ELECTRICITY FOR TRADES (LOOSELEAF)
3rd Edition
ISBN: 9781260437454
Author: Petruzella
Publisher: MCG
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Chapter 3.2, Problem 5RQ
To determine
Explain the reason why polarity is normally identified on DC but not AC voltage sources.
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Find Laplace transform and the corresponding ROC for
x(t) = e−3sin(2t) u(t)dt
Three similar coils, connected in star, take a total
power of 1.5 kW, at a power factor of 0.2, from a
three-phase, 400 V, 50 Hz supply. Calculate: (a) the
resistance and inductance of each coil; (b) the line
currents if one of the coils is short-circuited.
(a) Three 20 µF capacitors are star-connected across
a 400 V, 50 Hz, three-phase, three-wire supply.
Calculate the current in each line. (b) If one of the
capacitors is short-circuited, calculate the line cur
rents. (c) If one of the capacitors is open-circuited,
calculate: the line currents and the p.d. across each
of the other two capacitors.
ANS:
1.525 A; 2.64 A, 2.64 A, 4.57 A; 1.32 A, 1.32 A, 0;
210 V
Chapter 3 Solutions
ELECTRICITY FOR TRADES (LOOSELEAF)
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- A three-phase delta-connected load, each phase of which has an inductive reactance of 40 Ω and a resistance of 25 Ω, is fed from the secondary of a three-phase star-connected transformer which has a phase voltage of 230 V. Draw the circuit diagram of the system and calculate: (a) the current in each phase of the load; (b) the p.d. across each phase of the load; (c) the current in the transformer secondary windings; the total active power taken from the supply and its power factor.arrow_forwardDerive the numerical relationship between the line and phase currents for a balanced three-phase delta connected load. Three coils are connected in delta to a three-phase, three-wire, 400 V, 50 Hz supply and take a line current of 5 A 0.8 power factor lagging. Calculate the resistance and inductance of the coils. If the coils are star-connected to the same supply, calculate the line current and the total power. Calculate the line cur rents if one coil becomes open-circuited when the coils are connected in star. Ans: 110.7 Ω, 0.264 H; 1.67 A, 926 W; 1.445 A, 1.445 A, 0arrow_forwardDerive, for both star- and delta-connected systems, an expression for the total power input for a balanced three-phase load in terms of line voltage, line current and power factor. The star-connected secondary of a transformer supplies a delta-connected motor taking a power of 90 kW at a lagging power factor of 0.9. If the volt age between lines is 600 V, calculate the current in the transformer winding and in the motor winding. Draw circuit and phasor diagrams, properly labelled, showing all voltages and currents in the transformer secondary and the motor. ANS: . 96.2 A, 55.6 Aarrow_forward
- Deduce the relationship between the phase and the line voltages of a three-phase star-connected generator. If the phase voltage of a three-phase star-connected generator is 200 V, what will be the line voltages: (a) when the phases are correctly connected; (b) when the connections to one of the phases are reversed? ans: 346 V; 346 V, 200 V, 200 Varrow_forwardQ2: Given a discrete sequence x(n) = [1,-1,2,4, 1, 3,-1,1] and its spectrum X(k): i. Show that: X(k) =G(k)+WH(k) k=0,1,2,3,4,5,6,7 where G(k) = DFT[g(n)] and H(k) = DFT[h(n)], g(n) and h(n) are even and odd components of x(n) respectively. ii. Compute G(k) and H(k) using the method of computation of DFTS of two real sequences. (15 marks)arrow_forwardQ2: Given a discrete sequence x(n) = [1,-1,2,4, 1, 3,-1,1] and its spectrum X(k): i. Show that: X(k) = G(k)+WH(k) k = 0,1,2,3,4,5,6,7 where G(k) = DFT[g(n)] and H(k) = DFT[h(n)], g(n) and h(n) are even and odd components of x(n) respectively. ii. Compute G(k) and H(k) using the method of computation of DFTS of two real sequences.arrow_forward
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