LABORATORY WORK № 6

PERFORMANCE OF A DC GENERATOR

 

1.1.         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.

 

1.2.         Task

Obtain by simulation the main characteristics of the generator:

1.       open circuit characteristic;

2.       short-circuit characteristic;

3.       voltage regulation characteristic.

 

1.3.         Performance of a DC generator

A single conductor, shaped in the form of a loop, is positioned between the magnetic poles. As the loop does not move, the magnetic field has no effect. If we rotate the loop, the loop cuts through the magnetic field, and an EMF (voltage) is induced into the loop. When we have relative motion between a magnetic field and a conductor in that magnetic field, and the direction of rotation is such that the conductor cuts the lines of flux, an EMF is induced into the conductor. The magnitude of the induced EMF depends on the field strength and the rate at which the flux lines are cut. The stronger the field or the more flux lines cut for a given period of time, the larger the induced EMF in the armature:

 

                                                        (6.1)

 

where E is generated voltage; k is fixed constant, depending on a number of active conductors on armature, a number of parallel paths in armature winding and a number of field poles; F is flux per pole, n is rotational speed in rpm.

The direction of the induced current flow can be determined using the «left-hand rule» for generators. This rule states that if you point the index finger of your left hand in the direction of the magnetic field (from North to South) and point the thumb in the direction of motion of the conductor, the middle finger will point in the direction of the current flow.

In a generator, the induced EMF is the internal voltage avail-able from the generator; when the generator supplies a load the armature carries a current and develops a torque. This torque opposes a prime-mover (such as a diesel engine, other type motor) torque.

 

1.4.         Method of testing

1.       Short circuit characteristic .

2.       Enter generator parametrs to model blocks.

3.       Enter coefficient «0» to Voltage regulation block. That corresponds to field current equal to zero.

4.       Enter coefficient «0» to load block R_L. That corresponds to short circuit of the generator.

5.       After that simulate the model and put values of the obtained field and armature current.

6.       Increase field voltage every 5 % and renew simulation. Put the new field and armature current values to Table 6.1.

7.       Plot short circuit characteristic .

Table 6.1.

Short circuit experiment data

	0	0.05	0.1	0.15	0.2	0.25

 

1.5.         Content of report

1.       Objective, task and model.

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

Ø open circuit characteristic;

Ø short circuit characteristic;

Ø 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?