ECE320, Spring 2005, Exam 2 25th February, 2005

Name
PID Number
Problem / Max Points / Points
1 / 40
2 / 40
3 / 20
Total / 100

Instructions:

  • Closed book, but you are allowed to have one-page single-sided handwritten notes.
  • Read all the problems first.
  • Attempt to solve first the problems you can, don’t spend too much time on a problem.
  • Read carefully the statement and/or circuits of each problem.
  • Write your solutions neatly and box your answers (and steps for partial credits)

Problem 1. (40 points) In the magnetic circuit below, assume that the relative permeability of iron is , the air-gap is g = 10 mm, i = 50 A, N = 250 turns, cross section of the iron everywhere A = 60 mm*50 mm.

a) Calculate the flux density in the air-gaps. (15 points)

b) What would be flux density in the air gaps

if the current is i = 50 sin (377 t) A. (10 points)

c) For the above (b), what would be the voltage induced

in the coil? (15 points)

Problem 2. (40 points) A 100 kVA, 10kV/250 V, 60Hz, single-phase transformer gave the following test result: Open-circuit test (open the high voltage side): 250V, 10A, and 500 watts measured on low voltage side and Short-circuit test (short the low voltage side): 300V, rated current, and 1000 watts measured on high voltage side.

a)Draw the simplified high voltage side referred equivalent circuit. (10 points)

b)Calculate the above equivalent circuit parameters. (15 points)

c)A load is connected to the low voltage side, drawing 80 kW with pf =.866 lagging at 250 V. Using per-unit values to calculate high voltage side voltage. (15 points)

Problem 3. (20 points)

Use your own words and sketches to describe the basic principle of a dc machine. (Your description has to be in full sentences not broken phrases.)