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Professor Miller, Manchester Metropolitan University, Electrical Engineering, Power Electronics, 2000 2001 Exam, buck boost, convertor, transformer, capacitor, bipolar, transistor, power , dc motor,hyristor chopper, chopping frequency, current ripple,IRF150 HEXFET, drain current, junction temperature, three phase, delay angle, triac,induction motor, rectifier, dc link, voltage, rms, PWM, EMC, electromagnetic radiation
Typology: Exams
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Exam ination for th e H ND ELECTR O NIC ENGINEER ING YEA R TW O
W ednesday 9 M ay 2001
9 .30 am to 11.30 am
Instructions to Candidates
A ttem pt ALL questions.
Th e m ark s for each question are sh ow n in parenth eses.
06/09 /02 continued
(b ) A th yristor dissipates 20 W. Th e th erm alresistance b e tw een junction and case of th e th yristor is 0. 6 °C/W. Th e th yristor is in an am b ient air tem perature of 25°C. Calculate th e m axim um th erm alresistance of a h eat sink so th at th e operating tem perature of th e th yristor junction does not exceed 125°C. [10]
(b ) W ith th e aid of a diagram describ e th e principle of operation of a th yristor ‘crow b ar’for th e protection of electronic circuits. [6]
(c) Snub b e r circuits are used to protect pow er electronic devices from b e ing destroyed. W ith th e aid of a diagram explain th e principle of operation of:
(i) a voltage snubber used to protect a th yristor; [4] (ii) a flyw h eeldiode used to protect a transistor. [4]
(a) determ ine th e approxim ate required a.c. input voltage to th e b ridge; [4]
(b ) determ ine th e m inim um sm ooth ing capacitance value to b e connected at th e output term inals of th e b ridge. Th e a.c.supply frequency is 50 H z; [6]
(c) sk etch fully lab e lled representative w aveform s of input and output, voltages and currents, of th e rectifier; [6]
(d) explain th e effect th at th e w inding inductance w illh ave on th e current in th e field w inding. [4]
(b ) A converter is connected to a 230 V, 50 H z, sinusoidalsupply voltage and tak es pow er of 1035 w att. H ow ever, th e input current is a square w ave and th e Fourier series for th e current, i(ωt) is sh ow n b e low :
(^) ω + ω + ω + ω + π
i (^) ω t = sin t sin t sin t sin 7 t etc.... 7
(^20 1) am pere
Using only th e first th ree term s of th e h arm onic series, determ ine;
(i) th e r.m .s. input current for each of th e h arm onics; [4] (ii) th e totalr.m .s. input current; [4] (iii) th e pow er factor of th e convertor; [4] (iv) th e totalh arm onic distortion (TH D ) of th e current. [4]
