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EE 255 - Chapter 9f

EE 255 - Chapter 9f - plot(i_l,v_t'b'LineWidth,2.0...

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265 plot(i_l,v_t,'b-','LineWidth',2.0); xlabel('\bf\itI_{L} \rm\bf(A)'); ylabel('\bf\itV_{T} \rm\bf(V)'); string = ['\bfTerminal Characteristic of a Cumulatively ' ... 'Compounded DC Generator']; title (string); hold off; axis([ 0 50 0 120]); grid on; The resulting terminal characteristic is shown below. Compare it to the terminal characteristics of the cumulatively compounded dc generator in Problem 9-28 and the shunt dc generators in Problem 9-25 (d) . 9-29. A cumulatively compounded dc generator is operating properly as a flat-compounded dc generator. The machine is then shut down, and its shunt field connections are reversed. (a) If this generator is turned in the same direction as before, will an output voltage be built up at its terminals? Why or why not? (b) Will the voltage build up for rotation in the opposite direction? Why or why not? (c) For the direction of rotation in which a voltage builds up, will the generator be cumulatively or differentially compounded? S OLUTION (a) The output voltage will not build up, because the residual flux now induces a voltage in the opposite direction, which causes a field current to flow that tends to further reduce the residual flux. (b) If the motor rotates in the opposite direction, the voltage will build up, because the reversal in voltage due to the change in direction of rotation causes the voltage to produce a field current that increases the residual flux, starting a positive feedback chain. (c) The generator will now be differentially compounded.
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266 9-30. A three-phase synchronous machine is mechanically connected to a shunt dc machine, forming a motor- generator set, as shown in Figure P9-11. The dc machine is connected to a dc power system supplying a constant 240 V, and the ac machine is connected to a 480-V 60-Hz infinite bus. The dc machine has four poles and is rated at 50 kW and 240 V. It has a per-unit armature resistance of 0.04. The ac machine has four poles and is Y-connected. It is rated at 50 kVA, 480 V, and 0.8 PF, and its saturated synchronous reactance is 2.0 Ω per phase. All losses except the dc machine’s armature resistance may be neglected in this problem. Assume that the magnetization curves of both machines are linear. (a) Initially, the ac machine is supplying 50 kVA at 0.8 PF lagging to the ac power system. 1. How much power is being supplied to the dc motor from the dc power system? 2. How large is the internal generated voltage A E of the dc machine? 3. How large is the internal generated voltage A E of the ac machine? (b) The field current in the ac machine is now increased by 5 percent. What effect does this change have on the real power supplied by the motor-generator set? On the reactive power supplied by the motor- generator set? Calculate the real and reactive power supplied or consumed by the ac machine under these conditions. Sketch the ac machine’s phasor diagram before and after the change in field current.
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