Construction of Three-phase Induction Motor:
There are two main parts of the three phase induction motor:
i) Stator which is the stationary part
ii) Rotor which is the rotating part
The stator is composed of thin circular, ring-shaped laminations of good quality silicon steel in which slots are provided round the inner periphery. Silicon steel has low hysteresis loss. The laminations are insulated from each other to reduce eddy current loss. These laminations are stacked together and fitted inside the frame of the motor to appear like a hollow cylinder. A three-phase stator winding is uniformly distributed in the slots as shown in the sectional view. The winding is wound for specific number of poles, say 2 or 4 or 6 etc. It is connected either in star or delta and the terminals are brought out to connect to the 3-phase supply.
2) Rotor: There are two types of rotors:
i) Squirrel cage rotor
ii) Wound rotor or slip-ring rotor
i) Squirrel Cage Rotor: This rotor is formed by a laminated cylindrical core having semi-closed slots provided around its outer periphery. This rotor is mounted on the shaft and placed in the central space of the stator as shown in the sectional view. Thick copper or aluminium bars are driven through the slots and shortcircuited at the two ends by the two thick end rings as shown in the figure (a). These bars are the rotor conductors. They are brazed or welded to the end-rings to produce the short-circuited rotor winding. The appearance of the rotor bars and the end rings is like a squirrel cage and hence the rotor is called by that name. The rotor slots are not exactly parallel to the shaft but skewed slightly to avoid magnetic noise and direct magnetic locking between rotor and stator teeth.
ii) Slip-Ring Rotor (Wound Rotor): This rotor is formed by a cylindrical laminated core having slots on its outer periphery to carry a three-phase uniformly distributed winding, wound for the same number of poles as that of the stator winding. The rotor winding is inherently connected in star and the terminals are connected to the three copper slip-rings mounted on the shaft. External resistance can be inserted in each phase of the rotor winding through brushes and slip-rings. In this rotor also the slots are not exactly parallel to the shaft but slightly skewed.