Task 1 - DC Motors [P7] a. State how the torque and speed developed by a d.c. motor varies when we i. decrease the field excitation ii. increase the armature current. b. The output of a series d.c. motor when connected to a 230V supply and running at 1320rev/min is 3.8kW. The supply current for this loading is 20.5A. Calculate the i. torque developed ii. operating efficiency c. Describe an application for d.c. series machine in a building d. Sketch the speed/load current characteristic for a series‑wound d.c. motor. e. The output of a d.c. shunt motor when supplied at 200V and running at 960rev/min is 2.5kW. The supply current for this load condition is 14A. The armature and field resistances are 0.4Ω and 160Ω respectively. Calculate for this condition the i. armature current ii. back e.m.f f. Explain why shunt motors are suitable for fan and pump applications Task 2 - DC Generators [P8] a. State the function of the commutator in a d.c. generator. b. List FOUR losses that occur in a d.c. machine. c. Sketch circuit diagrams showing field connections for the following d.c. shunt generators, i. self‑excited ii. separately excited d. A d.c. self-excited shunt generator has a field resistance of 160Ω. The generated emf is 420V and it supplies a load of 55kW Calculate the armature resistance when the generator terminal voltage is 408V. e. Explain, with the aid of a graph, why the terminal voltage of the self-excited shunt generator, driven at constant speed, falls when the load is increased. f. Give an example where a self excited d.c. shunt generator could be used and what its advantage is over a separately excited d.c. machine.