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An electric motor is a device which converts an electrical energy into a mechanical energy. This mechanical energy then can be supplied to various types of loads. The motors which are all operating under DC supply is called DC motor. The motors which are all operating under 1Φ AC supply or 3Φ AC supply is called AC motor. Out of all these motor, 3Φ AC supply motors are widely used as conventional current itself AC supply.
Basically, the electrical motor consist of two main parts, they are
Stator is a stationary part of any electrical motor, to start any electrical motor, the supply must be given to the stator.
The part which rotates and is connected to the mechanical load through shaft called rotor. There are two types of rotor construction, they are
The stator is made up of laminated stamping which are 0.4 mm to 0.5 mm of thickness. The stampings are slotted on its periphery to carry the stator winding. The stampings of stator are insulated from each other to minimize the iron loss. The material used for stampings are generally silicon steel, which is highly capable of minimizes the hysteresis loss. For 3Φ motor, the slots on the outer surface of the stator core carries 3Φ winding, connected eitheir in star or delta. This 3Φ winding on the stator is called stator winding.
The outer core of the rotor is slotted and cylinderical. The rotor consist of uninsulated copper or aluminium bars called as rotor conductors. These aluminium bars are placed in the slots of the rotor. These aluminium bars are permanently shorted at each end with the help of copper ring called end ring. The aluminium bars are usually brazed to the end rings to provide good mechanical strength. As the name itself implies that the rotor looks like a cage, forming a closed electrical circuit. Hence the rotor is called squirrel cage rotor.
Slip ring rotor construction is very much likely to the stator. Slip ring rotor carries a 3Φ star or delta connected, distributed winding, wound for same number of poles as that of stator. Slip ring rotor construction is laminated and slotted. The slots of the rotor contains the rotor conductor or rotor winding. The three ends of 3Φ winding, available after connecting the winding in star or delta, are permanently connected to the slip rings, which is mounted on the same shaft. As we know that slip ring is an electromechanical device that allow us to connect external stationary circuit to the internal rotating circuit. So by using slip ring rotor, it is possible to add external resistance to the circuit by means of rheostat. Slip ring rotor is also called as Wound rotor.
|Squirrel Cage Rotor||Slip Ring Rotor|
|Slip ring and brushes are absent.||Slip rings and brushes are present to add external resistance.|
|Rotor resistance starter cannot be used.||Rotor resistance starter can be used.|
|Speed control by rotor resistance is not possible.||Speed control by rotor resistance is possible.|
|Squirrel cage rotor is very simple in construction.||Slip ring rotor is very complex is construction.|
|Due to simple construction, the rotors are cheap.||Due to complexity in construction, the rotors are very costly.|
|Construction is robust and maintenance free.||Construction is delicate and due to brushes and external resistance, maintenance is necessary in frequent.|
|As permanently shorted, external resistance cannot be added.||Resistance can be added externally due to the presence of slip ring and brushes.|
|In industry, 95% of Squirrel cage induction motors are get used.||In industry, only 5% of slip ring induction motors are get used.|
|Rotor copper losses are less hence have higher efficiency.||Rotor copper losses are high hence poor in efficiency.|
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