60 Chapter 4 • Alternating current motors
(secondary), an emf is induced in these conductors, in the same way that an emf is induced in the secondary winding of a transformer, by the flux set up by the currents through the primary.
Since the rotor circuit is completed through the end-rings of the squirrel-cage rotor, the induced emf causes a current to flow through the rotor conductors. Te current-carrying rotor conductors in the stator field thus have a force exerted on them.
Fig. 4.3 represents a section of the stator and the rotor of an induction motor. Te magnetic field is rotating in a clockwise direction. For the flux direction and motion shown, an application of Fleming’s right-hand rule shows the direction of the induced current in the rotor conductor to be out of the paper.
At the instant being considered, with the current-carrying conductors in the magnetic field as shown, force is exerted upward on the conductors. One-half cycle later the stator field direction will have reversed, but the rotor current will also have reversed, so that the force on the rotor is still upward. Likewise, rotor conductors under other stator field poles will have a force exerted on them, all tending to turn the rotor in a clockwise direction.
4.5 Speed and slip
An induction motor cannot run at synchronous speed (speed equal to that of the rotating magnetic field). If it were possible, by some means, for the rotor to attain synchronous speed, the rotor would then be standing still with respect to the rotating flux. No emf would be induced in the rotor, no rotor current would flow and therefore there would be no torque developed.
Te rotor speed, even at no-load, will always be slightly less than synchronous speed in order that current may be induced in the rotor which will produce a torque large enough to overcome friction and wind resistance. Tis difference between rotor speed and synchronous speed is called slip.
Slip may be expressed in revolutions per minute but is more commonly expressed in terms of the synchronous speed as either a percentage or as a per unit value. Where the per unit form is used;
S = N – Nr
_____ N
or S = n – nr
_____ n
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