Construction of DC Generator

CONSTRUCTION OF DC GENERATOR

The construction of DC generator and DC motor are the same. Any DC generator can be run as a DC motor and vice‒versa.

All DC machines (generator or motor) have a Stator and Rotor.

The Stator consist of

(i) Yoke or Magnetic frame

(ii) Field system‒Poles, Interpoles, Field winding

The Rotor has the following parts.

(iii) Armature ‒ Armature core, Armature winding

(iv) Commutator

(v) Brushes, Bearing

(i) Yoke or Magnetic Frame

1. Functions

(i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine.

(ii) It carries the magnetic flux produced by the poles.

2. Choice of Material

To provide low reluctance path, it must be made up of some magnetic material usually made up of cast‒iron because it is cheapest. But for large machines rolled steel (or) cast steel, silicon steel is used which provides high permeability. i.e., low reluctance and gives good mechanical strength.

(ii) Field system

Field system consists of two main parts. Poles and Field winding.

1. Poles

Each pole is divided into two parts namely: (i) Pole core (ii) Pole shoe. This is shown in Fig. 2.4.

(i) Functions of pole core and pole shoe

(a) Pole core carries field winding which is necessary to produce the flux.

(b) Pole shoes spread out the flux in the air gap, and also, being of larger cross‒section, to reduce the reluctance.

(c) They support the exciting coils (or field coils).

(ii) Choice of material

It is made up of magnetic material like cast‒iron or cast‒steel.

As it requires a definite shape and size, laminated construction is used. The laminations of required size and shape are stamped together to get a pole which is then bolted to the yoke.

2. Inter Poles or Commutating Poles

Interpoles or the commutating poles are fixed to the frame in between main poles. They are in small size compared with the main poles. They are used for sparkless commutation (to improve commutation).

3. Field Winding

The field winding is placed on the pole core.

(i) Functions

To carry the current and to produce the magnetic flux.

(ii) Choice of material

It is made up of aluminium or copper.

(iii) Armature

It is further divided into two parts namely:

1. Armature core

2. Armature winding

1. Armature core

It is cylindrical in shape with slots on its outer periphery. It is mounted on the shaft.

(i) Functions

It provides the house for armature conductors.

(ii) Choice of Material

It is made up of cast‒iron or cast‒steel. It is made up of laminated construction to keep eddy current‒loss‒as low as possible.

2. Armature Winding

The armature windings are placed into the slots on the armature surface. The ends of the coils are soldered with commutator segments.

(i) Functions

(a) When the armature rotates an emf is induced in the armature conductors in case of generators.

(b) To carry the current supplied in case of DC motors.

(ii) Choice of material

Armature winding has to carry the entire current so it is made up of conducting material, which is copper.

(iv) Commutator

The basic nature of emf induced in the armature conductor is alternating. This needs rectification in case of DC generator, which is made possible by a device called commutator.

1. Functions

(i) To facilitate the collection of current from the armature conductors.

(ii) To convert the alternating current induced in the armature conductors into  unidirectional current in the external load circuit.

2. Choice of material

As it collects current from armature, it is also made up of copper segments.

(v) Brushes and Bearings

The function of brushes is to collect current from commutator. They are usually made up of carbon or graphite and in the shape of rectangular block. These brushes are housed in brush‒holders usually of the box‒type variety.

The brushes are made to press over the commutator with spring tension. The tension of the spring can be adjusted by changing the position of lever in the notches.

A flexible copper wire called pigtail is casted at the top of the brush. It conveys current from the brush to the terminal box mounted on the yoke.

Ball bearings are frequently employed because of their reliability. But for heavy duty machines, roller bearing are preferable.