Synchronous Motor Behavior: Constant & Varying Excitation

For supply \(V\) \(\Rightarrow\) back emf \(E_b\) \(\Rightarrow\) resultant across armature \(V-E_b\) \(\Rightarrow\) \(I_a = (V-E_b)/R_a\)

The value of \(E_b\) depends on \(N\)

Mechanical power developed- \(E_bI_a\)

\(E_b\) set up in armature by rotor flux which oppose \(V\)

\(E_b\) depend on rotor excitation only, and not on \(N\) as in dc motors

The net voltage in armature is vector difference of \(V\) and \(E_b\) (not arithmetic difference like dc motors)

\(I_a\) obtained by dividing the vector difference of voltages by armature \(Z\) (not \(R_a\) as in dc machines)

field excitation makes \(E_b=V\)

vector difference of \(E_b\) and \(V\) is zero

motor intake is zero

\(E_b\) falls back by small angle \(\alpha\)

\(E_R\) and \(I_a\) brought into existence and supplies losses

rotor further fall back in phase by a greater \(\alpha\) - called load or coupling angle

\(E_R\) and \(I_a\) increased at slightly decreased P.F.