In this article we are going discuss about why rotating field is used over rotating armature in alternators or synchronous machine.In the alternator construction we said in large AC generators we use stationary armature and rotating field.There are some advantages are with stationary armature arrangement want to know what are they? here we go....advantages of stationary armature in synchronous machines.
Advantages of rotating field over rotating armature
1) We use AC power in our daily life which has generation level around 11 kV to 33 kV,it is difficult to get induced emf in armature because we can't mount as many as conductors as required on rotating armature hence we prefer stationary armature.
2) As said in above point large conductors posses high centrifugal forces while rotating.So there will be chance of conductors slipping out from slots.So by using rotating field over rotating armature we can reduce mechanical and electrical stresses.
3) 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.
4) It is not that much easy to collect larger currents at very high voltages from rotating armature.
4) 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 dc.) and hence can be easily supplied with the help of slip ring and brush assembly by keeping it rotating.
5) The ventilation arrangement for high voltage side can be improved if it is kept stationary.
6) It is better to rotate low inertia system than high inertia system so we use low voltage on rotor side so that inertia can be reduced along with insulation on the rotor side which also reduces cost of the system
7) As we collect power from armature a rotating field type makes it easier to collect power from the armature,and greater output can be collected at low losses compared to that of rotating armature.
8) 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.
By considering the all above reasons we can say for very high voltages, rotating field type of arrangement in alternators is better than stationary field type. For small voltage rating alternators rotating armature arrangement may be used.