Construction and Working Principle of DC Generator

---

---

DC Generator

A DC generator is an electromechanical energy conversion device that converts mechanical power into DC electrical power through the process of electromagnetic induction.

A DC generator operates on the principle of electromagnetic induction i.e. when the magnetic flux linking a conductor changes, an EMF is induced in the conductor. A DC generator has a field winding and an armature winding.

The EMF induced in the armature winding of a DC generator is alternating one and is converted into direct voltage using a commutator mounted on the shaft of the generator. The armature winding of DC Generator is placed on the rotor whereas the field winding is placed on the stator.

Construction of a DC Generator

Here is the schematic diagram of a DC Generator

A DC generator consists of six main parts, which are as follows

Yoke

The outer frame of a DC generator is a hollow cylinder made up of cast steel or rolled steel is known as yoke. The yoke serves following two purposes

• It supports the field pole core and acts as a protecting cover to the machine.
• It provides a path for the magnetic flux produced by the field winding.

Magnetic Field System

The magnetic field system of a DC generator is the stationary part of the machine. It produces the main magnetic flux in the generator. It consists of an even number of pole cores bolted to the yoke and field winding wound around the pole core. The field system of DC generator has salient poles i.e. the poles project inwards and each pole core has a pole shoe having a curved surface. The pole shoe serves two purposes

• It provides support to the field coils.
• It reduces the reluctance of magnetic circuit by increasing the cross-sectional area of it.

The pole cores are made of thin laminations of sheet steel which are insulated from each other to reduce the eddy current loss. The field coils are connected in series with one another such that when the current flows through the coils, alternate north and south poles are produced in the direction of rotation.

Armature Core

The armature core of DC generator is mounted on the shaft and rotates between the field poles. It has slots on its outer surface and the armature conductors are put in these slots. The armature core is a made up of soft iron laminations which are insulated from each other and tightly clamped together. In small machines, the laminations are keyed directly to the shaft, whereas in large machines, they are mounted on a spider. The laminated armature core is used to reduce the eddy current loss.

Armature Winding

The insulated conductors are put into the slots of the armature core. The conductors are suitably connected. This connected arrangement of conductors is known as armature winding. There are two types of armature windings are used - wave winding and lap winding.

Commutator

A commutator is a mechanical rectifier which converts the alternating emf generated in the armature winding into the direct voltage across the load terminals. The commutator is made of wedge-shaped copper segments insulated from each other and from the shaft by mica sheets. Each segment of commutator is connected to the ends of the armature coils.

Brushes

The brushes are mounted on the commutator and are used to collect the current from the armature winding. The brushes are made of carbon and is supported by a metal box called brush holder. The pressure exerted by the brushes on the commutator is adjusted and maintained at constant value by means of springs. The current flows from the armature winding to the external circuit through the commutator and carbon brushes.

Working Principle of DC Generator

The working principle of DC generator is based on the Faradays law of electromagnetic induction. According to this law, when the magnetic flux liked to a conductor or coil changes an EMF is induced in the conductor or coil. The magnitude of this induced EMF is given by,

$$\mathrm{\mathit{e}\:=\:\mathit{N}\frac{\mathit{d\phi }}{\mathit{dt}}\:\cdot \cdot \cdot (1)}$$

Where, $\phi$ is the flux linkage of the coil and N is the number of turns in the coil.

In case of a DC generator, the magnetic flux ($\phi$) remains stationary and the coil rotates. The EMF induced where the coil is rotating and flux is stationary, is known as dynamically induced EMF.

In order to understand the working principle of a DC generator, we consider a single loop DC generator (i.e. N = 1) as shown in above figure. Here, the coil is rotated by some prime mover (a source of mechanical energy), and there is a change in the magnetic flux linkage to the coil.

Let $\phi$ be the average magnetic flux produced by each magnetic pole of the machine, then the average induced EMF in the generator is given by,

$$\mathrm{\mathit{E_{av}}\:=\:\frac{\mathit{d\phi }}{\mathit{dt}}\:=\:\mathrm{Flux\: cut\: per\:sec\:by\: the \:coil}}$$

$$\mathrm{\Rightarrow \mathit{E_{av}}\:=\:\mathrm{Flux\: cut\: in \:one \:rotation\:\times \:No.\:of\: rotations\: per\: sec}}$$

$$\mathrm{\Rightarrow \mathit{E_{av}}\:=\:\mathrm{\left ( Flux\:per\:pole\times No.\:of\:poles \right )}\:\times \:\mathrm{No.\:of \:rotations \:per\: sec}}$$

$$\mathrm{\therefore \mathit{E_{av}}\:=\:\mathit{\phi \:\times P\:\times \:n}\:\cdot \cdot \cdot (2)}$$

Where, P is the total number of poles in the generator and n is the speed of the coil in rotation per second. The expression in the Equation-(2) gives the average induced EMF in a single loop DC generator.

The following points explain the working principle of a DC generator −

• Position 1 − The induced EMF is zero because, the movement of coil sides is parallel to the magnetic flux.
• Position 2 − The coil sides are moving at an angle to the magnetic flux, and hence a small EMF is generated in the loop.
• Position 3 − The coil sides are moving at right angle to the magnetic flux, therefore the induced EMF is maximum.
• Position 4 − The coil sides are cutting the magnetic flux at an angle, thus a reduced EMF is induced in the coil sides.
• Position 5 − No flux linkage with the coil side and the coil sides are moving parallel to the magnetic flux. Therefore, no EMF is induced in the coil.
• Position 6 − The coil sides move under a pole of opposite polarity and hence the polarity of induced EMF is reversed. The maximum EMF will induce in this direction at position 7 and zero when it is at position 1. This cycle repeats with rotation of the coil.

In this way, EMF is induced in a DC generator. Though, this induced EMF is alternating in nature, which is then converted in the unidirectional EMF by using a device called commutator.

The direction of induced EMF in the armature conductor of the DC generator is determined by the Fleming right hand rule (FRHR) which we discussed in the module-1 (basic concepts) of this tutorial.