_CE.3121 Assignement 9

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California Polytechnic State University, Pomona *

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3121

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Electrical Engineering

Date

Jan 9, 2024

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Kay Thai 12.4 What is the standard voltage rating for convenience power (plug-in receptacles) in the United States? In Europe? US- 120 V, 60 Hz Europe- 220 V, 50 Hz 12.6 What is Ohm’s law of electrical circuits? What will happen when a 120-V appliance is connected to a 240-V system? Ohm’s law states the relationship between electric current and potential difference. The current that flows through most conductors is directly proportional to the voltage applied to it and the temperature should be constant. If we use 240 instead of 120V the load will draw more current so more heat will produce that may damage the device. 12.8 What are the line-to-line and line-to-neutral voltages in a 120/240-V single-phase AC system? In a 120/208-V three-phase system? (Use system diagrams shown in Fig. 13.3(b) and (d) in Chapter 13.) For 120/240V=240V For 120/208V=208 30 ◦ 12.9 What is power factor? What is the power factor of a DC system? Power factor is a measure of the efficiency of electrical power utilization in an AC (alternating current) electrical system. It is a dimensionless quantity that represents the ratio of the real power (in watts) to the apparent power (in volt-amperes) in an AC circuit. In other words, it quantifies how effectively electrical power is being converted into useful work in a circuit. In a pure DC (direct current) system, the concept of power factor is not applicable because there is no alternating current or phase angle between voltage and current as there is in AC systems. DC systems don't have reactive power or power factor issues. Therefore, the power factor of a DC system is always 1, indicating that all of the apparent power is real power, and there is no energy wasted due to phase differences between voltage and current. 12.10 Is it desirable to maintain a high power factor in an electrical system? Can power factor be improved? If so, how? Improving the PF can maximize current-carrying capacity, improve voltage to equipment, reduce power losses, and lower electric bills. The simplest way to improve power factor is to add PF correction capacitors to the electrical system 12.11 What is the standard AC system frequency in the United States and in some other countries? The standard AC frequency in the United States is 60 Hz. Most other countries use 50 Hz.

12.12 Name the two major advantages of an AC system over a DC system in building applications. AC can be transmitted across long distances without much energy loss and are easier to be implemented for AC motors. 12.13 Can AC be converted to serve DC loads? If so, how? Yes however, Some appliances need dc instead of the mains ac so it has to be rectified to only flow in one direction be using a diode. 12.14 What is the advantage of a three-phase system over a single-phase system? More efficient power transmission More constant power delivery Higher power handling capability Balanced loading 12.15 Owing to the resistance and/or reactance of wiring, voltage is always lower at the end of a distribution system than at its beginning. This is called voltage drop. A well-designed system should limit voltage drop to a minimum. Answer the following: What is the recommended maximum overall voltage drop for a combination lighting and power system? The recommended maximum overall voltage drop for a combination lighting and power system is 3% For an electrical heating system? For an electrical heating system, the maximum recommended voltage drop is 5%. What is the nominal voltage rating of single-phase appliances in the United States? The nominal voltage rating for single-phase appliances in the United States is 120 volts. 12.16 An electrical load is rated for 1150 W at 115-V, single phase. What is the load current if the power factor is 1.0, or 100%? 10 amps 12.17 If the current of the load in Question 12.16 is 12 A, what is the power factor? 0.83 12.25 Describe the difference between power and energy. Power is the rate at which energy is transferred or transformed. It represents potential. Energy is the ability to do work. It represents the actualization of power 12.26 If a single-phase electrical resistance load operates at 115 V, how much current is required to consume 800 W of power? 7 AMPS

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