Question

A 800 kW, 11 kV, 3-phase star-connected synchronous motor has a synchronous reactance of $35\Omega$/phase and negligible resistance. The motor is operating on full-load at 0.8 pf leading. The efficiency is 95%. The excitation emf per phase is _______kV.

• A. 7.668

Answer 1

The correct option is A 7.668
Given,
$\text{efficiency,}\eta =0.95$

$\text{Input power}=\frac{800kW}{0.95}=842.10kW$

$I_a=\frac{842.10}{\sqrt{3}\times 11\times 0.8}=55.25Aat0.8p.f.leading$

$V_t=\frac{11000}{\sqrt{3}}=6351V$

$\overrightarrow{E_f}=\overrightarrow{V_t}-j\overrightarrow{I_a}{X}_s$

$=6351-j35\times 55.25\angle co{s}^{-1}0.8$

$=7668.90\angle -{11.637}^{\circ }$

$|E_f|=7.668kV$