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8.1 Introduction
The popularity of 3 phase induction motors on board ships is because of their simple,
robust construction, and high reliability factor in the sea environment. A 3 phase
induction motor can be used for different applications with various speed and load
requirements. Electric motors can be found in almost every production process today.
Getting the most out of your application is becoming more and more important in
order to ensure cost-effective operations. 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. 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.
8.1.1 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.
8.1.2 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.
8.2 Construction
The three phase induction motor is the most widely used electrical motor. Almost
80% of the mechanical power used by industries is provided by three phase induction
motors because of its simple and rugged construction, low cost, good operating
characteristics, absence of commutator and good speed regulation. In three phase
induction motor the power is transferred from stator to rotor winding through
induction. The Induction motor is also called asynchronous motor as it runs at a speed
other than the synchronous speed. Like any other electrical motor induction
motor also have two main parts namely rotor and stator. 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. The main body of the Induction Motor comprises of two major parts as shows
in Figure 1 :
i. Shaft for transmitting the torque to the load. This shaft is made up of steel.
ii. Bearings for supporting the rotating shaft.
iii. One of the problems with electrical motor is the production of heat during its
rotation. In order to overcome this problem we need fan for cooling.
iv. For receiving external electrical connection Terminal box is needed.
v. There is a small distance between rotor and stator which usually varies from
0.4 mm to 4 mm. Such a distance is called air gap.
Stator
Stator: As its name indicates stator is a stationary part of induction motor. A stator
winding is placed in the stator of induction motor and the three phase supply is given
to it. Stator is made up of number of stampings in which different slots are cut to
receive 3 phase winding circuit which is connected to 3 phase AC supply. The three
phase windings are arranged in such a manner in the slots that they produce a rotating
magnetic field after AC supply is given to them. The windings are wound for a
definite number of poles depending upon the speed requirement, as speed is inversely
proportional to the number of poles, given by the formula:
Ns= 120f/p
Where Ns= synchronous speed
f = Frequency
p = no. of poles
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 [See Fig.
(8.2)]. 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.
8.2.1.1 Stator of Three Phase Induction Motor
The stator of the three phase induction motor consists of three main parts :.
i. Stator Frame
It is the outer most part of the three phase induction motor. Its main function is to
support the stator core and the field winding. It acts as a covering and it provide
protection and mechanical strength to all the inner parts of the induction motor. The
frame is either made up of die cast or fabricated steel. The frame of three phase
induction motor should be very strong and rigid as the air gap length of motorist very
small, otherwise rotor will not remain concentric with stator, which will give rise to
unbalanced magnetic pull.
Stator Core
The main function of the stator core is to carry the alternating flux. In order to reduce
the eddy current loss, the stator core is laminated. These laminated types of structure
are made up of stamping which is about 0.4 to 0.5 mm thick. All the stamping are
stamped together to form stator core, which is then housed in stator frame. The
stamping is generally made up of silicon steel, which helps to reduce the hysteresis
loss occurring in motor.
iii. Stator Winding or Field Winding
The slots on the periphery of stator core of the motor carries three phase windings.
This three phase winding is supplied by three phase ac supply. The three phases of the
winding are connected either in star or delta depending upon which type of starting
method is used. The squirrel cage motor is mostly started by star – delta stator and
hence the stator of squirrel cage motor is delta connected. The slip ring three phase
induction motor are started by inserting resistances so, the stator winding of slip ring
induction can be connected either in star or delta. The winding wound on the stator
of three phase induction motor is also called field winding and when this winding is
excited by three phase ac supply it produces a rotating magnetic.
8.2.2. Rotor
The rotor is a rotating part of induction motor. The rotor is connected to the
mechanical load through the shaft. Rotor consists of cylindrical laminated core with
parallel slots that carry conductor bars. Conductors are heavy copper or aluminium
bars which fits in each slots. These conductors are brazed to the short circuiting end
rings. The slots are not exactly made parallel to the axis of the shaft but are slotted a
little skewed for the following reason, They reduces magnetic hum or noise and They
avoid stalling of motor. 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: Squirrel cage type and Wound type
8.2.2.1 Squirrel cage rotor.
Squirrel cage three phase induction motor: The rotor of the squirrel cage three phase
induction motor is cylindrical in shape and have slots on its periphery. The slots are
not made parallel to each other but are bit skewed (skewing is not shown in the figure
of squirrel cadge rotor beside) as the skewing prevents magnetic locking of stator and
rotor teeth and makes the working of motor more smooth and quieter. The squirrel
cage rotor consists of aluminum, brass or copper bars. These aluminum, brass or
copper bars are called rotor conductors and are placed in the slots on the periphery of
the rotor. The rotor conductors are permanently shorted by the copper or aluminum
rings called the end rings. In order to provide mechanical strength these rotor
conductor are braced to the end ring and hence form a complete closed circuit
resembling like a cage and hence got its name as “squirrel cage induction motor”. The
squirrel cage rotor winding is made symmetrical. As the bars are permanently shorted
by end rings, the rotor resistance is very small and it is not possible to add
external resistance as the bars are permanently shorted. The absence of slip ring and
brushes make the construction of Squirrel cage three phase induction motor very
simple and robust and hence widely used three phase induction motor. These motors
have the advantage of adapting any number of pole pairs. The below diagram shows
squirrel cage induction rotor having aluminum bars short circuit by aluminum end
rings. 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. (8.3)]. 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.