Types of DC Generators

TYPES OF DC GENERATORS

DC generators are usually classified according to their methods of field excitation.

The types of DC generators are

i) Separately excited generators.

ii) Self excited generators.

Separately Excited DC Generators

If the field winding is excited by a separate DC supply, then the generator is called separately excited DC generator. Fig.3.12 shows the diagram of a such generator.

From the fig 3.12

Armature current Ia = Load current IL

Ra = Armature winding resistance.

Terminal voltage V = E - IaRa - Vbrush

Vbrush = Voltage drop at the contacts of the brush.

Vbrush is neglected because of very low value.

Generated emf Eg = V + IaRa + Vbrush

Power developed = EgIa

Power delivered to load = VIa

Self Excited DC Generators

If in a dc generator field winding is energized by the current produced by the generator themselves is called a self exited dc generator. Due to residual magnetism there is some flux always present in the poles. When the armature is rotated, a small emfis produced in the armature winding because of the residual flux. This emf produces a field current in the field winding. The self excited generators can be classified depending upon how the field winding is connected to the armature. There are three types.

i) Series generator

ii) Shunt generator

iii) Compound generator

i) Series generator

Figure 3.13 shows the connection diagram of dc series generator.

The field winding is connected in series with the armature. This type of DC generator is called DC series generator. The field winding has less number of turns of thick wire or strips. It has low resistance and is denoted by Rse.

Here armature, field and load all in series. So they carry the same current.

Ia= Ise = IL

Generated emf

Eg = V + IaRa + IaRse + Vbrush

Where V = Terminal voltage o

IaRa = Voltage drop in the armature resistance

IaRse = Voltage drop in the series field winding resistance.

Vbrush = brush drop.

Terminal voltage V = Eg - IaRa - IaRse - Vbrush

Power developed in armature = EgIa

Power delivered to load = VIa

ii) Shunt Generator

Fig.3.14 shows the connection diagram of a DC generator.

In a dc shunt generator field winding is connected across the armature. The load is also connected across the armature. The shunt field winding has more number of turns of thin wire. It has high resistance.

Terminal voltage V = Eg - IaRa

Shunt field current Ish = V/ Rsh

Armature current Ia = IL +Ish

Power developed by armature = EgIa

oto Power delivered to load = VIL.

iii) Compound Generator

The compound generator has both shunt field and series field windings. Depending upon the shunt field and series field connections compound generator can be classified as

(i) Long shunt compound generator

(ii) Short shunt compound generator.

(i) Long Shunt Compound Generator

Fig 3.15 shows the connection diagram of a long shunt compound generator. Here shunt field winding is connected across both series field and armature windings

From the fig.3.15

Series field current Ise = Ia = IL + Ish

Shunt field current Ish = V/Rsh

Generated emf Eg = V +Ia (Ra+ Rse) + Vbruch

Terminal voltage V = E - I (Ra + Rse) - Vbruch

Power developed in armature = EgIa

Power delivered to load = VIL

 (ii) Short Shunt Compound Generator

Fig.3.16 shows the connection diagram of short shunt compound generator. Here shunt field winding is connected in parallel with the armature and this combination is connected in series with the series field winding.

Series field current = Ise = IL

Load current = IL

Armature current Ia = Ish + Ise

Generated emf Eg =V + IaRa + IseRse + Vbrush

Voltage across shunt field winding = Ish Rsh

Ish Rsh = Eg - IaRa - Vbrush

= V + IaRa + IseRse + Vbrush - IaRa - Vbrush

Ish Rsh = V + IseRse

Shunt field current

Ish = V + Ise Rse / Rsh

Terminal voltage V = Eg - IaRa - IseRse - Vbrush

Power developed in armature = EgIa

Power delivered to load = VIL

Example: 3

A shunt generator delivers 450A at 230V and the resistance of the shunt field and armature are 50Ω and 0.032Ω respectively calculate the generated emf.

Given data

Load current IL = 450 A

Terminal voltage (V) = 230 V

Shunt field resistance (Rsh) = 50 Ω

Armature resistance (Ra) = 0.03 Ω

Solution:

Shunt field current Ish = V/Rsh

230/50 = 4.6 A

Armature current Ia = IL + Ish

= 450 +4.6

= 454.6 A

Armature drop = IaRa = 454.6 × 0.03 = 13.6 V

Generator emf Eg = V + IaRa

= 230 + 13.6

= 243.6 V

Example: 4

A separately excited generator has induced emf of 250 V and a full load terminal voltage of 240 V. If the value of R= 0.01Ω. Find the full load current and output of the generator. Neglect armature reaction and brush drop.

Given data:

Ra = 0.01Ω

Generated emf= Eg =250 V

Terminal voltage V = 240 V

Solution:

Eg = V + IaRa

IaRa = Eg - V

Ia = Eg - V / Ra

= 250 - 240 / 0.01

Ia = 100 A

Ia = IL = 100 A

Output power = V IL

= 240 × 100

= 24000 W

= 24 KW

Example: 5

A long shunt compound generator delivers a load current of 50 A at 500 V and has armature, series field and shunt field resistances of 0.05Ω 0.03Ω and 250Ω respectively. Calculate the generated voltage and the armature current. Allow Ivolt per brush for contact drop.

Given data:

L = 50 A ; V = 500 V

Rsh = 250Ω ; Ra = 0.05Ω

Rse = 0.03Ω ; Vdrop = 1V/brush

Solution:

Shunt field current = Ish = V/Rsh =500/250 = 2A

Ish = 2A

Armature current Ia = IL + Ish = 50 + 2 = 52A

Voltage drop on series field IseRse = IaRse = 52 × 0.03 = 1.56 V

Armature voltage drop Ia Ra = 52 × 0.05 = 2.6 V

Drop at brushes = 2 × 1 = 2V

Generated EMF = V + IaRa + IseRse + Vbrush

= 500 + 2.6 + 1.56 + 2

= 506.16 V

Example: 6

A short shunt compound generator delivers a load current of 30A at 220V and has armature, series field and shunt field resistance of 0.05Ω, 0.3Ω and 200Ω respectively. Calculate the induced emf and the armature current. Allow 1V per brush for contact drop.

Given data:

V = 220V ; Rse = 0.05Ω ; Rsh = 200Ω ;

Rse = 0.3Ω ;  IL = 30A

Solution:

IL = Ise = 30A

Armature current Ia = Ish + Ise = 200 +30 = 230A

Generated emf Eg = V + IaRa + IseRse + Vbrush

=220 + 230 × 0.05 + 30 × 0.3 + 1 × 2 = 242.5 V