MODULE: ELECTRICAL ENGINEERING TECHNOLOGY IV/ELECTRICAL MACHINES & CONTROL II
UNIT 1: THREE PHASE A.C MOTOR
INTRODUCTION
The three-phase induction motors are the most widely used electric motors in industry. They run at essentially
constant speed from no-load to full-load. However, the speed is frequency dependent and consequently these motors
are not easily adapted to speed control. We usually prefer d.c. motors when large speed variations are required.
Nevertheless, the 3-phase induction motors are simple, rugged, low-priced, easy to maintain and can be
manufactured with characteristics to suit most industrial requirements.
Three-Phase Induction Motor
Like any electric motor, a 3-phase induction motor has a stator and a rotor. The stator carries a 3-phase winding
(called stator winding) while the rotor carries a short-circuited winding (called rotor winding). Only the stator
winding is fed from 3-phase supply. The rotor winding derives its voltage and power from the externally energized
stator winding through electromagnetic induction and hence the name. The induction motor may be considered to be
a transformer with a rotating secondary and it can, therefore, be described as a “transformer type” a.c. machine in
which electrical energy is converted into mechanical energy.
Advantages
i) It has simple and rugged construction.
ii) It is relatively cheap.
iii) It requires little maintenance.
iv) It has high efficiency and reasonably good power factor.
v) It has self-starting torque.
Disadvantages
(i) It is essentially a constant speed motor and its speed cannot be changed easily.
(ii) Its starting torque is inferior to d.c. shunt motor.
CONSTRUCTION
A 3-phase induction motor has two main parts;
(i) stator and
(ii) rotor.
The rotor is separated from the stator by a small air-gap which ranges from 0.4 mm to 4 mm, depending on the power
of the motor.
Stator
It consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations of silicon steel to
reduce hysteresis and eddy current losses. A number of evenly spaced slots are provided on the inner periphery of the
laminations. The insulated connected to form a balanced 3-phase star or delta connected circuit. The 3-phase stator
winding is wound for a definite number of poles as per requirement of speed. Greater the number of poles, lesser is
the speed of the motor and vice-versa. When 3-phase supply is given to the stator winding, a rotating magnetic field
of constant magnitude is produced. This rotating field induces currents in the rotor by electromagnetic induction.
Prepared by Omondi Ferdinand – 0735 766 013/0712 747 442 1
Engineering Trainer – KMTC - Kisumu Campus
, MODULE: ELECTRICAL ENGINEERING TECHNOLOGY IV/ELECTRICAL MACHINES & CONTROL II
UNIT 1: THREE PHASE A.C MOTOR
Figure 1: Stator
Rotor
The rotor, mounted on a shaft, is a hollow laminated core having slots on its outer periphery. The winding placed in
these slots (called rotor winding) may be one of the following two types:
(i) Squirrel cage type
(ii) Wound type
(i) Squirrel cage rotor.
It consists of a laminated cylindrical core having parallel slots on its outer periphery. One copper or aluminum bar is
placed in each slot. All these bars are joined at each end by metal rings called end rings [See Fig. (8.2)]. This forms a
permanently short-circuited winding which is indestructible. The entire construction (bars and end rings) resembles a
squirrel cage and hence the name. The rotor is not connected electrically to the supply but has current induced in it
by transformer action from the stator.
Those induction motors which employ squirrel cage rotor are called squirrel cage induction motors. Most of 3-phase
induction motors use squirrel cage rotor as it has a remarkably simple and robust construction enabling it to operate
in the most adverse circumstances. However, it suffers from the disadvantage of a low starting torque. It is because
the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit
to have a large starting torque.
Figure 2 Figure 3
(ii) Wound rotor.
It consists of a laminated cylindrical core and carries a 3- phase winding, similar to the one on the stator Fig.3. The
rotor winding is uniformly distributed in the slots and is usually star-connected.
The open ends of the rotor winding are brought out and joined to three insulated slip rings mounted on the rotor shaft
with one brush resting on each slip ring. The three brushes are connected to a 3-phase star-connected rheostat as
shown in Fig.4. At starting, the external resistances are included in the rotor circuit to give a large starting torque.
These resistances are gradually reduced to zero as the motor runs up to speed.
