An AC motor uses an alternating current to generate a rotating magnetic field in the stator that interacts with the rotor. The two main types are induction and synchronous motors. Induction motors rely on electromagnetic induction to generate a current in the rotor from the stator's rotating magnetic field, causing the rotor to turn at a slower synchronous speed. Squirrel cage rotors have embedded conductors in a striped pattern and are simple, reliable, and cheaper but have poor starting torque. Wound rotors have coils connected through slip rings that allow reducing starting current and enabling speed control.

2. AC MOTORS
An AC motor is an electric motor driven by an alternating
current (AC). The AC motor commonly consists of two basic
parts, an outside stationary stator having coils supplied with
alternating current to produce a rotating magnetic field, and
an inside rotor attached to the output shaft producing a
second rotating magnetic field. The rotor magnetic field may
be produced by permanent magnets, reluctance saliency, or
DC or AC electrical windings.

3. The two main types of AC motors are classified as
induction and synchronous. The induction
motor (or asynchronous motor) always relies on a
small difference in speed between the stator
rotating magnetic field and the rotor shaft speed
called slip to induce rotor current in the
rotor AC winding.

4. ASYNCHRONOUS MOTORS
If a motor is asynchronous, the speed of
its shaft doesn’t equal the motor’s
synchronous speed. To be more specific,
the rotor of a asynchronous motor turns
at a speed less than the synchronous
speed.

6. ELECTROMAGNETIC INDUCTION
If a conductor is brought into a region with a
changing magnetic field, it receives a difference in
voltage across its surface. This phenomenon is
referred to as electromagnetic induction , and the
voltage is called the induced voltage. The
magnitude of the voltage is proportional to how
quickly the magnetic field is changing.

7. When a current-carrying conductor is placed
inside a magnetic field , it receives a force that
causes the conductor to move. This is the
fundamental principle behind all
asynchronous motors, which are commonly
called induction motors or AC induction
motors (ACIMs).

9. The three basic operation of an
induction motor in three steps:

10. 1. The stator of an AC Motor creates a changing
magnetic field in response to polyphase
power.

11. 2. The rotor of an induction motor has an
induction motor has conductors instead of
magnets. When these conductors enter
the stator, each receives an induced
voltage.

12. 3. The induced voltage produces a current
in each conductor. As a result, a force is
exerted on each conductor, and this force
turns the rotor.

13. The relationship between an asynchronous motor’s speed
and its synchronous speed is called a slip. Denoting the
motor’s speed as n, its slip can be computed with the
following equation:
Slip is usually expressed as a percentage of the
synchronous speed.

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15. An asynchronous motor requires a significant amount
of current to start its operation. This increased current
means increased torque, and in some asynchronous
motors, the starting torque can be two or three times
larger than the torque exerted in full-loading
conditions.
CURRENT AND TORQUE

17. SQUIRREL-CAGE ROTOR
The oldest and most popular type of asynchronous
motor has a cylindrical rotor with conductors
embedded into its surface in a striped pattern. This
rotor looks like a running wheel used by gerbils and
hamsters, so its called a squirrel-cage rotors.

19. Advantages:
1. Simple and Reliable
2. Can be easily fixed or replaced
3. Much cheaper than motors who have
permanent magnets
4. Can be constructed at large sizes.

20. Disadvantages:
1.Have poor starting torque and high
starting currents
2.More sensitive to the supply voltage
fluctuations
3.Speed control is not possible

21. Applications:
Squirrel Cage Induction Motors are suitable for
applications where the drive requires constant
speed, low starting torque and no speed control
drives.

22. Pumps

23. Blowers

24. Heaters

25. WOUND ROTOR
A motor whose motor contains of conductive wire
instead of conductive bars. This type of motor is
called a wound-rotor or slip-ring motor. Also called
WRIM’s (wound-rotor induction motors).

26. Difference of WRIM to brushed DC motor.
1.Instead of a brush, the coils of a WRIM connect
to external circuitry through slip rings on one
end of the rotor. There is one slip ring for each
phase.
2.Unlike the coils in brushed DC motor, the
WRIM’s coils don’t receive power from outside
the motor.

28. Increasing the resistance of the WRIM’s coils
reduces the strength of the stator’s magnetic
field. This significantly reduces the amount of
starting current required by the motor.
For large machines, this can result to saving
great deal of power and can reduce the chance
of start-up failure.

31. Advantages:
1. The locked-rotor current can be reduced by inserting
external resistances in series with the rotor
windings.
2. The speed can be varied by varying the external
resistances.
3.The wound rotor motor is ideally suited to accelerate
high inertia load.

32. Disadvantages:
1. Has slip rings,brushes,brush holders & rotor terminals;
thus greater operating troubles and require frequent
maintenance.
2.It has large overhang leakage,more copper loss,thus less
power factor and inferior efficiency compared to squirrel
cage motor.
3.Compared to squirrel cage rotors,wound rotor motors are
expensive, so it is not so common in industry applications.

33. Applications of Polyphase Wound Rotor Induction Motors
•Wound rotor motors are suitable for loads requiring high
starting torque and where a lower starting current is
required.
•The Wound rotor induction motors are also used for
loads having high inertia, which results in higher energy
losses.
•Used for the loads which require a gradual buildup of
torque.

34. •Used for the loads that require speed control.
•The wound rotor induction motors are used in conveyors,
cranes, pumps, elevators and compressors.
•The maximum torque is above 200 percent of the full load
value while the full load slip may be as low as 3 percent. The
efficiency is about 90 %.

35. COMPARISON OF SQUIRREL CAGE AND WOUND ROTOR

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