                            Electrical Machines - Basic Vocational Knowledge (IBE - Deutschland; 144 pages)  Introduction  1. General information about electrical machines  2. Basic principles   2.1. The magnetic field    2.1.1. Definition and presentation of the magnetic field    2.1.2. Magnets Magnetic field    2.1.3. Magnetic field of a current-carrying conductor    2.1.4. Magnetic field of a current-carrying coil    2.1.5. Magnetic fields in electrical machines   2.2. Measurable variables of the magnetic field   2.3. Force action of the magnetic field   2.4. Voltage generation through induction  3. Execution of rotating electrical machines  4. Synchronous machines  5. Asynchronous motors  6. Direct current machines  7. Single-phase alternating current motors  8. Transformer

#### 2.1.5. Magnetic fields in electrical machines

Field types

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
- alternating field
- rotating field

Constant field

A constant field results from a permanent magnet or through a coil saturated by direct current. Figure 9 - Constant field

(1) Rotor excitation through current flow
(2) Stator excitation through current flow
1 Field winding, 2 Rotor, 3 Magnetic flow, 4 Stator
A constant field denotes a temporally constant magnetic field in an air gap.

Alternating field

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. Figure 10 - Magnetic alternating field

1 Alternating current, 2 Induction and current, 3 Induction sequence, 4 Current sequence

Rotating field

Definition of term:

A rotating field may be compared to the magnetic field of a rotating, permanent magnet. Figure 11 - Emergence of a rotating field through rotation of a 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. Figure 12 - Emergence of a rotating field in the stator of a rotating electrical machine

(1) Stator with three spatially displaced windings
(2) Commensurate temporally shifted currents, t1; t2; t3 Instantaneous times

The current directions are arbitrarily indicated thus:  Figure 13 indicates each coil with a winding and in its veritable spatial position. A clear-cut picture of current distribution for the moments t1, t2 and t3 emerges once the current directions in the individual conductors are entered into a line diagram. Figure 13 - Explanation for the emergence of a rotating field, t1; t2; t3 Instantaneous times

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. Figure 14 - Four-pole machine

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.   