Synchronous machines
In synchronous machines, the armature winding either exports A.C. power (synchro-nous generator) or imports A.C. power (synchronous motor), whereas the field winding is always energised from a D.C. source. In other words, the synchronous machines are doubly excited energy-conversion devices. The generation of e.m.f, in general, depends on the relative motion between field flux and armature winding. In view of this, an A.C. generator, alternator or synchronous generator may have either rotating field poles and stationary armature, or rotating armature and stationary field poles. Nevertheless, synchronous machines are in variably constructed with high-power armature winding on the stator and low-power field winding on the rotor; though small synchronous machines with the reverse arrangement may also be built.
The advantages of providing the field winding on rotor and armature winding on the stator are given below:
(1) More Efficient. With armature winding on the stator and field winding on the rotor, only two slip rings are required in a synchronous machine. There are, therefore, reduced slip ring losses and a more efficient synchronous machine.
(2) Better Insulation. Stationary armature windings can be insulated satisfactorily for higher voltages, allowing the construction of high-voltage, say 33 kV, synchronous machines.
(3) Eficient Cooling. Stationary armature winding can be cooled more efficiently, thus l permitting the construction of large synchronous machines, say 1000 MW or above.
(4) More Output. Low-power field winding on the rotor gives a lighter rotor and,therefore, low centrifugal forces. In view of this, higher rotor speeds are permissible, thus
increasing the synchronous machine output for given dimensions.
(5) Lesser Rotor Weight and Inertia. Field winding on the rotor requires less amount of copper and insulation.This reduces overall weight of rotor and its inertia. Reduced rotor
weight allows the use of low-priced bearings and also their longer life because of minimal wear and tear.
(6) Rigid and Convenient Construction. Three-phase armature winding, capable of handling high voltage and high current, can be more easily braced against electromagnetic forces when it is placed in stator slots In addition, flexible water tube connection for water cooling can be installed more conveniently on stator than on the rotor. This all results in a rigid and convenient construction of a synchronous machine.
(7) More Armature Tooth Strength. High-current synchronous machines require more armature copper for each slot. Greater amount of copper can be accommodated by making the slots deeper so that wider and stronger teeth are prepared for the armature. Armature on stator would have wider and stronger teeth whereas the armature on rotor would lead to narrower and weaker teeth. Strong teeth also results in less noise due to vibration and are less likely to be damaged during fabrication and use. Therefore, armature winding must be provided on the stator and field winding on the rotor.
Synchronous machies are of two types depending upon the geometrical structure of the rotor,
(a) Salient-Pole type rotor
(B) Cylindrical-rotor, round rotor or non-salient Pole type rotor.
(a) Salient-Pole type rotor – Is Salient means standing out, Sticking out or Projecting and Four salient poles are shown on the rotor. The field winding on the salient poles is a concentrated winding.
(B) Cylindrical-rotor, round rotor or non-salient Pole type rotor – In This rotor the field winding is a distributed winding housed in the rotor slots. The air-gap is uniform throughout, neglecting the slot-openings.
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