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Q1— Section header —
1.1Name THREE types of variable losses in a DC machine (3\
1.2A 65 kW, 44 DC shunt motor has an armature resistance of 0,25 ohms and a field resistance of 250 ohms respectively. The armature takes 120 A when the motor is operating at full load Calculate the resistance that should be inserted into the armature circui
1.3List THREE ways in which the speed of a DC motor may be adjusted (3\ [14]14
Q2— Section header —
2.1otential difference between the star point of the load and the neutral point of the alternator (7\
2.2otential difference across each phase of the load (3\
2.3urrent in each line (3\ [13]13
Q3— Section header —
3.1Give TWO effects of harmonic voltages in transformers (2\
3.2Define the term voltage regulation of a transformer (3\
3.3A 85 kVA, 2 200/240 V, single phase transformer has a percentage impedance of (1,5 + j4\%. Calculate the following at a power factor of 0,75 lagging:
3.3.1ull load efficiency if the iron losses amount to 0,94 kW (5\
3.3.2aximum efficiency (3\
3.3.3pplied primary voltage to circulate full load current on short circuit (3\
3.3.4The percentage regulation (2\ [18]18
Q4— Section header —
4.1Give THREE reasons for using distributed windings in a three phase alternator (3\
4.2A 750 kVA, 11 kV, three phase, short chorded alternator has a percentage impedance of (3 + j15\ percent. The power is 0,7 lagging. Calculate the following:
4.2.1The ohmic value of the phase resistance and reactance (6\
4.2.213
Q5— Section header —
5.1Explain wit h the aid of phasor diagrams, that the power factor of a synchronous motor working on a constant mechanical loads, depends on its excitation. (6\
5.2A 5 kVA, 480 V, three phase, 50 Hz, star connected synchronous motor is full loaded and draws 4 kW at a leading power factor. The machine has a percentage impedance of (4 + j40\ %. Calculate the following
5.2.1The power factor (1\
5.2.2The resistive and reactive voltage drops (3\
5.2.3The EMF to which the machine is excited (3\
5.2.4The load angle in electrical degrees (1\ [14]14
Q6— Section header —
6.1What is the function of an external rotor resistance starter, coupled to the slip ring of a wound induction motor? (2\
6.2A 480 V, six pole, three phase, star connected induction motor, has a rotor impedance of (0,16 + j0,7\ ohms per phase at stand still. The stand still EMF between slip rings is 290 V. Calculate the following:
6.2.1The torque developed at a full load slip of 5% (7\
6.2.2The full load power output if the friction and windage losses are (3\
6.2.314
Q7— Section header —
7.1State TWO advantages that a power factor improvement has for the consumer. (2\
7.2The voltage supply to a consumer is 400 V, 50 Hz, three phase. The consumer has a lighting load of 2 kW at unity power factor and a 30 kW induction motor operating at a power factor of 0,8 lagging. The efficiency of
7.2.1The total kVA of the load (5\
7.2.2The power factor of the load (1\
7.2.3The value of the line current to a delta connected capacitor bank which, when connected in parallel with the load, will limit the current taken from the mains to 60 A (6\ [14] TOTAL:14