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