in a three-phase machine
Representation as a sum vector. The tip of the outer vector describes a circle.
Another possible way of generating a rotating field is the two-phase alternating current which forms by the temporal offset of two alternating currents by 90 °, a rotating field. Due to the worse in the two- phase alternating current utilization of the conductor cross-sections in the energy transfer it plays no significant role. However, it is used in many electrical machines, such as the two-phase synchronous motor for generating a rotating field.
Mathematical approach
The pole pairs are offset in time by alternating in turn magnetized so that idealized produces a rotating magnetic field which corresponds to the speed of which the frequency of the alternating current:
Multiplying the number of pole pairs per phase reduces the speed:
The maximum speed at the usual mains frequency of 50 Hz:
This results in a rotating field speed per minute of 3000 min -1.
Rotating field as a sum of three alternating fields
The rotating magnetic field of a one-pole three-phase machine can be generated by summation of three 120 degree spatially and temporally phase-shifted alternating fields as shown in the animation (above). Accordingly, in the space vector representation is:
The sum of the three alternating fields obtained by mathematical transformation the total field:
The magnetic induction of the three phase rotary field produced thus has the time constant 1.5 times the amount of induction of each alternating fields and rotates with the angular frequency.
Special applications
Self employed -starting AC motors produce a rotating field by a phase shift. A phase shift of the electric current is achieved by a capacitor, a phase shift of the magnetic flux through short-circuiting rings to the iron core (see capacitor motor, shaded pole motor ).
Polyphase meter
Be In order to detect the orientation of the field rotating field measuring instruments used. Early specimens were equipped with a small electric motor, current devices operate on an electronic basis and must be reported to the loss of one phase in the situation.