Synchronous Machine

The outline of the pole shoes is shaped in such a way that the vectorial lines have such lengths to provide an almost sinusoidal induction run, along the corner covered by a polar couple, and with p periods along the whole air gap circumference F = armature e.m.f.   Ai = interpolar axis

C = excitation currents  Ap = polar axis

NO-LOAD OPERATION

This function is called excitation characteristic or no-load characteristic of the synchronous machine and it presents the typical run shown in Figure. To obtain modest no-load voltages U0, small values of polar flux and of induction are enough; therefore the rotor and stator iron present a high permeability, so the reluctance of the magnetic circuit is essentially determined by the air gap and it is linear (air gap characteristic). 

OCC

On the contrary to obtain greater no-load voltages, we need greater polar flux and induction, that imply saturation conditions in the iron, by reducing its permeability; the contribution of the iron sections becomes therefore relevant to the total reluctance of the magnetic circuit, that becomes strongly non linear: increments of  Ie are necessary more than proportional as to the increments of U0. The machines generally present a rated voltage Un, corresponding to a condition of modest saturation, with deviation of the 15- 30% as to the air gap characteristic.If the machine has operated at least once, the iron presents, with a null excitation current, a residual magnetism due to its hysteretical behaviour. Therefore for Ie=0 è there is a residual flux enough to produce a small no-load voltage U0.

LOAD OPERATION

The load operation is obtained by connecting the armature terminals to a mains in three-phase sinusoidal speed, so that at the terminal themselves there are sinusoidal currents; having assumed the star connected armature windings, their currents coincide with the ones at the terminals, whose effective value is I=Ii. The triad of the winding currents, or armature currents, can be expressed as :

ARMATURE REACTION

The armature currents are outgoing in the upper pole and incoming in the lower on

Flux distribution by magnetic poles only; as in OCC

Armature Reaction

Flux distribution by magnetic poles only; as in OCC

The armature currents are incoming in the upper pole and outgoing in the lower one

Armature ReactionThe armature currents are outgoing in the two left half poles and incoming in the right ones

Flux distribution by magnetic poles only; as in OCC

Armature ReactionThe armature currents are incoming in the two left half poles and outgoing in the right ones

Flux distribution by magnetic poles only; as in OCC

Armature Reaction in Motors

Is it Generator or Motor ?

Armature Reaction in Motor

Armature Reaction Simulation

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Armature Reaction

Conclusions(i) With brushes located along G.N.A. (i.e., θ = 0°), there is no

demagnetizing component of armature reaction (Fd = 0). There is only distorting or crossmagnetizing effect of armature reaction.(ii) With the brushes shifted from G.N.A., armature reaction will have both demagnetizing and distorting effects. Their relative

magnitudes depend on the amount of shift. This shift is directly proportional to the armature current.

(iii) The demagnetizing component of armature reaction weakens the main flux. On the other hand, the distorting component of

armature reaction distorts the main flux.(iv) The demagnetizing effect leads to reduced generated voltage

while crossmagnetizing effect leads to sparking at the brushes

SCC

The short circuit operation carries out when, with excited machine (Ie0), we short circuit the armature terminals, so to nullify the chained voltages and therefore the E =0:

therefore at these terminals we have the short circuit currents that are equal to: