DC & AC

(4A)
Types of DC Generators

Field Excitation When a DC voltage (excitation voltage) is applied to the field windings of a dc generator, current (exciting current) flows through the windings and sets up a steady magnetic field.

This DC excitation voltage can be supplied by an external source or produced by the generator itself.

A generator that supplies its own field excitation is called a Self-Excited generator and a generator that relies to an outside source is called a Separately Excited Generator.

The magnitude of the exciting current (field current) is directly proportional to the amount of flux produced.


I. Separately Excited DC Generator
➢ A DC generator whose field magnet winding is supplied from an independent external DC source (e.g., battery, dc-power supply, etc.)

II. Self Excited DC Generator
➢ A DC generator whose field magnet winding is supplied from the output of the generator itself.
➢ Categorized based on how the field winding/s is/are connected.
a) Series Wound – field winding (series field) is connected in series to the armature
b) Shunt Wound – field winding (shunt field) is connected in parallel to the armature
c) Compound – has both series and shunt field windings
i. Long Shunt Compound – series field is connected in series to the armature
ii. Short Shunt Compound – series field is connected in series to the load

There are two types of field windings used in DC Machines:

The Series Field winding (Rse) is made with a few turns of large diameter wire. It has a low resistance and is designed to be connected in series with the armature.

The Shunt Field (Rf ) winding is made with many turns of small diameter wire. It has a high resistance and is designed to be connected in parallel with the armature. Since the shunt field is connected in parallel with the armature, the line voltage or the load is connected across it. The current through the shunt field is, therefore, limited by its resistance.

Presence of Residual Magnetism:

The field of a DC Generator is an electromagnet which is in a form of an inductor. Hence it can store energy as an electro-magnetic field.

This field will not all disappear completely even when generator is turned-off, some magnetic energy will remain, as residual magnetism or residual flux.

Due to the residual flux in the field, it will enable the armature to develop the residual voltage, which causes a small current to flow through the field windings.

As the generated voltage rises, the field current also rises, which in turn causes more flux to be developed, and further increase the generated voltage.

This process will continue until the generator reaches its rated value.

Should the field lose its residual flux, it is to be connected to a separate DC source for it to produce and retain small amount of flux. This method is called flashing the field

Saturation Curve of DC Generators:

The field current is directly proportional to the flux created and the flux created is directly proportional to the generated voltage.

But there will come a time that even with the increase in field current, there will be minimal or none at all increase to the generated voltage.

This this due to the saturation of the core of the field windings that even when the excitation current is increased, the core cannot anymore produce more flux.

Brush Contact Drop:

It is the voltage drop over the brush contact resistance when current passes from commutator segments to brushes and finally to the external load.

Its value depends on the amount of current and the value of contact resistance. This drop is usually small and includes brushes of both polarities.

However, in practice, the brush contact drop is assumed to have following constant values for all loads. 0.5-V: for metal-graphite brushes. (pair) 2.0-V: for carbon brushes. (pair)