The various advantages of rotating field can be stated as
1)This avoids the interaction of mechanical and electrical
2) It is easier to collect larger currents at very high voltage
from a stationary member than from the slip ring and brush
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
2) The winding can either be single layer or double
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
• Wide range of RPM
• Lighter in weight
• More delicate
• Needs field current
• More complicated
• Narrow RPM range
• Heavier for output
• Needs to turn
• Very simple system
The output frequency of an alternator depends on the number of poles and the rotational
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
•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
•Synchronous motors requires dc excitation which must be supplied from external
•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