The Principle of the Induction Machine and Slip
What happens to the torque if the rotor reaches synchronous speed?
Current is induced in the blue rotor conductors and torque arises only when there is relative motion, with the gold rotating field sweeping past them. Push the rotor speed with the slider toward synchronous speed (1800 rpm) and the relative motion shrinks, and so does the induction; below that, you can first pick what the torque does at the very moment the rotor reaches synchronous speed exactly. That answer and its reason are the question of this lesson.
Induction needs relative motion
When the rotating field sweeps past rotor conductors that are stopped or turning more slowly, the conductors cut flux and pick up an emf (Faraday). Current flows in the short-circuited rotor conductors, and this current feels a force in the rotating field (F = BIl) that drags the rotor in the direction the field turns. By Lenz's law, the force made by the induced current acts to reduce the relative motion that causes it, so the rotor speeds up in the same direction as the field. A rotor whose conductors are aluminum bars tied together by rings at both ends looks like a squirrel cage, hence the name squirrel-cage rotor.
Synchronous speed is the hare it cannot catch
As the rotor speeds up, its relative motion with the rotating field shrinks, the conductors cut flux more slowly, and the induced current and torque fall together. If the rotor reaches synchronous speed, the relative motion is 0 and emf, current, and torque are all 0; at that moment the rotor cannot overcome friction and load, so it slows down again and the relative motion and torque come back. So when running as a motor, the rotor always settles at a speed slightly below synchronous, which is why an induction machine is called an asynchronous machine. Conversely, if the rotor is driven from outside faster than synchronous speed, the slip turns negative and the machine sends power out as an induction generator, a scheme some wind turbines use.
Slip measures everything
How far the rotor lags is measured by the slip s = (Ns - N)/Ns. At standstill s = 1, and at synchronous speed s = 0. The frequency of the flux seen by the rotor conductors, the slip frequency, is f2 = s·f, and the emf and reactance induced in the rotor are also proportional to the slip. At the moment of starting, s = 1 so the rotor frequency equals the supply frequency, while in normal running the slip is usually a few percent, so the rotor frequency is low, a few hertz. The slip alone sets the electrical state of the induction machine.
Back to the first screen
When the rotor reaches synchronous speed, the torque becomes 0. Induction comes from the relative motion of the rotating field sweeping past the rotor conductors, and at synchronous speed that relative motion vanishes. If the conductors cannot cut flux there is no emf, current, or torque; with no torque the rotor cannot even overcome friction and slows down again, and then the relative motion returns and so does the torque. So the rotor never reaches synchronous speed and always settles behind it by the slip s = (Ns - N)/Ns. The slip is the price an induction machine pays to produce torque.
Slip is the handle for analysing the induction machine. Since the rotor circuit’s EMF and reactance both carry slip, treating slip as the variable lets you fold the rotor, seen from the stator, into a single equivalent circuit. The key is writing the rotor resistance as R2/s — this one term holds both the real electrical loss and the mechanical output the rotor delivers. The next unit builds that equivalent circuit (MC-C3).