Download EE 340 – Fall 2007 Homework-1 (80 points) Solve following

EE 340 – Fall 2007
Homework-1 (80 points)
Solve following Problems:
1.18
1.19
2.2
2.3
2.6
Additional Problems:
I.
A single-phase motor pool pump is supplied from a 240 V, 60 Hz AC supply.
A full power, the motor draws 2.5 kW at 75% power factor lag. The circuit is
protected by a 15 A circuit breaker. The customer wants to connect an extra
load (rated at 240 V, 1 kW, PF = 100%) to the same circuit that serves the
pool pump.
a) Will he be able to do so? Explain.
b) Calculate the minimum size of a shunt capacitor (in uF) that is needed to
allow the customer to serve both loads without tripping the circuit breaker.
II.
A single-phase load has an equivalent impedance of 16 + j 12 Ω. Although
this load is rated at 120 V, it can operate at lower voltage, but no less than 110
V. If the load is supplied by a 120 V source through a cable that has a total
impedance (i.e., both wires) of (4 + j 2) mΩ per foot. Calculate the maximum
length of the cable that will assure proper operation of the load.
III.
A remarkable method to balance a circuit: A resistive load with R = 0.242
Ω is connected between phases a-b of a 440V, 3-phase, positive sequence
supply. Let Vab = 440 ∟0o V, Vbc = 440 ∟-120o V, Vca = 440 ∟+120o V
Calculate the load currents and line currents. Repeat these calculations when a
capacitor with impedance -jXC = -j√3R= -j0.419 Ω, and an inductor with
impedance jXL = j√3R = j0.419 Ω are connected across phases b-c and c-a,
respectively.