Category: THREE-PHASE INDUCTION MOTORS

Total flux produced by the rotor currents does not link with the stator winding. The part of rotor flux that links the rotor conductors but not with the stator winding is called leakages flux and hence develops leakage inductance (L2). The leakage flux and hence the inductance is very small, if the rotor conductors are…

Since the rotor winding is made of some conducting material (copper or aluminium), it has a definite resistance (R = ρ× l/a). Its value remains constant and is denoted by R2.

The revolving magnetic field set up in the stator by polyphase currents is common to both stator and rotor winding. This field induces emfs in both windings. The stator-induced emf per phase is given by the relation   E1 = 4.44 × kw1 × T1 × f × ɸm        (12.1) Where kw1 = winding factor, that is, product or coil span factor kc and distribution factor kd. T1 = No.…

When three-phase currents are supplied to the stator winding of a polyphase induction motor, a resultant field is set up, which rotates at a constant speed called synchronous speed (Ns = 120 × f/P). This rotating field induces polyphase emfs in the rotor winding, and if rotor winding is closed, polyphase currents circulate in it. These currents…

The frequency of rotor currents depends upon the relative speed between rotor and stator field. When the rotor is stationary, the frequency of rotor currents is the same as that of the supply frequency. But once the rotor starts to rotate, the frequency of rotor currents depends upon slip speed (Ns− N). Let at any speed N,…

The rotor of an induction motor always rotates at a speed less than synchronous speed. The difference between the flux speed (Ns) and the rotor speed (N) is called slip. It is usually expressed as a percentage of synchronous speed (Ns) and is represented by symbol S. Mathematically, % slip, or fractional slip, Rotor speed,  …

In Figure 12.2, it has been observed that a revolving field is set up in the stator of a three-phase induction motor, when three-phase supply is given to its winding and the direction of rotation depends upon the supply sequence. In Figure 12.3, it has been observed that rotor of a three-phase induction motor rotates in the…

When three-phase supply is given to the stator of a three-phase wound induction motor, a revolving field is set up in the stator. At any instant, the magnetic field set-up by the stator is shown in Figure 12.9. The direction of resultant field is marked by an arrow head Fm. Let this field is rotating in an anticlockwise…

Consider a stator on which three different windings represented by three concentric coils a1a2, b1b2, and c1c2, respectively, are placed 120° electrically apart. Let a three-phase supply, as shown in Figure 12.5, is applied to the stator. Three-phase currents will flow through the three coils and produce their own magnetic fields. The positive half cycle of the alternating current (AC)…

A three-phase induction motor consists of two main parts, namely stator and rotor.