Construction of DC Machine
DC generator and DC motor have the same general construction. Any DC generator can be run as a DC motor and vice versa. The Construction of DC machine is described below.
In DC machine yoke serve double purposes:
- It handles magnetic flux (Φ ⁄ 2 ) produced by the pole.
- It provides mechanical support to the whole machine.
In the small machine, the yoke is made of cast iron. But for large machine usually cast steel and rolled steel is employed. Generally, the yoke is not laminated because it carries a stationary flux. But in some modern DC machine where motor operates in power electronics converter, the yoke is also laminated to reduce eddy current losses.
Field pole consists of two parts pole core and pole shoes.
Generally, pole core is not laminated but in modern DC machine, both pole core and pole shoes are laminated. The pole shoes serve two purposes.
- They spread out the flux in the air gap, being of large cross-section, reduce the reluctance of the magnetic path.
- They support the field coil.
The pole is fastened to the pole face by countersunk screws and the thickness of the laminations from 1mm to 0.25mm.
The armature core keyed to the shaft and rotates between the field pole. The armature core is made by silicon steel lamination (about 0.4mm to 0.6mm) to reduce eddy current loss. These laminations are perforated for air ducts which permit axial flow of air for cooling purpose.
The armature winding can be made by two types of winding: one is- lap winding and other is- wave winding.
- Lap winding – In lap winding the number of the parallel path (P) is equal to the conductor (A).
For low voltage rating and high current rating, lap winding prefers.
- Wave winding – In wave winding, the number of the parallel path is equal to two irrespective number of the pole of the machine.
For high voltage rating and low current rating wave winding prefer.
A commutator act as a mechanical invertor or mechanical rectifier which convert AC into DC (in case of dc generator) and DC into AC (in case of motor condition). The commutator segment is made by hard-drawn copper and insulated with each other by (0.8mm) thick mica sheets. The number of segments is equal to the number of armature coil. Each commutator segment is connected to the armature conductor by means of copper lugs.
The brush is stationary sliding contact whose function is to collect the armature current from the commutator segment and supply to the load (in case of the generator) or feed the current into the commutator segment (in case of a motor). The brush is usually made of carbon or graphite. These brushes are kept in brush holder and mounted on a spindle. The brush can slide in the rectangular box open at both ends. The springs are adjusted from the brush pressure. If the brush pressure is very large, friction produces heating of the commutator and the brush. On the other hand, if the brush pressure is too small the imperfect contact with the commutator may produce excessive sparking.