ENB120 Electrical Energy and Measurements GRW/2014

DC CIRCUIT DESIGN CHALLENGE

Sam Sailor wants to use a solar panel on his boat to power his cabin lights, cabin electronic equipment (radio, GPS, fish finder) and bilge pump. The solar panel acts principally as a current source, while the bilge pump motor and battery are voltage sources. [Note: solar panels and electric motors are actually more complex than this, but assume that they behave this way for this assignment]. The cabin electronics equipment draws a constant current regardless of voltage. Sam has connected all of these sources and loads in parallel, but is having some unexpected problems. He has asked you to analyse the system, and to provide a recommendation for an improved design. You note that the solar panel, cabin lights and electronics equipment are all mounted on the cabin roof, while the battery and bilge pump are below deck. Sam has only used thin cables to connect the system together, with the resistances marked on the diagram.

+

a

b V

8 A

BatterySolarPanel

+

f

g V

Bilge pump electric motor

d

e W

Cabin Lights

0.5

0.5

c A

Cabin Electronics Equipment

Oddly, the parameters of these sources and loads can be derived from your student number. The parameters of the elements are determined using your student number as follows: Circuit Element Symbol Formulae Example value for

student # 4726315 Battery internal series resistance a (Digit 1 + 20)/100 (4 + 20)/100 = 0.24 Battery internal voltage b V (Digit 2 + 125)/10 (7+ 125)/10 = 13.2 V Cabin electronics current consumption c A (Digit 3 + 12)/10 (2 + 12)/10 = 1.4 A Solar panel shunt resistance d (Digit 4 + 18) (6 + 18) = 24 Cabin lights power rating at 12 V e W (Digit 5 + 6) x 2 (3 + 6) x 2 = 18 W Bilge pump internal series resistance f (Digit 6 + 40)/10 (1 + 40)/10 = 4.1 Bilge pump internal voltage (Back EMF) g V (Digit 7 + 85)/10 (5 + 85)/10 = 9.0 V

Take Home Exam Perform this take home exam on your own. Tutors (including peer tutors) and academic

staff are not able to help you with this assessment item. Your submission coversheet must include the statement that you have performed this

work without the help of others.

Note: The values provided in the fourth column show the calculation of the parameters for student number 4726315. Do not use these values, they are an example only. Derive values from your own student number. Explanations of steps used are essential, and will be carefully accounted for in assessment. Best marks are awarded to numerically correct solutions, but partial credit will be given to partially complete solutions or incorrect solutions where errors have carried through the calculations. State any and all assumptions that you make.

(a) Calculate the resistance of the cabin lights, based on their power rating at 12 V. (1 mark)

(b) Draw a circuit diagram of the system described above. Where possible, simplify the

diagram by noting series and parallel resistances and sources. (2 marks)

(c) Using a circuit analysis technique of your choosing, calculate the power balance of the

system. Show all working. Comment on the effectiveness of power transfer from the solar panel to the cabin lights, cabin electronics, the battery and the bilge pump.

(4 marks)

(d) Use circuit simulation software to verify your calculations in (b). Show screen shots of the software that verify your results.

(2 marks)

(e) Sam always leaves his electronic equipment and bilge pump turned on, day and night. He leaves the cabin lights on day and night too! During a sunny day, the solar panel delivers 8 A for 7 hours. How many Amp-hours are stored in the battery? When the sun is not shining for the other 17 hours, the solar panel produces no current, but its shunt resistance remains connected. How many Amp-hours are drawn from the battery during this time? Show all working. Use circuit simulation software to verify your calculations, supported by screen shots.

(5 marks)

(f) During the day the radio stops working because its voltage supply is too high. You suggest thicker cables with lower resistance will solve the problem. If its maximum allowed radio voltage is 15.0V, calculate the maximum acceptable cable resistance.

(3 marks)

(g) Show Sam how a diode can be used to improve the efficiency of the system during the night. Quantify the efficiency gains made from your design using either circuit simulation software or manual circuit. Look online for an appropriate diode to use in this application. Ensure that your diode model in the circuit is equivalent to the real diode that you have chosen.

(4 marks) (h) Sam complains that his battery keeps going flat, which is unacceptable as he must have his

radio and other electronic equipment running 24 hours a day. Using your suggested design improvements above, and if desired other sensible design changes, demonstrate a way that Sam can operate his system which will allow him to always have radio power.

(4 marks)

Total: (25 marks)