Causes of voltage drop: Drop due to armature resistance, Ia Ra drop Drop due to leakage reactance, Ia XL drop Drop due to armature reaction, Ia Xa drop Synchronous reactance Xs = XL + Xa Synchronous impedance Zs = Ra + j Xs Vector diagram for the synchronous motor on lagging load: [ alt="F1 Vijay 19-02-21 Savita D10" src="//storage.googleapis.com/tb-img/production/21/02/F1_Vijay_19-02-21_Savita_D10.png" style="height: 517px; width: 475px;"> Where, E is the induced EMF Ia is the armature current ψ is the angle between E and Ia also called an internal power factor angle. ψ = ϕ ± δ Where, ' + ' is for lagging load ' - ' is for leading load Air gap voltage Er = V + Ia Ra + j Ia XL For synchronous motor: The air gap excitation mmf lags the armature mmf by π/2 – ψ For synchronous Alternator: The air gap excitation mmf lags the armature mmf by π/2 + ψ