What you are looking here at is an asynchronous (induction) motor. The difference to a synchronous is in the fact that the rotor is usually just a (sqirrel) cage and some iron.
For a synchronous motor the rotor has either a magnet or has geometrically strongly defined(salient) poles and pole gaps. Sometimes it has both!
Now why is one synchronous and the other asynchronous?
- The synchronous motor has a constant excitation in the rotor (or has a constant difference between inductances). Therefore the field rotates in synchronicity with the rotor. $$f_{rotor}=0$$
- The asynchrnous motor on the other side has an induced field in the rotor that is dependent on the speed of rotation.
$$f_{rotor}>0$$
The rotational speed is calculated from the difference of the stator and rotor flux frequencies.
Why is the speed 1460? The stable part of the torque/rotational speed charachteristic for a asynchronous motor is near the synchronous speed. If you want to find the number of poles round it to the nearest synchronous rotational speed and calculate it normally. In some special working regimes this won't be true as the asynchronous motor could operate at for example 900 rotations if you make some adjustments. In your case however it will have p=4.
Read the wikipedia articles, or google asynchrnous and synchronous motors.
It is a tradeoff, hence actual wound-rotor motors expose the rotor windings so that a resistance can be added during start, and tapered off or just removed when at speed. The resistance added is actually not tremendous, usually from 1 to 5 times the DC resistance of the rotor.
The initial problem, is that at at start, a typical induction motor can draw up to 1000% rated current. For large (> 200 HP motors) this can severely stress the supply. Pullout torque (torque required to either start the rotor, or drop it out from near synchronous speed) is directly related to slip. Increasing resistance in the rotor, effectively increases the slip, which increases the torque.
As more resistance is added to the rotor, the peak torque curve is moved closer and closer to zero speed. That is the sweet spot for starting a wound rotor motor; lowest inrush current, highest slip, highest torque. Adding more resistance will reduce the available torque, as the slip begins again to decrease.
You can run Wound Rotor motors in a variable speed mode, if you can control the resistance, but that is generally very inefficient.
Consider that basically, an induction motor at standstill is a transformer, with an essentially shorted secondary. When power is applied to the stator, a voltage is induced into the rotor, which, being shorted develops the current which creates the magnetic field to pull the rotor along with the stator's rotating field.
Okay, since the induction motor is a transformer when starting (at zero speed), there are reactances to deal with. The reactance actually causes the induced voltage (and current, and generated magnetic field) to be out of phase with the stator field, generally lagging by about 90 degrees, so the magnetic interaction between the rotor and stator is fairly weak.
If pure resistance is added to the rotor circuit, the phase lag starts to grow smaller. (Note that different constructions of the rotor bars, with different resistances are used to permanently affect the torque curves of many motors). Add enough resistance, and the phase lag reduces to the point of what the motors design slip is, which corresponds to its maximum design torque.
The problem with leaving the resistance in circuit (aside from power dissipation) is that as the motor speeds up, and approaches its synchronous speed - slip, now you have advanced the rotor/stator magnetic phase, resulting in reduced torque at the shaft.
Best Answer
During half of a cycle of the power frequency, the position of a rotor pole moves from one stator pole to the opposite pole of the same pair. If the motor has only two poles, that rotor pole moves back to the original position during the next half cycle. So there is one revolution per cycle of the power frequency. If there are more than two poles, the rotor moves from one pair of poles to another pair of poles during one cycle of the power frequency. Thus Speed(RPM) = 120 f(Hz) / P(poles).
Slip
In an induction motor, the mechanical speed of the rotor is less than the speed of the magnetic fields described above. The difference allows the stator to induce current in the rotor. The difference is very small when the motor has no external mechanical load. For the most common 3-phase motor designs, the slip is about 2 or 3 percent at full-load torque. For single-phase motors, the full-load slip is a little higher.
Power is speed multiplied by torque. Therefore a motor of a given power rating will have a higher torque rating if the speed rating is lower.
The physical size, weight and cost of a motor are more closely related to its torque rating than to power its power rating.