# Question 8.7: CASE STUDY−−Selecting Electric Motors to Power an Assembly L...

CASE STUDY−−Selecting Electric Motors to Power an Assembly Line A large computer manufacturing company is expanding its operations in response to an increase in demand. To meet the new production goals, the company needs to upgrade its assembly line. This upgrade requires additional electric motors that will be able to deliver a total of 1,550 horsepower output. There are two types of motors available, induction and synchronous, whose specifications are listed in the accompanying table. Both motor types are three phase, and the quoted voltage is rms line-to-line. If the synchronous motors are operated at a power factor (pf) of 1.0, their efficiency is reduced to 80%, but the local power utility will provide electricity at a discounted rate of $0.025 per kilowatt-hour for the first year  Type of Motor Specifications Induction Synchronous (pf = 0.9 lag) Synchronous (pf = 1.0) Rated horsepower 400 500 500 Efficiency 85% 90% 80% Line voltage 440 440 440 Capital investment$17,640 $24,500$24,500 Maintenance cost (First year) $1,000$1,250 $1,250 Electricity cost (First year)$0.030/kWh $0.030/kWh$0.025/kWh

Historical data indicate that maintenance costs increase at the rate of 4% per year and that the cost of electricity increases at the rate of 5% per year. The assembly line must operate two eight-hour shifts per day, six days per week, for 50 weeks per year. An eight-year study period is being used for the project, and the company’s market-based cost of capital (MARR$_{m}$) is 8% per year. Your engineering project team has been tasked with determining what combination of motors should be purchased to provide the additional power requirements of the upgraded assembly line.

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