220 THREE-PHASE CIRCUITS AND RESIDENTIAL WIRINGIf the system voltage is 100 V, find the per-phase
impedance of the load. In this problem, is it pos-
sible to specify the capacitive or inductive nature
of the impedance?
4.3.4Two wattmeters are used, as shown in Figure 4.3.1,
to measure the power absorbed by a balanced
delta-connected load. Determine the total power in
kW, the power factor, and the per-phase impedance
of the load if the supply-system voltage is 120 V
and the wattmeter readings are given by
(a)WA=−500 W, andWC=1300 W.
(b)WA=1300 W, andWC=−500 W.
*4.3.5Referring to Problem 4.2.15(b), with a phase volt-
age as the reference phasor and with a positive-
phase-sequence supply system, two single-phase
wattmeters are used to measure the power deliv-
ered to the load. The wattmeter current coils are
arranged as in Figure 4.3.1 to carry the currentsIA
andIC, whereas the potential coils share a common
connection at the nodeB. Determine the readings
of the two wattmetersWAandWC, and verify that
the sum of these readings equals the total power
in the load.
4.4.1Referring to Figure 4.4.1, letVBN = VRN =
120 V rms magnitude, andVBR=240 V rms
magnitude. Write down expressions forvBN(t),
vRN(t), andvBR(t), and sketch them as a function
of time.
*4.4.2Consider a 240-V supply feeding a resistive load
of 10 kW through wires having a total resistance
ofR= 0. 02 . For the same load, let a 120-V
supply be used with a total wire resistance ofR/2
=0.01. Compute theI^2 Rloss in the lines for
both cases and compare.
4.4.3A person, while driving a car in a cyclone and
waiting at an intersection, hears a thumping sound
when a power line falls across the car and makes
contact with the chassis. The power line voltage to
ground is 2400 V. Instinctively he steps out onto
the wet ground, while holding the door handle,
to check out the external situation. Assuming his
body resistance to be 10 kand negligible auto-
chassis resistance, comment on what might hap-
pen to the individual. Would it have been safer
for him to remain in the car until some help ar-
rived?