Yoke:- in small machine yoke is generally made of cast iron and in large machine it is made of silicon steel or fabricated steel.

There is no need to laminate the yoke because flux in DC machine is stationary and hence no eddy current are produced.

Requirements of yoke

1 high permeability.

2 low reluctance.

3 high mechanical strength.

Function of yoke

  • It provide mechanical support to the entire machine.
  • Protect entire machine.
  • It carry half of the flux per pole and provides low reluctance path for completing close loop of flux.
Field magnet

Pole core:- pole core and pole shoe are both made of cast iron or cast steel pole core is not laminated but pole shoe laminated in order to reduce eddy current loss. 

Pole shoe:-

  • Pole shoe provide mechanical support to field winding.
  • It distributed the magnetic field uniformly to the air gap.
  • Provides low reluctance path to the field flux.

Pole pitch

The angular distance between two adjacent pole or angular span of each pole is called pole pitch.

Pole pitch is always is 180 degree electrical.180 degree electrical pole pitch implies that while going from one pole to other their is a shift of 180 electrical in flux density which means flux density reverses.

Pole arc


The actual angular span of each pole at the centre is called as pole arc.

Field winding

Field winding is made up of copper and it is concentric winding.

The field winding of DC machine can be connected to armature winding either in series or parallel.

Commutator 

Commutator in a DC machine is responsible of conversion of 

1 AC to DC in generator.

2 DC to AC in motor.

Commutator is generally made of hard drawn copper and it between commutator segment we have mica as insulation.

Commutator keep armature mmf stationaBrushes

Bushes

Brushes in a ratating machine are used to make rotating terminal stationary when terminal are brought out of machine.

In small DC machine brushes are made of carbon in large machine they are made of metal graphite, electro graphite and carbon graphite.

Carbon has following properties which are useful design brushes.

  1. Self lubrication.
  2. Friction less.
  3. Negative temperature coefficient.
  4. It improves commutation.
Armature core

Armature core is generally made of silicon steel and it is laminated in order to reduced eddy current loss.

Armature core provide mechanical support to the armature winding by placing the winding in the slot.

Armature core only carries half of the main field flux per pole 

Armature winding 

1 Concentrated winding

all the turn are present in only two slot weight is not uniformly distriwinding

2 Distributed winding

The turn are distributed through out the periphery. Weight is uniformly distributed.