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Electrical Interview Questions With Anwers:DC Machines 3

Electrical Question 1: What are the different types of DC Generators ?

Answer:
DC 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
  1. A Permanent magnet
  2. Field Coils excited by the external source
  3. Field Coils excited by the same source.
Based on the method of excitation flux produced DC Generators are Characterized by

Separately Excited DC Generator:

In 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:

In 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 turns.

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:

Compound 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?

Answer:

Magnetic or Open circuit characteristics:

This is also called no load characteristics. It gives the relation between the generated emf in the armature on load and field current at constant speed.

Internal or total Characteristics:

This gives the relation between the emf generated in the armature and the armature current

External Characteristics:

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 ?

Answer:
Saturation 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 ?

Answer:
Magnetic 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
Under constant speed the emf generated of the dc generator proportional to the flux Φ.Which is produced with the amount of field current.
Therefore 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 ?

When 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 ?

Answer:
Critical 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 ?

Answer:
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
  1. No Residual magnetism
  2. Reversal of Field connections ( destroys the residual magnetism when connections reversed)
  3. Resistance of the field winding is more than the Critical Resistance

Electrical Question 8: Explain the applications of the DC Generators ?

Answer:
  1. Separately excited Generators are used in Ward Leonard Systems of speed control because self excitation at lower voltage would be unstable.
  2. Series Wound generators are used in regenerative braking operation of the electrical dc locomotives. They are used in series lighting.
  3. Shunt 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.
  4. Cummilative compound generators are used in lighting and power supply requirements
  5. Differential compound generators are used in applications where generators are required related to the arc welding
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