12. The measurements taken in compiling data to plot the open circuit characteristics of a shunt-wound machine driven at 72 rad/s are listed below:
Field Current, $I_f$ (Amperes) | 1 | 2 | 3 | 4 | 5 | 6
------- | -------- | -------- | -------- | -------- | -------- | --------
Generated emf, E (Volts) | 50 | 94 | 124 | 141 | 152 | 158
Plot the characteristic and hence determine:
(a) the field current and the field resistance so the machine built up to 120V at 72 rad/s,
(b) the emf generated if the field current is set at 4.5A but the speed reduced to 60 rad/s.