This term refers to a particular type of generator the number of revolutions of which standard frequency is coordinated with the number of pairs of poles . The synchronous number of revolutions n1=60(f/p) . The standard frequency f=50 Hz .
Synchronous alternators of small power are sometimes built like d.c. generators equipped with stationary poles and a rotating armature . The difference is that not the commutator but the armature winding is connected to the slip-rings causing its alternating current to flow into the external circuit .
Taking into account that it is difficult to collect large power by means of sliding contacts , synchronous alternators are usually provided with rotating poles (the rotor) and a stationary armature (the stator). The power of such alternators is practically unlimited (1,400,000 kVa).
The stator is found in a steel frame with the core mounted inside and assembled of 0,35-0,5 mm electrical steel laminations . The stator winding is laid the core slots , its leads being brought out the terminal box . Synchronous alternators may be either one-phased or three-phased . A three-phase alternator has the stator windings either star- or delta-connected in the terminal box .
The greater part of shipboard alternators operated at the speed of rotation ranging from 500 to 1500 rev/min and has the rotor equipped with salient poles .
The rotor is a steel shaft on which the cores of the poles with mounted on them excitation windings are fixed , their supply being obtained through slip-rings and and brushes . The source of direct current is usually a shunt-wound generator (the exciter) which is mounted with a synchronous alternator on the same shaft .
The principal characteristics for synchronous alternators are the following :
No-load characteristic , which is the relationship between the alternator e.m.f. and the excitation current E=f(Iex) with the speed of rotation n=0 . This characteristic makes it possible to judge the degree of steel saturation .
External characteristic U=f(I) which detects the relationship between the alternator voltage and the load current with the speed of rotation n=const , the excitation current Iex =const AND POWER FACTOR COS f=const . The external characteristics show the change of the voltage when the load current value and character are varied .
Regulation characteristic Iex=f(I) which reveals the relationship between the excitation current and the load current with the alternator voltage U=const , the speed of rotation n=const , and the power factor COS f=const . The regulation characteristics indicate in what way the excitation should be varied with the alteration of the load current value and character in order to keep the voltage at the alternator terminals unchangeable .
Synchronous alternators in marine application are those with silicon rectifier in the excitation system , those with static excitation system , brushless alternators ets . Self-excited brushless alternators are the most widespread because they don’t have either a commutator , slip-rings or brushes , which makes them more reliable and easier to operate .
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