Three Phase Induction Motor:
The three phase induction motors are the most widely used electric motors in the industry.They run at essentially constant speed from no-load to full-load.However, the speed is frequency dependent and consequently, these motors are not easily adapted to speed control.We usually prefer dc motors when large speed variations are required.
Nevertheless, the 3-phase induction motors are simple, rugged, low-priced, easy to maintain and can be manufactured with characteristics to suit most industrial requirements.In this, we shall discuss working principle of 3-phase induction motors.
Like any electric motor, a 3-phase induction motor has a stator and a rotor.The stator carries a 3-phase winding (called stator winding) while the rotor carries a short-circuited winding (called rotor winding). Only the stator winding is fed from 3-phase supply.The rotor winding derives its voltage and power from the externally energized stator winding through electromagnetic induction and hence the name.
The induction motor may be considered to be a transformer with a rotating secondary and it can, therefore, be described as a “transformer type” ac machine in which electrical energy is converted into mechanical energy.
Construction of Three Phase Induction Motor:
The construction of three phase induction motor. A 3 phase induction motor has two mainq parts (i) stator and (ii) rotor.The rotor is separated from the stator by a small air-gap which ranges from 0.4 mm to 4 mm, depending on the power of the motor.
1. Stator :
It consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations of silicon steel to reduce hysteresis and eddy current losses. A number of evenly spaced slots are provided on the inner periphery of the laminations.The insulated connected to form a balanced 3-phase star or delta connected the circuit.
The 3-phase stator winding is wound for a definite number of poles as per requirement of speed.Greater the number of poles, lesser is the speed of the motor and vice-versa.When 3-phase supply is given to the stator winding, a rotating magnetic field of constant magnitude is produced.This rotating field induces currents in the rotor by electromagnetic induction.
The rotor, mounted on a shaft, is a hollow laminated core having slots on its outer periphery.The winding placed in these slots (called rotor winding) may be one of the following two types:
(i) Squirrel cage type
(ii) Wound type
(i) Squirrel cage rotor:
It consists of a laminated cylindrical core having parallel slots on its outer periphery.One copper or aluminum bar is placed in each slot.All these bars are joined at each end by metal rings called end rings.
This forms a permanently short circuited winding which is indestructible. The entire construction (bars and end rings) resembles a squirrel cage and hence the name.The rotor is not connected electrically to the supply but has current induced in it by transformer action from the stator.
Those induction motors which employ squirrel cage rotor are called squirrel cage induction motors.Most of 3 phase induction motors use squirrel cage rotor as it has a remarkably simple and robust construction enabling it to operate in the most adverse circumstances.
However, it suffers from the disadvantage of a low starting torque.It is because the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit to have a large starting torque.
(ii) Wound rotor:
It consists of a laminated cylindrical core and carries a 3-phase winding, similar to the one on the stator. The rotor winding is uniformly distributed in the slots and is usually star-connected.The open ends of the rotor winding are brought out and joined to three insulated slip rings mounted on the rotor shaft with one brush resting on each slip ring.
The three brushes are connected to a 3-phase star-connected rheostat as shown in Fig. At starting, the external resistances are included in the rotor circuit to give a large starting torque.These resistances are gradually reduced to zero as the motor runs up to speed.
The external resistances are used during starting period only. When the motor attains normal speed, the three brushes are short-circuited so that the wound rotor runs like a squirrel cage rotor.
Working Principle of Three Phase Induction Motor:
Speed of rotating magnetic field:
The speed at which the rotating magnetic field revolves is called the synchronous speed (Ns).Referring to the time instant 4 represents the completion of one-quarter cycle of alternating current Ix from the time instant 1.During this one-quarter cycle, the field has rotated through 90°. At a time instant represented by 13 or one complete cycle of current Ix from the origin, the field has completed one revolution.
Therefore, for a 2-pole stator winding, the field makes one revolution in one cycle of current.In a 4-pole stator winding, it can be shown that the rotating field makes one revolution in two cycles of current.In general, fur P poles, the rotating field makes one revolution in P/2 cycles of current.