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Transcript
Dr. G.H. Massiha and Alan Smith Department of Industrial Technology The University of Louisiana at Lafayette Lafayette, LA 70504 Tel. (337) 482-5719 Fax (337) 482-6661 [email protected] The Electric Bill Determining Watts and Kilowatthours. It is extremely helpful to know how to determine watts and kilowatthours. This knowledge is valuable when estimating the cost of operation of a new piece of equipment or in determining whether or not an appliance is functioning properly. It also makes one a better judge of the relative value of electrical appliances and increases the over-all understanding of electricity. Finally, and perhaps most important of all, this knowledge often results in a greater appreciation of the value received from the electric-energy dollar. Since power is the rate at which energy is used, power utilized over a period of time represents energy consumption. If you multiply power in watts and time in seconds, you have energy in joules, symbolized by W. W = Pt Because power companies deal in huge amounts of energy, the most practical unit is the kilowatt-hour (kWh). You use a kilowatt-hour of energy when you use the equivalent of 1000 W of power for 1 hr. For example, a 100 W light bulb burning for 10 hr uses 1 kWh of energy. W = Pt = (100 W)(10 hr) = 1000 Wh = 1 kWh There are several methods for determining the amount of energy used by an appliance: Method 1. Nameplate Data. The nameplate provides the equipment manufacturers with a means of conveying to the customer the rated electrical values and output capacities of the equipment. The nameplate may be a separate metal plate securely fastened to the equipment or it may be stenciled, stamped, or otherwise labeled directly on the equipment frame. The input voltage for which the equipment was designed is always given and the input watts, if not given, can be obtained by calculation. Various other data are included, depending upon the type of equipment in question. Several nameplates are shown in Figure 1. The wattage rating on the nameplate is interpreted as that value of watts used by the equipment when it is connected to its rated voltage and is operating at full capacity. For example, suppose an electric iron has a nameplate rating of 600 watts, 120 volts. This iron converts electrical energy at a rate of 600 watts when it is connected to 120 volts. In order to determine the kilowatthours it is necessary to know the hours of operation. For 4 hr of operation, 2.4 kWh is required (600 X (4/1,000)). Occasionally the nameplate data include only the volt and the ampere rating. For farm equipment, other than motors, the watts are calculated by multiplying the volts by the amperes. If two, voltage and two current values are given, multiply the largest value of one by the smallest value of the other. The power to an electric motor cannot be obtained by multiplying volts and amperes because the power factor of a motor is not 100 percent. A practical value for the power input to a motor is obtained by: a. Multiplying 1,200 by the horsepower rating of the motor if the motor is smaller than ½ hp. GE Long Life Light Bulb 100 Watt 120 Volts Nameplate on Light Bulb SONY Model CFD-Z500 120 Volts AC or DC 15 Watts Label on table-model radio AC Only 1200 Watts 120 Volts Model 17109R Nameplate on an electric iron Figure 1. Samples of nameplates found on electrical equipment. b. Multiplying 1,000 by the horsepower rating of the motor if the motor size is ½ hp or greater. The amount of energy required depends upon the operating time. Example 1. Determine the kilowatthours used for operating two motors at their rated capacity for 20 hr. The ratings of the motors are 1 and ¼ hp. 1,000 x 1 = 1,000 watts 1,200 x ¼ = 300 watts 1,300 watts 1,300 x 10 = 13 kWh 1,000 Method 2a. Voltmeter and Ammeter. The power used by an appliance with a large resistance such as light bulbs, irons, and toasters can be measured with the aid of a voltmeter and ammeter. Figure 2. Analog AC voltmeter and digital AC ammeter connected to a space heater. Method 2b. Wattmeter. The power usage by an appliance that uses motors can be measured with the aide of a wattmeter. When the consumer desires to know the exact amount of energy used by a certain appliance, he should request the power supplier to install a check meter. This meter is exactly the same as the regular kilowatthour meter. The kilowatthour value is read as described in the preceding section. If the number of watts is desired, multiply the kilowatthour readings by 1,000, then divide by the number of hours the appliance is on. 115-volt source The larger terminals are ammeter terminals Appliance being tested (including motors) This meter is a combination of ammeter and voltmeter Figure 3. Analog Wattmeter and an AC motor Method 3a. Meter-disk Revolutions. This method makes use of the regular kilowatthour meter that is installed on houses for recording the number of kilowatthours used each month. The meter’s Kh factor and metal disk are the significant items employed and these, together with a watch or clock, are all that is needed. The Kh factor is printed on the nameplate of the meter as shown in Figure 3 and for farm-type meters ranges from 1.5 to 5.0. For a particular meter the Kh value is constant. It is equal to the number of watthours per revolution of the disk. The disk revolutions are counted for a definite length of time (6, 10, or 12 min.) and the watthours are obtained by calculation. Figure 5 shows how to read the values from the dials on a kilowatthour meter. All appliances except the one being tested must either be disconnected Figure 4. Front view of kilowatthour meter. The Kh factor is shown on the nameplate. The first reading is 3,257 kWh and the second reading is 3,293 kWh. Figure 5. How to read the dials on a kilowatthour meter. or turned off. The desired values are obtained from these relationships: Watthours = Kh x no. of revolutions Watthours per hr = watthours x 60 min of count Watts = watthours per hr Example 2. All electrical equipment is off, except an electric motor. The kilowatthour-meter-disk revolutions are counted for a period of 6 min. The disk makes 20 revolutions and the Kh factor of the meter is 2.5. Determine the kWh that would be used by this motor if it were operated for 1 hr. What is the power input to the motor? Watthours = Kh x revolutions = 2.5 x 20 = 50 Watthours per hr = 50 x (60/6) = 500 kWh = 500/1000 = 0.5 Power input to the motor = watts = 500 Method 3b. This type of meter does not have dials like the above meter. To calculate the energy used over a period of time, take an initial reading from the meter, wait the specified period of time, and record the second reading. To calculate the kilowatthour usage, subtract the initial reading from the second reading and that value is the amount of energy used for that specific period. Figure 6. Kilowatthour meter. Listed below are some problems to calculate the power input to: A. An electric heater that draws 6 amp when operating from a 115-volt source. B. A lamp that draws 0.52 amps from the 115-volt source. C. A heater operating from the 230-volt source and having a resistance of 46 ohms. Conclusion Being able to calculate power and energy will allow students to check the energy being consumed by the devices that are used in everyday tasks. Three different methods for calculating power and energy are shown. From these methods students will begin to understand the relationship that power, energy usage, and money have with each other.