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Hi guys, can someone please have a look at the following problem that I am struggling with;

An 8.25 kW shunt DC motor is supplied with a terminal DC voltage of 300 V. The armature resistance of the motor Ra is 0.25 ohms, and the field resistance Rf is 40 ohms. The field winding consists of 1500 turns. An adjustable resistance Radj is connected in series to the field winding. Radj may be varied over the range from 0 to 80 ohms and is currently set at 20 ohms. Armature reaction may be ignored in this machine. The magnetization curve for this motor, taken at a speed of 1200 rpm, is shown in Figure Q1.

(i) What is the speed of the motor when operating at rated load with an armature current of 30 A?

(ii) If the motor is now unloaded with no changes in terminal voltage or Radj, what is the no-load speed of the motor?

(iii) Iron core losses are 250 W. What are the copper losses and mechanical losses in the motor at rated load? (ignore brush and stray losses)

(iv) What is the efficiency of the motor at rated load?

(v) If the terminal voltage is reduced to 260 V and the armature current stays at 30 A, calculate the required Radj in order to restore the speed of the motor to the value in (i).

An 8.25 kW shunt DC motor is supplied with a terminal DC voltage of 300 V. The armature resistance of the motor Ra is 0.25 ohms, and the field resistance Rf is 40 ohms. The field winding consists of 1500 turns. An adjustable resistance Radj is connected in series to the field winding. Radj may be varied over the range from 0 to 80 ohms and is currently set at 20 ohms. Armature reaction may be ignored in this machine. The magnetization curve for this motor, taken at a speed of 1200 rpm, is shown in Figure Q1.

(i) What is the speed of the motor when operating at rated load with an armature current of 30 A?

(ii) If the motor is now unloaded with no changes in terminal voltage or Radj, what is the no-load speed of the motor?

(iii) Iron core losses are 250 W. What are the copper losses and mechanical losses in the motor at rated load? (ignore brush and stray losses)

(iv) What is the efficiency of the motor at rated load?

(v) If the terminal voltage is reduced to 260 V and the armature current stays at 30 A, calculate the required Radj in order to restore the speed of the motor to the value in (i).

**I have attempted to calculate I(f), which I am getting a value of 5A.**

Also the mmf seems to be 7500 A.turns - (1500*5)

But I do not know where to go from here, any help would be appreciated.Also the mmf seems to be 7500 A.turns - (1500*5)

But I do not know where to go from here, any help would be appreciated.

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