The movement that we see around use like moving automobiles, movement of automatic doors in malls is the application of electric motors. It can be of two major types first one is alternating current motors and the other is direct current motors.
This article will be dedicated to Electrical motors and their types, but before we start our further learning let us look at some basic introductory topics which are prerequisite for better understanding.
If we want to convert electrical energy to mechanical energy we use electric motors and if we want to convert mechanical energy into electrical energy we use the electrical generators.
Fleming left-hand rule:
This rule is used to determine the direction of the force on a current-carrying conductor placed in a magnetic field, it is used in electrical motors.
The middle finger, the forefinger, and the thumb of the left hand are kept at a right angle to each other. The middle finger represents the direction of the current, the forefinger represents the direction of the magnetic field and thumb will indicate the direction of force acting on the conductor.
There are two types of laws that govern the phenomena of electromagnetic induction.
Faraday law: this law gives us the magnitude of induced electromotive force.
Lens law: this law gives us the direction of induced electromotive force.
This law states that the direction of the induced current is such that it opposes the cause which produces it that is it opposes the change in magnetic flux. It is a consequence of the law of conservation of energy.
For the construction of the motor we require the following:
The basic working of an electric motor:
According to the principle of magnetism, opposite attracts and similar repel. Two magnetic bars of different polarities are placed opposite to each other that will attract each other on the other hand, the north end will repel the north end of the other and in a similar way south will repel south. These attracting and repelling forces will create rotational motion.
The field system is the uniform magnetic field and the field strength can vary. Electric current is supplied externally through a commutator. Magnetic force (F) = ILB acts perpendicular to both wire and magnetic fields.
Armature winding is of two types: lap winding and wave winding. In lap winding the armature winding is divided into numbers of sections equal to the number of poles and wave winding is used where low current output and high voltage are required.
AC motor classification:
AC motor is classified into two types:
Synchronous motors are called so because the speed of the rotor of this motor is the same as the rotating magnetic field.it’s basically a hard and fast speed motor because it’s just one speed, which is synchronous speed and thus no intermediate speed is there or in other words, it’s in synchronism with the availability frequency. The synchronous speed is given by
Where f is supply frequency and p is that the number of poles
Main features of synchronous motors:
Synchronous motors are inherently not self-starting. They require some external means to bring their speed close to synchronous speed before they’re synchronized.
The speed of operation is in synchronism with the availability frequency and hence for constant supply frequency, they behave as constant speed motor regardless of load condition.
This motor has the unique characteristics of operating under any electric power factor. This makes it’s utilized in electric power factor improvement.
Application of synchronous motor:
An asynchronous motor having no load connected to its shaft is used for power factor improvement. Owing to its characteristics to behave at any electrical power factor, it is used in a power system in situations where static capacitors are expensive.
Synchronous motors find application where operating speed is less and high power is required. For power requirements from 35kilo watt to2500 kilowatt, the size, weight, and cost of the corresponding three-phase induction motor are very high. Hence these motors are preferably used. Example- reciprocating pump, compressor, rolling mills extra.
A typical induction motor consists of two parts namely stator and rotor like another type of motor.
An outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field.
An inside rotor attached to the output shaft that is given a torque by the rotating field.
Principle of operation:
An alternating current is applied in the stator armature which generates flux in the stator magnetic circuit. This flux induces an electromotive force in the conducting bars of the rotor as they are “cut” by the flux while the magnet is being moved ( E=BVL(faraday’s law)). Current flows in the rotor circuit due to induced electromotive force, which in term produces a force that can be changed to the torque as the output.