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Transcript
Applied Electricity PHY 222 2.0 Tutorial Number 01 1. Express in rectangular and polar notations the phasors for the following quantities. a.) i = 10 sin ωt b.) i = 5 sin (ωt +π/3) c.) v = 40 sin (ωt +π/6) d.) v = 60 sin (ωt +π/4) Draw the phasor diagram representing the above voltage currents. 2. Express each of the following phasors in polar notations. a.) 10 + j5 b.) 3 – j8 c.) 4+ j3 d.) 20 – j10 3. A voltage , v = 150 sin(314t + 30) volts, is maintained across a coil having a resistance of 20Ω and an inductance of 0.1 H. derive expression for the r.m.s values of the voltage and current phasors in a.) Rectangular notation, b.) Polar notation. Draw the phasor diagram. 4. Define the r.m.s value, peak value and average value of an alternating current. Explain why r.m.s value is more generally employed in a.c measurements than either average or the peak value. 5. A coil of 200 turns is rotated at 3000r/min in a magnetic field having a uniform density of 0.1T, the axis of rotation at right angles to the direction of the flux. The mean area per turn is 40cm2. Calculate The frequency, the period, the maximum value of the generated e.m.f.. The value of the generated e.m.f. when the coil has rotated through 30. from the position of zero e.m.f. 6. A single phase motor take 8.3 A at a power factor of 0.866 lagging when connected to 230V, 50Hz supply. Two similar capacitors are connected in parallel with each other to form a capacitance bank. This capacitance bank is now connected in parallel with the motor to raise the power factor to 0.95. Determine the capacitance of each capacitor. 7. Show by a phasor diagram that the sum of three phase balanced current is zero and hence no current with flow in the neutral connection of balanced star connected system. 8. A balanced star-connected load of (8 + j6)Ω per phase is connected to a threephase, 230V supply. Find the line current, Power factor, active, reactive and total power. 9. A balanced three-phase delta connected load of 160kW takes a line current of 100A with a line voltage of 11,000V 50Hz. Find the circuit constant of the load for phase. 10. A load of 20kW (balanced) with 0.7 pf lagging is fed from a 400V, three phase supply as shown in figure below. Three equal capacitors are connected in delta across the load to improve the overall pf to 0.85 lagging. Find the resultant current drawn from the supply and the capacitance of each capacitor. A C B C C 20kW 0.7 pf Lagging C 11. A delta connected unbalanced loads is arranged as shown in figure below and is connected to balanced supply voltage of 415V. The phase sequence is RYB. Calculate the phase and line currents. R Y B 12. A three phase load draws a current of 10A at a lagging pf of 0.65 from a 600V, 50Hz source. The line current reduces to 7.2A when a capacitor bank is connected across in ; a) Star-connected , and b) delta connected. Find the kVAR rating of the bank in each case. 13. Briefly describe the electrical power generation and distribution using a block diagram. ********************************************