THE OPEN UNIVERSITY OF SRI LANKA

BACHELOR OF TECHNOLOGY – LEVEL 5

ACADEMIC YEAR 2013/2014

ASSIGNMENT # 01

MEX5270 POWER ELECTRONICS AND MOTOR DRIVES

INSTRUCTIONS

1. Answer all questions.

2. Send your answer scripts under registered cover to the address given below or put it into the

assignment box available at the Research Laboratory (New Science and Technology Building)

of Mechanical Engineering Department or upload the assignment to the OUSL virtual

classroom in PDF format on or before 17th

January 2014.

THE COURSE COORDINATOR – MEX5270

DEPARTMENT OF MECHANICAL ENGINEERING

THE OPEN UNIVERSITY OF SRI LANKA.

P.O.BOX 21, NAWALA, NUGEGODA.

3. There will be penalty imposed of five marks deduction for each day’s delay up to a maximum

of 10 days, after which a mark of zero will be given.

4. Copying among students is not allowed and if found a zero mark will be given.

5. Direct copying from web resources or Books is strictly prohibited.

6. You may use Harvard Referencing or any other standard method for referencing.

7. If you are submitting the assignment using the virtual classroom, first click the ‘Assignment’

module and select the appropriate assignment in the given item list. Then answer the particular

assignment through the virtual class. If you do not follow the instructions, then your

submission can be seen by anyone in the MEX5270 class.

Please note the following:-

Indicate all the steps of solving a problem clearly in your own words.

Question 1

A transistor junction has a total power loss of 26W, and the junction to case thermal resistance

is 0.90C/W. The case is mounted on heat sink no. 7 with a 75m thick mica insulation with

thermal grease resulting in the case to heat sink thermal resistance of 0. 40C/W. The air

temperature inside the converter cabinet can be as high as 550C. Determine the maximum

junction temperature in this device.

Question 2

The turn ON and the turn OFF switching transition times for the power semiconductor switch S

in Figure Q2 are respectively tsw(on) and tsw(on). Assume linear variation of current and voltage

during the switching transitions. The ON state voltage drop across the switch is VD. Show that

the average switching power loss across the switch is

SoffSWonSWD

SW fttIVVI

P )()(36

Where V is the off state voltage across the switch, I is the ON state current through the

switch and Sf is the switching frequency.

Figure Q2

Question 3

The convertor and snubber circuit shown in Figure Q3 has Vs = 50V and IL= 5A, C=0.05F, R

=5 and tf = 0.5s. The switching frequency is 120kHz with a duty ratio of 0.4.

1) Determine expressions for iQ, ic and vc during turn off.

2) Determine the transistor turn off losses in the switch and the snubber.

Figure Q3

Question 4

a. Consider the single phase controlled half wave bridge rectifier shown in Figure Q4.

The firing delay angle is 450. Draw the waveforms of the half wave bridge rectifier for

the following clearly labeling the graphs.

1) Supply voltage ( vs )

2) Output voltage ( vo )

3) Output current ( i )

4) Voltage across the SCR (vSCR)

Figure Q4

b. A controlled half wave rectifier has an ac source of 240Vrms at 60 Hz. The load is a

25 resistor. Determine the following

1) delay angle such that the average load current is 3A.

2) power absorbed by the load.

3) power factor.

Question 5

The single phase full bridge rectifier shown below has an AC source with Vm =100V at

60Hzand a series R-L load with RL=10Ω and L= 10mH.

Determine

1) the average current in the load

2) the power absorbed by the load

3) the average and rms current in the diode.

Figure Q5

END