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Introduction
➢An induction machine can be used as either an
induction motor or an induction generator.
➢Induction motors are popularly used in industries.
➢Focus is given to three-phase induction motor
➢Main features: cheap and low maintenance.
➢ Induction motors are also used worldwide in many residential,
commercial, and utility applications.
➢ It can be part of a pump or fan, or connected to some other form
of mechanical equipment such as a winder, conveyor, or mixer.
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Introduction
➢ Three-phase induction motors are the most
common and frequently encountered machines in
industry
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simple design, rugged, low-price, easy maintenance
wide range of power ratings: fractional horsepower to
10 MW
run essentially as constant speed from zero to full load
speed is supply-frequency dependent
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not easy to have variable speed control
requires a variable-frequency power-electronic drive for
optimal speed control
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Induction Machines: Some Applications
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Construction
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➢ An induction motor has two main parts
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Stator
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consisting of a steel frame that supports a hollow, cylindrical core
core, constructed from stacked laminations (why?), having a
number of evenly spaced slots, providing the space for the stator
winding
Stator of IM
Construction
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Construction
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Rotor
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Construction
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Cutaway View
Brushes
Cutaway in a
typical woundrotor IM.
Notice the
brushes and the
slip rings
Slip rings
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Construction
Cutaway View
Cutaway in a
typical
squirrel-cage
rotor IM.
Construction - Stator
➢ The stator is the stationary electrical part of the motor.
➢ The stator core of a motor is made up of several hundred thin
laminations.
➢ Stator laminations are stacked together forming a hollow cylinder. Coils
of insulated wire are inserted into slots of the stator core.
➢ Electromagnetism is the principle behind motor operation. Each
grouping of coils, together with the steel core it surrounds, form an
electromagnet. The stator windings are connected directly to the power
source.
MZS
FKEE, UMP
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Construction-Enclosure
➢ The enclosure consists of a frame (or yoke) and two end
brackets (or bearing housings). The stator is mounted
inside the frame. The rotor fits inside the stator with a
slight air gap separating it from the stator. There is NO
direct physical connection between the rotor and the
stator.
• The enclosure also protects the electrical
and operating parts of the motor from
harmful effects of the environment in which
the motor operates. Bearings, mounted on
the shaft, support the rotor and allow it to
turn. A fan, also mounted on the shaft, is
used on the motor shown below for
cooling.
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Nameplate
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Rotating Magnetic Field
Rotating Magnetic Field
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Rotating Magnetic Field
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Rotating Magnetic Field
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Rotating Magnetic Field
➢ Balanced three phase windings, i.e.
mechanically displaced 120 degrees form
each other, fed by balanced three phase
source
➢ A rotating magnetic field with constant
magnitude is produced, rotating with a speed
120 f e
ns =
p
Where fe is the supply frequency and P is the
no. of poles and ns is called the synchronous
speed in rpm (revolutions per minute)
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Operational Principle
➢ The rotating magnetic field cuts the rotor windings and
produces an induced voltage in the rotor windings
➢ Due to the fact that the rotor windings are short circuited, for both
squirrel cage and wound-rotor, and induced current flows in the rotor
windings
➢ The rotor current produce another magnetic field
➢ A torque is produced as a result of the interaction of those two
magnetic fields
Induction motor speed
➢ At what speed will the IM run?
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Can the IM run at the synchronous speed, why?
If rotor runs at the synchronous speed, which is the same
speed of the rotating magnetic field, then the rotor will
appear stationary to the rotating magnetic field and the
rotating magnetic field will not cut the rotor. So, no
induced current will flow in the rotor and no rotor
magnetic flux will be produced so no torque is generated
and the rotor speed will fall below the synchronous speed
When the speed falls, the rotating magnetic field will
cut the rotor windings and a torque is produced
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Induction motor speed
➢ So, the IM will always run at a speed lower than
the synchronous speed
➢ The difference between the motor speed and the
synchronous speed is called the Slip
nslip = ns −n
Where nslip= slip speed
ns= speed of the magnetic field
n = mechanical shaft speed of the motor
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Equivalent Circuit Model
Equivalent Circuit Model of a three-phase machine helps
solving for voltages, current , powers, ….. etc.
It is made per phase assuming the machine is connected in Y.
Only machines with symmetric polyphase windings exited by
balanced polyphase voltages are considered. It is helpful to
think of three-phase machines as being Y-connected.
