DC generators are electromechanical energy conversion devices that convert mechanical energy into direct current electricity. But DC generators do not produce 100% direct current. Actually, they produce alternating current in armature coils which are rectified to direct current using a commutator.
Any electromechanical energy conversion machine has two parts i) stator and ii)rotor.
The stator consists of the yoke, pole core, pole shoe, field winding, brushes, interpole, interpole winding.
Made of cast iron for small DC machines and fabricated steel for large dc machines.
Perform two tasks
Provides a path for the pole flux and carries half of it.
Provides mechanical support to the whole machine.
Pole core is welded or bolted to yoke.
Two types of construction are available for pole core.
Solid pole core, where it is made of a solid single piece of cast iron or cast steel.
Laminated pole core, where it made of numbers of thin, limitation of annealed steel which are riveted together. The thickness of the lamination is in the range of 0.04inches to 0.01 inches.
The main field flux is produced inside it and travel.
The pole shoe or pole face is attached to pole core.
The cross-section area of the pole shoe is more than that of the pole core. It has two benefits
The reduced cross-section of the pole core requires less copper for the field winding.
The large pole shoe area increases the flux per pole entering the armature, due to a reduction in the air gap reluctance.
Pole shoe provides mechanical strength and support to the field winding.
The material which is used for making pole core is also used for making pole shoe.
The field winding is concentrated winding. It is made up of insulated copper wire.
The winding is wound over the pole core.
The cross-section area and numbers of turns of field winding depend on the type of generators.
For shunt, DC generators wire have more number of turns with less cross-section area. For the reason to increase the resistance of the shunt winding.
For series, DC generators wire with fewer numbers of turns with large cross-section area is used in order to decrease resistance.
And for compound DC generators both types of wire are present.
Brushes connect the internal circuit to the external circuit. It enables us to take generated power out of the generator.
These brushes can be of
Interpole and interpole winding
Interpoles are narrow poles place at the GNA and fitted to the Yoke. It is also known as Commutating poles or compoles.
For generators, the polarity of the interpoles must be the same as that of the main pole ahead of it in the direction of rotation.
Interpole reduces the effect of the armature reaction and in addition, it produces some extra mmf in the interpolar zone.