Machine components

1. UNIT: III
SYNCHRONOUS GENERATORS

2. INTRODUCTION:
• The machine which converts mechanical power into a.c.
electrical power is called synchronous generator or alternator.
However the same machine can be operated as a motor.

3. SYNCHRONOUS MACHINE:
• A machine in which the following relation is maintained for its
satisfactory operation is called a synchronous machine (The machine
may work as a generator or motor):
Ns = 120 f /P rpm
• where NS is the synchronous speed in rpm; f is the supply frequency
and P is the number of poles of the machine.
• When the machine is to work as a generator, it has to run at
synchronous speed (Ns ) to generate power at certain frequency (f),
called power frequency.
• In India its value is 50 Hz, whereas in the USA it is kept at 60 Hz.
• When the machine works as a motor, it can rotate only at
synchronous speed (Ns ) since the magnetic poles are locked with the

4. SYNCHRONOUS MACHINE(CONTD.):
• When connected to an electric power system, a synchronous machine
always maintains this relationship. If a synchronous machine working
as a motor fails to maintain this average speed(Ns) the machine will
not develop sufficient torque to maintain its rotation and will stop.
Then the motor is said to be pulled out of step.
• In case, a synchronous machine is operating as a generator, it has to
run at a fixed speed called synchronous speed to generate power at a
particular frequency since all the appliances and machines are
designed to operate at this frequency.

5. BASIC PRINCIPLES:
• A synchronous machine is just an electro-mechanical
transducer which converts mechanical energy into electrical
energy or vice-versa. The fundamental phenomenon which
make these conversions possible are :
• (i) the law of electro-magnetic induction and
• (ii) law of interaction.
•

6. GENERATOR ACTION:
(i) Law of electromagnetic induction: This relates to the
production of emf, i.e., emf is induced in a conductor whenever
it cuts across the magnetic field. This is called Faraday’s first law
of electromagnetic induction.

7. MOTOR ACTION:
(ii) Law of interaction: This law relates to the phenomenon of
production of force. or torque i.e., whenever a current carrying
conductor is placed in the magnetic field, by the interaction of
the magnetic fields produced by the current carrying conductor
and the main field, force is exerted on the conductor and torque
is developed.

8. PRODUCTION OF SINUSOIDAL
ALTERNATING E.M.F:
• When a conductor or coil cuts across the magnetic field an emf
is induced in it by the phenomenon called electromagnetic
induction. This can be achieved either by rotating a coil in the
stationary magnetic field or by keeping the coil stationary and
rotating the magnetic field. (The magnetic field can be rotated
by placing the field winding on the rotating part of the machine

9. • For illustration see Figs. (a) and (b), two positions of a coil
rotating in a stationary magnetic field are shown. Whereas, in
Figs. (c) and (d), two positions of a rotating electro-magnet in a
coil placed on stationary armature are shown. At first instant,
the emf induced in the coil is zero since flux cut by the coil is
zero. However, at second instant, the emf induced in the coil is
maximum (say positive). The two instants T1 and T2 are
marked on the wave diagram shown in Fig. (e). In one
revolution the induced emf completes one cycle and its wave
shape is shown in Fig. (e).

10. CONSTRUCTIONAL FEATURES
OF SYNCHRONOUS MACHINES:
• The important parts of a synchronous machine are given below:
• 1. Stator 2. Rotor 3. Miscellaneous
• 1. Stator: The outer stationary part of the machine is called stator; it has the
following important parts:
• (i) Stator frame: It is the outer body of the machine made of cast iron and it
protects the inner parts of the machine. It can be also made of any other
strong material since it is not to carry the magnetic field. Cast iron is used
only because of its high mechanical strength.
• (ii) Stator Core: The stator core is made of silicon steel material. It is made
from number of stamping which are insulated from each other. Its function
is to provide an easy path for the magnetic lines of force and accommodate
the stator winding.
• (iii) Stator Winding: Slots are cut on the inner periphery of the stator core in

11. CONSTRUCTIONAL FEATURES OF
SYNCHRONOUS MACHINES:
• 2. Rotor: The rotating part of the machine is called rotor. From
construction point of view, there are two types of rotors named
as
• (i) Salient pole type rotor;
• (ii) Non-salient pole type rotor

12. • (i) Salient pole type rotor: In this case, projected poles are provided on the
rotor. The cost of construction of salient pole type rotors is low, moreover
sufficient space is available to accommodate field winding but these cannot
bear high mechanical stresses at high speeds. Therefore, salient pole type
construction is suited for medium and low speeds and are usually employed
at hydro-electric and diesel power plants as synchronous generators. Since
the speed of these machines (generators) is quite low, to obtain the required
frequency, the machines have large number of poles as shown in Figs. To
accommodate such a large number of poles, these machines have larger
diameter and small length. For a speed of 200 rpm (alternators coupled with
water turbines) the diameter of the machines is as large as 14 metre and
length is only 1 metre. The salient pole type rotor has the following
important parts:

13. • (a) Spider: Spider is made of cast iron to provide an easy path
for the magnetic flux. It is keyed to the shaft and at the outer
surface, pole core and pole-shoe are keyed to it.
• (b) Pole core and pole shoe: It is made of laminated sheet
material. Pole core provides least reluctance path for the
magnetic field and pole shoe distributes the field over the
whole periphery uniformly to produce sinusoidal wave form of
the generated emf.

