A $25$ kVA, $400$ V$,3-$phase synchronous generator delivers rated kVA at rated voltage at $0\mathrm{.}8$ pf lagging. The per phase $($star basis$)$ armature resistance and synchronous reactance respectively are $0\mathrm{.}66$ and $7\mathrm{.}1.$ The field winding is supplied $10\mathrm{.}6$ A at $110$ V$.$ The friction and winding loss is estimated to be $480$ W and iron loss as $580$ W$.$ Calculate: $($a$)$ the full$-$load efficiency. $($b$)$ the terminal voltage when the load is thrown off.