9-72: A motor with a 200-horsepower output is needed in the factory for intermit
ID: 1234737 • Letter: 9
Question
9-72: A motor with a 200-horsepower output is needed in the factory for intermittent use. A Graybar motor costs $7000 and has an electrical efficiency of 89%. A Blueball motor costs $6000 and has 85% efficiency. Neither motor would have any salvage value, since the cost to remove it would equal its scrap value. The annual maintenance cost for either motor is estimated at $300 per year. Electric power costs $0.072/kWh (1 hp = 0.746 kW). If a 10% interest rate is used in the calculations, what is the minimum number of hours the higher initial cost Graybar motor must be used each year to justify its purchase?Explanation / Answer
Difference in cost is $1,000. Interest is 10%. So we need to save $100 per year to just pay the interest on the difference. It would be nice to know when we can apply the energy savings; monthly or yearly? But as long as we pay interest on the difference in price as often as we apply the energy savings, the numbers work out the same. 89% efficiency 200*0.746 = 149.2 kW is what we need x*0.89 = 149.2 KW x = 149.2/.89 = 167.6404494 this is kW electricity needed to generate 149.2 kW motion 85% efficienct x = 149.2/0.85 = 175.5294118 difference = 7.888962327 kW electric company charges for energy. Power times time = energy kW is power h = hours = time Each hour the more efficent motor saves us 0.0072 $/kWh*7.89kW = $0.0568 $100/$0.0568/h = 1,760.5 hr each year (or about 34 hours per week).
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