Disadvantages of 3 phase Induction motor: — During starting, it draws high initial starting current when attached to a heavy load. For speed control of motor having poor efficiencies under normal operation. Speed control by varying the slip frequency when the stator is fed from a constant voltage, constant frequency mains. The above discussion makes it clear that a given motor gets derated differently when it is driving different. There are two types of rotor.
It may be constructed for single phase, three phase or even poly phase motors. Principle of operation 3 phase induction motor: When a three-phase supply is connected to the stator terminals of the squirrel-cage motor, the currents in the stator winding give rise to a rotating magnetic field of constant magnitude that induces voltage in the stationary rotor conductors that are short-circuited and so both circulate currents in the rotor conductors that produce some flux and the interaction of the stator and rotor fluxes give rise to a torque and the rotor begins to rotate in the direction of the magnetic field due to the winding of the stator. The stator core is pressed directly into the frame. Guarded Guarded motors have mesh or wire over the openings to prevent objects being pushed or reaching into the motor. These resistances are gradually reduced to zero as the motor runs up to speed. The rotor is separated from the stator by a small air-gap which ranges from 0. Related Reading: By using a soft starter for an induction motor, the mechanical and electrical stresses are reduced, protecting the motor during the time of starting.
Starting torque The starting torque is low. The stator winding of the three-phase induction motor is very similar to the armature winding of a three-phase synchronous machine. The derating is also affected by the efficiency of the. The rotor windings open ends all are brought together and connected to three insulated slip rings fixed on the shaft of rotor with a brush resting on every slip ring. The ends of these bars are shortened at two sides through two rings of the same material. Using a in the rotor circuit to recover slip energy. A number of evenly spaced slots are provided on the inner periphery of the laminations.
As the teeth of the rotor and the stator attract each other and lock. Related Reading: Protections for 3 phase induction motor Single phasing protection: To tackle this problem, protection devices are used for 3 phase induction motor. A mild steel shaft is passed through the center of the rotor and is fixed to it. Even though the method is simple and economical, the motor losses increase with increase in slip. The core of the stator is laminated and made of perforations of sheet steel and is grooved on its internal cylindrical surface and the winding consists of recessed coils arranged in exactly the same way as those of a synchronous rotary field generator for the same amount of poles. The Stator: The stator composes of magnetic strips of magnetic iron. Three phase induction motor construction.
Afterwards I will cover the working principle and features of Squirrel Cage Induction Motor. Mounted on the motor shaft and engraved with a number of sewers to install the winding. These parallel slots carry rotor conductors. Stator is same in both types of motors. Squirrel cage rotor type The squirrel cage rotor is very similar to the rotor in the wound rotor type in terms of mechanical structures. The squirrel cage motor is mostly started by star — delta stater and hence the stator of squirrel cage motor is delta connected. This value decreases as the lower limit of speed decreases.
It contains a steel frame that surrounds a hollow cylindrical shaped core made of skinny laminations of silicon steel to cut back. Some very small rotors which operate on the basis of eddy current, have their rotor as solid steel without any conductors. What is an Induction Motor? Thus the outer cage has a high resistance with low reactance to resistance ratio and the inner cage has low resistance but high reactance to resistance ratio. The outer periphery of the rotor has a semi-closed slot which carries a 3 phase insulated windings. When 3-phase supply is given to the stator winding, a rotating magnetic field of constant magnitude is produced.
At starting, the resistance are connected in rotor circuit and is gradually cut out as the rotor pick up its speed. An induction motor can be used for different applications with various speed and load requirements. All these bars are joined at each end by metal rings called end rings. The terminals of the rotor winding are connected to insulated slip rings mounted on the shaft. The is achieved by means of phase control of the antiparallel thyristors, connected as shown in Fig.
One important point to be noted is that the end rings and the rotor conducting bars are permanently short-circuited, thus it is not possible to add any external resistance in series with the rotor circuit for starting purpose. Squirrel cage motors are widely used on ships due to their rugged construction and simple design, few e. The bearings shown there are ball bearings, which have less friction than cheaper sleeve bearings. Construction of Squirrel Cage Induction Motor All induction motors have a rotor and a stator. The resistor enables the variation of each rotor phase resistance to serve the following purposes given below.
The brushes are used to carry to and from the rotor winding. Each slot contains uninsulated bar conductor of aluminium or copper. The stampings are fixed to the stator frame. These windings are connected as either star or delta depending upon the requirement. This rotating magnetic field makes currents within the rotor by due to. Thanks for watching my learning videos.