Separately excited generators
Separately excited generator runs on DC power. The field magnet winding current which is required for the separately excited DC generator to run, is supplied separately by an external source of DC voltage. Hence, the name suggests the fact. The source of the power may be:
- Another DC generator
- Controlled rectifier
- Diode rectifier
As, the generators require a separate source of power to start, it is expensive than self excited DC generators. When, the self excited generators are unable to satisfy the need and efficiency of the output, separately excited DC generators are used to replace the former.
Self exciting Dc generators may sometimes fail to start itself due to low voltages. For instance, in Ward Leonard systems of controlling the speed, separately excited DC generators are used, due to availability of lower voltages.
Separately excited DC generators are quick in nature and also provide accurate response to alterations that occur in the field of current. So, these generators are usually used where the characteristics of quickness and responsive are demanding and important during work.
Shunt generators are DC generators. In this generator the field winding and the armature winding are connected in parallel. The armature winding supplies both the field and load current to the shunt generator. The generator may not use a permanent magnet so it has to start by a DC source. Shunt generator may be separately excited or self excited.
These generators act in conjunction with automatic regulators. The regulators are exciters. They help in supplying the current required by the generator to start. The work of the regulator is to control the voltage. It cuts in and controls the resistance of the shunt field rheostat.
The regulator helps in maintaining the workable voltage, which is demanded for the efficient working of the electrical appliances. This is an important application of shunt generator.
Shunt generators help in charging batteries. When a battery is discharged, the voltage drops.Hence, a battery can stand large changes in current when it is discharged, but not when it is fully charged.
Having similar graphical representation of voltage, both shunt generator and lead batteries have the same drooping characteristics. As such, shunt generators are well suited for battery charging services. Shunt generators even works when connected in parallel. It is similar to many components connected by parallel operating wiring systems.
Series generator is a DC generator. Unlike other generators, the field windings, armature windings and the load are all connected in series. Hence, the name shows it characteristics. A series generator carries a constant amount of current for a constant load. So, it is also known as constant current source.
With the increase in current flow the voltage also increases through the series generator circuit. After reaching a certain limit, the voltage starts falling, due to excess demagnetizing effect of armature winding. Hence series generator has limited applications.
- Series arc lighting.
- Series incandescent lighting.
- Series booster, that helps in increasing the voltage.
- Provider of field current for DC locomotives.
A compound generator is a type of DC generator. It is a combination of both Series DC generator and Shunt DC generator. So, it is named as a compound DC generator.
It contains two set of rods. One pole is winded by a thick wire few turns and connected in series. The other is winded by a thin wire with more turn compared to the former, and connected in parallel.
Usually it is connected in two ways,
- Long shunt compound generator.
- Short shunt compound generator.
Compound generator is widely used. It is applied in:
- It can automatically adjust, and can maintain an almost constant voltage.
- It helps in regulating the voltage and makes it constant for a broad variety of load range.
- It is easy to compensate the drop in resistance by adjusting the characteristics of the generator. Hence it helps in providing a constant voltage supply.
- It is used in arc welding. During welding, when the electrode touches the metal it gets short circuited. So, it is used to regulate the voltage and current flow.
Compound generators are used to supply powers to the following;
- Railway circuit.
- Motors of electrified rail road.
- Industrial motors in many industries.
- Elevator motors.
- Incandescent lamps.