Electrical Question 1: What are the different types of DC Generators ?
Generators are characterized by the method in which the flux is
produced in the magnetizing circuit ( Field Winding ) of the DC
generator. The flux required for the DC Generator is produced by
- A Permanent magnet
- Field Coils excited by the external source
- Field Coils excited by the same source.
Based on the method of excitation flux produced DC Generators are Characterized by
Separately Excited DC Generator:
Separately excited DC Generator the field winding excitation current
(which produces the flux) of the DC Generator is controlled by the
External source such as battery or current from a small DC machine.
Series Wound Generator:
Series Wound Generator the field winding of the DC Generator is
connected in series of the armature winding of the DC machine. The
current which flows out from the DC Generator through the armature
winding the same current flow through the series field winding of the DC
Generator. Series field winding consists of thick coils of lesser
Shunt Wound Generator:
In Shunt wound Generator
the field winding of the DC Generator is connected in parallel to the
armature winding of the DC Generator. In Shunt type of construction the
current produced at the armature is divided between the load and field
winding. Some part of the current flow through the field winding and
remaining through the load. Shunt field winding consists of high
resistance winding with thin coils of more turns
Compound Wound Generator:
wound generators contain both series and shunt type of field windings.
They are connected as long shunt and short shunt. In long shunt the
shunt field winding is connected parallel to the series winding and
armature winding. In Short shunt type of arrangement the shunt field
winding will be placed just parallel to the armature winding alone. This
type of machines has the both the characteristics of series wound
machine and shunt wound machine.
Electrical Question 2: What are the Characteristics of DC Generator?
Magnetic or Open circuit characteristics:
is also called no load characteristics. It gives the relation between
the generated emf in the armature on load and field current at constant
Internal or total Characteristics:
This gives the relation between the emf generated in the armature and the armature current
This curve gives the relation between the terminal voltage and armature current under constant speed and excitation.
Electrical Question 3: Why Saturation Curve for the DC Generator does not start with zero ?
curve or Magnetization curve in a DC Generator gives the relation
between the Field Ampere turns (X axis) required to produce the flux per
pole ( Y axis ). This curve starts from a point which is slightly
higher than the origin representing that there is some flux produced by
the field poles even no current is passed through the field windings.
This flux produced is because of the property called residual magnetism
which always exists in the DC Generator with out which DC Generator
cannot be started.
Electrical Questions 4: What is Open Circuit Characteristics of DC Generator ?
or Open Circuit Characteristics of a DC Generator is the plot between
the Field Current in Amps ( X axis) required to produce or build up emf
in the generator terminals ( Y axis) of a DC Generator.
EMF generated E=KΦN.
Where K is the constant
Φ is the flux and
N is the speed of the DC Generator
constant speed the emf generated of the dc generator proportional to
the flux Φ.Which is produced with the amount of field current.
it is a curve drawn between the generating emf produced under no load
and field current when the machine is running at constant speed. It is
the curve drawn under no load,so it is called open circuit
characteristics of the machine.
Electrical Questions 5: Given the OCC curve and what happen when the field resistance of the generator increases ?
the field resistance of the DC generator increases then the slope of
the OCC characteristics of the dc generator increases, i.e, the line
will move to the left towards the vertical position. This can be
explained with the help of Ohms law.
Electrical Questions 6: What is Critical Field Resistance ?
Resistance of a DC Generator is defined as the maximum field resistance
required to start the dc generator. Beyond this resistance the
generator will not able to build up the voltage (EMF) and the motor
fails to start. So care should be taken that field resistance of the dc
generators should be less than the critical resistance.
Electrical Questions 7: What are the causes of failure to build up voltage in DC Generator ?
In a DC Generator voltage will be build up in a step wise manner.
Some of the reasons for the generator failure to build up the voltage are
- No Residual magnetism
- Reversal of Field connections ( destroys the residual magnetism when connections reversed)
- Resistance of the field winding is more than the Critical Resistance
Electrical Question 8: Explain the applications of the DC Generators ?
excited Generators are used in Ward Leonard Systems of speed control
because self excitation at lower voltage would be unstable.
Wound generators are used in regenerative braking operation of the
electrical dc locomotives. They are used in series lighting.
wound generators with field regulators are used in the light and power
supply requirements.They have the property of providing constant voltage
at any load.
- Cummilative compound generators are used in lighting and power supply requirements
- Differential compound generators are used in applications where generators are required related to the arc welding