Construction Detail of a DC Generator

A d.c. generator consists of three main Parts.
 1. Magnetic-field system 2. Armature   3. Commutator and brushgear

Magnetic-field system 

The magnetic field system is stationary part of the machine. It produces main magnetic Flux. 

Yoke: the outer frame or yoke is a hollow cylinder of cast steel or rolled steel.An even number of pole cores are bolted to the yoke.

Yoke Serves the following two purposes 

• It supports the pole cores and acts as protecting cover to the machine
• It forms a part of magnetic circuit.

Poles: The poles project inwards they are called salient poles.Each Pole core has a pole shoe having curve surface.

The poles shoe serves two purposes.

• It supports the field coils.
• It increase the cross sectional area of the magnetic circuit and reduces its reluctance.

Pole core: the pole core are made of sheet steel laminations that are insulated from each other riveted together. The poles are laminated to reduce eddy-current losses.Each pole core has one or more field coils placed over it to produce a magnetic field.

Field Coils: The field coils are made copper wire and Placed over the poles core. It is connected in series with one another such that when current flows through the coils , alternate north and south pole are produced in the direction of rotation.  

  

Armature 

The rotating Part of the d.c. Machine is called the armature.The armature consists of a shaft upon which a laminated cylinder Armature core.The armature has a following parts

Armature core: The armature core has grooves or slots on it outer surface. laminations  are insulated from each other and tightly clamped together. The insulate conductor put in the slot of the armature core. the conductor are wedged and band s of steel wire are fastened round the core to prevent them flying under centrifugal force. the conductor are suitably connected . this connected arrangement of the conductor is called armature winding.  

Armature winding: The Armature winding is distributed on the armature slots in which emf is induced . Two types of the winding  are used - Wave and lap.

• Wave winding: the end coil are connected to commutator segments some distance apart, so that only two parallel paths are provided between the positive and negative brushes. that is , for Wave winding A=2. Wave winding is used in high voltage , low current machines.
• Lap winding: the end coil are connected to commutator adjacent segments on the commutator so that the total parallel paths is equal to the total number of poles. That is, for LAP winding A=P.The lap winding is used in low voltage, high current machines 

Commutator and Brushgear 

Alternating voltage is produced in coil rotating in magnetic field. To obtain direct current in the external circuit a commutator is needed.

Commutator:The commutator is rotates with the armature , is made of from a number of wedge-shaped hard-drawn copper bars or segments insulated from each other and from shaft.The segments from a ring around the shaft of the armature . Each commutator segment is connected to end of the armature coils.

Brushes: Brushes are collected the current from the armature winding by means two or more carbon brushes mounted on the commutator. Each brushes supported in a metal box called a brush box or brush holder.the pressure exerted by the brushes on the commutator can be adjusted and is maintained at a constant value by means of spring. Current produced in the armature winding is passed on the commutator and then to collected by external circuit means brushes.