AC motor is a device which converts alternating current into a mechanical device by using an electromagnetic induction phenomenon. The AC motor consists of two basic parts an outside stationary stator having coils supplied with an 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 is an electric conductor which is suspended inside a magnetic field. Since the rotor is constantly rotating there is a change in magnetic field. According to the Faraday’s law, this change in magnetic field induces an electric current inside a rotor.
Types of AC Electric Motor
The AC motors can be basically classified into two categories, synchronous, and asynchronous motors
Classification of AC Motor
These motors operate at a synchronous speed and convert AC electrical power into the mechanical power.
Parts of the Synchronous Motor
When the power supply is applied to the synchronous motor, a revolving field is set up. 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. As a result, 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 is the rotor winding which is supplied by a DC supply. Thus, two magnetic fields are produced in a synchronous motor.
The 3-phase winding produces 3-phase magnetic flux and rotor winding produce constant flux. The 3-phase finding produces a magnetic field which rotates at a speed called synchronous speed.
When rotor and stator start rotating, at some point the rotor and stator have the same polarity causing a repulsive force on the rotor and for the next second, they cause an attractive force. But rotor remains in standstill condition due to its high inertial moment. Therefore, the synchronous motor is not self-starting.
- The motor speed is constant irrespective of the load.
- Electromagnetic power of the synchronous motor varies linearly with the voltage.
- Compared to an induction motor, it operates at higher efficiencies at lower speeds
- It is not self-starting. It needs some arrangement for starting and synchronizing.
- Since its starting torque is zero, it cannot be started while having a load
- It cannot be used for applications which require frequent starting and when self-starting is required.
- Conveyor systems
- Variable transformers
- Cryogenic pumps
The induction motor is also named as Asynchronous not because it always runs at a speed lower than the synchronous speed. The induction motor can be classified into mainly two sub-categories. The single-phase induction motor and the 3-phase induction motor.
In an induction motor, the single armature winding acts both as an armature winding as well as a field winding. The flux is produced in the air gap whenever the stator winding is supplied to the Air Gap. This flux will rotate at a fixed speed. Therefore, it will induce a voltage in the stator and the rotor winding.
The current 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. Due to the relative speed between the stator RMF and rotor conductor, an induced EMF is developed in the rotor conductor. A rotor current is then produced due to this induced EMF.
This induced current lags behind the stator flux.
The direction of the induced current is in such a way that it tends to oppose the source of its production. The source of the production is the relative velocity between rotor stator flux and rotor. The rotor will try to rotate in the same direction as a stator in order to reduce the relative velocity.
The speed of rotating magnetic field is given by
Single phase induction Motor
AC electric motor which utilizes single phase power supply is called single phase induction motor.it is commonly used in the domestic and industrial application.it consists of stator and Rotor part. A single-phase power supply is given to the stator winding. A squirrel cage rotor laminated with the iron core is connected to a mechanical load with the help of the shaft.
Single phase induction Motor
Principle of operation
When the single-phase supply is given to the stator winding an alternating flux will produce in the stator winding.
A squirrel cage motor is attached to the mechanical load with the help of the shaft. Due to the rotating flux in the stator, an alternating electromagnetic field is induced in the rotor. But this alternating flux did not provide required rotation to the rotor. That is why the single phase motors are not self-starting.
In order to achieve self-starting convert this single phase motor into a two-phase motor for temporarily. This can be achieved by introducing a starting winding
- Efficient transmission
- Fewer substations required
- Cannot handle the overload
- No uniform Torque
- High insulation cost
- Portable drills
Three Phase Induction Motor
When a three-phase supply is connected to the stator winding, this type of motor is called three-phase induction motor. Just like a single phase motor, it has also both stator and rotor winding. The stator wounded by a 3-phase winding supplied by a 3-phase supply produces an alternating flux which rotates at a synchronous speed.
Three Phase Induction Motor
When AC supply is given to the 3-phase winding of the stator, it produces an alternating flux which revolves with synchronous speed. This rotating magnetic field induced an EMF in the rotor which in turn produced an induced current which flows in a direction which opposite that of the rotating magnetic field, produce a torque in the rotor. The speed of the rotor will not be same as that of the stator. If speed matches no torque will produce
- Simple and rugged construction
- High efficiency and good power factor
- Minimum maintenance
- Self-starting motor.
- Speed decreases with increase in load
- Speed control is difficult
- Having poor starting torque and high rush current.
- Large capacity exhaust fans
- Driving lathe machines