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5. What are the characteristics of DC generators? (What are the characteristics of DC generator in hindi)

# What are the characteristics of DC generators? (What are the characteristics of DC generator in hindi)

Hello friends, in this article we will know that what are the characteristics of DC generator? And how many types are there? And learn about many facts related to it.

Each dc generator has the following three main characteristics –

## Magnetic or open circuit characteristics – E0/I (Magnetic or open circuit characteristics of DC generator, OCC) –

The constant speed of the generator and its field current If and (with the armature open) he induced emf0 The curve obtained between them is called the generator magnetic characteristic. It is also known as no-load saturation characteristics. This curve shows how the flux changes to the density state when the field excitation changes from a maximum value to zero, similar to the magnetization curve of a material of electromagnets. The form of this characteristic is almost the same in all types of generators.

## Intrinsic or complete characteristics –

This is the characteristic of the armature current at a constant speed of the generator and the actual induced emf in the armature E (= E)0 shows the relationship between the voltage drop due to the armature reaction. When current flows in the armature conductors of the generator, an electromagnetic field is produced in the armature which acts in the opposite direction to the main field. This field is called the demagnetizing field. Due to its origin, the electromotive force induced in the generator starts decreasing.

(So, E = ZNP/(60A))

## External Characteristics – V/L (External characteristics of DC generator) –

The curve drawn between the terminal voltage V and the load current IL at a constant speed of the generator is called the load characteristics. Here the value of V is equal to (E – IaRa), where IaRa is the voltage across the conductors of the armature. This performance characteristics or sometimes called the voltage regulationc curve because it also includes the calculation of the voltage across the armature. Since V < E, the external characteristic curve is the internal characteristic curve. This characteristic is important to test the suitability of the generator for a particular task.

### This can be achieved in two ways –

• By taking measurements at the same time from a suitable voltmeter and ammeter on the generator under load.
• With the help of graphical method from OCC. The condition is that the armature and field resistance are known and the demagnetizing effect of the armature reaction is also known (by short path test) under the prescribed load conditions.

## Magnetic, Internal and External characteristics of DC Series Generator

With the help of first drivers like electric motor, petrol engine, diesel engine, steam turbine, gas turbine, steam engine etc., the generator is made to run at a constant speed.

Thereafter the value of the load R is gradually decreased from the maximum. The value of the observed current and voltage at each step is marked on the graph. The external or load characteristics (V/I curve) of a DC series generator are shown in the figure.

The value of flux in a series generator depends on the load or armature current. Due to residual magnetism, the curve at zero current starts slightly above the zero value from the voltage axis. If the demagnetization effect of the armature reaction is not included, then that curve gives the value of the total flux, while the generator is providing current to the external load, but due to the armature reaction, the actual flux is slightly below this curve. called intrinsic characteristic.

The terminal voltage V of the generator is equal to the value obtained by subtracting the voltage drop across the machine from the electromotive force E. If Rs and Ra are series field and armature resistance respectively, then the voltage drop will be I(Ra + Rse) and the terminal voltage,

V = E – I(Ra +Rse)

The voltage drop or ohmic drop is graphically represented by a straight line passing through the origin. Curve (III) is an external feature. It is clear that as the load current increases, the terminal voltage first increases according to an almost linear law, then gradually increases in a curvilinear manner and finally decreases.

Burden stream IL As the current (or armature current Ia) increases, the flux in the generator also increases. If the armature reactance and armature resistance are negligible, then the generated voltage and the terminal voltage will have the same value and the external characteristics will be similar to the magnetization curve of the machine.

Looking at the load or external characteristics, it is clear that the DC series generator is a generator with rising voltage characteristics, but the terminal voltage of the generator decreases at high load currents. This is because the armature at high armature current The reaction is overwhelming. In fact, as the load current increases from IL, the terminal voltage of the generator gradually decreases and the terminal voltage at the load current OB decreases to zero.

If the generator works on the initially straight line part of the external characteristic, it provides voltage in proportion to the load current. Booster is a series generator of this type. If work is done on the drooping part PB of the external character and if the generator is designed so that this part is almost vertical, the generator will provide a nearly constant current, independent of the resistance of the external circuit. This type of series generator can be used to light multiple series connected arc lamps.

## Internal and load characteristics of separately excited generator –

Generally, the characteristics of the DC generator are plotted at a constant speed. In an externally excited generator, the excitation is independent of the load current, so if the armature reactance is zero, the flux will be constant as shown by the curve I in the figure.

The curve is slightly drooping due to armature reaction (Curve II). The second curve gives the induced emf in the armature. Hence it is an overall or intrinsic characteristic. The terminal voltage is obtained by subtracting the armature voltage (in the direction of the load) from the total electromotive force generated.

Curve III shows the external characteristics. It is clear from the curve that there is a slight decrease in the terminal voltage due to increase in load as well as due to armature feedback. This shortcoming can be easily made up by increasing the excitation current.

## Magnetic or O/C Characteristics of Shunt Generators –

The magnetic characteristics of a shunt generator are shown in the figure. The initial part of the curve is linear because at low flux density the iron path reluctance is negligible and the total reluctance is of the air gap.

which is situated. On increasing the flux density i.e. field current, the magnetic poles get saturated and the curve starts to become flat and the electromotive force generated remains almost constant.

## External or Load characteristics of Shunt Generator

The external characteristics of a shunt generator are shown in the figure. If the load on the generator is gradually increased, the value of current will increase due to decrease in resistance. As the load current increases, the terminal voltage decreases further. Thankfully, the current gets further reduced.

When the current attains a certain value (much higher than the normal full load in modern generators), the load resistance field short-circuits the coil to that extent. that the terminal voltage decreases more rapidly than the load resistance, in such a case a further decrease in the external resistance actually results in a decrease in the current. The characteristic curve is droop at that time and when the armature is short circuited then this current cuts the axis at some point A. This is the reason why the shunt generator fails to excite when needed again due to severe short circuit.

There are three main reasons for drop in terminal voltage of shunt generator under load condition –

## Armature Resistance Drop

As the load current increases, more and more voltage is used across the resistance of the armature circuit. Hence the terminal voltage V = E – IaRa decreases.

### Armature Reaction –

Due to the demagnetizing effect of the armature reaction, the pole flux gets weakened and the emf in the armature decreases.

The drop in terminal voltage due to armature reactance and armature resistance decreases the field current, due to which the induced or induced electromotive force becomes even greater.

## Internal Characteristics of Shunt Generator –

The internal characteristics of the shunt generator are shown in the figure, these characteristics are obtained by adding the armature voltage to the external characteristics.

## Characteristics of Compound Generator –

The characteristics of a compound generator are shown in the figure, the characteristics of a shunt generator are drooping and the characteristics of a series generator are of a rising nature. Combining the two characteristics together, a curve is obtained that shows a nearly constant voltage over a large area of ​​the load current. These characteristics are similar to those of compound generators.

### Level Compound Generator –

In this type of generator, almost constant voltage is obtained from zero load to full load current.

### Over Compound Generator –

In this generator, the value of terminal voltage increases as the load current increases.

### Under Compound Generator –

The characteristics of this type of generator are more drooping nature and with increase in load current the terminal voltage drops rapidly.

Type of DC Generator