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Thursday, December 24, 2015

Types of DC Generator and Their Characteristics


Types of DC Motor

D.C. generators are classified according to the method of their field excitation. These groupings are:
  1. Permanent magnet dc generators, where a permanent magnet is used to establish flux in the magnetic circuit.
  2. Separately-excited generators, where the field winding is connected to a source of supply other than the armature of its own machine.
  3. Self-excited generators, where the field winding receives its supply from the armature of its own machine, and which are sub-divided into
    1. Shunt
    2. Series, and
    3. Compound wound generators.
When the field winding of a d.c. machine is connected in parallel with the armature, as shown in Fig, the machine is said to be shunt wound. If the field winding is connected in series with the armature, then the machine is said to be series wound. A compound wound machine has a combination of series and shunt winding.

Characteristics

(a) Separately-excited generator


The two principal generator characteristics are the generated voltage/field current characteristics, called the open-circuit characteristic and the terminal voltage/load current characteristic, called the load characteristic. Typical separately-excited generator characteristics are shown in Figure.

When a load is connected across the armature terminals, a load current Ia will flow. The terminal voltage V will fall from its open-circuit e.m.f. E due to a volt drop caused by current flowing through the armature resistance, shown as Ra.
Terminal voltage,
E − IaRa

(b) Shunt wound generator

In a shunt wound generator the field winding is connected in parallel with the armature as shown in Figure. The field winding has a relatively high resistance and therefore the current carried is only a fraction of the armature current.
Terminal voltage, V = E - IaRa
Ia = If + I from Kirchhoff’s current law, where,
Ia = armature current,
If = field current = V/Rf
and I = load current

As the load current on a generator having constant field current and running at constant speed increases, the value of armature current increases, hence the armature volt drop, IaRa increases. The generated voltage E is larger than the terminal voltage V and the voltage equation for the armature circuit is V = E - IaRa. Since E is constant, V decreases with increasing load.


(c) Series-wound generator

In the series-wound generator the field winding is connected in series with the armature

Thus E is proportional to flux. For values of current below magnetic saturation, the flux is proportional to the current, hence E α I. For values of current above those required for magnetic saturation, the generated e.m.f. is approximately constant. The values of field resistance and armature resistance in a series wound machine are small; hence the terminal voltage V is very nearly equal to E.

In a series-wound generator, the field winding is in series with the armature and it is not possible to have a value of field current when the terminals are open circuited, thus it is not possible to obtain an open-circuit characteristic.

(d) Compound-wound generator

In the compound-wound generator two methods of connection are used, both having a mixture of shunt and series winding, designed to combine the advantages of each. Fig.(a) shows what is termed a long-shunt compound generator, and Fig.(b) shows a short-shunt compound generator. The latter is the most generally used form of D.C. generator.
 
In cumulative-compound machines the magnetic flux produced by the series and shunt fields are additive. Included in this group are over-compounded, level-compounded and under-compounded machines – the degree of compounding obtained depending on the number of turns of wire on the series winding.



Review Questions

1. With the increases in field excitation of a DC generator, its generated emf ________________
  1. decreases.
  2. increases.
  3. remains constant.
  4. increases up to a limit and then remains almost constant. 

2. Which of following DC generator will be in a position to build up without any residual magnetism in the field?
  1.  Compound. 
  2. Shunt.
  3. Series.
  4. None of them

3. Which of the following DC generators has rising V-I characteristics?
  1. Compound. 
  2. Shunt.
  3. Series.
  4. None of them.

4. The ___________________ generator has the poorest voltage regulation.
  1. under-compounded.
  2. differential compounded. 
  3. shunt. 
  4. over compounded.

5. The voltmeter connected across a generator reads voltage same at no load and at full load (rated). The generator is of the type
  1. level compound.
  2. series generator.
  3. short - shunt generator.
  4. shunt generator.

     

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