In DC Motors current is drawn from the supply and conducted into the armature conductors through the brushes and commutator. When the armature conductors carry current in the magnetic field established by the field, a force is exerted on the conductors which tends to move them at right angles to the field.
But in the case of an Induction motor, currents are induced in the rotor circuit and the rotor conductors carry current in the stator magnetic field, and thereby a force is exerted on the conductors which tends to move them at right angles to the field.
Three-phase Induction motor working Principle
When the stator or primary winding of a 3-phase Induction Motor is connected to a 3-phase ac supply, a rotating magnetic field is established which rotates at the synchronous speed.
The direction of revolution of this field will depend upon the phase sequence of the primary currents and therefore will depend upon the order of connection of the primary terminals to the supply. The direction of rotation of the field can be reversed by interchanging the connection to the supply of any two leads of a 3-phase Induction motor. The number of magnetic poles of the revolving field will be the same as the number of poles for which each phase of the primary or stator winding is wound.
Synchronous speed: The speed at which the field produced by the primary currents will revolve is the synchronous speed of the motor and is given by
When the rotating field produced by the primary currents sweeps across conductors, an emf is induced in these conductors as same as an emf induced in the secondary winding of a Transformer by the flux set up by the primary current. Some external resistance is used for short or closed rotor winding, the emf induced in the secondary by the revolving field causes a current to flow in the rotor conductors.
A section of an Induction motor stator and rotor, with the magnetic field assumed to be rotating in a clockwise direction and with the rotor stationary. The relative motion of the rotor with respect to the stator field is counter-clockwise. When we apply the right-hand rule, the direction of induced emf or current in the rotor conductor is outward. So, the direction of the flux due to the rotor current alone is anticlockwise.
Applying the left-hand rule or by the effect of combined field, the rotor conductor experiences a force tending to move the conductor to the right. We here consider only one rotor conductor but other adjacent rotor conductors in the stator filed likewise carry current in the same directions as the conductors shown and also have a force exerted upon them tending to move them towards the right.
one-half cycle later, the stator field direction will have reversed, but the rotor currents will have also reversed so that the force on the rotor is still the same. Similarly, rotor conductors under other stator field poles will have a force exerted upon them all tending to turn the rotor in the clockwise direction.
If the developed torque is great enough to overcome the resisting torque of the load, the rotor will accelerate in the clockwise direction or in the same direction as the rotation of the stator field.
Why an Induction Motor cannot run at synchronous speed?
According to Lenz`s law, the direction of induced emf would be in such a direction that it would try to oppose the very cause for which it is due. The cause producing the induced currents is because of the relative speed between the rotating magnetic field and stationary rotor conductors. Hence, they circulate in such a manner that a torque produced in the rotor causes it to follow the rotating magnetic field and thus reduces the relative speed.
when the rotor is stationary and about to start, the frequency of induced emf in the rotor is equal to that of the supply fed to the stator as the relative motion is at synchronous speed. As the motor starts to pick up, the relative motion between the rotor and the synchronously rotating magnetic field becomes less and the frequency of emf induced in the rotor decreases. The motor runs at synchronous speed in case the relative motion is Zero. Hence, there would be no induced emf and no current in the rotor conductors and no rotor field, and hence no torque. Thus Induction Motor cannot run at synchronous speed.
Production of Torque
The working principle is illustrated diagrammatically below:
When an induction motor is running at no load will have a speed very close to the synchronous speed and hence emf in the rotor winding is very small. The small emf just provides a small current developing torque sufficient to overcome the losses due to friction and maintain the rotor in motion.
Since the mechanical load is applied on the motor shaft, it must slow down because the torque developed at no load will not be sufficient to keep the rotor revolving at no load speed against the additional opposing torque of load. As a result, the motor slows down and the relative motion between the magnetic field and the rotor is increased which results in greater rotor currents and greater developed torque.
If the load is increased, the motor slows down until the relative motion between the rotor and the rotating magnetic field is just sufficient to result in the development of the torque necessary for that particular load.
- In the case of small and medium size induction motors the decrease in speed from no load to full load is usually to 4 to 5 percent.
- For large Size motors, it varies from 2 to 2.25 percent.