ELL203:ELECTROMECHANICS
TUTORIAL SHEET 3
Note: Make suitable assumptions wherever necessary
• The following test results are obtained for a 20kVA, 50Hz, 2400/240V distribution
transformer
– OC Test(LV): 240V, 1.066A, 126.W
– SC Test(HV): 57.5V, 8.34A, 284W
(a) When the transformer is operated as a step down transformer with the output
voltage to 240V, supplying a load at unity PF, determine the maximum efficiency and
the unity UPF load at which it occurs.
• In a 25kVA, 2000/200V transformer, the iron and copper losses are 300 and 400W
respectively. Calculate the efficiency on unity power factor at (i)full-load (ii)half-load.
Also determine the load for maximum efficiency and the iron and copper loss in that
case.
• Open and short-circuit tests performed on 500kVA, 6600/2300V, 50Hz transformer
yielded the following data:
– No-load loss= 3kW
– Full-load short-circuit loss=4kW.
Calculate the load KVA at which the transformer efficiency would be maximum for a
given power factor. Calculate this maximum efficiency for a pf of 0.85.
• A 1 − ϕ, 10kVA, 460/120V, 60Hz Transformer has an efficiency of 96% when it delivers 9kW at 0.9 pf. The transformer is connected as an autotransformer to supply
load to a 460V circuit from a 580V source. Determine the maximum kVA that the
autotransformer can supply to the load. Also determine the efficiency for full-load at
0.9 pf.
• Two identical 1 − ϕ transformer each of 250kVA, 230/460V, are connected in opendelta to supply a balanced three-phase load at 460V and a power factor of 0.8 lagging.
Determine the maximum secondary line current without overloading the transformer
and the power delivered by each transformer in the above case. Also determine the
primary line currents.
• If a third transformer is added to complete the ∆, find the percentage increase in the
real power that can be supplied assuming the load voltage and power factor remaining
unchanged.
• Three 1ϕ, 10kVA, 460/120V, 60Hz transformers are connected to form a 3ϕ, 460/208V
transformer bank. The equivalent impedance of each transformer referred to HV side is
1+j2Ω. The transformer delivers 20kW at 0.8 pf leading. Draw the schematic showing
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the transformer connection. Determine the currents flowing through the primary and
secondary of all three transformers. Also determine the voltage regulation.
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