Types of GeneratorsACCORDING TO EXCITATION

Separately Excited DC Generator

• A dc generator whose field magnet winding is supplied from an independent

external d.c. source (e.g., a battery etc.)

Separately Excited DC Generator

• It may be noted that separately excited d.c.generators are rarely used in practice.

• The d.c. generators are normally of self excited type.

Self Excited DC Generator

• A d.c. generator whose field magnet winding is supplied current from the output of the generator itself

Types of Self Excited DC Generator

• Shunt Wound

• Series Wound

• Compound Wound

Presence of Residual Magnetism

• The field is in the form of an inductor, hence, it can store

energy as an electro-magnetic field.

• This field will not all disappear once the generator is

turned off, some will remain, as residual magnetism or

flux.

Presence of Residual Magnetism

Due to the residual flux in the field, it will enable to armature

to develop the residual voltage, which causes a small current

to flow through the field windings.

Presence of Residual Magnetism

As the generated voltage rises, the field current also rises,

which in turn causes more flux to be developed, and still a

larger voltage. This process will continue until the voltage

reach its proper value.

Presence of Residual Magnetism

Should the field loses its residual flux, the field is connected

to a separate DC source in order for it to produce small

amount of flux, this method is called flashing the field.

Shunt Wound DC Generator

The field windings are

connected across or in parallel

with the armature.

They have the full voltage of

the generator applied to them.

Shunt Wound DC Generator

The shunt field is usually

constructed of many turns

of fine wire.

The shunt field is a constant

flux field, therefore, this type

of generator is of the

constant voltage type.

Equivalent Circuit of Shunt Generator

Eg

- generated voltage

Vt

- terminal voltage

ia=ish+ iL

E g=V t+ ia Ra

V sh=V t=i sh Rsh

Pg=E g ia

Pd=V t iL

Where:

Vsh

- shunt field voltage

ish

- shunt field current

ia

- armature winding current

iL

- line current

Rsh

- shunt field resistance

Ra

- armature winding resistance

Pg

- power generated / developed

Pd

- power delivered

Series Wound DC Generator

The field windings are

connected in series with the

armature winding.

The current passing through

the series field depends upon

the load, thus, the flux

produced is variable.

Series Wound DC Generator

• They consist of relatively

few turns of thick wire or

strips.

Series Wound DC Generator

• Due to the variable flux, the

voltage generated by this

type of generators are also

variable.

• Such generators are rarely

used except for special

purposes, ie. Boosters.

Equivalent Circuit of Series Generator

ia=is=iL

E g=V t+ ia Ra+ is Rs

Pg=E g ia

Pd=V t iL

Eg

- generated voltage

Vt

- terminal voltage

Where:

is

- series field current

ia

- armature winding current

Rs

- series field resistance

Ra

- armature winding resistance

Pg

- power generated / developed

Pd

- power delivered

iL

- line current

Rs

Compound Wound DC Generator

• Combines both the series field and shunt field.

Can be connected as (a) short shunt or as (b) long shunt.

a) Short shunt b) long shunt

Types of Compound Wound Generator according to flux

I. Cumulative Compound Generator – the series field is aiding the shunt field to supply power and lighting loads.

1. Under Compounded – the full load terminal voltage is less than the no-load voltage. It is used when the load is located near from it.

Types of Compound Wound Generator according to flux

2. Flat Compounded – the full-load terminal voltage is the same as the no-load voltage. It is used when the load is at a medium distance from it.

3. Over Compounded – the full-load terminal voltage is greater than the no-load voltage. It is used when the load is far from it.

Types of Compound Wound Generator according to flux

II. Differential Compound Generator – the series field flux opposes the shunt field flux. It is used in electric welding.

Equivalent Circuit of Long Shunt Compound Generator

ia=ish+ iL

E g=V t+ ia(Ra+ Rs)

V sh=ish Rsh

Pg=E g ia

Pd=V t iL

i s=ia

Rs

Equivalent Circuit of Short Shunt Compound Generator

ia=ish+ iL

E g=V t+ ia Ra+ is Rs

V sh=ish Rsh

Pg=E g ia

Pd=V t iL

is=iLRs

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.

Brush Contact Drop

• 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.

• 2.0 V for carbon brushes.

Other Reminders

• The following may be included in the design of DC Machines:

➢Interpole/ Commutating pole winding

– used to correct the objectionable commutation effects of armature reaction. This winding is permanently connected in series with the armature.

Other Reminders

➢Compensating winding

– used for the purpose of neutralizing the effect of armature reaction in the zones outside the influence of the interpoles. This winding is also connected in series with the armature.

Other Reminders

➢Diverter

– a low resistance shunt connected directly across the series field of a compound generator for the purpose of adjusting the degree of compounding.

Problems

• An 8-pole d.c. shunt generator with 778 wave-connected armature conductors and running at 500 r.p.m. supplies a load of 12.5 Ω resistance at terminal voltage of 250 V. The armature resistance is 0.24 Ω and the field resistance is 250 Ω. Find the armature current, the induced e.m.f. and the flux per pole.

• Answer: 21 A, 255.04 V, 9.83 mWb

Problems

• A short-shunt compound generator delivers a load current of 30 A at 220 V, and has armature, series-field and shunt-field resistances of 0.05 Ω, 0.30 Ω and 200 Ω respectively. Calculate the induced e.m.f. and the armature current. Allow 1.0 V per brush for contact drop.

• Answer: 232.56 V, 31.145 A

Problems

• The following information is given for a 300-kW, 600-V, long-shunt compound generator : Shunt field resistance = 75 Ω, armature resistance including brush resistance = 0.03 Ω, commutating field winding resistance = 0.011 Ω, series field resistance = 0.012 Ω, divertor resistance = 0.036 Ω. When the machine is delivering full load, calculate the voltage and power generated by the armature.

• Answer: 625.4 V, 317.7 kW

Problems

• A separately excited d.c. generator, when running at 1200 r.p.m. supplies 200 A at 125 V to a circuit of constant resistance. What will be the voltage when the speed is dropped to 1000 r.p.m. and the field current is reduced to 80% ? Armature resistance, 0.04 Ω and total drop at brushes, 2 V.

• Answer: 90V

Voltage Regulation

• It is the percentage rise in the terminal voltage of the generator when the generator load is removed.

%𝑉𝑅 =𝑉𝑁𝐿−𝑉𝐹𝐿

𝑉𝐹𝐿× 100

Where

𝑉𝑁𝐿= no-load terminal voltage

𝑉𝐹𝐿=full-load terminal voltage

Problems

• The voltage of a 100-kw 250-volt shunt generator rises to 260 volts when the load is removed. What full-load current does the machine deliver, and what is its per cent regulation?

• Answer: 400 A, 4%

Problems

• A 25-kw 230-volt shunt generator has a regulation of 8.7 percent. What will be the terminal voltage of the generator at no load?

▪Answer: 250 V

Problems

• If the change in voltage is assumed to be uniform between no-load and full-load kilowatts, calculate the kilowatt output of the generator when the terminal voltages are 240 and 235 volts.

• Answer: 12.5kW, 18.75kW