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close this bookElectrical Machines - Basic Vocational Knowledge (IBE - Deutschland; 144 pages)
View the documentIntroduction
Open this folder and view contents1. General information about electrical machines
Open this folder and view contents2. Basic principles
Open this folder and view contents3. Execution of rotating electrical machines
close this folder4. Synchronous machines
close this folder4.1. Operating principles
View the document4.1.1. Synchronous generator
View the document4.1.2. Synchronous motor
Open this folder and view contents4.2. Constructional assembly
Open this folder and view contents4.3. Operational behaviour
Open this folder and view contents4.4. Use of synchronous machines
Open this folder and view contents5. Asynchronous motors
Open this folder and view contents6. Direct current machines
Open this folder and view contents7. Single-phase alternating current motors
Open this folder and view contents8. Transformer
 

4.1.1. Synchronous generator

Alternating-voltage generator

A sine-shaped alternating voltage can be generated very simply by utilising the arrangement set out in Figure 30 by means of the induction effect (U0 = c • Φ • n)

The sine-shaped voltage is attained through a conductor loop in the parallel homogeneous magnetic field. The conductor loop ends are connected to the slip ring and the voltage is fed to the operating means by carbon brushes.


Figure 30 - Model of an alternating voltage generator (inner-pole machine)

1 Current direction

The same effect is produced if a stationary induction coil is shifted to within the sphere of a rotating magnet.


Figure 31 - Model of an alternating voltage generator (external pole machine)

The voltage induction in the synchronous generator can be attained by the generation of a magnetic flow in

- stationary stators and rotating induction winding (external pole machine), or

- in the rotating magnetic stand and stationary induction winding in the stator (inner-pole machine).

Every rotation of the conductor loop induces a period of alternating voltage. Where the rotation ensues within a second there is one period per second, that is to say, a frequency of one Hz. Given n rotations per minute, that is to say n/60 rotations per second, there is initially a frequency of

This equation, moreover, shows that proportionality prevails between the frequency of the generated voltage and the speed. This explains the name “synchronous generator”.

Where a four-pole arrangement (two north poles along with two south poles) is employed, there arises a period of alternating voltage in the event of a semi-rotation of the magnets.


Figure 32 - Interdependence of pole pair number and frequency in a synchronous generator

(1) Two-pole generator, (2) Four-pole generator

1 Winding beginning, 2 Winding end, 3 Voltage, 4 A cycle, 5 A rotation = two cycles, 6 A rotation = one cycle, 7 Coil, 8 Coil connection

The following then applies:


p = pole pair number
p = one two-pole machine
p = two four-pole machine etc.

Thus, the greatest speed at which f = 50 Hz is therefore n = 3000 rpm (p = 1).

Three-phase generator

Figure 33 initially depicts the basic arrangement of a two phase alternating voltage generator.


Figure 33 - Principle of the two-phase alternating voltage generator

1 Casing, 2 Stator, 3 Field spider, 4 Beginning winding one, 5 End winding one, 6 Beginning winding two, 7 End winding two

Two coils (resp. four half coils) are positioned spatially within 90 degrees on the circumference of a common stator ferromagnetic circuit.

By means of a rotating electromagnet (field spider) out-phased voltages of like amplitude and frequency are induced temporally within 90 degrees in these windings. These can be dropped off directly at the windings.


Figure 34 - Two-phase alternating voltage

1 Voltage, 2 Winding voltage, 3 Voltage of winding two

Where three coils are shifted spatially within 120 degrees in an alternating voltage generator and distributed within the range of a common stator circuit, a rotating (electro)magnet induces three displaced voltages temporally within 120 degrees.


Figure 35 - Principle of the three-phase alternating generator

1.1. Winding one beginning
1.2. Winding one end
2.1. Winding two beginning
2.2. Winding two end
3.1. Winding three beginning
3.2. Winding three end


Figure 36 - Three-phase alternating voltage

1 Winding one voltage, 2 Winding two voltage, 3 Winding three voltage, 4 Voltage

This principle was first cited in 1885 by Ferraris. (Galileo Ferraris, 1847-1897, Italian physicist and electrical engineer).

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