If you open the terminals of the 3 phase rotor. the induction motor acts as a transformer and the same theory applied in the transformers are applied to the induction motor if the rotor is blocked

i mean V1/V2=N1/Ne

If secondary(rotor circuit) is open no torque will developed, and rotor will not rotate. The voltage developed will follow the same relation of transformer(Vs/Vr=Ns/Nr). Read the book A.C machines by M.G.Say.

If the rotor of a wound rotor indution motor is open, as already said above by the colleagues, a voltage appears. This voltage can be desribed in terms of transformer theory but some differences are present. The main ones are: 1)the presence of an air gap causes a higher leakage inductance; 2) the phase angle of the voltage at the rotor terminal can be continously varied (this is not possible in a transformer) in comparison to the phase of the input voltage; 3)theharmonic content of the voltage at the open terminals has a behaviour slightly different from the one of a "normal" transformer.

I would like to add that when the rotor circuit is closed (Short circuit or through external resistances- method we use for starting up ) in the case the stator flux induce an electromotive force in the rotor and since the rotor is closed there will be a current in the rotor (conductors in the slots) and the interaction rotating field with rotor current create the force of rotation.

Refer Induction machine by Alger.

When rotor terminals are open, the stator is connected to three phase supply- the transformer induced voltage on open circuit rotor voltage is inevitable at standstill condition. When rotor is also being rotated then at any slip the rotor voltage as well as rotor frequency both may either decrease to sE2 and sf or increase to (2-s)E2 and (2-s) f depending on direction of rotation. Consider a case of slip ring induction motor running as a motor with one rotor phase open. This is causing George's phenomenon forcing the machine to run at slip of 0.5, in such cases the voltage on rotor terminal will be higher than nominal case. A very high voltage will appear on the open phase of rotor terminal if certain other faults on stator terminals are also present.

Rotor circuit is open no torque will developed, means the torque sufficient to take mechanical load. Due to flux linking to rotor core material(stampings) close circuit, rotor may rotate, at no load, and can be stop with hand, cannot be considered as useful torque as motor!!!!!!!!!!!!!!!!.

I agree with my colleagues that a voltage is induced in the rotor when the rotor is still or rotating. Here, I should add a remark: What kind of voltage is induced in the rotor? Transformer or speed voltage?

I believe that we can say both; If we consider the 3 phases of stator as a whole, then we have a rotating magnetic filed with constant amplitude. So, we have speed (and not transformer) voltage.

On the other hand, if we take into account the interaction of a rotor phase with each of the three phases of the stator, then we have a transformer voltage.

I have written the relation and have derived the result that the summation of these 3 transformer voltages is equivalent to the effect of a speed voltage (Which is the result of the net rotating magnetic field due to contribution from all the 3 stator phases).

I think this is a nice derivation.

I am eager to know your valuable view points.

Refer AC Machines by M G Say

I do not have the book, but could you please let me know if you confirm my deduction?

In my opinion, in AC machines involving rotation (exclude case of induction regulator) both transformer and speed induced EMF are present in rotor winding. This is debatable. As far as expressions are concerned (page 76 M G Say AC Machines ELBS 1983), both the EMF are present. The presence of both the EMF- transformer EMF at supply frequency and speed induced EMF at a frequency decided by mutual speed between rotor and rotating magnetic field ultimately results in concept of slip in induction machines- a concept of relative speed.

This concept does not speak of its Speed EMF.

The AC speed induced EMF and its effect is quite less.

Practically, if rotor is sheet rotor of conducting material running at large speed and stator is excited using DC current the braking force is very large.

But if we excite with AC stator current, a very feeble braking force or No braking force is produced.

This can be tested even in normal induction motor by applying DC dynamic braking and then replacing DC source by AC source. In latter case, you will find that machine braking torque is almost NIL.

As such we can can say on the basis of expressions that both types of EMF are present but their net effect is the rotating magnetic field and concept of slip.

Even when you are analyzing the induction motor by solving Max\well's equations the Poission's equation applicable to conducting medium, will have both types of speed and pulsating terms. Though for initial learning the concept of rotating magnetic field and concept of slip should be given with least or no reference to speed induced EMF although you may using expressions of speed and transformer induced EMF for derivation for RMF and slip.