In case of synchronous machines Armature is always placed on stator and field winding on armature as there are following advantages:-
1. Better economy:-
It is always economical to place armature winding on Stator and field winding on rotor in synchronous machines as this can be illustrated by example as below:-
Consider a 3-Phase Star connected 500 MVA, 11KV synchronous machine, now its line current will be = 500X103
Which Will be = 26244 A
Now If we place armature winding on rotor than we will required 3 slip rings each will be capable of handling such a high current of 26244A. Further Slip rings needed to be insulated from shaft for a voltage= 11/ = 6.35 KV.
Also Star point needed to be brought out by using fourth slip ring which will be grounded through a resistance.
Now consider a case where we will place field winding on rotor and armature winding on Stator, in that case field winding need to handle lower current and voltages. Now consider 2 MW at 500V then current required to handle field current of= 2000/0.5= 4000A . So we needed 2 brushes to handle 4000A each instead of 4 brushes carrying such a high current as in earlier case.
Also Slip rings needed to be insulated for voltage of 500V only. Thus we see that how it will be cheaper to have armature winding on stator.
2. Low insulation levels required:-
As we have seen in both cases if we place armature winding on stator than slip rings needed to be insulated for lower voltages. Thus this type of construction will allow construction even upto 33KV synchronous machines
3. Lower losses:-
When armature winding is placed on stator than we will require only 2 carbon brushes which means lower losses due to slip rings.
4. Higher Output power:-
Since field winding of lighter weight is placed on field winding so this will leads to low centrifugal forces. So higher RPM of rotor will be possible and which will result into increased output for given dimensions.
5. More rigid and easy construction:-
When armature winding is place on stator than it will be capable of handling higher voltages and currents , also it will be easier to place heavy winding on Stator slots. It will easier to install water tubing for cooling on stator than rotor. These all add to rigidness and convenience of synchronous machines.
6. Higher Armature tooth Strength:-
Higher current synchronous machines will require more armature copper for each slot. For accommodating more copper by making slots deeper on armature. It will be easier to cut deeper slots in when armature is on stator than on rotor. As if armature winding to be placed on rotor than rotor would have narrower and weaker teeth.
Stronger the teeth lower will be the noise and vibration.
7. Lower rotor weight and lighter bearings:-
Field winding on rotor will leads to lower quantity of copper and insulation on rotor. This means lower weight of rotor and which means low priced bearings which means longer life due to minimum wear and tear.