Gust_Electric Power

Gust_Electric Power - Power and Electric Energy Power and...

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1 Power and Electric Energy Power and Electric Energy z Voltage(Work ) * Current(Charge ) = Power(Work ) Charge Time Time z Power(Watts) = Potential(Volts) * Current(Amperes) = EI Energy Calculation z What is the energy required to operate a 3000 W heater for 20 minutes? ¾ Energy = Power * Time ¾ E = 3000 W * 1200 s ¾ E = 3,600,000 J
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2 Resistance z The relative difficulty with which current can be transmitted in a material is defined as the electrical resistance of the material. z Two quantities of voltage and current can be related through the physical parameter, resistance. The voltage supplies the potential force in an electrical system. Flow of charge or current is the desired result. Ohm’s Law z I = E / R Where I = current in amperes, A E = potential in volts, V R = resistance in ohms, ohm z R = ρ (L / A) Where R = resistance, ohm L = length A = cross-sectional area ρ = resistivity Gustafson, Fundamentals in Electricity for Agriculture, 1988
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3 Resistance of a Conductor z R t = R i (1 + α T) ohm Where R t = resistance at specified temperature, ohm R i = resistance at reference temperature, ohm α = temperature coefficient of resistance, 1/ o C T = difference temperature between specified and reference, o C Direct and Alternating Current z Electrical systems are generally classed into two categories by the form of the current. z Direct current (dc) is characterized by current flow in only one direction at all times. Batteries, thermocouples, solar cells, and rotating dc generators are all examples of sources for direct current systems. z Alternating current (ac) is characterized by alternating flow in two directions. Gustafson, Fundamentals in Electricity for Agriculture, 1988
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4 Direct and Alternating Current con’t z An alternating current system is one in which the direction of flow changes periodically. ¾ e = E m sin θ ¾ Where e = instantaneous voltage ¾ E m = maximum voltage ¾ θ = angle Amplitude of Sine Waves z One of the most frequently measured characteristics of the sine wave is its amplitude. z Peak voltage is the maximum amplitude of either the positive or negative part of the cycle. z The effective or rms (root mean square) of a sine wave is the value equivalent to the constant dc magnitude that would provide the same amount of power. z For example, many conventional residential wiring systems operate at 120 V effective or rms. This means that a light bulb would glow at the same brightness on 120 V rms AC or connected to 120 V DC source. Amplitude of Sine Waves con’t z E rms = E Peak / 2 0.5 z I rms = I Peak / 2 0.5
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5 Phase Relations and Power in AC Circuits z When a sine wave voltage is imposed on a load, a sine wave current will result. For the case of a resistive load, the voltage and the current waves are “in-phase” with each other. The term “in-phase” means the current and the voltages go through zero and through their peak values at the same time. Gustafson, Fundamentals in Electricity for Agriculture, 1988
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Gust_Electric Power - Power and Electric Energy Power and...

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