Construction of Induction Motor
Theconstruction of induction motor differs from the DC motor byfollowingfactors:
- its short air-gap
- absence of a commutator
- simple windings
- Laminated stator.
Parts of Induction Motor
Following figure shows all the parts of an induction motor:
The two main parts are Stator and Rotator that are as follows:
It consists of frame and stator core as follows:
The frame for small motors is made up of a single piece of small,hollow cylinder cast-iron.But for medium and large sized induction motors a fabricatedframe is used. Cylindrical shape is formed by the joining of four or more arc-shaped rolled steel pieces where the outer surface is provided withcooling fins.The frame is used forthe absolute support and protection of other parts.
ii. Stator Core
The stator core makes availablesome space for stator windingsand the rotating magnetic field. It is formed by thinsheets of steel (called stampings or laminations) that is insulated by paper. The thickness of the sheet is from0.35 mm to 0.65 mm.There is air gap in it with open, semi-closed or completely closed slots. Commonly the statorcore has semi-closed slots that can be represented by following formula:
S = n. (3 p)
WhereSisthe number of slots,p is for which the induction motor is designed andconstructed andn = 1, 2, 3 etc.
Stator winding of the induction motor is done on the stator core.
- Single layer mesh windings are used for smallcapacity motors.
- Double-layer lap windings are used for medium capacity motors.
- Single layer concentric windings are used forlarge capacity motors.
All the winding coils are placed in the slots, the slots are closed with wooden wedgesor paper insulator, which keeps the coil sides intact. The whole stator winding maybe impregnated with a thermosetting varnish and baked.
There are two types of Rotor Core that are:
Rotor Core for Squirrel Cage Induction Motor
Lamination is made up of circular or rectangular shaped closed slots.First the rotor core is heaped by the laminations then rotor shaft is introduced at thecentre.
- For small and mediumsized motorsmolten aluminumis passed from the slots to form rotor bars and endrings.
- For larger sized motors copper or brass bars are passed through the slots and the ends are welded or silversoldered collectively to form end rings.
The rotor bars are slightly inclined to the shaft axis due to the ‘skew’ provided while stacking the rotor stampings. Skewing is done because:
- It helps the motor to run quietly by reducing magnetic hum.
- It reduces the locking tendency of the rotor.
Rotor Core for Slip Ring Induction Motor
Here lamination is done by semiclosed slots where as closed slots are used incagerotors.Here molten aluminum or copper bars are not used.
In squirrel cage motors,there is no rotor winding.But in slip ring motors rotor winding on the rotor cage is called the wound rotor motors.The winding isconnected in star shape and the consequential three terminals are connected to threeslip rings provided on one end of the shaft.
for more tutorial
What is Induction Motor?