Electrical Engineering ⇒ Topic : Elementary ThreePhase Alternator

William
 
Elementary ThreePhase Alternator In an actual 3phase alternator, the three windings or coils are stationary and the field **rotates. Fig. a (i) shows an elementary 3phase alternator. The three identical coils A, B and C are symmetrically placed in such a way that e.m.f.s induced in them are displaced 120 electrical degrees from one another. Since the coils are identical and are subjected to the same uniform rotating field, the e.m.f.s induced in them will be of the same magnitude and frequency. Fig. a (ii) shows the wave diagram of the three e.m.f.s whereas Fig.a (iii) shows the phasor diagram. Note that r.m.s. values have been used in drawing the phasor diagram. Thus E_{A} is the r.m.s. value of the e.m.f. induced in coil A. The equations of the three e.m.f.s are e_{A} = E_{m} sin ω t
figure (a) It can be proved in many ways that the sum of the three e.m.f s at every instant is zero. (i) Resultant = e_{A}+e_{B}+ e_{c} _{} (2) Referring to the wave diagram in Fig. a (ii), the sum of the three e.m.f.s at any instant is zero. For example, at the instant P, ordinate PL is positive while the ordinates PN and PH are negative. If you make actual measurements, it will be seen that PL + ( PN)+ ( PH) = 0 (3) Since the three windings or coils are identical, E_{A} = E_{B }= E_{C }= E (in magnitude). As shown in Fig. B, the resultant of E_{A} and E_{B} is E_{r} and its magnitude is = 2E cos: 60° = E. This resultant is equal and opposite to E_{c}. Hence the resultant of the three e.m.f.s is zero. (4) Using complex algebra, we can again prove that the sum of the three e.m.f.s is zero. Thus taking E_{A} as the reference phasor, we have, figure (b) E_{A}+ E_{B}+ E_{C} = (E +j0) + E ( 0.5  j 0.866) +E (0.5 + j 0.866) = 0 The reader may wonder how we can get power from the terminals of a 3phase alternator if the sum of the voltages it delivers is zero at every instant ? This will be explained when we come to the connections of the three phases or windings.  
 
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