1. DEK 3143 2010/2011
Q1 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. DEK 3143 2010/2011
Q3 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
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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)
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4. DEK 3143 2010/2011
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