MECH2406: Fundamentals of Electrical Engineering 3. (a) 250 turns of coil are wound on a steel ring of mean diameter 150 mm and cross-sectional area 300 mm. The relative permeability of the steel is 1200. A 1 mm air gap is cut in the steel ring. (b) (i) Calculate the reluctance of the magnetic circuit. (3 marks) (ii) Calculate the flux in the steel ring when a current of 4A flows in the coil. (4 marks) Permeability of free space is 4 x 107 Wb A m. A 50 Hz, 6,600V / 220V 11kVA step down transformer was tested on open-circuit and short-circuit. The test results are: Open-circuit test Primary voltage: 6,600V, power input = 1kW Short-circuit test Secondary current: 50A, power input = 3kW. (i) What are the full-load primary and secondary currents? (3 marks) (ii) What are the core loss and full-load copper loss? (4 marks) (iii) Calculate the efficiency of the transformer when it is supplying 75% full-load at 0.8 power factor lagging. (6 marks)