DC Generator

Introduction

We all have attended a lot of marriage parties or any celebration and unfortunately sometimes we face issues of power cut in between from the power grid. At that time, we need some power back up so that our work or celebration will not get stopped. So one of the very popular backups around us is the electric generator.

Electric Generator - A Power Back-up

Electric generator is a device which generates electricity with the help of mechanical energy. It means that the electric generator is the machine which converts mechanical energy into electrical energy. It means that the electrical energy is not created by the electric generator.

Electric generators are of two types, namely -

• AC (Alternating current) Generator

• DC (Direct current) Generator

In this tutorial we will study about the construction, working, application etc., of DC Generator.

Principle of DC Generator

The basic principle of a DC generator is Faraday's law of electromagnetic induction. According to this principle an EMF will be generated in a conducting coil when there is any change in the magnetic flux linked with that coil. The change in magnetic field may happen due to -

• Fluctuation in the magnetic field

• Change in the area of the coil

• Change in the angle between the magnetic field and area vector of the coil.

Now let us check which factor is involved in the case of the DC generator which causes the generation of EMF in the coil.

Different Parts of DC Generator

From Faraday's law of electromagnetic induction, we can conclude that we need a conductor coil, a magnetic field and many more parts to convert mechanical energy into electrical energy.

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Figure - 1

Let us start the discussion of different parts of DC Generator one by one in detail.

• Stator: It is the stationary magnet in the DC generator which is providing a magnetic field to the coil. It consists of two magnets of opposite polarity and they are facing each other as shown in figure - 1.

• Rotor: It is a laminated armature coil of cylindrical shape with slots. The lamination is of iron and the purpose of the lamination is to decrease the eddy losses.

• Armature Windings:  It is the winding of insulated conducting wire and the power conversion takes place because of this.

• Yoke: It is the external structure of the generator which is in hollow cylindrical shape. It gives support to the poles. A low reluctance path to the magnetic fields is provided by this.

• Poles: As shown in the figure that it is holding the field winding and increasing the cross-sectional area of the winding and results in uniform spread of magnetic flux. The coils are connected either in parallel or series with the armature winding which are wound over the poles.

• Pole shoe: It is fixed to the yoke and utilized for the spreading of flux and prevents the falling of field coils.

• Commutator: It is of cylindrical shape and located on the shaft. The main function of the commutator is to convert AC to DC and apart from this it connects the external circuit which is stationary to the rotating armature coil with the help of brushes.

• Brushes: It helps to establish the connection between commutators with the external circuit.

Working of DC Generator

To understand the working of the DC generator in a better way, we will follow Figure 2.

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Figure - 2

In this figure, we are having magnets with opposite polarity and a single turn loop ABCD. Let us assume that this loop will rotate with uniform speed in clockwise direction. Since the magnetic field is uniform, the area enclosed by the coil is constant but the angle between magnetic field and area vector is continuously changing or the projected area is changing when the loop is rotating. So it will change the magnetic flux linked and result in the induction of EMF. Now see the figure -2, in the position -1 the angle between the area vector and the magnetic field is 90^o, so the flux linked will be zero. As it starts rotating the angle will change and the flux linked is continuously changing or increasing. Now at position -3 the area vector is making 0^o or 180^o with the magnetic field which results in the maximum linkage of the flux. Again at position - 5 the flux linkage is zero. After the position-5 the loop starts moving under the opposite polarity which reverses the polarity of the generated EMF. In this rotation maximum EMF will be generated at position -7 and zero at position - 1 again. This cycle continues and alternating nature of EMF will be generated which results in alternating current. Here, the role of commutator comes into picture which converts the AC into DC.

Mathematical Equation of Emf in Dc Generator

The equation of emf generated is given by -

$$\mathrm{EMF \:Generated\:=\:\frac{P \phi Z N}{60.A}}$$

where,

Z = Number of armature conductor

P = Number of poles

A = Number of parallel paths within the armature

N = Armature coil’s rotational speed (in r.p.m)

$\phi$ = Magnetic flux produced by each pole

EMF Generated = Generated EMF across any parallel lane

Types of Losses in DC Generator

As we know that none of the machines in this universe is 100 % efficient, there must be some loss in that.

Similarly, in DC Generator there are some losses to consider -

• Copper loss: This type of losses is in the windings when current flows through it. Copper losses are of three types - armature losses, winding losses and losses due to resistance of brushes.

• Core losses: This happens when the induced current flows in the armature and it causes hysteresis and eddy losses. This is also called magnetic or iron losses.

• Frictional losses: This happens when one surface rubs against each other.

Application of DC Generator

Following are some of the applications of DC Generators:

• It is used for the power back up and lighting purposes.

• It is also used to give DC supply to welding machines.

• It is used to compensate for any kind of voltage drop.

FAQs

Q1. How many types of electric generator are there on the basis of output current?

Ans. There are two types of electric generator based on the output -

• AC generator
• DC generator

Q2. What is a DC generator?

Ans. DC generator is the machine which converts mechanical energy into electrical energy.

Q3. What is the use of poles in DC Generator?

Ans. Poles are used for holding the field winding and increasing the cross-sectional area of the winding and results in uniform spread of magnetic flux.

Q4. State the working principle of DC Generator.

Ans. The principle of a DC generator is Faraday's law of electromagnetic induction. It states that the emf will be induced in a conductor when there will be change in the magnetic flux linked with the conductor.

Q5. What is the working of commutators in DC Generator?

Ans. The commutator is used to convert the induced AC to DC in the DC generator.

Q6. Name the different types of losses in DC generators.

Ans. The different types of losses in the DC generators are -

• Copper losses
• Iron losses
• Friction losses