3. Synchronous machine
Week 5
3.1 Introduction:
Synchronous motors are motors that always run at the same speed regardless of load.
Synchronous motors are somewhat more complex than squirrel-cage and wound rotor motors and,
hence, are more expensive. There is no slip in a synchronous motor, that is, the rotor always moves
at exactly the same speed as the rotating stator field.. The machine consists of three main parts:

Stator, which carries the three phase winding,

Rotor, with one DC winding or permanent magnets

Slip rings or excitation machine (exciter) (in case of electrical excitation).
Synchronous motors are used whenever exact speed must be maintained or for power factor
correction. Synchronous motors are more expensive than other types for the lower horsepower
ratings, but may possibly be more economical for 100 hp and larger ratings.
3.2 Stator construction:
The stator of a synchronous generator holds a three-phase winding where the individual phase
windings are distributed 120° apart in space and is sometimes called the armature winding. The
stator must be made of laminated iron sheets in order to reduce eddy currents.
3.3 Rotor construction:
The rotor holds a field winding,which is magnetized
by a DC current( the field current).
The rotating
field winding can be energized through a set of slip
rings and brushes (external excitation), or from a
diode-bridge mounted on the rotor (self-excited). The
rectifier-bridge
is
fed
from
a
shaft-mounted
Metal frame
Laminated iron
core with slots
Insulated copper
bars are placed in
the slots to form
the three-phase
winding
alternator, which is itself excited by the pilot exciter.
In externally fed fields, the source can be a shaftdriven dc generator, a separately excited dc generator, or a solid-state rectifier. Several variations to
these arrangements exist. There are two types of rotors:

Salient-pole rotor (Fig.2) for low-speed machines (e.g.hydro-generators)

Cylindrical rotor (Fig.3) for high-speed machines (e.g. turbo-generators).
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3. Synchronous machine
Week 5
3-Phase Stator Winding
Rotor Field
Winding
Brushes
-
+
Slip Rings
Cylindrical
Pole Rotor
Field current
a- Schematic diagram showing a cylindrical rotor of a synchronous machine
Steel
retaining
ring
Shaft
Shaft
Wedges
DCcurrent
current
DC
terminals
terminals
b- cylindrical rotor of a synchronous machine
Figure3.1: cylindrical rotor of a synchronous machine
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3. Synchronous machine
Week 5
3-Phase Stator Winding
Rotor Field
Winding
Brushes
-
+
Slip Rings
Salient Pole
Rotor
Field current
a- Schematic diagram showing a salient-pole rotor of a synchronous machine
Slip
rings
Pole
Fan
DC excitation
winding
b- salient-pole rotor of a synchronous machine
Figure3.2: Salient-Pole rotor of a synchronous machine
3.4 Principle of operation of the synchronous generator:
When the 3phase rotor is rotated (by an external prime-mover) the rotating magnetic
flux(induced by DC current) induces voltages in the stator windings. These voltages are sinusoidal
with a magnitude that depends on the field current, and also differ by 120° in time and have a
frequency determined by the angular velocity of the rotation.
3.5 Principle of operation of the synchronous motor:
The stator is supplied with three phase supply in order to develop a rotating magnetic field. Also
the rotor is supplied with DC supply to produce constant
magnetic field. As a result of the interaction of these two fields, the rotor will start to move.
However, the synchronous motor is not self started. Consequently, it usually equipped with squirrel
cage windings that mounted on the pole faces of the synchronous motor rotor. These rotor windings
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3. Synchronous machine
Week 5
are frequently referred to as damper or amortisseur windings. Thus, the synchronous motor starts as
an induction motor. When the motor accelerates to near synchronizing speed (about 95%
synchronous speed), DC current is introduced into the rotor field windings. This current creates
constant polarity poles in the rotor, causing the motor to operate at synchronous speed as the rotor
poles "lock" onto the rotating AC stator poles.
3.6 Excitation Methods
Two methods are commonly utilized for the application of the direct current (DC) field current to
the rotor of a synchronous motor.

Brush-type systems apply the output of a separate DC generator (exciter) to the slip rings of the
rotor.

Brushless excitation systems utilize an integral exciter and rotating rectifier assembly that
eliminates the need for need for brushes and slip rings.
3.7 Method of Synchronization
There are three basic method of synchronizing two or more machine:
. Bright lamp method
. Dark lamp method
. automatic method
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