122 Three-phase a.c. circuits5.7 PROBLEMS
1 A balanced delta-connected load is supplied from a symmetrical three-
phase, 400 V source and takes a phase current of 20 A. Determine (a) the
line current and (b) the total power consumed.2 Three identical coils are connected in star and take a total of 3 kW from a
three-phase, 200 V supply at a lagging power factor of 0.8. If the same coils
are connected in delta and supplied from the same source determine (a) the
line current taken and (b) the total power consumed.
3 A balanced star-connected load consisting of a resistance of 1 1~ in series
with an inductance of 15 mH in each phase is supplied from a 230 V, 50 Hz
supply. Determine the total power consumed in complex form.
4 Two wattmeters connected to measure the power in a three-phase circuit
for which the line voltage is 400 V read 40 kW and -10 kW. Determine (a)
the power consumed, (b) the power factor and (c) the line current.
5 The power in a three-phase circuit is measured using the two-wattmeter
method. Both wattmeters read positively, one reading being twice as big as
the other. Calculate the power factor of the circuit.
6 A three-phase motor operates at full load with an efficiency of 90 per cent
when supplied from a 2 kV, 50 Hz source. Two wattmeters connected to
read the power taken from the supply read 300 kW and 100 kW. Calculate
(a) the power input to the motor, (b) the power factor, (c) the line current,
and (d) the useful power output from the motor.