2. Three phase induction motor
Week 3
2.1 Introduction
Three-phase AC induction motors are widely used in industrial and commercial applications. They
are classified either as squirrel cage or wound-rotor motors.
These motors are self-starting and use no capacitor, start winding, centrifugal switch or other
starting device.
They produce medium to high degrees of starting torque. The power capabilities and efficiency in
these motors range from medium to high compared to their single-phase counterparts.
Popular applications include grinders, lathes, drill presses, pumps, compressors, conveyors, also
printing equipment, farm equipment, electronic cooling and other mechanical duty applications.
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Simple and rugged construction
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Low cost and minimum maintenance
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High reliability and sufficiently high efficiency since there is no losses in brush contacts
or mechanical friction
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Needs no extra starting motor and need not be synchronized
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Need only one source of power
2.2 Construction of Induction Motor:
An Induction motor has basically two parts, Stator and Rotor. Also some of other parts were
acknowledged in the following section
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2. Three phase induction motor
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2.2.1 Stator construction:
Figure2.1: Diagram construction of the stator
The stator is made up of several thin laminations of aluminum or cast iron. They are punched and
clamped together to form a hollow cylinder (stator core) with slots as shown in Figure 2.1. Coils of
insulated wires are inserted into these slots.
The iron core on the figure has paper liner insulation placed in some of the slots.
Each grouping of coils, together with the core it surrounds, forms an electromagnet (a pair of poles)
on the application of AC supply. The number of poles of an AC induction motor depends on the
internal connection of the stator windings. The stator windings are connected directly to the power
source. Internally they are connected in such a way, that on applying AC supply, a rotating magnetic
field is created
2.2.2 Rotor construction:
There are two main types
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2. Three phase induction motor
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Week 3
Squirrel cage type
Wound rotor
2.2.2.1 Squirrel cage rotor:
This rotor has a laminated iron core with slots(Figure2.2), and is mounted on a shaft. Aluminum
bars are molded in the slots and the bars are short circuited with two end rings. The bars are skewed
on a small rotor to reduce audible noise. Fins are placed on the ring that shorts the bars. These fins
also work as a fan and improve cooling.
Most motors use the squirrel-cage rotor because There are no commutators, slip rings or brushes.
Hence this is a most rugged and maintenance-free construction.
Rotor bars) slightly skewed(
End ring
Figure2.2:Squriell cage rotor
2.2.2.2 Wound rotor:
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2. Three phase induction motor
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Wound Rotor
Brush
Slip Rings
External Rotor
Resistances
Figure 2.3: Schemtic diagram showing Induction motor, wound rotor type
The wound rotor or slip-ring induction motor differs from the squirrel-cage motor only in the rotor
winding. The rotor winding consists of insulated coils, grouped to form definite polar areas of
magnetic force having the same number of poles as the stator. The ends of these coils are brought
out to slip-rings. By means of brushes, a variable resistance is placed across the rotor winding
(Fig.2.3). By varying this resistance, the speed and torque of the motor is varied. The wound rotor
motor is an excellent motor for use on applications that require an adjustable-varying speed (an
adjustable speed that varies with load) and high starting torque.
Terminals
Slip rings
Bearings
Fan
Laminated core
Shaft
Coils
Figure 2.4:Wound rotor
2.2.3 Enclosure
Enclosure or the frame (Figure2.5) ,its main
application to hold the parts together. also it Helps
with heat dissipation. In some cases, protects internal
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2. Three phase induction motor
Week 3
components from the environment. A cooling fan is attached to the shaft at the left-hand side. This
fan blows air over the ribbed stator frame
Figure2.5:Induction Motor enclosure
2.2.3
Bearings:
There are two main types, the sleeve bearings and ball bearings .Ball (Roller) Bearings
(Figure2.6a) Support shaft in any position. Its Grease lubricated and required no maintenance
The Sleeve Bearings(figure2.6b) are Standard on most motors. They are only used with horizontal
shafts and its oil lubricated.
(a)
Figure2.6: a)Ball Bearings
b)Sleeve bearings
(b)
b
2.2.5 Conduit Box
Point of connection of electrical power to the motor’s stator windings.
2.2.6 Eye Bolt
Used to lift heavy motors with a hoist or crane to prevent motor damage, as it can be seen in the
following figure.
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2. Three phase induction motor
Week 3
Figure2.7: Sectional view of Induction Motor
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