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 See Sec. 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:. 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.
The rotor winding is uniformly distributed in the slots and is usually star-connected. Various methods are available to reduce slip, VFDs often offering the best solution The rotor of three phase induction motor consists of a cylindrical laminated core with parallel slots that can carry conductors. The conductors are heavy copper or aluminium bars fitted in each slot and short-circuited by the end rings.
The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed because this arrangement reduces magnetic humming noise and can avoid stalling of the motor. As the rotor winding in an induction motor are either closed through an external resistance or directly shorted by end ring, and cut the stator rotating magnetic field, an Emf is induced in the rotor copper bar and due to this Emf a current flows through the rotor conductor.
The working principle of three phase induction motor, it may be observed that the rotor speed should not reach the synchronous speed produced by the stator. If the speeds become equal, there would be no such relative speed, so no Emf induced in the rotor, and no current would be flowing, and therefore no torque would be generated. Consequently, the rotor cannot reach the synchronous speed. The difference between the stator synchronous speed and rotor speeds is called the slip.
The rotation of the magnetic field in an induction motor has the advantage that no electrical connections need to be made to the rotor. Advantages of Induction Motor 1.
The most important advantage of an induction motor is that its construction is quite simple in nature. The working of the motor is independent of the environmental condition. This is because the induction motor is Robust and mechanically strong.
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:. For explaining the principle of operation of a three-phase induction motor, consider a portion of the three-phase induction motor as shown in the figure. The working of the three-phase induction motor is based on the principle of electromagnetic induction. When three-phase stator winding of an induction motor is energized from a 3 phase supply, a rotating magnetic field is set up which rotates around the stator at synchronous speed N s.
For more details about rotating magnetic field, read Production of rotating magnetic field. This rotating field passes through the air gap and cuts the rotor conductors, which are stationary.
An EMF gets induced in every rotor conductor due to the relative speed between the rotating magnetic flux and the stationary rotor. Since the rotor circuit is short-circuited, currents start flowing in the rotor conductors.
The current-carrying rotor conductors are placed in the magnetic field produced by the stator. Consequently, a mechanical force acts on the rotor conductors. The sum of the mechanical forces on all the rotor conductors produces a torque which tends to move the rotor in the same direction as the rotating field. The fact that the rotor is urged to follow the stator field i. Now, the cause producing the rotor currents is the relative speed between the rotating field and the stationary rotor conductors.
Hence to reduce this relative speed, the rotor starts running in the same direction as that of the stator field and tries to catch it. This is how a three-phase induction motor starts running. We have seen above that the rotor rapidly accelerates in the direction of the rotating magnetic field. In practice, the rotor can never reach the speed of stator flux. Since the rotor circuit is short-circuited, a current starts flowing in the rotor conductors. Now, the current carrying rotor conductors are in a magnetic field created by the stator.
As a result of this, mechanical force acts on the rotor conductors. The sum of mechanical forces on all the rotor conductors produces a torque which tries to move the rotor in the same direction as the RMF.
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