$220$ V$,4-$pole, $25$ hp$,1000$ rpm DC machine has the

open$-$circuit characteristic given below, and is used as a

separately$-$excited generator. The armature resistance is

$0\mathrm{.}2.$

$($a$)$ If the machine is driven at rated speed with a field

current of $3$ A$,$ determine the variation in terminal voltage

between no load and full load.

$($b$)$ Determine the field current required to deliver rated

current at rated voltage with the machine driven at rated

speed.

$($c$)$ Repeat part $($b$)$ but assume that the peed of the prime

mover drops by $5\%$ between no load and full load