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Fee 542: Power Electronics And Variable Speed-Drives B Question Paper

Fee 542: Power Electronics And Variable Speed-Drives B

Course:Bachelor Of Science In Electrical And Electronic Engineering

Institution: University Of Nairobi question papers

Exam Year:2015

SECOND SEMESTER EXAMINATIONS 2014/2015
FIFTH YEAR EXAMINATIONS FOR THE DEGREE OF BACHELOR OF SCIENCE IN ELECTRICAL AND ELECTRONIC ENGINEERING
FEE 542: POWER ELECTRONICS AND VARIABLE SPEED-DRIVES B

DATE: APRIL 20, 2015 TIME: 08.30 A.M. - 10.30 A.M.
INSTRUCTIONS TO CANDIDATES
Answer any THREE of the following Five questions.
All questions carry equal marks.
Show all working clearly in the answer sheets.
Do NOT attempt more than three questions.

QUESTION 1

Figure Q1 shows a three-phase dc-to-ac converter (inverter). It employs a three-SCR conduction-module such that each SCR conducts for an angle of 180o, where Q1, Q2, Q3 are positive-conducting SCRs and Q4, Q5, Q6 are negative-conducting SCRs.

By drawing the conducting periods of positive-conducting and negative-conducting
SCRs, derive the three-SCR conducting sequence and show the gate-firing sequence. ( 4 Marks )
(b) Derive the phase-voltage waveforms of the load voltages eao, ebo respectively. ( 5 Marks )
(c) Hence, derive the line-to-line voltage waveform of the load voltage Vab ( 3 Marks )
(d) By applying Fourier analysis, show that amplitude of the phase-voltage of any nth harmonic is given by ( 4 Marks )

(e) Given that Vd = 300 V, determine the values for eao(rms) for; ( all answers to zero decimal place )
i) the fundamental,
ii) the second harmonic,
iii) the third harmonic. ( 4 Marks )

QUESTION 2

An HVDC transmission scheme has the following specifications between station-A and station-B:
Station-A: 3-phase, 900 kV, 50Hz is the ac input to the rectifier-inverter bridge, 720 kVdc is the dc output voltage of the rectifier-module.
Station-B: 3-phase, 1,080 kV, 60 Hz is the ac output of the inverter-rectifier bridge at station-B.
The dc-wire resistance of the transmission line is 20 ?.
1,800 Mw of power is to be transmitted from station-A to station-B.
By means of a neat block diagram of the dual-link scheme, determine;
the firing angle aa of the rectifier-module at station-A,
the firing angle ßb of the inverter-module at station-B. ( 12 Marks )

If 1,200 MW of power is now transmitted from station-B to station-A with 480 kVdc
being the dc output voltage of the rectifier-module at station-B, determine;
the firing angle ab of the rectifier-module at station-B,
the firing angle ßa of the inverter-module at station-A ( 8 Marks )

QUESTION 3
From the operation and characteristics of dc motors, show from first principles that; for a separately-excited dc motor,

the power developed by the armature is given by

Pd = ? x Ia x If x Kt = ?2 B + ? TL, watts

the operating speed is expressed as

N = (30 Rf ( Va-Ia Ra ))/(p Kv Vf) , rpm ( 10 Marks )

A 22.5-Hp, 360-V, 1800-rpm separately-excited dc motor controls a mechanical load requiring a torque of TL = 75 N-m at a speed of 1,200 rev/min. The field circuit resistance is Rf = 135 ?, the armature circuit resistance is Ra = 0.20 ?, and the voltage constant of the motor is Kv = 0.7032 V/(A-rad/s). The field voltage is
Vf = 360 V. Assume that the viscous friction and the no-load losses are negligible, and also that the armature current is continuous and ripple free. Determine;
the back emf voltage, Eg
the required armature voltage, Va ,
the rated armature current, Ia of the motor ( 10 Marks )

QUESTION 4

With the aid of neat circuit diagrams and suitable waveforms, describe;
single-phase semi-converter drive
single-phase dual-converter drive ( 6 Marks )

The speed of a separately-excited dc motor is controlled by a single-phase semi-converter drive. The field current is also controlled from another semi-converter drive and the value of field current is set to the maximum. The ac supply to the armature converter and the field converter is single-phase, 240 V, 50 Hz. The armature resistance is Ra = 0.225 ?, the field resistance is Rf = 325 ? and motor voltage constant is Kv = 1.050 V/(A-rad/s). The load torque is TL = 125 N-m at a speed of 1,350 rev/min. The armature current and field current are continuous and ripple free. Determine;
the value of field current If ,
the delay angle, aa of the converter in the armature circuit,
the input power factor ( PF ) of the armature circuit. ( 8 Marks )

With suitable neat circuit diagrams and waveforms, describe;
3-phase, semiconverter drive,
3-phase, dual-wave converter drive, ( 6 Marks )

QUESTION 5

(a). With the aid of a neat, simple block diagram, explain the basic operation of a variable speed drive. ( 6 Marks )
(b). Give the benefits of using a variable speed-drive. ( 4 Marks )
(c). A three-phase, 11.2 kW, 1750-rpm, 460-V, 50-Hz, star-connected induction motor has the following parameters:

The motor is controlled by varying both the voltage and frequency. The volts/hertz ratio, which corresponds to the rated voltage and rated frequency, is maintained constant.
(i). Calculate the maximum torque Tm and the corresponding speed ?m for 50 Hz and 25 Hz.
(ii). Repeat (i) if Rs is negligible. ( 10 Marks )

Fee 542: Power Electronics And Variable Speed-Drives B question paper

Fee 542: Power Electronics And Variable Speed-Drives B Question Paper

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