B - A - 3.8 - Power PDF

Module 3: Electrical Fundaments Topic 3.8: Power  INTRODUCTION On completion of this topic you should be able to: 3.8.1 Define Power and describe the following in regard to power: Work Energy (kinetic...

Module 3: Electrical Fundaments Topic 3.8: Power  INTRODUCTION On completion of this topic you should be able to: 3.8.1 Define Power and describe the following in regard to power: Work Energy (kinetic and potential) 3.8.2 Describe dissipation of power by a resistor and state the formula for power. 3.8.3 Perform calculations involving power, work and energy. 30-03-2024 Slide No. 2  WORK When a force acts on an object and sets it in motion – work is performed. Work – defined as product of force times distance. Work (W) = Force (F) x Distance ( D). W=FD eg. A 400 lb engine is lifted 10 feet W = 400 lbs. x 10 ft W = 4,000 ft/lbs. English system – typically foot-pounds (ft/lbs.) Metric SI system – unit is the joule 1 joule = 1 Newton x 1 metre Work (W) = F (Newtons) x D (metres) 30-03-2024 Slide No. 3  WORK AND POWER Work is done whenever a force causes motion. When a force is used to lift or move a weight – work is performed. Voltage is an electrical force – voltage forces current to flow in a closed circuit. When voltage exists but circuit is open – No current flows – No work is done. When voltage causes electrons to move, work is done. Power pertains to the rate at which work is done. Horsepower (hp) – unit of power – rating often found on electrical motors. 1 horsepower is equal to 746 watts – 1 hp = 746 watts 30-03-2024 Slide No. 4  POWER Power – defined as the rate at which work is done. Electrical unit of power (P) is the Watt (W). With DC: 1 Watt = 1 volt x 1 amp Watts = volts x amps P=VxI 1 kilowatt = 1,000 Watts 30-03-2024 Slide No. 5  JOULE James Prescott Joule (1818-1889) England Inventor Established the principle of conservation of energy. “Energy is neither created nor destroyed, but only converted from one form to another” 1 joule – equivalent of 1 watt of power radiated or dissipated for 1 second. 1 joule – the work done by 1 watt of power in 1 second. 30-03-2024 Slide No. 6  ENERGY Energy provides the capacity for work to be done. The SI unit of energy – joule. One joule of energy – can do one joule of work. Therefore 1 joule of energy could provide 1 watt of power for 1 second. How long would 1.5 kilojoules of energy provide 25 watts of power? Answer = 1 minute 30-03-2024 Slide No. 7  ENERGY – KINETIC AND POTENTIAL Energy associated with position or internal state is called potential energy. Even though an object is not doing work, it can still be capable of doing work. Energy associated with motion is called kinetic energy. All forms of energy are inter-convertible by appropriate processes. A battery has potential energy stored in chemical form. Connected – chemical potential energy is converted to electrical kinetic energy. 30-03-2024 Slide No. 8  POWER When current flows through a resistance – electrical energy is converted into heat. Obvious in an electric torch where lamp filament heats up and glows white hot. Exact same process occurs when current flows through any electronic component. Result may be less evident in some components. Power – rate of change of electrical energy to some other form of energy. Power is measured in watts (W) Calculated 30-03-2024 from P = EI Slide No. 9  POWER RATING A total amount of work may be done in different lengths of time. E.g. A number of electrons are moved in 1 second or in 1 hour – work RATE varies. In both cases, total work done is the same. However, when work done faster – wattage (instantaneous power rate) is greater. 30-03-2024 Slide No. 10  POWER RATING Electrical components are often given a power rating (in watts). Indicates rate at which device converts electrical energy into another form of energy. Some electrical devices indicate maximum power device is designed to use. Some devices indicate normal power (eg,100-watt lamp uses 100 watts normally). Resistor power rating (watts) – rating is maximum power rating. Most circuits, actual power used by resistor is considerably less than power rating. Typically 50% safety factor is used – e.g.. if resistor used 2 watts – use 3 W resistor. 30-03-2024 Slide No. 11  DISSIPATION OF POWER How much power is used by resistor with 6 Volts applied and current of 100 mA? 0.6 Watts of heat is generated in this resistor. To prevent overheating, heat must be dissipated to surroundings at the same rate. A resistor's ability to lose heat depends to a large extent upon its surface area. A small resistor with a limited surface area cannot dissipate heat quickly. A small resistor is likely to overheat if large currents are passed through it. Larger resistors dissipate heat more effectively. 30-03-2024 Slide No. 12  RESISTOR POWER RATINGS 1 Watt ¼ Watt 2 Watt ½ Watt ⅛ Watt Comparisons of most common sized resistors. Compare physical sizes to thumb. 30-03-2024 Slide No. 13  HEAT SINKS Ways to increase dissipation of heat from a resistor: Size Airflow Heat sinks 30-03-2024 Slide No. 14  FORMULA WHEEL Unknown: Inside circle Known: Outside circle Ohm’s Law: V = IR Power Law: P = VI P = Power in Watts R = Resistance in ohms V = Voltage in Volts I = Current in amps 30-03-2024 Slide No. 15  POWER CALCULATIONS How much power will be dissipated by load? P = EI P = 12 volts x 2 amps P = 24 watts 30-03-2024 Slide No. 16  POWER CALCULATIONS R = 20 Ω E = ????? I = 4 amps How much power will be dissipated by load? P = I 2R P = (4 amps x 4 amps) x 20 Ω P = 320 watts 30-03-2024 Slide No. 17  POWER CALCULATIONS 200 watts E = 12 volts I = ???? What is the current draw of 2 x 100 watt driving lights in a typical car? I = P/E I = 200 watts ÷ 12 volts I = 16.66 amps 30-03-2024 Slide No. 18  POWER CALCULATIONS P = ??? E = 12 volts I = 20 amps What is the maximum power load on a circuit with wiring rated to 20 amps? P = EI P= 12 volts x 20 amps P = 240 watts 30-03-2024 Slide No. 19  POWER CALCULATIONS 1.5 hp Motor E = 28 volts I = ??? Approximately how many amps will this 1.5 hp 28 VDC motor draw? I=P÷E I = (1.5 hp x 746 watts) ÷ 28 volts I = 40 amps (approx.) 30-03-2024 Slide No. 20  CONCLUSION Now that you have completed this topic, you should be able to: 3.8.1 Define Power and describe the following in regard to power: Work Energy (kinetic and potential) 3.8.2 Describe dissipation of power by a resistor and state the formula for power. 3.8.3 Perform calculations involving power, work and energy. 30-03-2024 Slide No. 21  This concludes: Module 3: Electrical Fundaments Topic 3.8: Power