Types and Characteristics of a DC generators

TYPES AND CHARACTERISTICS OF GENERATOR

The generators are classified into two types based on the excitation method. The below figure shows types of generators.

Generally characteristics mean behavior of the particular device with respect to the parameters. For generators the characteristics are classified into two types shown below:

1. Magnetizing characteristics

2. Load characteristics

MAGNETIZING CHARACTERISTICS:

The characteristics which are obtained from the O.C.C curve are named as Magnetizing characteristics. These characteristics are arrived from emf equation of a generator by increasing or decreasing the flux value.

Eo = φZNP/60

Eo α φ

Hence magnetizing characteristics are purely dependent on flux produced by the winding when DC supply is given.

*As If increases, φ increases then Eo increases, at certain point φ becomes constant where Eo is said to be saturates.

2.LOAD CHARACTERISTICS:

The load characteristics of DC generators drawn with terminal voltage Vt and load current IL

Due to armature reaction the no load rated voltage Eo falls to E (known as Internal characteristics) and further falls to terminal voltage Vt (known as External characteristics).

SEPARATELY EXCITED DC GENERATOR:

The generator which fails to self excitation is excited by using prime mover such generators are named as separately excited generators. The circuit diagram of this generator are shown below:

In these separately excited DC generators, EMF completely depends on flux φ. The field regulator is necessary to control If.

Armature current Ia = IL

Terminal voltage V = Eg – IaRa + Brush contact drop

Eg = V + IaRa

But Eg = φZNP/60A

Power delivered to the load P = V*I

= (Eg – IaRa)*Ia

= EgIa – Ia^2 Ra

SHUNT GENERATOR:

* In shunt generator the winding is in parallel to the armature. The induced EMF is same but field current If is different.

* The load characteristics of DC generator is given below:

E = Vt + IaRa

As per EMF equation E = φZNP/60

E α φ α Ia

If the load increases IaRa drop increases by increasing Ia

IaRa = 1/Vt

* If the voltage drop increases the terminal voltage decreases due to very small armature resistance the Vt as load current changes from no-load to full load is also very small. Hence DC shunt generator is called constant generator.

Armature current Ia = IL + Ish

Ish = V/Rsh

The general emf

Eg = V + IaRa + Brush contact drop

Eg = V + ( IL + Ish)Ra + Brush contact drop

Terminal voltage V = Eg – IaRa + brush contact drop

SERIES GENERATOR:

In series generator the winding is in series with the armature. The induced EMF is differ but If is same near winding. The load is connected in parallel to the series generator.

Armature current Ia = Ise = IL

Where Ia = Armature current

Ise = Series field current

IL = Load current

Eg = V + Ia(Ra + Rse) + Brush contact drop

The equation for DC series generator is

E = Vt + Ia(Ra +Rse)

In this generator the current Ia = Ise = IL (observe circuit diagrams)

Ia α Ise

Ise α φ

Φ α E (From EMF equation)

From the above three relations all are directly proportional to each other i.e. if one of the parameter increases the other parameters are also increases. So the internal characteristics increase in nature.

From equations E = Vt + Ia(Ra + Rse)

Vt = E – Ia(Ra + Rse)

The external characteristics are also increasing in nature as E increases but due to the presence of IaRa drop the characteristics are take position below the internal characteristics.

COMPOUND GENERATORS:

The characteristics of compound generator depends on the type of generator i.e. cumulative or Differential compound generator.