MODULE: ELECTRIC ENGINEERING TECHNOLOGY IV/ELECTRICAL MACHINES & CONTROL II.
UNIT 1: THREE PHASE SYNCHRONOUS MOTOR
INTRODUCTION
It may be recalled that a d.c. generator can be run as a d.c. motor. In like manner, an alternator may operate as a
motor by connecting its armature winding to a 3-phase supply. It is then called a synchronous motor. As the name
implies, a synchronous motor runs at synchronous speed (Ns = 120f/P) i.e., in synchronism with the revolving field
produced by the 3-phase supply. The speed of rotation is, therefore, tied to the frequency of the source. Since the
frequency is fixed, the motor speed stays constant irrespective of the load or voltage of 3- phase supply. However,
synchronous motors are not used so much because they run at constant speed (i.e., synchronous speed) but
interesting is that they possess other unique electrical properties.
THE CONSTRUCTION OF SYNCHRONOUS MOTOR
A synchronous motor is a machine that operates at synchronous speed and converts electrical energy into
mechanical energy. It is fundamentally an alternator operated as a motor. Like an alternator, a synchronous motor
has the following two parts:
(i) a stator which houses 3-phase armature winding in the slots of the stator core and receives power from a 3-
phase supply [See (Fig. (2.1)].
(ii) a rotor that has a set of salient poles excited by direct current to form alternate N and S poles. The exciting
coils are connected in series to two slip rings and direct current is fed into the winding from an external
exciter mounted on the rotor shaft. The stator is wound for the same number of poles as the rotor poles. As
in the case of an induction motor, the number of poles determines the synchronous speed of the motor:
Figure 2.1
An important drawback of a synchronous motor is that it is not self-starting and auxiliary means have to be used for
starting it.
Unique Features of Synchronous Motor
Some salient features of a synchronous motor are:
(i) A synchronous motor runs at synchronous speed or not at all. Its speed is constant (synchronous speed) at all
loads. The only way to change its speed is to alter the supply frequency (N s = 120 f/P).
(ii) The outstanding characteristic of a synchronous motor is that it can be made to operate over a wide range of
power factors (lagging, unity or leading) by adjustment of its field excitation. Therefore, a synchronous motor
can be made to carry the mechanical load at constant speed and at the same time improve the power factor of
the system.
Prepared By Mr. Omondi Ferdinand – 0712747442
Electrical Trainer - Kisumu 1
, MODULE: ELECTRIC ENGINEERING TECHNOLOGY IV/ELECTRICAL MACHINES & CONTROL II.
UNIT 1: THREE PHASE SYNCHRONOUS MOTOR
(iii) Synchronous motors are generally of the salient pole type.
(iv) A synchronous motor is not self-starting and an auxiliary means has to be used for starting it. We use either
induction motor principle or a separate starting motor for this purpose. If the latter method is used, the machine
must be run up to synchronous speed and synchronized as an alternator.
THE OPERATION OF SYNCHRONOUS MOTOR
Operating Principle
The fact that a synchronous motor has no starting torque can be easily explained.
(i) Consider a 3-phase synchronous motor having two rotor poles NR and SR. Then the stator will also be
wound for two poles NS and SS. The motor has direct voltage applied to the rotor winding and a 3-phase
voltage applied to the stator winding. The stator winding produces a rotating field which revolves round the
stator at synchronous speed Ns (= 120 f/P). The direct (or zero frequency) current sets up a two-pole field
which is stationary so long as the rotor is not turning. Thus, we have a situation in which there exists a pair
of revolving armature poles (i.e., NS - SS) and a pair of stationary rotor poles (i.e., NR - SR).
(ii) Suppose at any instant, the stator poles are at positions A and B as shown in Fig. (2.2 (i)). It is clear that
poles NS and NR repel each other and so do the poles SS and SR. Therefore, the rotor tends to move in the
anticlockwise direction. After a period of half-cycle (or ½ f = 1/100 second), the polarities of the stator
poles are reversed but the polarities of the rotor poles remain the same as shown in Fig. (2.2 (ii)). Now S S
and NR attract each other and so do NS and SR. Therefore, the rotor tends to move in the clockwise
direction. Since the stator poles change their polarities rapidly, they tend to pull the rotor first in one
direction and then after a period of half-cycle in the other. Due to high inertia of the rotor, the motor fails to
start.
Figure 2.2
Hence, a synchronous motor has no self-starting torque i.e., a synchronous motor cannot start by itself.
Making Synchronous Motor Self-Starting
A synchronous motor cannot start by itself. In order to make the motor self-starting, a squirrel cage winding (also
called damper winding) is provided on the rotor. The damper winding consists of copper bars embedded in the pole
faces of the salient poles of the rotor as shown in Fig. (2.3). The bars are short-circuited at the ends to form in effect
a partial squirrel cage winding. The damper winding serves to start the motor.
To start with, 3-phase supply is given to the stator winding while the rotor field winding is left unenergized.
The rotating stator field induces currents in the damper or squirrel cage winding and the motor starts as an
induction motor.
