Synchronous Machines: Generalities

The synchronous machine is a particular type of rotating electrical machine having a rotation speed strictly connected to the frequency of the sinusoidal electrical quantities at the terminals. 
In the operation as a generator it is called alternator; when it operates as a motor it is called synchronous motor. 
There are no fundamental differences between the two conditions and often the same machine can operate both as an alternator and as a motor. 
Even if the motor has important uses, in any case the fundamental application of the synchronous machine is as an alternator. As a matter of fact, nearly the whole of the electrical power used in the world is produced with alternators.
Usually the inductor is set in the rotor and the armature in the stator.


Fig. 1 - Synchronous machine (cross section)
Fig. 2 - Synchronous machine (longitudinal section)
S = stator P = salient pole
Ci = slots and armature conductors Sc = pole shoe
T = air gap Ce = excitation conductors
L = induction vectorial lines R = rotor
A = shaft Cr = rotor crown
C = frame Cr = rotor crown
S = stator A = shaft
Ci = armature conductors Ac = conducting rings
T = air gap So = brushes
Sp = pole shoe Me = excitation terminals
Np = polar core Mi = armature terminals
Ce = excitation conductors
Rotor
The rotor can have salient poles, generally for a number of polar couples p>3, or it can be smooth (generally for p<2). In the first case the rotor includes a massive steel crown with polygonal section integral to the shaft. On it we have mounted the 2p polar cores in massive steel, and the pole shoes, that can be of massive steel or with bars, because their induction has time fluctuations. Around the polar cores the inductor or excitation conductors are wound, to form 2p coils identical between them.

The smooth rotor has been carried out in massive steel and it is equipped with longitudinal slots housing the inductor or excitation conductors; the slot distribution and the connections between the conductors are such to carry out two (or four) identical coils.

The coils of the consecutive poles are always connected in antiseries and the inductor winding resulting from it is supplied through creeping contacts made up of brushes and rings; the constant excitation current Ie is provided by a specific d.c. generator (exciter), that is oftern made up of a rotating generator, keyed on the same shaft of the synchronous machine.

Sometimes, in machines having not a very big power, the poles are magnetized by means of permanent magnets instead of excitation coils: in this case, being the excitation current not required, the machines are without rings and brushes and they are called brushless.
There are also synchronous machines (of small power) where the inductor is set in the stator, that is therefore equipped with salient poles, excited by coils or permanent magnets.

 
Stator
The stator, having the shape of a cylindrical crown, is carried out in bar iron, being the seat of periodically variable magnetic induction.
On its internal edge there are longitudinal slots of length l, evenly spaced, housing the armature conductors. 
These are connected to carry out the armature windings that, connected among them, build up one or, more often, three armature windings, according to the fact that the machine is single phase or three-phase. The winding terminals are connected to the main terminals of the machine, and later they have to be connected to a mains in single phase or three-phase sinusoidal speed.

 
Inductor and armature conductors
The armature and inductor conductors are generally made of copper, isolated among them and as to the iron parts. 
In the inductor (operating in d.c.) conductors of high section can be used; on the contrary in the armature (operating in a.c.), to reduce the skin effect, we apply to thinner elementary conductors, reciprocally isolated, that are transposed among them and parallel connected to the edges.

Rated values

Similarly to the transformer ones, the rated values of a synchronous machine locate the levels of the main quantities with which we obtain the optimum operation of the machine. 
The main rated values of the synhcronous machine are listed in the Table. 

Pn Rated power [VA, W]
Un Armature rated voltage  [V]
In Armature rated current [A]
fn Rated frequency  [Hz]
Ie Excitation current  [A]
Ue Excitation voltage  [V]
nn Rotation rated speed  [rpm]
Cn Rated couple at the shaft [Nm]

Armature rated voltage and current have the meaning of effective values.
In the case of alternators the rated power has the meaning of apparent delivered electrical power and it is expressed in VA; in the case of motors it has on the contrary a meaning of power returned to the shaft and it is expressed in W. 
For single phase and three-phase alternators the following relationships are respectively valid:

on the contrary for the synchronous motors we have: