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Imagine that as an engineering intern you have been assigned the task of selecting a motor for a pump. After reviewing motor catalogs, you narrow your choice to two motors that are rated at \(1.5 \mathrm{~kW}\). Additional information collected is shown in an accompanying table. The pump is expected to run 4200 hours every year. After checking with your electric utility company, you determine the average cost of electricity is about 11 cents per \(\mathrm{kWh}\). Based on the information given here, which one of the motors will you recommend to be purchased? \begin{array}{lll} \text { Criteria } & \text { Motor X } & \text { Motor Y } \\ \hline \text { Expected useful life } & 5 \text { years } & 5 \text { years } \\\ \text { Initial cost } & \$ 300 & \$ 400 \\ \begin{array}{l} \text { Efficiency at the } \\ \text { operating point } \end{array} & 0.75 & 0.85 \\ \begin{array}{l} \text { Estimated } \\ \text { maintenance cost } \end{array} & \$ 12 \text { per year } & \$ 10 \text { per year } \\ & & \end{array}

Step by step solution

01

Calculation of Electricity Cost

The first step is to calculate the cost of the electricity that each motor consumes. This cost can be calculated by using the motor power rating and conversion factors to find the yearly energy usage in kWh. Then with the given price of electricity, the annual electricity cost can be calculated.

02

Calculation of Total cost for motor X

Then, the total projected cost for the motor X is calculated by adding the initial cost, the estimated maintenance cost for 5 years and the estimated electricity cost for 5 years.

03

Calculation of Total cost for motor Y

Similarly, the total projected cost for the motor Y is calculated by adding the initial cost, the estimated maintenance cost for 5 years and the estimated electricity cost for 5 years.

04

Comparison and Decision

Finally, the total costs for both motors are compared and the motor with the lower total cost is the more cost-effective choice and should be recommended for purchase.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Total Cost Calculation

Understanding total cost calculation plays a crucial role in engineering decision-making, particularly when it comes to selecting equipment. It's not just about the upfront cost; it's the sum of all expenses associated with the purchase and operation over the item's useful life.

In the context of motor selection, total cost includes the initial purchase price, the cost of the energy the motor consumes (calculated from the power rating and operational hours), and the maintenance costs over the motor’s expected life. Engineers must calculate these costs precisely to assess which option yields the most economic value in the long-run. This holistic approach ensures a cost-effective purchase, avoiding the common pitfall of focusing solely on the purchase price.

Electrical Efficiency

When it comes to motor selection, electrical efficiency is a key factor. Electrical efficiency is the percentage of electrical energy that is converted into mechanical energy without being lost as heat or other forms of energy waste.

The higher the efficiency of the motor, the lower the electrical consumption for the same output, resulting in lower operating costs over time. Engineers must balance the efficiency with the initial cost, as high-efficiency motors might cost more up front but save money in energy costs. The efficiency at the operating point is particularly important, as it reflects the motor's performance under typical operating conditions.

Engineering Economics

The field of engineering economics is concerned with the analysis of the costs and benefits associated with engineering projects and decisions. This includes studying how to allocate resources, estimate costs, and make financial decisions that affect the overall economic performance of engineering operations.

In the case of the motor selection exercise, engineering economics principles are applied to compare the total costs of each option—a combination of initial costs, operating energy costs, and maintenance costs. This economic analysis enables engineers to recommend the most financially prudent motor, factoring in all possible costs over its entire lifecycle.

Maintenance Cost Estimation

Accurate maintenance cost estimation is essential for comprehensive total cost calculations. Maintenance costs can vary widely depending on the equipment's design, quality, and the conditions under which it operates.

In the realm of motor selection, engineers estimate these costs based on historical data, manufacturers' information, and operating conditions. Lower maintenance costs can significantly reduce the total cost of ownership, so a slightly more expensive motor with lower anticipated maintenance costs may be the better option in the long term. Careful estimation of maintenance costs will lead to more effective cost management and less downtime due to unexpected repairs or maintenance.