Prepared by:
Dr. Taghreed Said
Higher technological institute
(H.T.I)
Introduction
Construction
Theory of operation
Types
Equivalent circuit
Power flow
Power torque characteristics
Performance characteristics
Starting method of induction motor
Speed control and braking method of induction motor
In 1882 Tesla discovered the
rotating magnetic field.
So Tesla discovered Ac
induction motor which
considered as one of the 10
greatest discoveries of all time
The three-phase induction
motors are the most widely
used electric motors in
industry.
About 90% of used electric
motors in industry are
induction motors.
About 50% of world
consumption of electricity
due to induction motors.
low-price, easy
maintenance
simple design, wide range of
rugged, power ratings,
has self-
has high
starting
efficiency
torque.
reasonably
good power
factor.
IM can be manufactured with characteristics to suit most industrial
requirements
The three-phase induction motors are the most widely used
electric motors in industry.
Advantages
simple design, rugged, low-price, easy maintenance
wide range of power ratings: fractional horsepower to 10MW
has high efficiency and reasonably good power factor.
has self-starting torque.
Disadvantages
run essentially as constant speed from no-load to full load
Its speed depends on the frequency of the power source
• not easy to have variable speed control
• requires a variable-frequency power-electronic drive for optimal
speed control
starting torque is inferior to D.C. shunt motor.
D.C motors are usually preferred when large speed variations are
required. And higher starting torque
single phase induction motor Three phase induction motor
widely used in domestic Prime mover if industry
applications
1. Trains
1. Blinder and juice makers 2. plains
3. cars
2. Washing machines 4. Motorcycle
3. Small fans 5. Ships
4. Vacuum cleaners 6. Submarines
7. elevators
5. Refrigerators 8. escalators
6. Air conditioners 9. Cranes
7. Drilling machines 10. pumps
11. compressors
12. Mills
Electrical Machines are Electromechanical
energy conversion devices.
Electrical motor coverts electrical energy into
mechanical energy and electrical generators
are vice versa.
Like any electric motor, a 3-phase induction
motor has a stator and a rotor.
Stator
Rotor(2-types)
Drive shaft
Frame/yoke
Bearing
Stator Winding
Terminal box
Rotor Winding
Cooling Fan
Name Plate
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
An induction motor has two main parts
1- a stationary stator
consisting of a steel frame that supports
a hollow, cylindrical core
core, constructed from stacked
laminations (why?)
core having a number of evenly spaced
slots, providing the space for the stator
winding
The insulated conductors are connected
to form a balanced 3-phase star or
delta connected circuit.
An induction motor has two main parts
1- a stationary stator
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.
1- a stationary stator
When a 3-phase winding is energized from a 3-phase supply, a rotating magnetic
field is produced.
2- a revolving rotor
It is housed on the shaft of the
induction motor.
It has two ends, one is called Driving
end and another is called non-
Driving end.
Mechanical load is connected on
driving end while cooling fan is
connected on non-driving end.
Both the ends are connected with bearings for
free rotation means of reduced friction losses.
composed of punched laminations, stacked to
create a series of rotor slots, providing space
for the rotor winding
According to rotor construction it can be Classified in two
category according to rotor Construction:-
1. Squirrel cage induction motor 2. Wound (Slip ring) induction motor
2- a revolving rotor
Two basic design types depending on the rotor design
1. squirrel-cage: copper or aluminum bars are placed in slots and
shorted at both ends by end rings (shorting rings).
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.
2- a revolving rotor
Two basic design types depending on the rotor design
2. wound-rotor: complete set of three-phase windings exactly
as the stator.
◦ Usually Y-connected, the open ends of the three rotor wires
are connected to 3 slip rings on the rotor shaft with one
brush resting on each slip ring.
◦ In this way, the rotor circuit is accessible.
Squirrel cage Induction Motor:- Slip ring Induction Motor:-
1-Construction Simplest and most rugged in Construction is not simple.
construction.
2-Cost Cheaper cost. Cost is slightly higher.
3-Mainantence Requires little maintenance. Requires high maintenance.
4-Efficiency Higher efficiency. Comparatively less efficiency.
5- Stating torque Low starting torque. It has 1.5 time High starting torque. It can be
full load torque. obtained by adding external
resistance in the rotor circuit.
6- Speed Control Speed control by rotor resistance is Speed control by rotor resistance
not possible. is possible.
7-Starting Starting current is 5 to 7 times the Less starting current compared to
Current full load current. squirrel cage Induction Motor.
8-Applications Lathe machine, Compressors, Crane, hoist, lift and where high
centrifugal pump, in agriculture etc. starting torque is required
where cost is most important factor
, with low mechanical power
applications
A Rotating Magnetic field (rotates at
synchronous speed Ns= 120fs/P.) is
setup in the stator when a 3-Phase
supply is given.
At Stand still
The stationary rotor cut by the
revolving field and due to
electromagnetic induction an e.m.f. is
induced in the rotor conductor.
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 produces another magnetic field
which rotates round the rotor at synchronous
speed Nr= 120 fr/P.
At stand still , the stator and rotor windings form
the equivalent of a transformer (Ns= Nr, fs=fr )
•A torque is produced as a result of the interaction
of those two magnetic fields
The cause producing the rotor currents is the relative speed between
the rotating field and the stationary rotor conductors.
At running
According to Lenz’s law, the direction of rotor currents will be such that
they tend to oppose the cause producing them.
the rotor starts running in the same direction as that of stator field and
tries to catch it with speed N to reduce this relative speed
If rotor runs at the synchronous speed???
When the speed falls, the rotating magnetic field
will cut the rotor windings and a torque is
produced.
So, the IM will always run at a speed lower than
the synchronous speed (Asynchronous machine)
The difference between the motor speed and the
synchronous speed is called the Slip speed (𝑁𝑠𝑙𝑖𝑝
= 𝑁𝑠 − 𝑁=𝑁𝑟 ).
120𝑓𝑠 120𝑓𝑟 120𝑓𝑠 𝑓𝑟
𝑁 = 𝑁𝑠 − 𝑁𝑟 = − = (1 − )
𝑃 𝑃 𝑃 𝑓𝑠
120𝑓𝑠 120𝑓𝑟 120𝑓𝑠 𝑓𝑟
𝑁 = 𝑁𝑠 − 𝑁𝑟 = − = (1 − )
𝑃 𝑃 𝑃 𝑓𝑠
The ratio between rotor frequency and stator frequency is called
slip; S, so
𝑓𝑟
𝑆= 𝑁 = 𝑁𝑠 (1 − 𝑆)
𝑓𝑠
𝑁𝑠 − 𝑁
𝑆=
𝑁𝑠
Slip: is the difference between the synchronous speed Ns of the
rotating stator field and the actual rotor speed N is expressed as a
percentage of synchronous speed.
120𝑓𝑠 𝑤𝑠 : Synchronous speed [rad/sec]
𝑁𝑠 =
𝑃 𝑁𝑠 : Synchronous speed [rpm]
2𝜋
𝑤𝑠 = 𝑁𝑠 𝑃: Number of pole
60
𝑓𝑠 : Supply frequency [HZ]
𝑁𝑠 − 𝑁 𝑓𝑟
𝑆= = 𝑓𝑟 : Rotor frequency [HZ]
𝑁𝑠 𝑓𝑠
𝑁: Motor Speed
𝑁 = (1 − 𝑆)𝑁𝑠
𝑆: Slip
No. of poles 2 4 6 8 10 12
Frequency
3000 1500 1000 750 600 500
f=50HZ
Frequency
3600 1800 1200 900 720 600
f=60HZ