Prepared by Omondi Ferdinand – 0735 766 013/0712 747 442 2
Engineering Trainer – KMTC - Kisumu Campus
UNIT 1: THREE PHASE A.C MOTOR
INTRODUCTION
The three-phase induction motors are the most widely used electric motors in industry. They run at essentially
constant speed from no-load to full-load. However, the speed is frequency dependent and consequently these motors
are not easily adapted to speed control. We usually prefer d.c. motors when large speed variations are required.
Nevertheless, the 3-phase induction motors are simple, rugged, low-priced, easy to maintain and can be
manufactured with characteristics to suit most industrial requirements.
Three-Phase Induction Motor
Like any electric motor, a 3-phase induction motor has a stator and a rotor. The stator carries a 3-phase winding
(called stator winding) while the rotor carries a short-circuited winding (called rotor winding). Only the stator
winding is fed from 3-phase supply. The rotor winding derives its voltage and power from the externally energized
stator winding through electromagnetic induction and hence the name. The induction motor may be considered to be
a transformer with a rotating secondary and it can, therefore, be described as a “transformer type” a.c. machine in
which electrical energy is converted into mechanical energy.
Advantages
i) It has simple and rugged construction.
ii) It is relatively cheap.
iii) It requires little maintenance.
iv) It has high efficiency and reasonably good power factor.
v) It has self-starting torque.
Disadvantages
(i) It is essentially a constant speed motor and its speed cannot be changed easily.
(ii) Its starting torque is inferior to d.c. shunt motor.
CONSTRUCTION
A 3-phase induction motor has two main parts;
(i) stator and
(ii) rotor.
The rotor is separated from the stator by a small air-gap which ranges from 0.4 mm to 4 mm, depending on the power
of the motor.
Stator
It consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations of silicon steel to
reduce hysteresis and eddy current losses. A number of evenly spaced slots are provided on the inner periphery of the
laminations. The insulated connected to form a balanced 3-phase star or delta connected circuit. The 3-phase stator
winding is wound for a definite number of poles as per requirement of speed. Greater the number of poles, lesser is
the speed of the motor and vice-versa. When 3-phase supply is given to the stator winding, a rotating magnetic field
of constant magnitude is produced. This rotating field induces currents in the rotor by electromagnetic induction.
Prepared by Omondi Ferdinand – 0735 766 013/0712 747 442 1
Engineering Trainer – KMTC - Kisumu Campus
, MODULE: ELECTRICAL ENGINEERING TECHNOLOGY IV/ELECTRICAL MACHINES & CONTROL II
UNIT 1: THREE PHASE A.C MOTOR
Figure 1: Stator
Rotor
The rotor, mounted on a shaft, is a hollow laminated core having slots on its outer periphery. The winding placed in
these slots (called rotor winding) may be one of the following two types:
(i) Squirrel cage type
(ii) Wound type
(i) Squirrel cage rotor.
It consists of a laminated cylindrical core having parallel slots on its outer periphery. One copper or aluminum bar is
placed in each slot. All these bars are joined at each end by metal rings called end rings [See Fig. (8.2)]. This forms a
permanently short-circuited winding which is indestructible. The entire construction (bars and end rings) resembles a
squirrel cage and hence the name. The rotor is not connected electrically to the supply but has current induced in it
by transformer action from the stator.
Those induction motors which employ squirrel cage rotor are called squirrel cage induction motors. Most of 3-phase
induction motors use squirrel cage rotor as it has a remarkably simple and robust construction enabling it to operate
in the most adverse circumstances. However, it suffers from the disadvantage of a low starting torque. It is because
the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit
to have a large starting torque.
Figure 2 Figure 3
(ii) Wound rotor.
It consists of a laminated cylindrical core and carries a 3- phase winding, similar to the one on the stator Fig.3. The
rotor winding is uniformly distributed in the slots and is usually star-connected.
The open ends of the rotor winding are brought out and joined to three insulated slip rings mounted on the rotor shaft
with one brush resting on each slip ring. The three brushes are connected to a 3-phase star-connected rheostat as
shown in Fig.4. At starting, the external resistances are included in the rotor circuit to give a large starting torque.
These resistances are gradually reduced to zero as the motor runs up to speed.
Prepared by Omondi Ferdinand – 0735 766 013/0712 747 442 2
Engineering Trainer – KMTC - Kisumu Campus
