Induction Motor

Slip.

If it were identical, then the motor would be doing very little work.

The stator has two windings (or sets of windings), one of which lags in phase behind the other. The result is that the stator magnetic field rotates, in the same direction which the rotor turns.

From the rotor's point of view, once it starts turning, the magnetic field is still rotating but at a slower rate (because the rotor is basically "catching up" with the rotating field).

Rotating Field Animation

It's even simpler than that really. The rotor in an induction motor has no direct connection to the electrical power source, the energy in the rotor is made by virtue of the lines of magnetic force of the stator fields expanding and collapsing, thereby cutting across the rotor bars and "inducing" current to flow in them (hence the term "induction" motor), which THEN creates magnetic fields around the rotor bars. Those magnetic fields then interact with the rotating stator fields to make the rotor "chase" the rotation.

So the simpler issue is that the magnetic fields in the rotor must be created BY the magnetic fields in the stator first, so they inherently MUST lag.

so they inherently MUST lag

But lag does not mean frequency will be different, Its just a time delay.

The rotor current is a lower frequency because the rotor is following the rotating stator field but not turning as fast as the rotating field. The rotor current frequency would be the difference between the field rotating speed and the rotor speed.

I think it is a trick question. While the waveform of what is induced in the rotor does lag behind that applied to the stator, the frequency need not actually be slower. With reference to the stator the frequency will be the same.

If you reference the driving current to the stator and the induced current to the rotating rotor, then the frequency of the induced current will appear slower because the rotor is turning in the same direction as the magnetic field inducing the current.

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Think of it this way, when a truck pulls a trailer, it does lag behind, but it travels the same number of miles in an hour.

1) You must be knowing that In a motor, Torque is produced by the interaction of two magnetic fields i.e.,T=kBr x Bs, Br is Magnetic flux density of Rotor and Bs is Magnetic flux density of Stator. If rotor speed and revolving magnetic field speed will be same then no magnetic flux will be there So, No torque. 2) The difference of Ns and Nr is Slip. as Slip=(Stator Speed-Rotor Speed)/Stator Speed. 3) Due to rotation of the rotor. The induced voltage doesn`t have same frequency as supply frequency or stator frequency. 4) Now go back to the derivation of formula you mentioned. fr=S*fs, fr=P*n/120 => fr=p(Ns-Nr)/120 => fr=p(S*Ns)/120 => fr=S*fs

Friction and air drag losses don't let it go to synchronous speed, In a way motor is preloaded with a load that is not visible to you.

There is a variant of induction motors called "fluence motors". The iron in the rotor has lobes equal to the number of poles on the motor and follows the stator magnetic field with no slip. In effect the rotor locks into the stator magnetic field and run the same way as a synchronous motor. Thus a 3600 rpm fluence motor runs at 3600 rpm while a standard induction motor would run at 3450rpm. You don't see many of these motors around.

Shrinkage. Otherwise known as slip.