Induction motor basic theory question

Hi, i have 2 questions.

The first being, to the difference between a wound rotor and a synchronous machine. From what i have read, the excitation on a induction machine occurs because of the she stator rotating magnetic field induces a voltage and because it is shorted this creates a large current. There are 2 main types of induction being the short circuited squirrel cage and short circuited wound rotor. T I can't understand what the difference is between a short circuited wound rotor induction machine and simply shortening the coil on a synchronous machines?? why dont we just short the coils to start a synchronous machine, why does it have to be DC excited?

The second being, if like the synchronous machine, the rotor follows the roatating stator magnetic field, how come the induction machine isnt a constant speed machine. Okay i know its sort of constant in the linear region but the speed can change unlike the synchronous. surely the stator field and rotor field are travelling at the same speed. and if the stator field is travelling faster surley the induction motor would go out of sync like the synchronous machine no??? Have i got the basic principle of the machine wrong?

Any help appreciated,

Thanks,

Lee.

 

As the name suggests, the induction motor operates by means of magnetic induction. A fundamental characteristic of this motor, however, is slip. Expressed in a more formal capacity, this breed of machine can be referred so as an asynchronous motor.

On the converse, external d.c. excitation implemented in the synchronous machine, of which excites the armature, has a desire to "chase" the field's rotating magnetic field in such a way until it becomes locked at system frequency. In some cases, there are rotor bars sunk underneath the face of the armature. In this way, through magnetic induction, it will start as an asynchronous machine until close to synchronous speed, then the d.c. excitation will be applied. Maybe this could be what's causing confusion?

An interesting property of the synchrounous machine is power factor correction.