A question on DC Shunt motor

Hi all

I am doing EEE second year and having trouble with the question below. The question has been repeated on every past exam i looked at..I asked the teacher and he refused to answer it because apparently he puts this question to separate boys from men!?!

A 240 volt, 2 hp, 1200 rpm dc shunt motor drives a mechanical load which has a torque speed characteristic which is proportional. The armature resistance of the motor is 0.75 ohms. With the field current set to 1.0 amp, the motor draws a line current of 7.0 amps and rotates at 1200 rpm. Assume a linear relationship between E­_a and I_f and neglect armature reaction. Friction and windage losses are to be assumed constant and equal to 150 watts.

If the field current is now reduced to 0.7 amps by way of a series field resistance, calculate:

(a) The new operating speed of the motor

(b) The new line current

(c) The mechanical power delivered to the load

(d)The efficiency of the machine

please give some ideas on how to solve it

thanks

swin