•A synchronous machine is a most important type of electric machine. 

•Synchronous Machine used at generating stations are known as Synchronous Generators or Alternators

•Synchronous motors are widely used in Industries and are well known for their const. speed operation

•Electromechanical energy conversion occurs whenever a change in flux is associated with mechanical motion.

Introduction

It consists of:

StatorRotorField Windings (On rotor)Armature Windings (On Stator)

Construction of Synchronous machines

Diagram

It is a stationary member

It is the cylindrical portion inside which the rotor rotates

An air gap is provided between the stator and the rotor

Armature winding are 3 phased and are housed in the slots cut in the stator

It consists of cast iron stator frame, a cylindrical laminated , a cylindrical laminated and uniformly slotted core.

Stator

Rotor is the rotating part of the machine

Can be classified as: (a) Cylindrical Rotor and (b) Salient Pole rotor

Large salient-pole rotors are made of laminated poles retaining the winding under the pole head.

Rotor

Armature windings connected are 3-phase and are either star or delta connected

The windings are 120 degrees apart and normally use distributed windings

Armature Windings (On Stator)

The 3-phase armature winding is distributed in the slots along the armature air gap periphery

For example: Consider that we have 18 slots, 2-pole 3-phase winding..

Hence we have 9 slots/per pole as shown figure..

The winding diagram of phase ‘a’ can be shown as:

Similarly, phase ‘b’ and phase ‘c’ are distributed in same manner

This implies that per phase emf is getting divided in each phase.

When all the 3-phase are connected then mmf-phase graph for each phase is displaced by 120 degrees

The mmf-phase graph can be drawn as:

The field winding of a synchronous machine is always energized with direct current

Under steady state condition, the field or exciting current is given

Ir = Vf/Rf

Vf = Direct voltage applied to the field winding Rf= Field winding Resistance

Field Windings (on Rotor)

Mostly all the synchronous machines use Distributed winding

 Attempt is made to use all the slots available under a pole for the winding which makes the nature of the induced e.m.f. moe sinusoidal

Consider a sync. Machine with 3-phase winding,

Slots, s= 18 slots

Poles=2

slots per pole, g= s/p= 9

slots/pole/phase= g/3 = 3

Distribution Factor (Kd)

• Let E = Induced e.m.f. per coil and there are 3 coils per phase

•In concentrated type all the coil sides will be placed in one slot under a pole. So induced e.m.f. in all the coils will achieve maxima and minima at the same time i.e. all of them will be in phase. Hence resultant e.m.f. after connecting coils in series will be algebraic sum of all the e.m.f.s. as all are in phase

As against this, in distributed type, coil sides will be distributed, one each in the 3 slots per phase available under a pole as shown in the Fig.

Slot pitch , Y= ԓP/s = 20 deg.

In general, let there be

Slots per pole = n

slots/pole/phase= m

Let turns/phase= Nph

Turns per slot, N1 = Nph/ q

Induce emf/turn =E1 = Ec / N

Induced emf/ slot= E = E1 * N1

= Ec * Nph

Nph q

= Ec

q

q – slots for each phase under one pole

Since all the coils are connected in series, hence the resultant emf is phasor sum of all the emfs