5.6 Self-assessment test 1217 State the phase sequence of a system for which E~ = 240 sin ~ot;
E b = 240 (sin ~ot + 120~ Ec = 240 sin (cot - 120~
8 How may the phase sequence of a three-phase system be reversed?
9 If the emf generated in phase A of a three-phase system of phase sequence
ABC is given by e = EM sin wt volts, give an expression for the emf in
phase C.
10 Explain what is meant by a balanced three-phase system.
11 The phase current of a star-connected three-phase system is 25 A. What is
the line current?
12 State the phase displacement between the phase voltage and the line
voltage of a star-connected three-phase system.
13 Give the line current of a delta-connected three-phase system for which
the phase current is 10 A.
14 A three-phase delta-connected load has a line voltage of 415 V and a line
current of 22 A. The power factor of the load is 0.8 lagging. Determine the
power in the load.
15 Give an expression for the apparent power in a three-phase load.
16 The apparent power in a three-phase load is 35 kVA and its power factor
is 0.6 lagging. Determine the reactive power.
17 Two wattmeters are connected to measure the total power in a three-phase
load and the readings are 240 W and 100 W. What is the total power in the
load?
18 Two wattmeters are connected as follows to measure the total power in a
three-phase system: one has its current coil connected in the A line and its
voltage coil connected between the A and C lines; the other has its current
coil connected in the B line and its voltage coil connected between the B
and C lines. Give an expression for the reading on each meter in terms of
appropriate line voltages, line currents and phase angles.
19 Give an expression for the power factor of a three-phase load in terms of
the readings on two wattmeters connected to read the total power.20 The readings on two wattmeters connected to read the total power in a
three-phase system are 125 W and 65 W. Determine (a) the real power, (b)
the reactive power, (c) the apparent power, (d) the power factor.