Construction of DC machines
The D.C. generators and D.C. motors have the same general construction. Any d.c. generators can be run as d.c. motor and vice versa.
A d.c. machine consists of two main parts-
- Stationary parts-It is designed mainly for producing a magnetic flux.
- Rotating part- It is called the armature, where mechanical energy is converted into electrical (electrical generator) or conversely, electrical energy into mechanical energy (electric motor).
By an air gap stationary and rotating parts are separated from each other. The stationary part of d.c. machine consists of main poles, design to create the magnetic flux, commutating poles interposed between the main pole and design to ensure sparkles operation of the brushes at the commutator and a frame/yoke.
The construction details and main parts of a d.c. machine are shown in the following Fig.
The essential parts of DC machine are
- Magnetic frame or yoke
- Pole- Cores and Pole-Shoes
- Pole coils or Field coils
- Armature Windings or Conductors
- Brushes and bearings
The outer frame or yoke serves two main purposes. They are
- It supports the poles and acts as a protecting cover for the machine.
- It carries magnetic flux produced by the poles.
Yoke is made of cast iron. But for large machines cast steel or rolled steel is implemented. A steel slab round a cylindrical mandrel and then welding it at the bottom to forming yoke. The feet and the terminal box etc. are welded to the frame afterward. Such yokes process sufficient mechanical strength and have high permeability.
Pole- Cores and Pole-Shoes-
Pole cores and pole shoes together consist the field magnets. The pole shoes serve two purposes
- They spread out the flux in the air gap and also, being of larger cross-section, reduce the reluctance of magnetic path.
- They support the field coils.
Pole coils or Field coils-
The pole coils or field coils, which consist of copper wire or strip, are former wound for the correct dimension. Then the former is removed and wound coil is put into place over the core as shown in the Fig. When current is passed through these coils, they electromagnetism the poles which produced the necessary flux that is cut by revolving armature conductors.
It is further divided into two parts namely,
- Armature Core
- Armature Winding
Armature core is mounted on the shaft and it is cylindrical in shape. It consists of slots on its border and the air ducts permit the air flow through armature for cooling purpose.
Functions- Arature core provides house for armature winding i.e. armature conductors. To provide a low reluctance path to the magnetic flux which is produced by the field winding.
Choice material- As it has to provide a low reluctance path to the flux, it is made up of magnetic material like cast iron or cast steel. It is made up of laminated construction to keep eddy current loss as lo as possible. A single circular lamination used for the construction of an armature core is shown in Fig.
Interconnection of the armature conductors is called armature winding which is placed in the slots on the armature core border. When the armature is rotated, in case of generator, magnetic flux cut by the armature conductors and e.m.f. gets induced in them.
- In generator generation of e.m.f. takes place in the armature winding.
- To carry the current supplied in case of DC motors.
- To do the useful work in the external circuits.
Choice of materials-As armature winding carries entire current which depends upon external load, it has to be made up of conducting material, which is copper. Armature winding is generally former wound. The conductors are placed in the armature slots which are separate tough insulating material.
It is of cylindrical structure. It is built up of wedge-shaped segments of high conductivity hard-drawn copper to reduce its wear and tear. Segments are insulated from each other by 0.8 mm thick mica sheets. The segments are tapered so that their assembly results in circular shape.
- Convert alternating quantity to direct quantity or vice versa.
- Keep the armature and rotor m.m.f. stationary in space.
The first commutator function of rectifying the alternating quantities in the armature winding to direct quantities at the output brushes of a d.c. generator. In case of d.c. motor the direct quantities at the brushes are converted to alternating quantities in the armature winding. Thus a commutator can act both as a converter or rectifier and as an inverter.
Brushes and Bearings-
Brushes are housed in box-type brush holders attached to the stator end cover, or the stator yoke. A small spring keeps the brushes pressed on to the commutator surface. The brush pressure on the commutator surface must be carefully adjusted. Too small a brush pressure may lead to excessive arcing between the brush-commutator contact. If the brush pressure is too high, it may cause excessive wear of the commutator surface and the brushes.
Brushes are made of carbon for small dc machines, electrographite for all dc machines and copper-graphite for low voltage high current dc machines.
Also Read- Principle of operation of DC Generator