14. • (c) Field winding or Exciting winding: Field winding is wound on
the former and then placed around the pole core. DC supply is
given to it through slip rings. When direct current flows
through the field winding, it produces the required magnetic
field.
• (d) Damper winding: At the outermost periphery, holes are
provided in which copper bars are inserted and short-circuited
at both the sides by rings forming damper winding. Generally,
the segments on individual poles are joined together to form
common rings resulting in a short-circuited squirrel cage
winding similar to that used in induction machines with squirrel
cage rotors. Salient pole machines are frequently provided with

15. • (ii) Non-salient pole type rotor: A non-salient pole alternator is shown in fig. In
this case, there are no projected poles but the poles are formed by the current
flowing through the rotor (exciting) winding. Non-salient pole type construction
is suited for the high speeds. The steam turbines rotate at a high speed (3000
rpm). When these turbines are used as prime-mover for this machine working as
a generator, a small number of poles are required for given frequency. Hence,
these machines have smaller diameter and larger length.
• Non salient pole type rotors have the following parts:
• (a) Rotor core: Rotor core is made of silicon steel stampings. It is keyed to the
shaft. At the outer periphery slots are cut in which exciting coils are placed. It
provides an easy path to the magnetic flux.
• (b) Rotor winding or Exciting winding: It is placed in rotor slots and current is
passed through the winding in such a way that poles are formed according to
the requirement.

16. • 3. Miscellaneous Parts: The following are few important
miscellaneous parts; (i) Brushes: Brushes are made of carbon
and these just slip over the slip rings. DC supply is given to the
brushes. From brushes current flows to the slip rings and then
to the exciting winding.
• (ii) Bearings: Bearings are provided between the shaft and outer
stationary body to reduce the friction. The material used for
their construction is high carbon steel.
• (iii) Shaft: Shaft is made of mild steel. Mechanical power is
taken or given to the machine through shaft

17. ADVANTAGES OF ROTATING FIELD
SYSTEM OVER STATIONERY FIELD
SYSTEM:
Only in small synchronous machines the field system is placed
on stator and armature winding on rotor, but in larger machines,
the field winding is placed on the rotor and armature winding is
placed on the stator. The rotating field and stationary armature
system is preferred over stationary field and rotating armature
system.
Following are the important advantages of rotating field system
over stationary field system:
• (i) The armature winding is more complex than the field
winding. Therefore, it is easy to place armature winding on

18. ADVANTAGES OF ROTATING FIELD
SYSTEM OVER STATIONERY FIELD
SYSTEM:
• (ii) It is easier to build and properly balance high speed rotors
when they carry the lighter field system.
• (iii) The weight of rotor is small when field system is provided
on rotor and as such less friction losses are produced.
• (iv) Better cooling system can be provided when the armature is
kept stationary winding when it is placed on stationary
structure

19. WINDING FACTOR
• The combined effect of coil span factor and distribution factor
is known as winding factor. In fact, winding factor is the
product of coil span factor and distribution factor. Kw = Kc ×
Kd

20. GENERATION OF THREE-PHASE EMF:
• In a three-phase system, there are equal voltages (or emfs) of the same
frequency having a phase difference of 120°. These voltages can be
produced by a three-phase AC generator having three identical windings (or
phases) fixed on the some spindle and displaced by 120° electrical. When
these windings are rotated in a stationary magnetic field as shown in Fig. (a)
or when these windings are kept stationary and the magnetic field is rotated
[see Fig. (b)], an emf is induced in each winding or phase. These emfs are of
same magnetic and frequency but are displaced from one another by 120°
electrical.

21. • Consider three identical coils a1 a2, b1 b2 and c1c2 mounted on the rotor
as shown in (a) or placed on the stationary armature as shown in Fig. (b).
Here, a1 , b1 and c1 are the start terminals, whereas, a2, b2 and c2 are
finish terminals of three coils. It may be noted that a phase difference of
120° electrical is maintained between the corresponding start terminals a1 ,
b1 and c1 . Let the three coils mounted on the same axis be rotated (or the
magnetic field system be rotated keeping coils stationary) in anti-clockwise
direction at Z radians/second, as shown in Fig. (a) and (b) respectively.
Three emfs are induced in the three coils respectively. Their magnitude and
direction, at this instant are given below:
• (i) The emf induced in coil a1 a2 is zero and is increasing in the positive
direction as shown by wave a1 a2 in Fig. (c).
• (ii) The coil b1 b2 is 120° (electrical) behind the coil a1 a2 The emf induced
in this coil is negative and is becoming maximum negative as shown by the
wave b1 b2 in Fig. (c).
• (iii) The coil c1c2 is 120° (electrical) behind b1 b2 or 240° (electrical) behind
a1 a2 The emf induced in this coil is positive and is decreasing as shown by

22. • Phasor diagram: The emfs induced in three coils are of the
same magnitude and frequency but are displaced by 120°
(electrical) from each other as shown in phasor diagram [see
Fig. (d)].
• These can be represented by the equations:

23. E.M.F. EQUATION:

24. E.M.F. EQUATION:

25. SYNCHRONOUS MOTOR:

26. WORKING PRINCIPLE OF A
SYNCHRONOUS MOTOR:

27. USE OF DAMPER WINDING:

28. PITCH FACTOR & DISTRIBUTION
FACTOR:

29. ALTERNATOR ON LOAD:

30. VOLTAGE REGULATION:

31. DETERMINATION OF VOLTAGE
REGULATION:

32. PARALLEL OPERATION OF
ALTERNATORS: