AC Motors

An AC motor is an electric motor driven by an alternating current(AC).

As in the DC motor case, a current is passed through the coil, generating a torque on the coil. Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave. It is called a synchronous motor. More common is the induction motor, where electric current is induced in the rotating coils rather than supplied to them directly.

Classification of AC motor

Single phase AC motor:-

1. Shaded Pole

2. Split Phase Motors

3. Capacitor Motors

4. Capacitor Run

5. Capacitor Start / Induction Run Motors

6. Capacitor Start / Capacitor Run

Three phase AC motor:-

i. squirrel cage

ii. wound rotor

Construction of AC motor:

Stator Core : The stator is the stationary part of the motor's electromagnetic circuit. The stator core is made up of many thin metal sheets, called laminations. Laminations are used to reduce energy loses that would result if a solid core were used.

Stator Windings Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core.When the assembled motor is in operation, the stator windings are connected directly to the power source. Each grouping of coils, together with the steel core it surrounds, becomes an electromagnet when current is applied. Electromagnetism is the basic principle behind motor operation.

Rotor Construction:
The rotor is the rotating part of the motor's electromagnetic circuit. The most common type of rotor used in a three-phase induction motor is a squirrel cage rotor. Other types of rotor construction is discussed later in the course. The squirrel cage rotor is so called because its construction is reminiscent of the rotating exercise wheels found in some pet cages.

The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the magnetic field, the windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles.

Functioning of AC motor:

when an AC is applied to the stator windings, the electromagnetic flux is produced which gets induced in the rotor of the motor. Due to this AC a rotating magnetic field is generated which results in the rotation of rotor by the induction of rotating magnetic field.

Rotating magnetic field:The windings are connected in wye. The two windings in each phase are wound in the same direction. At any instant in time, the magnetic field generated by one particular phase will depend on the current through that phase. If the current through that phase is zero, the resulting magnetic field is zero. If the current is at a maximum value, the resulting field is at a maximum value. Since the currents in the three windings are 120° out of phase, the magnetic fields produced will also be 120° out of phase. The three magnetic fields will combine to produce one field, which will act upon the rotor. In an AC induction motor, a magnetic field is induced in the rotor opposite in polarity of the magnetic field in the stator. Therefore, as the magnetic field rotates in the stator, the rotor also rotates to maintain its alignment with the stator's magnetic field.

Important concepts:

1. AC motors are self starting morots. They donot require special starter like DC motors.
   
2. The speed of motor is always smaller than the rotating magnetic field.
   

Types of AC motors:

1. Three phase induction motor:
   in this kind of motor, it uses three phase supply for its functioning. Its functioning is similar to other AC motors.
   
2. Single phase ac motor:
   in this kind of motor, it uses single phase supply for its functioning. it is used in small applications like hand blowers, drill machine etc.
   

Applications:

• clothes dryers.
• air conditioners.
• air handlers, blowers, and fans (including ceiling fans)
• Printing machine
• Used in the Grinding machine.
• Section Straightening Machines in Rolling mills

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