A $3\backslash$phi $,10$ kVA, $220$ V$,$ Y$-$connected synchronous generator has Ra $=0\mathrm{.}25\backslash$Omega per phase

and Xs $=5\mathrm{.}0\backslash$Omega per phase. Determine the excitation voltage, Ef $,$ when the generator is

delivering full load at power factor of

a$)0\mathrm{.}85$ lagging.

b$)$ unity.

c$)0\mathrm{.}8$ leading.