Induction motors have two main components, the stator and the rotor. The stator carries a three-phase winding that receives power from the supply. The rotor carries a winding that is in the form of a set of single-bar conductors placed in slots just below the surface of the rotor. The slots have a narrow opening at the surface of the rotor, which serves to lock the conductor bars in position. Each end of each bar conductor is connected to a short-circuiting ring, one at each end of the rotor. The stator winding is a conventional type as found in three-phase generators and synchronous motors.
The three-phase stator winding produces a rotating field of constant magnitude, which rotates at the speed corresponding to the frequency of the supply and the number of poles in the motor. The higher the number of poles the lower the speed of the rotation. Assume that the rotor is stationary and the motor has just been energized. The magnetic flux produced by the stator passes through the rotor and in so doing cuts the rotor conductors as it rotates. Since the flux has a sinusoidal distribution in space its rotation causes a sinusoidal emf to be induced into the rotor conductors. Hence currents are caused to flow in the rotor conductors due to the emfs that are induced. The emfs are induced in the rotor by transformer action, which is why the machine is called an ‘induction’ motor. Since currents now flow in both the stator and the rotor, the rotor conductors will set up local fluxes which interact with the excitation flux from the stator. This interaction causes a torque to be developed on the rotor. If this torque exceeds the torque required by the mechanical load the shaft will begin to rotate and accelerate until these two torques are equal. The rotation will be in the direction of the stator flux since the rotor conductors are being driven by the stator flux.
Initially the speed is much less than that of the stator field, although it is increasing. Consequently the rate at which the stator flux cuts the rotor conductors reduces as the shaft speed increases.
The frequency and magnitude of the induced rotor emfs therefore decrease as the shaft accelerates.
The local flux produced by the rotor conductors therefore rotates at a slower speed relative to the rotor surface. However, since the rotor body is rotating at a slow speed, the combined effect of the body speed plus the rotational speed of the local rotor flux causes the resulting rotor flux to rotate at the same speed as the stator field.
The rotor currents are limited by the short-circuit impedance of the rotor circuit. This circuit contains resistance and reactance. The inductive reactance is directly proportional to the frequency of the induced emfs in the rotor. As the rotor accelerates two effects take place:-
a) The rotor impedance increases.
b) The rotor emf reduces.
These effects result in the supply current is being nearly constant during most of the run-up period.
The rotor speed cannot reach the same speed as that of the stator field, otherwise there would be no induced emfs and currents in the rotor, and no torque would be developed. Consequently when the rotor speed is near to the synchronous speed the torque begins to decrease rapidly until it matches that of the load and rotational friction and windage losses. When this balance is achieved the speed will remain constant.
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