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  1. 1. DEK 3143 2010/2011Q1 a) With the help of a drawing describe the process of the generation of DC signal. (7 marks) b) Give the reason why does the rotor core of the DC machines skewed. (3 marks) c) A separately excited generator has no-load voltage of 140 V when the field current is adjusted to 2A. The speed is 900 rpm. Assume a linear relationship between the field flux and current. Calculate : i) the generated voltage when the field current is increased to 2.5 A. Assume N1 = N2. ii) the terminal voltage when the speed is increased to 1000 rpm with the field current set at 2.2 A (15 marks)Q2 a) Based on the construction of the dc machines describe the operating principles of a dc motor. (10 marks) b) A 120V shunt motor has an armature resistance Ra = 0.5 Ω, a field resistance Rf = 120 Ω and rotational losses of 250 W. On full load the line current is 19.5A and the motor runs at 1400 rpm. Determine :- i) the developed electrical power ii) the output power iii) the output torque iv) the efficiency at full load (15 marks)
  2. 2. DEK 3143 2010/2011Q3 a) Explain briefly the derivation of the applied voltage and induced voltage transformation of an ideal transformer, current transformation of an ideal transformer and the impedance transformation of an ideal transformer. (10 marks) b) A step down transformer is rated 150kVA, 2400/240V and has an equivalent circuit parameter as shown in Table Q3(b) :- Table Q3(b) : Equivalent Transformer Circuit Parameter. Rp = 0.2 Ω Rs = 0.002 Ω Rm = 10000 Ω Xp = 0.45 Ω Xs = 0.0045 Ω Xm = 1550 Ω i) Find the equivalent winding impedance referred to the high voltage side. ii) Repeat Q3(b)(i) for the low voltage side. (15 marks)Q4 a) By referring to the per phase equivalent circuit of a three phase induction motor state down the equation of :- i) the gross output power of the induction motor ii) the output torque of the motor (8 marks) b) A 10 poles, 50 Hz, star connected 3-phase induction motor having a rating of 60 kW and 415 V has a slip of 5% at 0.6 lagging power factor. If the full load efficiency is 90%, calculate :- i) the input power ii) the line current and the phase current iii) the speed of the rotor (in rpm) iv) the frequency of the rotor v) the torque developed by the motor (assuming negligible friction and -2-
  3. 3. DEK 3143 2010/2011 windage losses ) (17 marks)Q5 a) With the help of a phasor diagram, briefly describe the three loading condition of the synchronous generator. (8 marks) b) A 3-phase star connected synchronous generator supplies a load of 10 MW at 0.85 lagging power factor and at a terminal voltage of 11 kV. The armature resistance is 0.1 Ω/phase and synchronous reactance of 0.66Ω/phase. Calculate :-. i) the armature current ii) the internal generated voltage iii) the voltage regulation (10 marks) c) Draw also the phasor diagram for the answer in Q5(b) above. (7 marks)Q6 a) Describe the construction and the operation of a Capacitor Start Induction Run Motor. (12 marks) b) With the help of diagram describe the fundamental operation and performance characteristic of a stepper motor. (13 marks) -3-
  4. 4. DEK 3143 2010/2011 -4-