Important formulae to remember




  1. The maximum efficiency at full load and unity power factor of a 1∅, 25 KVA, 50/1000V, 50Hz transformer is 98%. Determine the efficiency at (i) 75% load 0.9 p.f (ii) 50% load 0.9 p.f (iii) 25% load 0.6 p.f
  2. A 500KVA transformer has an efficiency of 92% at full load unity power factor and at half full load 0.9 p.f. Determine it’s efficiency at 80% full load and 0.95 p.f
  3. A 1∅, 20 KVA transformer has 1000 primary turns and 2500 secondary turns the net cross sectional area of the core is 100 cm^2. When the primary winding is connected to 550 V, 50Hz supply, Calculate (i) The maximum value of flux density in the core (ii) The voltage induced in the secondary winding (iii) The primary and secondary full load currents
  4. A 40 KVA, 1∅ Transformer has core loss of 450W and full load copper loss 850W. If the power factor of the load is 0.8, Calculate (i) Full load efficiency (ii) Maximum efficiency at UPF (iii) Load for maximum efficiency
  5. A 10KVA, 1∅ transformer has a primary winding of 300 turns and secondary winding of 750 turns, cross sectional area of 64 cm^2. If primary voltage is 440 V at 50 Hz, find maximum flux density in the core, EMF induced in the secondary of transformer. At 0.8 lag p.f, Calculate the efficiency of transformer if full load copper loss is 400W and iron-loss is 200W
  6. A 20KVA, 1∅ transformer has 200 turns on primary and 40 turns on secondary. The primary is connected to 1000V, 50Hz supply. Find the secondary voltage, rated current flowing through two windings and the maximum value of flux
  7. Find the efficiency of 150 KVA, 1∅ transformer at (i) Full load UPF (ii) 50% of full load at 0.8 p.f, if the copper loss at full load is 1600 W and iron loss is 1400W

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