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Cork Institute of Technology
Bachelor of Engineering (Honours) in Mechanical Engineering - Stage 3
(NFQ - Level 8)
Summer 2007
Electrical/Electronic Engineering
(Time: 3 Hours)
Instructions Answer FIVE questions.
Examiners: Dr. R.A. Guinee Mr. P. Clarke Prof. M. Gilchrist
Q1. (a) Describe how LabView works and discuss the function of the following items in such a programming environment. (i) Block Diagram (ii) Control Panel (4%)
(b) It is required to simulate the statistical temperature variation of a reading room in a library due to student occupancy. A uniform random number generator (RNG) is employed for this purpose to model the temperature excursion between a low temperature value of 15°C and a high of 25°C. The RNG supplied with LabView runs with a clock speed of 1ms. Design a LabView virtual instrument with block and front panel diagrams to accomplish this task with the following features: (i) The RNG is run continuously upon activation. (ii) A graphical display along with a digital readout of temperature variation. (iii) An input delay feature that retards the RNG output update to 10 minute intervals in order to simulate student arrival to and from the reading room. (iv) a display of the running mean temperature value. (10%)
(c) What changes are necessary in the Virtual Instrument (VI) of part (b) to monitor the room temperature for a fixed period of 3 hours? Redraw the block diagram configuration to accommodate this fixed time period. (6%) [20%]
Q2. (a) Explain the features of the following functions in LabView VI design: (i) For Loop (ii) Shift Register (4%) using simple sketches of their respective graphical icons.
(b) Design a VI, with block and front panel diagrams, that computes and displays the running average and instantaneous value from a pressure sensor with measurements ranging from 5 to 10 psi with the following in-built features: (i) a random number generator (RNG) is used to model the pressure variations at 5 sec intervals. Note the supplied LabView RNG operates with an internal clock speed of 1ms. (ii) 3 successive RNG outputs are used in the averaging process. (9%)
(c) Modify the VI block diagram in part (b) to output the average value and the maximum and minimum pressure occurrence over a one-hour period only. (7%) [20%]
Q3. (a) Describe the use of the following in VI design: (i) Case Structure (ii) Sequence Structure (iii) Formula node (6%)
(b) Design a VI that uses a case structure to model the following process output: If i/p X ≥ 0 then o/p Y=4X+5 ; Else o/p (^) Y= 6X^2 +2 ; (8%)
(c) Redesign the VI in part (b) using a formula node only and comment. (6%) [20%]
Q5.(a) Starting with the rotor flux density space wave given by B = Bm cos θ with rotor space
angle θ in Fig.Q5 for a 2-pole electrical machine with air gap radius r and length l :
show that for a 3-phase P-pole machine with distributed stator windings the rms generated EMF voltage per phase is given
Erms = 4. 44 fkwNph φ
where Nph is the number turns per phase, f is the electrical frequency, kw is the winding reduction factor and φ is the flux per turn. (9%)
(b) A 4-pole, 3ph alternator has 36 slots each wound with 10 conductors, having a distribution factor of 0.96. If the generated emf per phase is 380V at 50Hz, calculate (i) the speed of rotation and (ii) the flux per pole. (11%) [20%]
Rotor Flux
Density B
N-Pole
Rotor Space
Angle θ
S-Pole
N-Pole
Fig.Q5: Rotor Flux Density Space Wave
Q6. (a) Show for a balanced 3-phase Y connected system illustrated in Fig.Q6 with phase
voltages Vph given by e (^) R = Em sin ωt, eS = Emsin(ωt-120 o^ ) and e (^) T = Em sin (ωt-240 o^ ) that (i) the neutral current is zero and that (ii) the line-to-line voltage VL is given by VL = 3 V Ph. Sketch the phasor diagram showing the phase and line voltages. (10%)
(b) An unbalanced star-connected load is supplied from a 3-phase four-wire as per Fig.Q with a line voltage of 380V at 50Hz of phase sequence RST. If the load impedances are ZR= 10+j10Ω, ZS= 10-j10Ω and ZT= - j14.1Ω determine (i) the line currents and (ii) the neutral current. (10%) [20%]
Q7. (a) Draw the torque speed characteristic of an induction machine and discuss the various
modes of operation. Identify all quantities in the single phase equivalent circuit of Fig.Q for an induction motor and carefully explain how the developed mechanical power is represented in the circuit. (7%)
(b) A 3phase 6-pole induction motor with 440V supply at 50Hz develops a power output of 8kW with a 4% slip and p.f. of 0.85. Stator losses amount to 500W and friction losses to 400W. Determine (i) the synchronous and motor speeds (ii) the torque delivery, (iii) the rotor winding (copper) loss, (iv) the overall efficiency and (v) the input line current.(13%) [20%]
N
IR
IS
R
T S
eR e (^) T N eS ZS
ZR
ZT
IT
IN
3-Phase Generator
3-Phase Load Fig.Q
R 0 X 0
RS XS IS
ES ER^1 − ss^ Rr
Ir Rr Xr
NS Nr
Ii I^0 Im^ ωS
_IS_*
VS
Fig.Q7: Per Phase Equivalent Circuit of an Induction Machine
