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3.1 Three Phase Induction Motor
The most common type of AC motor being used throughout the work today is the "Induction
Motor". Applications of three-phase induction motors of size varying from half a kilowatt to
thousands of kilowatts are numerous. They are found everywhere from a small workshop to a
large manufacturing industry.
The advantages of three-phase AC induction motor are listed below:
• Simple design
• Rugged construction
• Reliable operation
• Low initial cost
• Easy operation and simple maintenance
• Simple control gear for starting and speed control
• High efficiency.
Induction motor is originated in the year 1891 with crude construction (The induction machine
principle was invented by NIKOLA TESLA in 1888.). Then an improved construction with
distributed stator windings and a cage rotor was built.
The slip ring rotor was developed after a decade or so. Since then a lot of improvement has taken
place on the design of these two types of induction motors. Lot of research work has been carried
out to improve its power factor and to achieve suitable methods of speed control.
3.2 Types and Construction of Three Phase Induction Motor
Three phase induction motors are constructed into two major types:
1. Squirrel cage Induction Motors
2. Slip ring Induction Motors
3.2.1 Squirrel cage Induction Motors
(a) Stator Construction
The induction motor stator resembles the stator of a revolving field, three phase alternator. The
stator or the stationary part consists of three phase winding held in place in the slots of a
laminated steel core which is enclosed and supported by a cast iron or a steel frame as shown in
Fig: 3.1(a).
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The phase windings are placed 120 electrical degrees apart and may be connected in either star
or delta externally, for which six leads are brought out to a terminal box mounted on the frame of
the motor. When the stator is energized from a three phase voltage it will produce a rotating
magnetic field in the stator core.
Fig: 3.1
(b) Rotor Construction
The rotor of the squirrel cage motor shown in Fig: 3.1(b) contains no windings. Instead it is a
cylindrical core constructed of steel laminations with conductor bars mounted parallel to the
shaft and embedded near the surface of the rotor core.
These conductor bars are short circuited by an end rings at both end of the rotor core. In large
machines, these conductor bars and the end rings are made up of copper with the bars brazed or
welded to the end rings shown in Fig: 3.1(b).In small machines the conductor bars and end rings
are sometimes made of aluminium with the bars and rings cast in as part of the rotor core.
Actually the entire construction (bars and end-rings) resembles a squirrel cage, from which the
name is derived.
The rotor or rotating part is not connected electrically to the power supply but has voltage
induced in it by transformer action from the stator. For this reason, the stator is sometimes called
the primary and the rotor is referred to as the secondary of the motor since the motor operates on
the principle of induction and as the construction of the rotor with the bars and end rings
resembles a squirrel cage, the squirrel cage induction motor is used.
The rotor bars are not insulated from the rotor core because they are made of metals having less
resistance than the core. The induced current will flow mainly in them. Also the rotor bars are
usually not quite parallel to the rotor shaft but are mounted in a slightly skewed position. This
feature tends to produce a more uniform rotor field and torque. Also it helps to reduce some of
the internal magnetic noise when the motor is running.
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(c) End Shields
The function of the two end shields is to support the rotor shaft. They are fitted with bearings and
attached to the stator frame with the help of studs or bolts attention.
3.2.2 Slip ring Induction Motors
(a) Stator Construction
The construction of the slip ring induction motor is exactly similar to the construction of squirrel
cage induction motor. There is no difference between squirrel cage and slip ring motors.
(b) Rotor Construction
The rotor of the slip ring induction motor is also cylindrical or constructed of lamination.
Squirrel cage motors have a rotor with short circuited bars whereas slip ring motors have wound
rotors having "three windings" each connected in star.
The winding is made of copper wire. The terminals of the rotor windings of the slip ring motors
are brought out through slip rings which are in contact with stationary brushes as shown in Fig:
3.2.
Fig: 3.2
THE ADVANTAGES OF THE SLIPRING MOTOR ARE
• It has susceptibility to speed control by regulating rotor resistance.
