A$3,10$ kVA, $220$ V$,$ Y$-$connected synchronous generator has Ra $=0$:$25$ per phase and Xs $=5$:$0$ per phase. Determine the excitation voltage, Ef$,$ when the generator is delivering full load at power factor of

$($a$)$

$0\mathrm{.}85$ lagging.

$($b$)$

$1\mathrm{.}0$ unity.

$($c$)$

$0\mathrm{.}8$ leading.