SYNCHRONOUS GENERATOR

Introduction

An alternating voltage is generated in a single conductor or armature coil rotating in a uniform magnetic field poles. An alternating voltage will also be generated in stationary armature conductors when the field poles rotate past the conductors.

Ac generators are usually called alternator. They are also called synchronous generator. Rotating machines that rotate at a speed fixed by the supply frequency and the number of poles are called synchronous machine.

Advantage of rotating field alternator

Most alternator have the rotating field and the stationary armature. The rotating field type alternator has several advantage over the rotating armature type alternator:

A stationary armature is more easily insulated for the high voltage for which the alternator is designed. The generated voltage may be as high as 33 kv.
The armature winding, being stationary, are not subjected to vibration and centrifugal forces.
The output current can be taken directly from fixed terminal on the stationary armature without using slip rings, brushes, etc.
The bulk and weight of the armature winding are substantially greater than the winding of the field poles. The size of the machine is, therefore, reduced.
The stationary armature may be cooled more easily because the armature can be made large to provided a number of cooling ducts.

Construction of three phase synchronous machine

Similar to other rotating machine, an alternator consist of two main parts namely, the stat-or and the rotor. The stat-or is the stationary part. IT carries the armature winding in which the voltage is generated. The rotor is the rotating part. The rotor produce the main field flux.

Stat-or construction

The various parts of the stat-or include the frame, stat-or core, Stat-or winding and cooling arrangement. The frame may be of cast iron for small size machine and of welded steel type for large size machines. In order to reduce hysteresis and eddy-current losses,the stat-or core is assembled with high grade silicon content steel lamination . A three-phase winding is put in the slots cut on the inner periphery of the stat-or as shown in figure. The winding of each phase is distributed over several slots.

Rotor construction

There are two types of rotor construction namely, the silent pole type and the cylindrical rotor type.

Salient pole rotor

The term salient means 'protruding' or 'projecting'. Thus, a salient pole rotor consist of poles projecting out from the surface of the surface of the rotor core. In figure shows the end view of a typical 6-poles salient pole rotor. Salient pole rotor are normally used for rotor with four or more poles.

Since the rotor is subjected to changing magnetic field, it is made of thin steel lamination to reduce eddy current losses. Poles of identical dimension are assembled by stacking laminating to the required length and then riveted together. After placing the field coil around each pole body, these poles are fitted by a dove tail joint to a steel spider keyed to the shaft. Salient pole rotor have connected winding on the poles. Damper bars are usually inserted in the poles faces to damp out the rotor oscillation during sudden change in load condition. A salient pole synchronous machine has a uniform air gap.

Salient pole alternator driven by water turbines are called Hydro-alternators or hydro-generator.

Cylindrical rotor

A cylindrical rotor machine is also called a non-salient pole rotor machine. It has its rotor so construction that it forms a smooth cylinder. The construction is such that there are no physical poles to be seen as in the salient pole construction. Cylindrical rotor are made from solids forgings of high grade nickel-chrome- molybdenum steel. In about two third of the rotor periphery, slots are cut at regular interval and parallel to the shaft. The d.c field winding are accommodated in these slots. The winding is of distributed type. The UN-slotted portion of the rotor forms two[or four] pole faces. A cylindrical rotor machine has the centrifugal forces. Thus, cylindrical rotors are particularly useful in high- speed machines. The cylindrical rotor type alternator has two or four poles on the rotor.

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