As the motor approaches the synchronous speed, the rotor is excited with direct current. Now the resulting
poles on the rotor face poles of opposite polarity on the stator and a strong magnetic attraction is set up
Prepared By Mr. Omondi Ferdinand – 0712747442
Electrical Trainer - Kisumu 2
UNIT 1: THREE PHASE SYNCHRONOUS MOTOR
INTRODUCTION
It may be recalled that a d.c. generator can be run as a d.c. motor. In like manner, an alternator may operate as a
motor by connecting its armature winding to a 3-phase supply. It is then called a synchronous motor. As the name
implies, a synchronous motor runs at synchronous speed (Ns = 120f/P) i.e., in synchronism with the revolving field
produced by the 3-phase supply. The speed of rotation is, therefore, tied to the frequency of the source. Since the
frequency is fixed, the motor speed stays constant irrespective of the load or voltage of 3- phase supply. However,
synchronous motors are not used so much because they run at constant speed (i.e., synchronous speed) but
interesting is that they possess other unique electrical properties.
THE CONSTRUCTION OF SYNCHRONOUS MOTOR
A synchronous motor is a machine that operates at synchronous speed and converts electrical energy into
mechanical energy. It is fundamentally an alternator operated as a motor. Like an alternator, a synchronous motor
has the following two parts:
(i) a stator which houses 3-phase armature winding in the slots of the stator core and receives power from a 3-
phase supply [See (Fig. (2.1)].
(ii) a rotor that has a set of salient poles excited by direct current to form alternate N and S poles. The exciting
coils are connected in series to two slip rings and direct current is fed into the winding from an external
exciter mounted on the rotor shaft. The stator is wound for the same number of poles as the rotor poles. As
in the case of an induction motor, the number of poles determines the synchronous speed of the motor:
Figure 2.1
An important drawback of a synchronous motor is that it is not self-starting and auxiliary means have to be used for
starting it.
Unique Features of Synchronous Motor
Some salient features of a synchronous motor are:
(i) A synchronous motor runs at synchronous speed or not at all. Its speed is constant (synchronous speed) at all
loads. The only way to change its speed is to alter the supply frequency (N s = 120 f/P).
(ii) The outstanding characteristic of a synchronous motor is that it can be made to operate over a wide range of
power factors (lagging, unity or leading) by adjustment of its field excitation. Therefore, a synchronous motor
can be made to carry the mechanical load at constant speed and at the same time improve the power factor of
the system.
Prepared By Mr. Omondi Ferdinand – 0712747442
Electrical Trainer - Kisumu 1
, MODULE: ELECTRIC ENGINEERING TECHNOLOGY IV/ELECTRICAL MACHINES & CONTROL II.
UNIT 1: THREE PHASE SYNCHRONOUS MOTOR
(iii) Synchronous motors are generally of the salient pole type.
(iv) A synchronous motor is not self-starting and an auxiliary means has to be used for starting it. We use either
induction motor principle or a separate starting motor for this purpose. If the latter method is used, the machine
must be run up to synchronous speed and synchronized as an alternator.
THE OPERATION OF SYNCHRONOUS MOTOR
Operating Principle
The fact that a synchronous motor has no starting torque can be easily explained.
(i) Consider a 3-phase synchronous motor having two rotor poles NR and SR. Then the stator will also be
wound for two poles NS and SS. The motor has direct voltage applied to the rotor winding and a 3-phase
voltage applied to the stator winding. The stator winding produces a rotating field which revolves round the
stator at synchronous speed Ns (= 120 f/P). The direct (or zero frequency) current sets up a two-pole field
which is stationary so long as the rotor is not turning. Thus, we have a situation in which there exists a pair
of revolving armature poles (i.e., NS - SS) and a pair of stationary rotor poles (i.e., NR - SR).
(ii) Suppose at any instant, the stator poles are at positions A and B as shown in Fig. (2.2 (i)). It is clear that
poles NS and NR repel each other and so do the poles SS and SR. Therefore, the rotor tends to move in the
anticlockwise direction. After a period of half-cycle (or ½ f = 1/100 second), the polarities of the stator
poles are reversed but the polarities of the rotor poles remain the same as shown in Fig. (2.2 (ii)). Now S S
and NR attract each other and so do NS and SR. Therefore, the rotor tends to move in the clockwise
direction. Since the stator poles change their polarities rapidly, they tend to pull the rotor first in one
direction and then after a period of half-cycle in the other. Due to high inertia of the rotor, the motor fails to
start.
Figure 2.2
Hence, a synchronous motor has no self-starting torque i.e., a synchronous motor cannot start by itself.
Making Synchronous Motor Self-Starting
A synchronous motor cannot start by itself. In order to make the motor self-starting, a squirrel cage winding (also
called damper winding) is provided on the rotor. The damper winding consists of copper bars embedded in the pole
faces of the salient poles of the rotor as shown in Fig. (2.3). The bars are short-circuited at the ends to form in effect
a partial squirrel cage winding. The damper winding serves to start the motor.
To start with, 3-phase supply is given to the stator winding while the rotor field winding is left unenergized.
The rotating stator field induces currents in the damper or squirrel cage winding and the motor starts as an
induction motor.
As the motor approaches the synchronous speed, the rotor is excited with direct current. Now the resulting
poles on the rotor face poles of opposite polarity on the stator and a strong magnetic attraction is set up
Prepared By Mr. Omondi Ferdinand – 0712747442
Electrical Trainer - Kisumu 2
