CH16_Problems - 8 02 C hapter 1 6 DC M achines 9. T he...

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802 Chapter 1 6 DC Machines 9. The magnetization curve of a de machine is a plot the induced armature voltage versus field current with the machine being driven at a constant speed. The equivalent circuit shown in Figure 16.14 on page 781, the magnetization curve, and the following equations provide the basis for analyzing de machines: EA = Kc/lwm Tctev = KcfJlA Pctev = EAIA = WmTdev 10. The equivalent circuit a shunt-connected de motor is shown in Figure 16.16 page 784. torque-speed characteristic is shown in Figure 16.18 page 785. Speed can be con- trolled either by using Ractj to vary the field or by inserting a variable resistance in series with armature. Variation V T is not an effective means speed control. 11. a separately excited de is shown in Figure 16.21 page 788. characteristic is identical that the shunt-connected machine shown in Figure 16.18 trolled by varying the field current, by varying source voltage Vr, by inserting additional resistance in series with the armature. 12. The characteristics a permanent-magnet de are similar those a separately excited motor, except no means is available vary Problems Section 16.1: Overview Motors P16.1. N arne principal parts a rotating elec- trical machine. P16.2. What types motors contain brushes a commutator? is function these parts? field. Speed control be achieved by vary- ing supply voltage 13. series-connected de is shown in Figure 16.22 page 788. In the normal range operation, its torque is almost inversely proportional to square its speed. The series-connected de is suitable for starting heavy loads. It reach dangerous speeds if load totally removed. 14. The universal is basically a series de designed for operation from ac source. is found in applications where a large power-to-weight ratio needed. However, due commutator wear, its service life limited. Its speed controlled by varying the applied voltage by inserting series resistance. 15. Historically, variable de voltages for speed con- trol de motors were obtained by employing de generators. Presently, the approach choice is to use rectifiers first convert ac into de. Then, electronic chopper circuits with adjustable duty factors provide variable average voltages. 16. Figures 16.26, 16.27, 16.28 pages 795,796 796 show how character- istics shunt-connected separately excited de motors changed by varying arma- ture resistance, supply voltage, field to achieve speed control. 17. De generators separately excited, shunt connected, compound connected. Analysis de generators for de motors. P16.3. List the two principal types three-phase ac motors. Which in more common use? P16.4. two types windings are used in elec- trical machines? Which type is used in permanent-magnet machines? Why not? * Denotes answers are contained in the Student Solutions files. See Appendix F for more information about accessing the Student Solutions.
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*Pl6.5. List two practical disadvantages of de mot- ors compared to single-phase ac induction motors for supplying power a ventilation fan in a home or small business.
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CH16_Problems - 8 02 C hapter 1 6 DC M achines 9. T he...

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