Why field winding on the rotor in the synchronous machine?

There are many advantages of providing the field winding on the rotor in the synchronous machine and armature winding on the stator. In the synchronous machine, the armature winding either export a.c. power (synchronous generator ) or import a.c. power (synchronous motor) whereas field winding is always energized from d.c. source. Synchronous machines are invariably constructed with high power armature winding on the stator and low power field on the rotor.

1. More Economical

It is economical to own armature winding on the stator and coil field winding on the rotor. So as to let’s say this, consider a 3-phase, star connected 200MVA, 11 kV synchronous machine. Its line current is \frac { 200\times 10^3 }{ \sqrt { 3 } \times 11 } =10,500A. If the armature winding is placed on the rotor, three slip rings must be properly insulated from the shaft for a voltage of 11/\sqrt { 3 }, i.e. 6.35 kV. The star-point of the three-phase winding must even be brought out through the fourth connecter so as to attach it to a grounded metal plate through resistance.

For low power consumption required for the field winding is 1 MW at 500 volts. Then the exciting field current is 1000/0.5=2000 A. Only two slip rings, each capable of handling 2000 A, are required. Also, each slip ring should be of the voltage of 500 volts only. This shows that its cheaper to own coil field winding on the rotor instead of the armature. Thus the construction of a synchronous machine with armature winding on the stator and coil field winding on the rotor is more economical.

2. More Efficient

From the viewpoint for more reliable the armature winding on the stator and field winding on the rotor, only two slip rings are installed in a synchronous machine. Therefore, less requirement of slip ring reduced slip ring losses and a more efficient synchronous machine.

3.Better Insulation

Stationary armature windings can be insulated safely for higher voltages, it’s allowing the synchronous machine construction of high-voltage 33 kV without any harm to anyone.

4.Efficient Cooling

Stationary armature winding can be cooled more easily and efficiently, thus its allow the construction of large synchronous machines, which can be 1000 MW or above.

5.More Output

Low-power field winding on the rotor provides a lighter rotor and, therefore, low centrifugal forces required to make balance. In view of this, it provides higher rotor speeds are permissible, thus increasing the synchronous machine efficiency for given dimensions.

6.Lesser Rotor Weight and Inertia

Field winding on the rotor requires less amount of copper and less Insulation also. This reduces the weight of the rotor and its inertia and makes a machine more stable. Reduced rotor weight allows low priced bearings can be used and also their longer life because of less wear and tear.

7.Rigid and Convenient Construction

Three-phase armature winding is capable of handling high voltage and high current can be more easily support against electromagnetic forces when it is Placed in stator slots. In addition, a flexible water tube connection for water cooling can be easily installed and more conveniently on stator than on the rotor. This all results in a simplified and convenient construction of a synchronous machine.

8.More Armature Tooth Strength

Synchronous machines with high current require more armature copper for each slot. A greater amount of copper can be required by making the slots deeper so that to make winder and stronger teeth are prepared for the armature. Armature on stator should wider and stronger teeth whereas the armature on rotor should be narrower and weaker teeth Strong teeth also results in less noise due to vibration and are less damaged during fabrication and use. so we can say that armature winding must be provided on the stator and field winding on the rotor.

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