The document discusses synchronous motors and synchronization of alternators. It provides information on the following key points: 1. Synchronous motors run at a constant synchronous speed, which can be modified by changing supply frequency or number of poles. They require an exciter to start and can operate at varying power factors. 2. For proper synchronization of alternators, their terminal voltages and frequencies must match the bus-bar voltage and frequency, and their phases must be identical. 3. Synchronous motors are started by first applying reduced voltage to start rotation, then applying weak excitation to pull it into synchronism before applying full voltage. Proper excitation allows operation at varying power factors.

1. Chapter-7
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Parallel Operation

2. # Synchronizing:
The operation of connecting an alternator in parallel with another
alternator or with common bus-bars is known as synchronizing.
# Synchronous Motor:
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Synchronizing and Synchronous Motor
A synchronous motor is an electrical machine which runs at
synchronous speed. It can be operated at constant speed. The speed
(𝑁𝑠) can be modified by changing the frequency (f) or changing the
number of pole (P) as follows
𝑁𝑠 =
120𝑓
𝑃
It is not self starting motor. An exciter is required to start the machine.
This type of motor can be operated by the wide range of power factor
(both lagging and leading).
Most synchronous motors are rated between 150 kW and 15 MW and
run at speeds ranging from 150 to 1800 r.p.m.

3. # Condition for Synchronization of Alternators
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Synchronizing and Synchronous Motor
For proper synchronization of alternators, the following three
conditions must be satisfied :
1. The terminal voltage (effective) of the incoming alternator
must be the same as bus-bar voltage.
2. The speed of the incoming machine must be such that its
frequency (= PN/120) equals bus-bar frequency.
3. The phase of the alternator voltage must be identical with the
phase of the bus-bar voltage. It means that the switch must be
closed at (or very near) the instant the two voltages have correct
phase relationship.

4. Comparison Between Synchronous and Induction Motors
4
The comparison between synchronous motor and induction motor can be
written as follows:
Synchronous Motor Induction Motor
The speed remain constant
when load is increased
The speed decreases with the
increase of load
It can be operated either leading
or lagging power factor
It can be operated only lagging
power factor
A synchronous motor is
inherently not self-starting
Induction motor contains self
starting
A d.c. excitation is required by
synchronous motor
A d.c. excitation is not required
by synchronous motor
Cost is high Cost is low
Suitable for below 300 r.p.m. Suitable for above 600 r.p.m.

5. Synchronous Motor Applications
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The application of synchronous motor are
mentioned as follows:
1. For the correction of power factor
2. It is very suitable where constant speed is
required.
3. In voltage regulation of transmission line.

6. Construction of V-curves
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The V-curves of a synchronous
motor show how armature
current varies with its field
current when motor input is
kept constant as shown in Fig.
1. In order to draw these curves
experimentally, the motor is
run from constant voltage and
constant-frequency bus-bars.
Power input to motor is kept
constant at a definite value.
Next, field current is increased in small steps and corresponding armature
currents are noted. When plotted, we get a V-curve for a particular
constant motor input.
Fig. 1

7. 7
General Characteristics of Synchronous Motor
A synchronous motor is electrically identical with an alternator or ac
generator. Most synchronous motors are rated between 150 kW and
15 MW and run at speeds ranging from 150 to 1800rpm. Some
characteristic features of a synchronous motor are worth noting:
i) It runs either at synchronous speed or not at all i.e. while running
it maintains a constant speed. The only way to change its speed is
to vary the supply frequency (because 𝑁𝑆 =
120𝑓
𝑃
).
ii) It is not inherently self-starting. It has to be run upto synchronous
(or near synchronous) speed by some means, before it can be
synchronized to the supply.
iii) It is capable of being operated under a wide range of power
factors, both lagging and leading. Hence, it can be used for power
correction purposes, in addition to supplying torque to drive
loads.

8. There are two types of excitations
1. Under Excitation:
When the back e.m.f. (Eb) is less
than supply voltage (v) of an
synchronous motor then it is
called under excitation. It bears
lagging power factor.
2. Over Excitation:
When the back e.m.f. (Eb) is
greater than supply voltage (v) of
an synchronous motor then it is
called over excitation. It bears
leading power factor.
Synchronous Motor with Different Excitations
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Fig. 2
Fig. 3

9. Starting Procedure of a Synchronous Motor
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The starting procedure of synchronous motor is mentioned as
follows:
1. First, main field winding is short-circuited.
2. Reduced voltage with the help of auto-transformers is applied
across stator terminals. The motor starts up.
3. When it reaches a steady speed (as judged by its sound), a
weak d.c. excitation is applied by removing the short-circuit
on the main field winding. If excitation is sufficient, then the
machine will be pulled into synchronism.
4. Full supply voltage is applied across stator terminals by
cutting out the auto-transformers.
5. The motor may be operated at any desired power factor by
changing the d.c. excitation.

10. Thanks to all
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