Construction of DC machines
- Stator frame or Yoke
- Field Poles
- Commutator
- Brushes
- Armature core
- Armature Winding
Stator frame:
The stator frame is made up of cast steel. The laminations of the cast steel are not required as there is no eddy current loss in the stator. The field poles are mechanically connected to the stator frame using rivets. Yoke provides the return path of the main flux. It provides mechanical support to the field poles.
Field poles:
The electromagnet is used in DC machines and for that a field core winding is placed. So silicon steel is used in the machine to reduce the eddy current.
In the rotor, the tooth offers very low reluctance so that the flux oscillates and flows through it whereas the air gap in the slots is large when compared to the teeth in the rotor. Therefore an emf is induced in the pole shoe so that the eddy currents are produced in the pole shoe or field poles due to heat.
The pole shoe is laminated to reduce eddy current loss. The field poles are made up of silicon steel. The pole shoe provides the place for the field winding excited by the DC voltage to create the required flux.
The pole shoe distributes the flux uniformly in the air gap. The pole shoe also provides mechanical support to the field winding.
The shape of the main flux is trapezoidal or flat-topped. Hence the emf induced in the armature is also flat-topped.
The number of pulses produced per half cycle is equal to the number of coils in the armature.
Commutator:
The commutator is made up of hard-drawn copper to withstand the friction between the brush and the material. The number of commutator segments is equal to the number of coils. The commutator converts the induced AC into DC in the case of a generator hence it is called a rectifier. In the case of the motor, it converts the applied DC into AC hence it is called an inverter.
Spark will be produced when there is a gap between the commutator and the brushes.
To reduce this we need spring to push the brush to attach to the commutator.
If there is no spring then due to heat mica insulations will burn as it is thin and then all the segments fire. This is called ring fire in a DC machine.
Brushes:
Brushes are made up of carbon material. It is a self-lubricating material. This carbon brush provides smooth powder so that the friction will be less.
Carbon resistance is high when compared to copper and because of this reason carbon is preferred.
The purpose of the brush is to collect the current from the armature in the case of a DC generator.
Due to the high resistance of the carbon brush, the commutation improves and the brush has high thermal stability.
Armature Core:
The armature core is made up of silicon steel and it must be laminated to reduce the eddy current.
It is used to place the armature winding with an open type of slot. It provides a low reluctance to the main flux.
In a DC machine, the pole shoe and the armature core must be laminated. With an open slot, the amount of leakage flux is less in the DC machine hence it is preferred.
Armature winding:
