In an induction motor, the speed of the rotor structure is always less than the speed of the stator field. However the rotor field rotates faster than the rotor structure so that the rotor and stator fields are synchronized with each other.
In a synchronous motor, the rotor magnetic field is produced by permanent magnets or by DC current in the rotor winding. In either case, the rotation of the magnetic field of the rotor is mechanically fixed to the motion of the rotor. For uniform torque to be produced, the both the rotor structure and the rotor field must move synchronously with the rotor field.
In other words, both synchronous and induction motors have synchronously turning magnetic field with torque produced in proportion to the angular displacement between the stator and rotor magnetic fields. In the induction motor, the rotor structure must turn at a slower speed than the magnetic fields while in a synchronous motor, the rotor structure must move synchronously.
Re: Question Edit
In a synchronous generator, the stator magnetic field rotates behind the rotor magnetic field with respect to torque angle. It is the relative motion between the rotor magnetic field and the stator windings that allows the magnetic field of the rotor to produce current in the stator. The current produced produces a rotating magnetic field in the stator that is synchronous with the rotor magnetic field but has a torque angle displacement.
For instance, is this the equivalent circuit model for stator?
That is the equivalent model of one phase of the entire motor. The stator part consists of Rs, Xs and Xm. Wr and Rr are the rotor components. Note that it has been simplified to remove the effect of the stator:rotor turns ratio. However you can assume that the rotor parameters given are the values referred to the stator circuit. Referral to the stator means that the stator frequency can be used to calculate both Xs and Xr. The speed of the rotor is the mechanical speed.
The stator current is calculated by dividing the phase voltage by the equivalent complex impedance of the entire circuit shown.
The speed of the stator magnetic field in radians per second is 4xPixf/poles. The speed of the rotor (mechanical speed) is the speed of the stator magnetic field minus the slip. The speed of the rotor magnetic field is the same as the speed of the stator magnetic field. In the other problem, I believe that the stator omega is the frequency of the power and the mechanical speed is the speed of the magnetic field.
To further explain the concept:
Conceptually, the mechanical structure must always be kept in mind. In analyzing motor performance, "no-load" is assumed to be operation with nothing connected to the shaft. Except for that special condition the speed of the rotor = speed of load. The torque developed in the rotor = torque of load plus mechanical losses in the motor consisting of bearing friction and aerodynamic drag on the rotor (windage). In this problem, mechanical losses seem to be considered to be part of the load or considered to be negligible. Stator speed = field speed = synchronous speed.
It appears that iron losses have been considered as negligible in this problem. In the equivalent circuit, iron losses (hysteresis and eddy-current losses) would be represented as a resistor in parallel with Xm. That is a rather large loss to neglect, but I it is probably necessary to neglect it in order to construct the problem the way is is constructed.
Motor & Load Torque vs Speed
This problem defines load torque demand, the torque required to drive the load at a given speed. The load torque demand is proportional to speed squared. That would be typical for a fan. Information is provided to define the motor torque capability as a function of speed. The steady-state operating speed and torque for a motor driving a load is the intersection of the motor torque capability curve and the load torque demand curve. If the actual operating speed is above or below the normal operating point as it is when the motor is initially energized, the excess motor torque capability is applied to accelerating the motor and load inertia to the normal operating speed.
Best Answer
Since the motor converts electrical energy to mechanical energy, the electrical input power must be equal to the mechanical power transmitted to the load plus power lost in the motor. Since the input voltage is constant, that would have to be reflected in the input current and power factor since power = voltage X current x power factor.
The mechanism by which electrical power is converted to mechanical power is explained using the equivalent circuit of the motor. Just as in a DC motor, a back EMF is generated in an AC synchronous motor. So the motor equivalent circuit is an AC back EMF generator in series with the internal impedance of the motor. The back EMF opposes the source voltage so that the stator current is proportional to the source voltage minus the back EMF divided by the internal impedance of the machine. With an AC machine, the voltage, current and impedance values are all complex numbers. The phase angle difference between the terminal voltage and the back EMF is determined by torque angle, the angle between the rotating stator and rotor magnetic fields. As the name implies, the torque angle is proportional to torque.
I don't think that is related to the synchronous motor question. The induction motor rotor current is ultimately supplied by the stator, but the magnetic fields in both the stator and rotor are pretty much constant as long as the applied voltage and frequency are constant and the ratio of voltage to frequency is constant.