Question

Why the armature as used as a stator?

Answer 1

This is because we have to give supply to the field winding and the generated current is taken from armature winding.

As the ratings of alternators are in kVA and MVA, the current and voltages proeuced will be very large.

So if the armature winding is placed in rotor, the generated large amount of current need to be collected from the rotating part by using slip rings or carbon brushes. And it is not feasible to do that.

More information:

In Synchronous Machine, the armature winding is provided on Stator and field winding on Rotor which is contrary to other Electrical Machines. In fact there are certain issues due to which we provide armature winding on Stator. The benefits of this are as follows:

More Economical:

It is more economical to provide Armature winding on Stator and Field winding on rotor. Let us consider a three phase Star connected 750 MVA, 21 kV Synchronous Machine.

Line Current = (750x10³)/(1.732x21) = 20,620 A

If the armature winding is placed on the Rotor, we need three slip rings each capable of handling 20,620 A. Only not this, we need to provide slip rings with an insulation of (21/1.732 = 12.12 kV). The star point for grounding is also to be taken out through the fourth slip ring.

Now, suppose the power requirement for the Field is 2 MW at 500 V. Then

Excitation / Field Current = 2000/0.5 = 4000A

In this case only two Slip Rings each capable of handling 4000 A is required. Also, the insulation requirement for slip ring in this case is only 500 V.

This means it is far better and cheaper to use Field winding on the Rotor instead of armature

More Efficient:

With armature on the Stator and Field winding on the Rotor, only two slip rings are required due to which losses associated with slip ring and carbon brushes are reduced and efficiency increases.

Better Insulation:

As armature winding is stationary, therefore it is possible to insulate the armature winding for higher voltage level and making it possible to generate power at higher voltage up to 33 kV.

Better Cooling:

Better cooling of Armature winding is possible as the winding is stationary. This is the reason that DM (Demineralized Water) cooling is done for armature winding.

More Power Output:

As the cooling is efficient because of stationary armature winding and rotor weight is also less, therefore higher rotor speed is possible, thus increasing the Synchronous Machine output for a given dimension.

More Armature Tooth Strength:

High current Synchronous Machine require more armature copper for each slot. Greater amount of Copper can be accommodated if the Slot / Tooth is deeper and strong enough. If the Armature would have been on Rotor then Slot / Tooth would have been narrow and weaker. Thus armature on the Stator allows stronger and deeper Slot / Tooth. Strong Teeth also reduces the noise due to vibration and are less likely to be damaged during fabrication and use.