7 a] With the help of neat diagram explain the constructional details of three phase induction motor.
Solution :
Construction Of Induction Motor
A three-phase Induction motor mainly consists of two parts
- Stator
- Rotor.
The stator is the stationary part of the induction motor, and the rotor is the rotating part.
The construction of the induction motor is explained below in detail
- Stator :
The stator is built up of high-grade alloy steel laminations to reduce eddy current losses.
It has three main parts, namely the outer frame, the stator core, and a stator winding.
Outer frame
It is the outer body of the motor. Its main function is to support the stator core and to protect the inner parts of the machine.
Stator Core
The stator core is built of high-grade silicon steel stampings. The stampings are fixed to the stator frame. Each stamping is insulated from the other with a thin varnish layer.
Slots are punched on the inner side of the stampings to carry the stator winding as shown in the figure below:
Stator windings
The core of the stator carries three-phase windings which are usually supplied from a three-phase supply system. These windings are either connected in star or delta. The six terminals of the windings are connected in the terminal box of the machine.
2. Rotor
Construction of Rotor
The rotor is also built of thin laminations of the same material as the stator. The laminated cylindrical core is mounted directly on the shaft. These laminations are slotted on the outer side to receive the conductors. There are two types of rotors.
i) Squirrel Cage Rotor
ii) Slip ring or phase wound rotor
i) Squirrel Cage Rotor
A squirrel cage rotor consists of a laminated cylindrical core. The circular slots at the outer periphery are semi-closed. Each slot contains an uninsulated bar conductor of aluminum or copper. At the end of the rotor the conductors are short-circuited by a heavy ring of copper or aluminum called end rings.
The diagram of the cage rotor is shown below:
ii) Slip Ring or Phase Wound Rotor
The phase wound rotor is also called a Slip Ring Rotor. It consists of a cylindrical core that is laminated. The outer periphery of the rotor has a semi-closed slot that carries 3 phase
insulated rotor windings. The three ends of 3-φ winding, available after connecting the winding in star or delta , are permanently connected to slip rings.
With the help of slip rings the external resistances can be added in series with each phase of rotor winding to control the rotor resistance per phase .Which helps to control starting torque speed of motor etc.
