construction of generator

An alternating current generator consists principally of a magnetic circuit, DC field winding and mechanical structure, including cooling & lubricating system.
Broadly the alternator parts can be classified as

i) Stationary part - comprising the stator & stator winding.
ii) Rotating part - comprising the rotor , rotor winding & rotor fan.

Main parts of a generator are described below

Stator Frame :
The stator frame with core & stator winding is the heaviest component of the entire generator. The stator frame is used for holding the armature stampings and windings in position. It is built up of steel plates, which are electrically welded together, instead of the more expensive cast-steel frames formerly used. Ventilating ducts are provided in the radial ribs for passage of cooling medium. The stator end covers are attached to the end flanges of the stator frame & also rest on foundation frame. The end covers are aluminium alloy castings.

Stator Core :
The stator core is stacked from insulated electrical sheet steel lamination with a low loss index and suspended in the stator frame from insulated dove tailed guide bars. Axial compression of the stator core is obtained by clamping fingers, clamping plates, and non magnetic through type clamping bolts, which are insulated from the core . The clamping fingers ensure a uniform clamping pressure, especially within the range of the teeth, and provide for uniform , intensive cooling of the stator core ends.

In order to minimise hysteresis and eddy current losses of the rotating magnetic flux, which interacts with the core, the entire core is built up of thin laminations. Each lamination is made up from a number of individual segments. The segments are punched in one operation from electrical sheet steel lamination having a high silicon content and then carefully de-burred. The stator frame is turned on end while the core is stacked with lamination segments in individual layers . The clamping bolts running through the core are made of non-magnetic steel and are insulated from the core and the clamping plates to prevent the clamping bolts from short circuiting the laminations and allowing the flow of eddy currents. The pressure is transmitted from the clamping plates to the core by clamping finger.

Stator Winding :

The stator winding or the armature winding in generators have open circuit winding unlike DC machines which have closed circuit winding. There is no closed path for the armature current in the winding itself. One end of the winding is joined to the neutral point & the other end is brought out(for a star connected armature).Each conductor consists of large number of separately insulated strands to reduce the skin effect losses. The high voltage insulation is provided with thermosetting system to prevent corona discharge. A coat of conducting varnish is applied to the surface of all the Bars. A final wrapping of glass fabric impregnated with epoxy resin serves as surface protection.The beginnings & ends of the three phase windings are solidly bolted to the output leads with flexibles.