• High starting torque of 200 to 250% of full load value.
• Low starting current of the order of 250 to 350% of the full load current.
Hence slip ring motors are used where one or more of the above requirements are to be met.
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3.2.3 Comparison of Squirrel Cage and Slip Ring Motor
Sl.No. Property Squirrel cage motor Slip ring motor
1. Rotor Bars are used in rotor. Winding wire is to
Construction Squirrel cage motor is be used.
very simple, rugged
and long lasting. No Wound rotor
slip rings and brushes required attention.
Slip ring and brushes
are needed also need
frequent
maintenance.
2. Starting Can be started by Rotor resistance
D.O.L., star-delta, starter is required.
auto
transformer starters
3. Starting Low Very high
torque
4. Starting High Low
Current
5. Speed variation Not easy, but could be Easy to vary speed.
varied in large steps by
pole changing or Speed change is
through smaller possible by inserting
incremental steps rotor resistance
through thyristors or
by frequency variation. using thyristors or by
using frequency
variation injecting emf
in the rotor
circuit cascading.
6. Maintenance Almost Requires
frequent
ZERO maintenance maintenance
7. Cost Low High
3.3 Principle of Operation
The operation of a 3-phase induction motor is based upon the application of Faraday Law and the
Lorentz force on a conductor. The behaviour can readily be understood by means of the
following example.
Consider a series of conductors of length l, whose extremities are short-circuited by two bars A
and B (Fig.3.3 a). A permanent magnet placed above this conducting ladder, moves rapidly to
the right at a speed v, so that its magnetic field B sweeps across the conductors. The following
sequence of events then takes place:
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1. A voltage E = Blv is induced in each conductor while it is being cut by the flux (Faraday
law).
2. The induced voltage immediately produces a current I, which flows down the conductor
underneath the pole face, through the end-bars, and back through the other conductors.
3. Because the current carrying conductor lies in the magnetic field of the permanent
magnet, it experiences a mechanical force (Lorentz force).
4. The force always acts in a direction to drag the conductor along with the magnetic field.
If the conducting ladder is free to move, it will accelerate toward the right. However, as it
picks up speed, the conductors will be cut less rapidly by the moving magnet, with the
result that the induced voltage E and the current I will diminish. Consequently, the force
acting on the conductors wilt also decreases. If the ladder were to move at the same speed
as the magnetic field, the induced voltage E, the current I, and the force dragging the
ladder along would all become zero.
Fig: 3.3
In an induction motor the ladder is closed upon itself to form a squirrel-cage (Fig.3.3b) and the
moving magnet is replaced by a rotating field. The field is produced by the 3-phase currents that
flow in the stator windings.
3.4 Rotating Magnetic Field and Induced Voltages
Consider a simple stator having 6 salient poles, each of which carries a coil having 5 turns
(Fig.3.4). Coils that are diametrically opposite are connected in series by means of three jumpers
that respectively connect terminals a-a, b-b, and c-c. This creates three identical sets of windings
AN, BN, CN, which are mechanically spaced at 120 degrees to each other. The two coils in each
winding produce magneto motive forces that act in the same direction.
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The three sets of windings are connected in wye, thus forming a common neutral N. Owing to
the perfectly symmetrical arrangement, the line to neutral impedances are identical. In other
words, as regards terminals A, B, C, the windings constitute a balanced 3-phase system.
For a two-pole machine, rotating in the air gap, the magnetic field (i.e., flux density) being
sinusoidally distributed with the peak along the centre of the magnetic poles. The result is
illustrated in Fig.3.5. The rotating field will induce voltages in the phase coils aa', bb', and cc'.
Expressions for the induced voltages can be obtained by using Faraday laws of induction.
Fig: 3.4 Elementary stator having terminals A, B, C connected to a 3-phase source (not shown).
Currents flowing from line to neutral are considered to be positive.
