AC ELECTRIC MOTORS

Working principle
AC motor is usually a device which converts alternating electric current into a mechanical device through the use of an electromagnetic induction phenomenon. The AC motor includes two basic parts another stationary stator having coils supplied with an alternating current to make a rotating magnetic field and an internal rotor attached to the output shaft creating another rotating magnetic field.

The rotor can be an electric conductor which is suspended in the magnetic field. Since the rotor is constantly rotating there exists a alter in magnetic field. According to the Faraday’s legislation, this change in magnetic field induces an electric current inside a rotor.

Types of AC Electric Motor
The AC motors could be basically categorized into two categories, synchronous, and asynchronous motors
Synchronous Motor
These motors operate at a synchronous rate and convert AC electrical power in to the mechanical power.
When the energy supply is applied to the synchronous engine, a revolving field is established. This field tries to drag the rotor towards it but due to the inertia of the rotor, it cannot do it. So, there will be no starting torque. Consequently, the synchronous motor is not a self-starting motor.

Principles of operation

This motor has two electrical inputs. One is the stator winding which is supplied by a 3-phase supply and the other one may be the rotor winding which is supplied by a DC supply. Thus, two magnetic areas are stated in a synchronous motor.

The 3-phase winding produces 3-phase magnetic flux and rotor winding produce constant flux. The 3-phase finding creates a magnetic field which rotates at a acceleration called synchronous speed.

When rotor and stator begin rotating, at some time the rotor and stator have the same polarity leading to a repulsive force upon the rotor and for another second, they cause an attractive push. But rotor remains in standstill condition due to its high inertial instant. Therefore, the synchronous electric motor is not self-starting.

Advantages

The motor speed is continuous irrespective of the strain.
Electromagnetic power of the synchronous engine varies hzpt motor linearly with the voltage.
Compared to an induction motor, it operates at higher efficiencies at cheaper speeds
Disadvantages

It isn’t self-starting. It requires some arrangement for starting and synchronizing.
Since its beginning torque is zero, it can’t be started whilst having a load
It cannot be used for applications which require frequent beginning so when self-starting is required.
Application

Dampers
Conveyor systems
Variable transformers
Cryogenic pumps
Induction Motor
The induction motor can be named as Asynchronous not because it always runs at a speed less than the synchronous speed. The induction motor can be classified into generally two sub-groups. The single-phase induction electric motor and the 3-phase induction motor.

In an induction engine, the single armature winding acts both as an armature winding as well as a field winding. The flux is definitely produced in the surroundings gap whenever the stator winding comes to Zthe Air Gap. This flux will rotate at a set speed. Therefore, it will induce a voltage in the stator and the rotor winding.
The existing flow through the rotor winding reacts with the rotating flux and produces the torque.

Basic Working Principle

When an AC supply is fed to the stator winding in an induction motor, an alternating flux will be produced. This flux rotates at an asynchronous speed and this flux is known as the rotating magnetic field. Because of the relative speed between your stator RMF and rotor conductor, an induced EMF is usually developed in the rotor conductor. A rotor current is usually then produced for this reason induced EMF.

This induced current lags behind the stator flux.

The direction of the induced current is so that it tends to oppose the source of its production. The foundation of the production is the relative velocity between rotor stator flux and rotor. The rotor will try to rotate in the same path as a stator in order to decrease the relative velocity.

The speed of rotating magnetic field is given by

DC motor

Single phase induction Motor
AC electric motor which utilizes single phase power supply is called single phase induction engine.it is commonly found in the household and industrial application.it contains stator and Rotor component. A single-phase power is given to the stator winding. A squirrel cage rotor laminated with the iron core is linked to a mechanical load by using the shaft.
Principle of operation

When the single-phase supply is directed at the stator winding an alternating flux will produce in the stator winding.

A squirrel cage engine is attached to the mechanical load by making use of the shaft. Because of the rotating flux in the stator, an alternating electromagnetic field is usually induced in the rotor. But this alternating flux didn’t provide required rotation to the rotor. That is why the single phase motors aren’t self-starting.

In order to achieve self-beginning convert this solitary phase motor into a two-phase motor for temporarily. This is often achieved by introducing a starting winding

Advantages

Lightweight
Efficient transmission
Fewer substations required
Disadvantages

Cannot handle the overload
No uniform Torque
High insulation cost
Application

Refrigerator
Pumps
Compressor
Portable drills
Three Phase Induction Motor
When a three-phase supply is connected to the stator winding, this kind of motor is named three-phase induction motor. Just like a single phase electric motor, it has also both stator and rotor winding. The stator wounded by a 3-stage winding supplied by a 3-phase supply generates an alternating flux which rotates at a synchronous speed.
Working principle

When AC supply is given to the 3-phase winding of the stator, it generates an alternating flux which revolves with synchronous rate. This rotating magnetic field induced an EMF in the rotor which created an induced current which flows in a direction which reverse that of the rotating magnetic field, produce a torque in the rotor. The acceleration of the rotor will not be identical to that of the stator. If acceleration fits no torque will produce

Advantages

Simple and rugged construction
High efficiency and good power factor
Minimum maintenance
Self-starting motor.
Disadvantages

Speed decreases with increase in load
Speed control is difficult
Having poor beginning torque and high hurry current.
Application

Large capacity exhaust fans
Driving lathe machines
Crushers