The constructional features of d.c. machines, it has been stated that field winding is a concentrated winding on salient poles bolted to the stator frame and armature winding is a distributed winding housed in the slots around the periphery of the cylindrical rotor. Basic principles underlying the torque production and e.m t Generation in d.c machines are also. The object of this article is to present the physical concepts regarding the steady-state behavior of d.c. machines.
Commutator forms the most important component of a dc machine. Stator of a dc machine consists of a yoke (or frame), field windings, inter poles, compensating winding, brushes, and end covers. Rotor consists of armature core, armature winding, commutator, and shaft.
Yoke has two functions
- It provides a path for the pole flux \phi and carries half of it \phi/2
- It provides mechanical support to the whole machine. Since the flux carried by yoke is stationary (i.e. constant), it is not laminated. As stated before, case iron is used for small dc machines and fabricated steel for large dc machines. In case dc motor is to be operated through a power-electronics converter, the yoke is laminated to reduce the eddy-current losses.
The field pole consists of pole core and pole shoe. The post center is produced using cast steel yet the shaft shoe is covered and fixed to the post center. Hence both shaft center and post shoe are produced using dainty overlays of sheet steel to diminish the vortex current misfortunes. The overlaid post is welded or darted to the burden.
The pole is excited by a winding wound around the pole core.This winding is called as field or exciting winding which is prepared from copper. The number of turns and cross-section of field winding depend upon the type of dc machine as under
- For dc shunt machine large number of turns of small cross-section are used.
- For dc series machine small number of turns of large cross-section are used
- For dc compound machine both shunt and series field winding are used.
These are fixed to the yoke in between the main poles of a dc machine.These are actually tapered with sufficient sectional area at root to avoid magnetic saturation.The interpoles winding consisting of a few turns of thick wire is connected in series with the armature so that its magnetomotive force is proportional to armature current.
These winding are replaced in the slot cut in the pole face of a dc machine. Compensating winding is also connected in series with the armature circuit. This winding is however used in large dc machines only.
Brushes are housed in box type brush holder joined to the stator end spread or the stator burden. A little spring keeps the brushes gave to the commutator surface. The brush pressure on the commutator surface must be painstakingly balanced. Too little a brush weight may prompt unreasonable arcing between the brush commutator contacts.
Brush are made of carbon for small dc machine electrographite for all dc machines and copper graphite for low voltage high current dc machine.
Armature core serves two purpose
- It housing the armature coils in the slots
- It providing the low reluctance path to the magnetic flux \phi/2. It is made from 0.35 to 0.50 mm thick lamination of silicon steel to keep down the iron losses.
The armature winding is made from copper. It consists of a large number of insulated coils each coil having one or more turns. The coil is usually a former wound. These are placed in slots and appropriately connected in series and parallel depending upon the type of winding required.
There are basically two winding types
- Lap winding
- Wave winding
Two coil ends of each coil are then connected to the riser of segment of a commutator.
It is of cylindrical structure.lt is built up of wedge-shaped segments of high conductivity hard drawn copper to reduce its wear and tear. Sections are protected from one another by 0.8 mm thick mica sheets.
On armature shaft are mounted
- Hub H of commutator
- Spider in big machines or armature core in small machines
End covers are connected to the yoke on one side and to the bearings and shaft on other side.