LABORATORY WORK № 5

TEST OF SEPARATELY EXCITED DIRECT

CURRENT GENERATOR

 

5.1. 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.

 

5.2. Task

Obtain by simulation the main characteristics of the generator:

·          open circuit characteristic;

·          short-circuit characteristic;

·          voltage regulation characteristic.

 

5.3. Direct current generators

An electric generator is a device that converts mechanical ener-gyto electrical energy. A generator forces electronsin the wind-ings to flow through the external electrical circuit. The source of mechanical energymay be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combus-tion engine, a wind turbine, a hand crank, compressed airor any type motor. The principle of DC generator is based on Michael Faraday’s law of induction.

In mathematical form, Faraday’s lawstates that:

 

                                                                 (5.1)

 

where e is the electromotive forceand F is the magnetic flux.

For the special case of the coil of wire, composed of N loops with the same area, the equation becomes:

 

                                                              (5.2)

 

Considering Faraday’s law, together with Ampère’s lawand Ohm’s law, Lenz’s lawstates: the EMF induced in an electric circuit always acts in such a direction that the current it drives around the cir-cuit opposes the change in magnetic flux whichproduces the EMF.

 

 

 

Fig. 5.1. An alementary generator (a) and output DC voltage at the brushes (b)

 

An elementary generator consists of magnets (or coils, produc-ing magnetic field) and a rotating coil (armature) which is connected to the external circuit through commutating unit and brushes.

 

5.4. Method of testing

1.       Generator characteristic at no-load . Enter values of the given generator parameters into model blocks.

2.       In order to get open circuit characteristics, the load resistance has to be set equal to infinity. For this the load block R_L should be opened and constant 1e 6 entered.

3.       Open circuit characterstic is obtained by changing field current.

4.       Field current is changed by changing field voltage supplying the field current winding and measuring the voltage.

5.       Change field current by varying Voltage regulator block coef-ficient from zero (0) (field voltage is equal to zero) to unity (field is equal to rated).

6.       At first the maximum excitation current is set (it corresponds to maximum value of voltage). Enter value «1» into the Voltage regulator block.

7.       Fulfil the simulation and put the values of excitation current and generator voltage in Table 5.1. Renew simulation by reducing voltage every 10 – 20 % (by changing Voltage regulator coef-ficients 0.8, 0.6, 0.4 etc.). In this way new values of generator field currents and voltages are found. Put the new results also in Table 5.2.

8.       Change polarity of the voltage. (Enter into the voltage reference block negative value of voltage.) Using approaching method, find point, where generator output voltage is equal to zero by adjusting the Voltage regulator.

9.       Increase excitation voltage every 10 – 20 % of the rated (by sub-stituting to Voltage regulator block coefficients 0.1, 0.2, 0.4, 0.6, 0.8 and 1) to get open circuit charactristic in the 3 quarter. Plot open circuit characteristic  .

 

Table 5.1.

Data of open circuit experiment

		0.8	0.6	0.4	0.2	0.1	0
	0.1744	0.1395	10.46	0.0697	0.3488	0.01744	2.5 e – 0.35
	99.88	90.8	76.85	56.85	33.2	19.6	6

 

Table 5.2.

Data of open circuit experiment

		0.8	0.6	0.4	0.2	0.1	0

 

5.5. 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.

5.6. 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?