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Electric Power and Energy Definition and Measurement, Exercises of Construction

University of California - Los Angeles (UCLA)Construction

Ac circuits dissipate some energy as light, heat, or motion. Other energy goes to charging and discharging capacitors and building.

Typology: Exercises

2022/2023

Uploaded on 03/01/2023

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4/6/2017
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Electric Power and Energy
Definition and Measurement
Lesson 20
EET 150
In this lesson you will:
see the definitions of work, energy and power
learn to measure and compute power in simple dc circuits
define power in Ac circuits
see how power varies with time in Ac circuits
learn how voltage and current relationships effect ac power
define electric energy
see the construction of a energy meter
learn to read an energy meter
Learning Objectives
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Electric Power and Energy

Definition and Measurement

Lesson 20

EET 150

 In this lesson you will:  see the definitions of work, energy and power  learn to measure and compute power in simple dc circuits  define power in Ac circuits  see how power varies with time in Ac circuits  learn how voltage and current relationships effect ac power  define electric energy  see the construction of a energy meter  learn to read an energy meter Learning Objectives

Work, Energy and Electric Power

Energy and work occur when a force acts on a mass

Force = (Mass)(Acceleration of gravity) = Weight Lifting a weight requires work and dissipates energy Work = (Force)(Distance) W (Joules) = F (Newtons) X D (Meters) D Mass (M) F Power is how fast work is done Rate of energy consumption Power = Work/Time P (Watts) = W (Joules)/ t (seconds)

Electric Power Dc Circuits

Source V Load Power = Voltage X V Current

I

P (Watts) = V (Volts) x I (Amps) If load is a resistor then:

P  I R

2

R

V

P

2

Know load current Know load voltage Where R is resistance in ohms (W)

Measuring Power in Series Dc Circuit Es = 60 V

R 1 =10 W PR R 2 = 5 W

  • (^) P R R 3 = 15 W + PR A V V I = 2 A VR1 = 20 V PR1 = VR1∙I =20∙2 = 40 W VR2 = 10 V PR2 = VR2∙I =10∙2 = 20 W VR3 = 30 V PR3 = VR3∙I =30∙2 = 60 W Es = 20 V R^1 =10^ W^ R^2 =5^ W^ R 3 =15^ W Measuring Power in Parallel Dc Circuits V A 1 A 2 A 3 V = 20 V I 1 = 2 A PR1 = V∙I 1 =20∙2 = 40 W I 2 = 4 A PR2 = V∙I 2 =20∙4 = 80 W I 3 = 1.33 A PR3 = V∙I 3 =20∙1.33 = 26.6 W

Power in Ac Circuits

Load V A Ac circuits dissipate some energy as light, heat, or motion. Other energy goes to charging and discharging capacitors and building magnetic fields in inductors P = active power (light, heat, motion) Q = Reactive power (Charging capacitors, inductor magnetic fields) S = V∙I Apparent Power 2 2

S  P Q

Ac Power

Ac power changes with time. For Ac sources and resistor

loads, this graph plots the power.

Power wave never negative Power wave is zero twice in a period.

Measuring Ac Power Using the Wattmeter

The Wattmeter has internal voltage and current measuring elements and displays Watts (W) Load Wattmeter Line Side Load Side To ac voltage source Current Coil Voltage (Potential) Coil Polarity Mark Circuit Implements Formula P = V∙I∙cos(q) Watts V = Rms value of voltage waveform I = Rms value of current waveform q = phase angle between V and I waves

Phase Shift and Ac Power

P=(70.7)(21.21)cos(45 deg)= 1060 W Vrms = 100(0.707) = 70.7 V Irms = 30(0.707) = 21.21 A q = 45 degrees Vrms = 100(0.707) = 70.7 V Irms = 30(0.707) = 21.21 A q = 23.5 degrees P=(70.7)(21.21)cos(23.5 deg)= 1375 W Phase shift

Electric Energy Measurement

ENERGY = (instantaneous power) x (time)

W = p x t where W = energy

p = instantaneous power

t = time

kWh meters sum power over time interval using a rotating disk. Number of revolutions, n, proportional to energy so n = Cp x P x t Cp = meter energy constant (units kWh/rev)

Energy Meter Construction

kWh meter measures the electric energy we all consume in our homes and businesses Current coil Potential Coil n Indicator dials n=number of disk (rotor) revolutions

END LESSON 20

EET 150

COMING NEXT: MORE ELECTRONIC CIRCUIT

CONSTRUCTION TECHNIQUES

Electric Power and Energy Definition and Measurement