cameroon gce A level 2020 automatic control of electrical machines 2

cameroon gce A level 2020 automatic control of electrical machines 2

cameroon gce A level 2020 automatic control of electrical machines 2

1. A 37.3KW, 460V d.c shunt motor running at a light load takes a current of 4A at a speed of 660rev/min. The armature resistance is 0.3Ω while that of the shunt field circuit is 270Ω. Neglect armature reaction. When the motor is running at full load, determine:
a) the input current (6 marks)
b) the speed of the motor (2 marks)
the armature current at which the efficient is maximum (2 marks)
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2. 3-phase Transformer
The following light-load, line input readings were taken on a 3-phase, 150KVA, 6600/440, delta/Star-connected transformer:
Open-circuit test: 1900W 440V, 16.5A
Short-circuit test: 2700W 315V, 12.5A
a) Calculate the equivalent-circuit parameters per phase, referred to the high volt side (Io, cosθo, Rm, Xm, cosθsc, Zes, Res, Xes). (6 marks)
b) Determine the secondary terminal line voltages when operating at rated and half-rated current if the load power-factor is 0.8lagging. Calculate also the efficiencies for these loads. (2 marks)
c) Determine the secondary terminal voltage when operating at rated current with a load power-factor of 0.8leading and calculate the efficiency for this condition. (2 marks)
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