LABORATORY WORK ¹ 7

TESTS OF GENERATOR. CHARACTERISTICS

 

7.1. Objectives

1.       Elaborate MatLab-Simulink model of a direct current separate-ly excited generator.

2.       Get acquainted with properties and characteristics of direct cur-rent separately excited generator.

 

7.2. Task

Obtain by simulation the main characteristics of the generator:

1.       open circuit characteristic;

2.       short-circuit characteristic;

3.       voltage regulation characteristic.

 

7.3.  Generator Characteristics

The no-load and load characteristics of DC generators are usually of in the determining their potential applications. As the names imply, no-load and load characteristics respectively correspond to the behavior of the machine when it does not deliver power (open circuited, in case of a generator) and when it delivers power to an external circuit. The only no-load (or open-circuit) characteristics are meaningful of the shunt and separately excited generators. For the separately excited generator, the no-load characteristic corresponds to the magnetization, or saturation, characteristic which is the variation of E (or V) under open-circuit conditions as a function of the field current If. The shunt generator has a characteristic similar to that of a separately excited generator, except for the cumulative effect. If the shunt generator is loaded beyond a certain point, it breaks down, in that the terminal voltage collapses. In a series generator, the load current flows through the field winding; this implies that the field flux, and hence the induced EMF increases with the load until the core begins to saturate magnetically. Thus, a load beyond a certain point would result in a collapse of the terminal voltage of the series generator, too. Compound generators have the combined characteristics of shunt and series generators. In a differential com-pound generator, the shunt and series fields are in opposition; hence, the terminal voltage drops very rapidly with the load. On the other hand, cumulative compound generators have shunt and series fields aiding each other. The two field MMFs may be adjusted so that the terminal voltage on the full load is less than the no-load voltage, as in an under-compound generator; or the full-load voltage may be equal to the no-load voltage, as in a flat-compound generator. Finally, the terminal voltage on the full load may be greater than the no-load voltage, as in an over-compound generator.

 

Tests of Generator

MatLab Simulink software is used for modeling any linear or nonlinear systems. Model of the tested circuit is developed graphically by choosing and connecting its separate parts. That system is also called S-model. Elaborated model is saved as file with extension *.mdl. Development of S-models is based on Drag-and-Drop technology, i.e. each one part of the model is taken from special library and dragged by mouse from that to work window.

 

 

Fig. 7.1. Model of separately excited generator

Model for simulation of separate excited generator is opened in this way:

·          launch software MatLab;

·          open file separ_exc_generator.mdl;

·          the model nepr_ zad_ gen appears in the opened window

The model will be used to simulate characteristics of separate excited direct current characteristics. Elements, entering a circuit as well as their purpose are not considered here. The elements, parameters of which will be changed, are considered separately.

Model of the separately excited generator is given in Fig. 7.1.

Resistance of field winding, armature and load are substituted blocks Gain. Block Gain can be opened by double click of the left mouse button on it. Dialog window shown below opens and the required value is entered in the frame Parameters.

For example, if we need to change load resistance, we do double click on the element, modeling load resistance, then the window Block parameters opens. In the window Parameters frame Gain we enter the required resistance value and press button OK.

Note, that model, elaborated for investigation of separately excited generator characteristics, is based on differential equations, rewritten in frequency domain. Elaboration of model is not considered.

When all parameters are entered, simulation is started. It can be started in some ways: in the Menu window choose command Start, or by pressing combination of buttons Ctrl + Tor pressing in the Toolbar corresponding icon, which looks like a black triangle.

 

7.4. Method of testing

1.       External characteristic .

2.       Enter the given generator parameters to model blocks.

3.       Enter coefficient 1e 6 to Load block RL. That corresponds to open circuit operation of the generator.

4.       Calculate gain of Voltage regulator, providing the given current. Voltage regulator gain is calculated in this way:

 

 

5.       Enter the calculated gain to Voltage regulation block. This gain should be not changed during the experiment. Note: Voltage regulation block gain cannot be greater than unity.

6.       External characteristic is obtained by gradually loading the generator. For this resistance of load rheostat modeled by Load resistance RL block is changed. Enter values from Table 4, simulate and put values of the given armature cur-rent and voltage.

7.       Calculate Voltage regulator gain to obtain other field cur-rent value. Fulfil actions, described in items 4 and 5. Put the results in Table 7.1.

8.       According to the obtained results plot external characteristics  at different field current values in the same coordinate system.

 

Table 7.1.

External characteristics at 

1000

500

250

150

100

75

50

40

30

 

 

 

 

 

 

 

 

 

 

V, V

 

 

 

 

 

 

 

 

 

 

 

Table 7.2.

External characteristics at 

1000

500

250

150

100

75

50

40

30

 

 

 

 

 

 

 

 

 

 

V, V

 

 

 

 

 

 

 

 

 

 

 

1.5.         Content of report

1.       Objective, task and model.

2.       Work results (tables of simulation results and curves):

·      open circuit characteristic;

·      short circuit characteristic;

·      external characteristics.

3.       Conclusions.

 

1.6.         Control questions

1.       Explain, how direct current voltage in separately excited gen-erator is produced.

2.       Write the main equations, characterizing performance of the generator.

3.       Why the characteristic of short circuit is straight line and the characteristic of open circuit is non-linear?

4.       Explain influence of armature reaction upon operation of direct current generator.

5.       Why does the voltage of generator output change with load?

6.       Explain the purpose and operation of additional poles.

7.       What windings does the generator include and what is their graphical and alphabetical notation?