Lab 3: Separately Excited DC Generator

(Open Circuit characteristics)

OBJECTIVE:

To determine open circuit characteristics of a Separate Excited DC Generator.

PRELAB:

1. What are the different types of DC machines?

Separately-excited generators are those whose field magnets are energized from an independent external source of DC current, i.e. another dc generator, a controlled rectifier, a diode rectifier, or a battery.

A large savings in the initial cost and complexity of a DC generator can be realized if the field circuit is connected across the armature terminals instead of to a separate source.

DC series generator

The magnetization curve of a series DC generator looks very much like the magnetization curve of any other generator.

At no load, however, there is no field current, so (VL) is reduced to a small level given by the residual flux in the machine. As the load increases, the field current rises, so (E) rises rapidly. The Ia.(Ra+Rf) drop goes up too, but at first the increase in (E) goes up more rapidly than Ia.(Ra+Rf) drop rises, so (VL) increase. After a while, the machine approaches saturation, and (E) becomes almost constant. At that point, the resistive drop is predominant effect, and VL starts to fall.

This type of characteristic is shown in fig. It is obvious that this machine would make a bad constant-voltage source. In fact, its voltage regulation is a large negative number.

Short Shunt type (DC compound generators)

Long Shunt type (DC compound generators)

Separately excited DC motors: The field winding is excited from a supply which is not connected to the armature winding. It may be noted that current flowing through the field winding is independent of load and is equal to V / Rf where Rf is the field circuit resistance. The flux produced is proportional to the field current i.e. If

Self-excited DC motors (shunt): In this type of excitation, armature and field windings are connected across a constant source of supply. The field current If is drawn from the same source as that of armature current.

Self-excited DC motors (series): The field winding is connected in series with the armature so that If= Ia= IL. Therefore field winding is made up of thick winding wire of less no. of turns as compared to that of shunt field winding so that armature current can flow through it without overheating.

Self-excited DC motors (Compound): There are two field windings, namely a shunt field winding and a series field winding. The shunt field winding is connected in parallel with the armature and series field winding is connected in series with the combination.

Short Shunt type (DC compound motors)

Long Shunt type (DC compound motors)

2. What do critical resistance and critical speed mean?

Critical resistance. Normally, the shunt generator builds up to a voltage determined by the intersection of the field resistance line and the saturation curve. If the field resistance is greater than critical resistance, the generator fails to build up and the voltage remains at the residual level. To solve this problem, the field resistance is reduced to a value less than critical resistance.

The speed for which the given field resistance acts as critical resistance is called the critical speed, denoted as NC.

3. What is meant by airline?

The two things that mostly give magnetic resistance are magnetic saturation of the soft iron material that the poles/rotor body are made of and the air gaps between the poles and rotor. The flux in the air gaps and so also through the rotor are very strongly determined by the air gap - the ratio is an inverse one -more gap gives less flux.

4. Why OCC starts from a voltage a little higher than zero? Under what conditions this may not be true?

Residual magnetism is left in the steel structure from previous use of the generator. Although this flux is responsible for only a small induced voltage, it fulfils an extremely important role in self-excited generators by providing the seed flux which starts the process of build-up to normal flux levels.

5. Why OCC is also known as Magnetization Characteristics?

OCC is also known as Magnetization characteristics because with respect to the curve which shows the relationships between the generated EMF at no load (Eo) and the field current (If) at a constant speed. The curve will be considered linear if the values for Eo and If are low, once these values increase the curve will no longer be considered a linear but will change due to the magnetic saturation.

6. Why the external characteristics droop?

i) The armature reaction weakens the main flux so that the actual EMF generated (E) on the load is less than that of the generated (Eo) on no load.

ii) Across the armature resistance there is no voltage drop. (Il*Ra = Ia*Ra)

7. Derive the EMF equation for a DC generator?

8. If the terminal voltage of a DC separately excited generator is 230 V at 1500 rpm when the terminals are open, what will be the terminal voltage at 750 rpm? Comment on the same with a considerable load current assumed.

9. Give a rough sketch of OCC and load characteristics.

10. A DC separately excited generator fails to excite. What may be the reason?

A shunt generator may not develop its rated operating voltage due to a loss of residual magnetism.

Critical field resistance: shunt generator may fail to reach its operating voltage even though its residual magnetic field is satisfactory. This failure may be due to excessive resistance in the field circuit. Any generator has critical field resistance. The presence of resistance in the field circuit in excess of this critical value causes the generator to fail to build up to its rated operating voltage. Since field rheostats are used to control the voltage output at rated speed, it's important to reduce the resistance of the field rheostats to a minimum value before investigating other possible faults in the event of failure to develop rated voltage.

Brush contact resistance: Contact resistance at the brushes is another reason for the failure of the generator to develop its operating voltage.

Discussion:

Conclusion:

The results and specifications have been met connecting each type of generator that were a separately excited DC generator with no load characteristics and a separately excited DC generator with a variable load characteristics. The data have been plotted and verified successful in both cases.