Distinct Between Synchronous Asynchronous Alternators AC Motors
The most general definition is that the speed of rotation of synchronous machines is directly a function of the frequency of the system on which they are applied. By contrast, the speed of rotation of asynchronous machines is not directly a function of system frequency, but is more nearly a function of load.
Definition Of Synchronous
Generators of both types of have an even number p of magnetic poles, ie p = 2, 4, 6,.. etc.. The synchronous speed of both types is equal to 3600 x 2/p on a 60 Hz system and 3000 x 2/p on a 50 Hz system. Synchronous generators run at exactly their synchronous speed corresponding to the pole number and supply frequency. Asynchronous generators run slightly faster than their synchronous speed. The design distinction between the two is that the synchronous generator (or alternator or turbogenerator) has a field winding on its rotor which is supplied with DC from an external source to magnetise the machine. The asynchronous (or induction) generator has a short-circuited rotor winding (often just a so-called squirrel cage) in which the excitation current is induced from the armature winding on the stator. This necessitates that the asynchronous generator run slightly (1 to 2% depending on load) faster (or an induction motor slightly slower) than its synchronous speed. Important practical differences are that an asychronous generator can only function if it has a supply voltage of stable frequency to work on. It cannot generate on its own, neither can it provide reactive VA to meet system demands as a synchronous generator can. Partly for the latter reason there are relatively few asynchronous units in power generation.
