Applied Electricity Paper 2 , May/June 2010

Questions:	1	2	3	4	5	6	7	8	Main

General Comments
Weakness/Remedies
Strength

Question 8
(a) State the function of commutator in a d.c. machine. (b) A shunt generator has an armature resistance of 0.60 a field resistance of 80, and a terminal voltage of 160 V. When it is supplying a load current of 30 A, calculate the: (i) field current; (ii) armature current; (iii) generated e.m.f
_____________________________________________________________________________________________________
Observation
The expected answers were: (a) The function of the commutator in a d.c. machine is to provide (i) excitation to the field windings (ii) provide an output from the machine through brushes (iii) rectify the a.c. in or out of the armature (b) (i) If= V/Rf = 160/80 = 2 A (ii) la = I - If = 30 - 2 = 28 A (iii)E = V + laRa = 160 + (28 x 0.6) = 176.8 V The question was on Electrical Machines (D.C.). The question was popular. The performance was relatively good. Many candidates could not state the function of commutator in a d.c. machine. However, as many of them that attempted the part (b), the calculation aspect, got it correctly.