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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
Open this folder and view contents4. Synchronous machines
Open this folder and view contents5. Asynchronous motors
close this folder6. Direct current machines
View the document6.1. Constructional assembly
close this folder6.2. Operating principles
View the document6.2.1. Power generation (direct current motor)
View the document6.2.2. Torque generation (direct current motor)
View the document6.2.3. Armature reaction (rotor reaction)
View the document6.2.4. Excitation
View the document6.2.5. Value relations
Open this folder and view contents6.3. Operational behaviour of direct current machines
Open this folder and view contents6.4. Circuit engineering and operational features of customary direct current generators
Open this folder and view contents6.5. Circuit engineering and operational features of customary direct current motors
Open this folder and view contents7. Single-phase alternating current motors
Open this folder and view contents8. Transformer
 

6.2.1. Power generation (direct current motor)

Initially (Cp. 4.1.1.) an alternating voltage is generated in the rotor windings of every generator. This alternating voltage can, however, be rectified so that current always passes through the external circuit in the same direction (Cp. Figure 30). Rectification of the generated alternating voltages takes place in the following manner: Instead of the two slip rings isolated from one another in Figure 30, only one slip ring appears in Figure 83. This latter slip ring has, however, been divided into two halves insulated from each other (lamella) and is known as a commutator.


Figure 83 - Model of a direct current generator

1 Bulb
2 Carbon brushes
3 Commutator

The beginning and end of the conductor loop (winding) have been connected to the lamella. The two brushes face each other. They always run from one lamella to another as the conductor loop passes through the neutral zone. The induced voltage in the conductor loop is just zero at this moment and changing its direction. Figure 84 shows the principle of mechanical rectification.


Figure 84 - Principle of mechanical rectification

1, 2 Conductor loop, 3 Rotational direction, 4 Neutral zone

The commutator ensures that polarity of the carbon brushes remains constant at all times. The course of the rectified voltage is shown in Figure 85. We can perceive that the generated voltage still ranges between zero and a top value whilst always evidencing the same direction.


Figure 85 - Sequence of the rectified voltage of a conductor loop

1 Voltage

Where a less pulsating voltage is required it is necessary to increase the number of conductor loops whereby they should be spatially positioned. The number of lamella must be increased for every additional conductor loop. Figure 86 sets out the basic drawing of a direct current generator with two conductor loops.

The commutator comprises four lamella. The generated voltage is shown in Figure 87.


Figure 86 - Direct current generator with two conductor loops resp. windings

1, 2 Conductor loop; 3, 4 Conductor loop


Figure 87 - Sequence of direct current in a generator with two windings

1 Voltage in the conductor loop ½, 2 Voltage in the conductor loop ¾, 3 Terminal voltage, 4 Voltage in volt

The resultant voltage no longer varies as much however it has the disadvantage that the carbon brushes only tap off a voltage from the conductor loops when they generate a peak voltage. The conductor loops are ineffective at all other times. However, the two conductor loops add together their voltages once they are series connected.


Figure 88 - Generator with series-connected conductor loops


Figure 89 - Voltage sequence of a generator in accordance with Figure 88 - Legend as for Figure 87

The voltage presented in Figure 89 also features an increased voltage alongside an even one. Moreover, the direct voltage can be improved still further by increasing the number of conductor loops. In practise coils with several windings are employed instead of conductor loops. Operating current IA flows when a consumer connects to the carbon brush terminals.

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