Equivalent Circuit - Stator
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Complete Equivalent Circuit
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Simplified Equivalent Circuit
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IEEE Recommended Equivalent Circuit
Thevenin Equivalent Circuit
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Developed mechanical torque
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Equivalent Circuit Parameters Determination
The parameters of the equivalent circuit, Rc, Xm, R1, X1, X2, and R2, can be determined
experimentally from the results of a no-load test, a blocked-rotor test, and
measurement of the DC resistance of the stator winding.
No-load Test: This test is performed by applying balanced 3-phase voltages to
the stator windings at the rated frequency (50 or 60 Hz). The rotor is kept
disconnected from any mechanical load. The small power measured at no load
is the core loss + the friction and windage loss.
Blocked-rotor Test: In this test the rotor is blocked, so that the motor cannot
rotate, and balanced 3-phase voltages are applied to the stator terminals. The
blocked-rotor test should be performed under the same conditions of rotor
current and frequency that will prevail in the normal operating conditions. The
IEEE recommends a frequency of 25% of the rated frequency for the blocked
rotor test. The leakage reactances at the rated frequency can then be obtained by
considering that the reactance is proportional to frequency.
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Equivalent Circuit Parameters Determination
Stator Resistance DC Measurement:
Blocked-Rotor Test Test:
No-load Test:
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Power and Power Losses in Induction Machines
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Power Relations
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Power Relations
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Power Flow in Induction Motor
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Induced Torque
Homework:
A three-phase Y-connected 220 V, 7.5 kW, 60 Hz, six
poles induction motor has the following parameters per
phase:
R1 = 0.294 
R2 = 0.144 
X1 = 0.503 
X2 = 0.144 
Xm = 13.25 
The total friction, windage and core losses may be
assumed to be constant at 403 W independent of the load.
For a slip of 2 %, calculate the speed, output torque and
power, stator current, power factor and efficiency at rated
voltage and frequency.
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Torque-Speed Characteristics
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Torque-Speed Characteristics
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Maximum
torque occurs
when the
power
transferred to
R'2/s is
maximum.
Typical torque-speed characteristics of induction motor
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Maximum Torque
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Maximum Torque
If the stator resistance R is small (hence, R is negligibly
small)
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th
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Maximum Torque
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Induction Motor Modes of Operation
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Effect of Rotor Resistance
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Effect of Rotor Resistance
Effect of rotor resistance on torque-speed characteristic
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Classes of Induction Motor
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Classes of Induction Motor
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Starting Techniques of Induction Motors
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4.
Direct ONLINE (DOL): IM is connected directly to the supply
Autotransformer: Reduced voltage at starting
Star-Delta: Using a Y- Switch
Soft-Starter: Power Electronic Switches.
Your Task is to explore the advantages and disadvantages
of each method as well as their operational principles
Starting Techniques of Induction Motors
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Example 1
• A 208-V, 10hp, four pole, 60 Hz, Y-connected induction motor has a full-load slip of 5% t
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What is the synchronous speed of this motor?
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What is the rotor speed of this motor at rated load?
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What is the rotor frequency of this motor at rated load?
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What is the shaft torque of this motor at rated load?
Solution
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Example 2
Solution
Example 3
Solution
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Example- 4
Solution
Example- 5
Solution
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Example- 6
A three-phase, 460 V, 60 Hz, six-pole wound-rotor induction motor drives a constant load of
100 N m at a speed of 1140 rpm when the rotor terminals are short-circuited. It is required to
reduce the speed of the motor to 1000 rpm by inserting resistances in the rotor circuit.
Determine the value of the resistance if the rotor winding resistance per phase is 0.2 ohms.
Neglect rotational losses. The stator-to-rotor turns ratio is unity.
Solution
From the equivalent circuits, it is obvious that if
the value of
remains the same, the
rotor current I2 and the stator current I1 will
remain the same, and the machine will develop
the same torque. Also, if the rotational losses are
neglected, the developed torque is the same as
the load torque. Therefore, for unity turns ratio,
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Example- 7
A 3φ, 15 hp, 460 V, four-pole, 60 Hz, 1728 rpm induction motor delivers full output power to a
load connected to its shaft. The windage and friction loss of the motor is 750 W. Determine the
(a) Mechanical power developed.
(b) Air gap power.
(c) Rotor copper loss.
Solution
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Example- 8
Solution
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Example- 9
Solution
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Example- 10
Solution
Example-11
Solution
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Example-12
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Solution
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Solution- cont…
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Solution- cont…
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