The various advantages of rotating field can be stated as follows:-

1)This avoids the interaction of mechanical and electrical stresses.

2) It is easier to collect larger currents at very high voltage from a stationary member than from the slip ring and brush assembly.

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

5)3-ɸ armature can be directly connected to load.

-It is a Iron Ring. -It has Slots on its periphery. -It is stationary part and is in stator.-Field Rotates in between thestator causing eddy current loss in the stator.-To minimise eddy current loss, stator is laminated

-The slots consits of coils which are wound on it.-These coils form armature winding.

1) First and most important property of an armature winding is, two sides of any coil should be under two adjacent poles. That means, coil span = pole pitch.

2) The winding can either be single layer or double layer.

3) Winding is so arranged in different armature slots, that it must produce sinusoidal emf.

Rotaion of pole over a rotating shaft is called Rotating Feild

A ring in a dynamo or electric motor which is attached to and rotates with the shaft, passing an electric current to a circuit via a fixed brush pressing against it.

Field coil: the electric coil around a field magnet that produces the magneto motive force to set up the flux in an electric machine.

-It is slow andmoderate speed Alternator-It is made up of large circular steel frame-Field Winding are connected in series-It is used in large power station because it causes noise pollution and it cannot withstand mechanical stresses.

-It is having no. ofslots along the outer periphery-Field Winding are connected in series to slip ring-High speed alternator use non salient type rotor because it is noiseless and better emf is produced

• Wide range of RPM

• Lighter in weight

• More delicate

• Needs field current

• More complicated

• Narrow RPM range

• Heavier for output

• Bulletproof

• Needs to turn

• Very simple system

The output frequency of an alternator depends on the number of poles and the rotational speed.

Rotor speed in r.p.m

No. of rotor poles

Frequency of e.m.f in Hz

-The rotor winding is energized from a DC supply -When the rotor rotates, emf is induced in the armature due to electromagnetic induction -The direction of induced emf can be found by fleming’s right hand rule and the frequency is given by

•In synchronous motor the speed remains constant irrespective of the loads. This characteristics helps in industrial drives where constant speed is required irrespective of the load it is driving. It also useful when the motor is required to drive another alternator to supply at a different frequency as in frequency changes

•Synchronous motors can be constructed with wider air gaps than induction motors which makes these motors mechanically more stable

•In synchronous motors electro-magnetic power varies linearly with the voltage

•Synchronous motors usually operate with higher efficiencies ( more than 90%) especially in low speed and unity power factor applications compared to induction motors

•Synchronous motors requires dc excitation which must be supplied from external sources•Synchronous motors are inherently not self starting motors and needs some arrangement for its starting and synchronizing•The cost per kW output is generally higher than that of induction motors•These motors cannot be used for variable speed applications as there is no possibility of speed adjustment unless the incoming supply frequency is adjusted (Variable Frequency Drives)•Synchronous motors cannot be started on load. Its starting torque is zero•These motors have tendency to hunt•When loading on the synchronous motor increases beyond its capability, the synchronism between rotor and stator rotating magnetic field is lost and motor comes to halt •Collector rings and brushes are required resulting in increase in maintenance•Synchronous motors cannot be useful for applications requiring frequent starting or high starting torques required

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