INTRODUCTION
The conversion of electrical power into mechanical power takes place in the rotating part of an electric motor. Depending upon the type of supply, we classify them as D.C and A.C motor. The d.c motor was called as conduction motor in the sense; the electric power was conducted directly to the armature through brushes and commutator. However, in a.c motors, rotor does not receive electric power by conduction but by induction in exactly the same way as transformer secondary receives its power from the primary. Hence those motors are called as induction motors. In most of the industrial drives, induction motors plays a vital role due to its advantages,
Advantages
1. The construction is very simple and extremely rugged.
2. Cost is low and it is very reliable.
3. It has a reasonably good power factor and hence efficiency is also sufficiently high.
4. It requires minimum maintenance.
CONSTRUCTION
An induction motor consists essentially of two main parts:
a) a stator
b) a rotor
a) Stator
The stator has a laminated type of construction made up of a number of stampings which are slotted on its periphery to carry the windings. The stampings are insulated from each other and they are stamped together to build the stator core. The winding in the slots on the periphery of the stator core is connected either in star or delta. This three phase winding is called stator winding. It is wound for a definite number of poles and dependent on its speed. This winding when excited by the three phase supply produces a rotating magnetic field. The radial ducts are provided for the cooling purpose.
b) Rotor
The rotor is housed inside the stator. The rotor core is laminated in construction and is made up of cast iron. It is cylindrical, with slots on its periphery. The rotor conductors or winding is placed in the rotor slots. Depending on the construction,
i) Squirrel cage rotor
Motors employing this type of rotor are known as squirrel cage induction motor.
ii) Phase wound (or) slip ring rotor Motors employing this type of rotor are known as phase wound (or) slip ring motors.
TYPES
Depending on the construction of rotor, there are two types of three phase
SQUIRREL CAGE ROTOR
Almost 90 percent of induction motors are squirrel cage type, because this type has the simplest and most rugged construction. The rotor core is cylindrical and slotted on its periphery. The rotor consists of uninsulated copper or aluminium bars called rotor conductors. The bars are placed in the slots. These bars are permanently shorted at each end with the help of conducting copper ring called end ring. The bars are usually brazed at the end rings to provide good mechanical strength. The entire structure loots like a cage, forming a closed electrical circuit. So the rotor is called squirrel cage rotor.
As the rotor bars are permanently shorted on themselves it is not possible to add any external resistance in series with the rotor circuit for starting purposes. So slip ring and brush assembly is not needed for this rotor. Hence the construction of this rotor is very simple. Fan blades are generally provided at the ends of the rotor core. This circulates the air through the machine while operation, providing the necessary cooling.
SLIP RING OR WOUND ROTOR
In this, the construction of rotor is similar to that of stator. The rotor carries a three phase star or delta connected, distributed winding wound for same number of poles as that of stator. The rotor construction is laminated and slotted. One end of the three phase windings are connected in star or delta, and the other end are connected to the slip rings. They are mounted on the same shaft. The slip rings are used to connect external stationary circuit to the internal rotating circuit. So, in this type of induction motor external resistances can be added with the help of slip ring and brush assembly. When the motor is under running condition, the slip rings are shorted. This is possible by connecting a metal collar which gets pushed and connects all the slip rings together, shorting them. The brushes can also be lifted from the slip rings. This avoids wear and tear of the brushes due to friction. The main advantage of this type of three phase induction motor is that external resistance can be added and easily used for starting purposes.
ROTATING MAGNETIC FIELD (RMF)
The rotating magnetic field can be defined as the field or flux having constant amplitude but whose axis is continuously rotating in plane with a certain speed. When a permanent magnet is rotated, then the resulting field is a rotating magnetic field. In three phase induction motors, the rotating magnetic field is produced by supplying currents to a set of stationary windings, with the help of three phase a.c supply.
See More: Construction of a D.C Machine
See More: Working Principle of D.C Generator
See More: Working Principle of D.C Motor
