Constructional Details of DC Generator

CONSTRUCTIONAL DETAILS

The major parts of the DC generator is given below and also shown in fig.2.2

1. Magnetic frame or yoke

2. Poles, interpoles, windings, pole shoes

3. Armature

4. Commutator

5. Brushes, bearings and shaft

Yoke

The outer frame or yoke serves two purposes.

(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.

In small machines where cheapness is the main consideration and weight is not a critical factor. The yoke is made up of cast iron. But for larger machines cast steel or rolled steel is used.

Poles

The poles consist of pole cores, pole shoes and pole coils. The pole cores and pole shoes form the field magnet. The pole shoes used to spread out the flux in the air gap and also reduce the reluctance of the magnetic path. The pole shoes also support the exciting coils or field coils. The pole core is made up of cast iron or cast steel. To minimize the eddy current losses the pole cores and pole shoes are laminated. Annealed steel is used for laminations. The thickness of laminations varies from 1 mm to 0.25 mm.

The pole coils are made up of copper wire or strip. When current is passed through these coils, the pole becomes an electromagnet and starts establishing a magnetic field in the machine. The flux distribution through the pole, air gap, armature core and yoke is shown in fig.2.3.

Interpoles

In modern DC machines commutating poles or interpoles are provided to improve commutatation. Like the field winding the commutating poles also have exciting coils which are connected in series with the armature.

Armature

The armature consists of an armature core and armature windings. The armature core houses the armature conductors or coils and causes them to rotate and cut the magnetic flux of the field magnets.

The armature core is cylindrical or drum shaped. It is made up of circular sheet steel. The armature core is laminated with the thickness of 0.5 mm. the lamination is used to minimize the eddy current loss. The laminations are known as stampings. In small machines these stampings are directly keyed on the shaft. In larger machines the stampings are first assembled and then keyed on an armature spider and the armature spider is then keyed on to the shaft.

The armature conductors are housed in the slots. The slots are rectangular in shape for large machines and circular for small machines.

The conductors are housed in slots in two layers. The slots are closed by fiber or wooden wedges to prevent the conductors from flying out due to centrifugal force when the armature rotates. The arrangement is shown in fig.2.3 and 2.4.

Commutator

The function of commutator is collection of current from the armature conductors and converts the alternating current induced in the armature conductors into unidirectional current in the external load circuit. It is cylindrical in structure and is built up of wedge shaped segments of high conduction hard-drawn copper. These segments are insulated from each other by thin layers of mica. The number of segments is equal to the number of armature coils.

Each commutator segment is connected to the armature conductor by means of a copper lag or strip or riser. To prevent them from flying out under the action of centrifugal forces, the segments have V-grooves, these grooves are insulated by conical micanite rings. The cross sectional view of commutator is shown in fig.2.5.

Brushes and Bearing

The brushes are made up of carbon or graphite. The function of brushes is collect the current from the commutator and conveys the current in to the external load resistance. They are rectangular in shape. These brushes are housed in brush- holders and mounted over brush holder studs. The brush holder studs are mounted on a brush yoke. The brush holder studs are insulated from the brush yoke by insulation sleeves.

Ball bearings are used for light machines and for heavy machines roller bearings are used. The bearings are packed in hard oil for quieter operation.