Advantages of Rotating Field Over Rotating Armature

Advantages of Rotating Field Over Rotating Armature AU : May-12

The various advantages of rotating field can be stated as, 

1)  As everywhere a.c. is used, the generation level of a.c. voltage may be higher as 11 kV to 33 kV. This gets induced in the armature. For stationary armature large space can be provided to accommodate large number of conductors and the insulation.

2) It is always better to protect high voltage winding from the centrifugal forces caused due to the rotation. So high voltage armature is generally kept stationary. This avoids the interaction of mechanical and electrical stresses.

3) It is easier to collect larger currents at very high voltages from a stationary member than from the slip ring and brush assembly. The voltage required to be supplied to the field is very low (110 V to 220 V d.c.) and hence can be easily supplied with the help of slip ring and brush assembly by keeping it rotating.

4) The problem of sparking at the slip rings can be avoided by keeping field rotating which is low voltage circuit and high voltage armature as stationary.

5) Due to low voltage level on the field side, the insulation required is less and hence field system has very low inertia. It is always better to rotate low inertia system than high inertia, as efforts required to rotate low inertia system are always less.

6) Rotating field makes the overall construction very simple. With simple, robust mechanical construction and low inertia of rotor, it can be driven at high speeds. So greater output can be obtained from an alternator of given size.

7) If field is rotating, to excite it by an external d.c. supply two slip rings are enough. One each for positive and negative terminals. As against this, in three phase rotating armature the minimum number of slip rings required are three and can not be easily insulated due to high voltage levels.

8) The ventilation arrangement for high voltage side can be improved if it is kept stationary.

Due to all these reasons the most of the alternators in practice use rotating field type of arrangement. For small voltage rating alternators rotating armature arrangement may be used.

Review Question

1. List the advantages of stationary armature in synchronous machines.

Construction

As mentioned earlier, most of the alternators prefer rotating field type of construction. In case of alternators the winding terminology is slightly different than in case of d.c. generators. In alternators the stationary winding is called 'Stator' while the rotating winding is called 'Rotor'. 

Key Point : So most of alternators have stator as armature and rotor as field, in practice.