2.1.5. Magnetic fields in electrical machines
Every rotating machine consists of a stationary section (stand) and a rotating section (rotor).
Stands and rotors are made up of magnetic materials and windings and generate magnetic fields in the air gap.
We differentiate between the following magnetic fields:
- constant field
A constant field results from a permanent magnet or through a coil saturated by direct current.
(1) Rotor excitation through current flow
A constant field denotes a temporally constant magnetic field in an air gap.
An alternating field is generated as alternating current passes through a winding.
A magnetic field which changes its size and direction according to the frequency is called an alternating field.
1 Alternating current, 2 Induction and current, 3 Induction sequence, 4 Current sequence
Definition of term:
A rotating field may be compared to the magnetic field of a rotating, permanent magnet.
A rotating field denotes a rotating magnetic field within a specific space.
Generating a rotating field:
As Figure 12 indicates, the simplest stator of a rotating machine features three spatially positioned coils at 120 degrees. These coils are saturated by three temporally displaced three-phase currents at 120 degrees.
(1) Stator with three spatially displaced windings
The current directions are arbitrarily indicated thus:
Figure 13 indicates clearly that a single magnetic field emerges with a north and south pole following the spatial displacement of the coils (motionless in the area) coupled with a temporal displacement of the currents.
Speed of the rotating field:
A stator winding where the three coils have been so switched as to only yield one north pole and one south pole is called a two pole machine or a machine with a pole pair (p = 1). A four pole machine thus has two pole pairs etc.
Given a two pole machine the rotating field runs once through for every period of the alternating current. Following a period the pole pairs only undertake a half rotation.
The speed of the rotating field depends on the frequency of the alternating current and the pole pair:
A maximum speed of 3000 rpm can be attained given a frequency of f = 50 Hz.
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