Quiz 1, EE 303, January 29, 2008, Dr. McCalley

Closed book, closed notes, no calculator, no computer, no communication device

Answer all questions on this sheet of paper.

  1. Short-answer (42): The rms voltage and current of an inductive load are 110V and 10A, respectively.The frequency of the voltage waveform is 60 Hz. The instantaneous power consumed by the load has no average value (i.e., has an average value of 0).
  2. (5) What is the frequency of the current, in radians/sec?

2π(60)

  1. (5) What is the frequency of the instantaneous reactive power, in radians/sec?

4π(60)

  1. (5) What is the peak-to-peak amplitude of the current?

2(√2)(10) A

  1. (5) What is the real (active) power consumed by the load?

Since the instantaneous power consumed by the load has average value=0, the real power=0.

  1. (5) What is the reactive power consumed by the load?

Q=VIsinθ=(110)(10)sinθ; since we know average power is 0, then load is entirely reactive, and so θ=90° or θ=-90°. But the problem statement also tells us the load is inductive, therefore current lags voltage, i.e.,

θ=θv-θi=90°, and so Q=VIsinθ=(110)(10)sin(90°)=1100vars

  1. (5) What is the power factor of the load?

pf=cosθ=cos90°=0.

  1. (6) Write down time-domain expressions for voltage and current.

v(t)= √2(110)sin(ωt); i(t)= √2(10)sin(ωt-π/2)

  1. (6) Write down phasor expressions for voltage and current.

V=110∟0°; I=10∟-90°

  1. True-false (50, 5 each):

(a)  A major difference between boiling water reactors & pressurized-water reactors is the latter has a steam generator to perform heat exchange in the containment structure.

(b)  Overhead circuits require that conductors be insulated to protect wildlife.

(c)  Underground circuits are used in large cities because the equipment necessary for underground is much less expensive than the equipment necessary for overhead.

(d)  The EMS is the software system that provides the interface between human operators and the power grid, allowing remote monitoring and control.

(e)  Tree-trimming, one of the most costly maintenance items for power companies, was a major issue behind the cascading sequence of events that brought on the 2003 blackout in the northeast United States.

(f)  There is no reason why within the next 10 years we cannot provide 100% of Iowa’s electricity needs using wind turbines.

(g)  The current US policy to address acid rain, which is to place strict, fixed limits on the amount of SO2 that each power plant may emit, is likely to also be applied for CO2.

(h)  One reason why tower insulators are built disk-shaped is to increase the flashover distance between the conductor and the tower.

(i)  Aircraft power systems utilize 10 Hz AC instead of 60Hz because it enables rotating machines necessary to deliver the lower frequency can be designed with less weight.

(j)  Almost all nuclear plants in operation today are fusion plants.