Operating Characteristics of DC Generators

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The operating characteristics represent the graphical relationships between different basic quantities related to operation of generator. These quantities include the generator terminal voltage V_t, generated emf E_g, armature current I_a, field current I_f, line current I_L and speed N.

There three important characteristics of DC generator as given under.

  1. Magnetization

Resistance commutation DC Machines

Resistance commutation DC Macnines (DC motors or DC generators)

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Resistance commutation in Direct Current machines is a method to improve commutation (both in DC motors and DC generators). It reduces the occurrence of sparking during the process of commutation. In this method the low resistance copper brushes are replaced by high resistance

Compensating Winding in DC Machines

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Function of Compensating winding

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The compensating winding are used in DC machines to neutralize the cross magnetizing effect of armature reaction.

These are generally used only large machines which are subjected to large fluctuations due to

DC Series Generator

A DC series generator is self-excited DC machine in which the field winding is placed in series with armature conductors and hence the current in the field winding will be equal to the load current of the generator. As the series field winding carries the full load current they are made of relatively less number of turns and thick wires are used.

The terminal voltage for the DC series generator has rising characteristics and it increases with increase in load current. But at high loads the voltage

DC Shunt Generator

A DC Shunt generator uses self-excitation for the production of necessary magnetic field. In such self-excited the machines they use the current generated in the armature itself to excite the field coils. They have no need of external DC sources to power the field coils.

In shunt generator the field winding is placed across

Separately Excited DC Generator

A generator requires magnetic field and moving conductors to produce e.m.f. The magnetic field can be produced by using electromagnets (field coils). The electromagnets produce magnetic field when an electrical supply given to them. The type of a generator depends on how these field coils are energised. 

In a Separately excited DC generator the field windings are energised by applying an

Dummy Coils

The dummy coils (also called as Idle Coils) are used in some of the electrical machines wounded with wave winding, to have mechanical balance.

When a machine is wound with simplex wave winding the following equation must be satisfied.

Coil Pitch or Coil Span

Coil Pitch also called coil span is a measure of distance (in terms of armature slots) between the two either sides of a coil.

Full Pitched Winding

If the coil pitch is equal to the pole pitch then such a type of winding is said to be full pitched winding. For such a winding, the coil pitch is 180 electrical degrees and the either sides

Pole Pitch

In an electrical machine, pole pitch is the distance between two adjacent poles. It can also be defined as the periphery of the armature divided by the number of poles in the machine.

Poles pitch gives the number of armature conductors per pole.

For example, if in an electrical machine there are 4 poles and 32 conductors then the poles pitch is 8. Since, \[\frac{armature conductors}{number of poles}= frac{32}{4}=8\]

EMF Equation of a DC Generator

Generator EMF Equation

The generator works on the principle of Faraday's laws of electromagnetic induction. When a conductor cuts the magnetic lines of flux e.m.f. (electro motive force) is induced in it. This emf

Types of DC Generators

Classification of DC Generators

DC Generators are classified based on the method in which the field winding is excited. The following provides the classification of the various types of DC generators.

Yoke in an electrical machine

The outer frame of an electrical machine either motor or a generator is called yoke.

The yoke provide two functions. First is providing the mechanical support and protection cover to the machine assembly. The second is it provides low reluctance path to carry the magnetic flux. The below figure show the diagram of an electrical machine with the location of Yoke, poles and rotor.

Electrical Machine diagram showing Yoke and the Poles

Yoke is generally made of cast steel or rolled steel for large electrical machines whereas for small machines cast iron may be used.

Electrical Generator

An electrical generator is an electro-mechanical energy conversion device that converts mechanical energy into electrical energy. The machine with converts electrical to mechanical energy is called motor. It works on the principle of faraday's law of Electromagnetic Induction “whenever a conductor cuts magnetic lines