Why oversizing motors shouldn't be your starting position

Posted by AuCom on 02/04/15 10:34

What are the trade-offs in oversizing a motor for your next application?

Oversizing motors for any particular application is becoming more popular, as engineers seek to provide some ‘insurance’ for a new system. Insurance against possible system failure (and resulting damage to their reputation), and insurance for unanticipated future demands on the application.

An Automation World study on oversizing has asserted that over half of engineers choose to over specify motors. It seems like a ‘no-brainer’ – put in place a motor with plenty of margin of error for rated load, and use a variable speed drive (VSD) to throttle it to meet the system’s actual power needs. Peace of mind guaranteed.

Unless you consider an application from an overall system perspective. In many applications, using a motor larger than required will actually lower the efficiency of the overall system. The EuP Lot 30: Electric Motors and Drives study – prepared for the European Commission – asserts that for most motors there is a significant decline in energy efficiency when operating at below 75% of rated load. Despite this, the study estimated the average load factor for electric motors across a range of industry sectors to be between 41% and 51%. In fact, some sectors were found to have motors running at as low as 24% of rated load.

Oversizing also results in higher motor costs, and often higher system costs e.g. the cost and installation of additional ducting or cabling required with a more powerful motor. Similarly, maintenance costs can be increased by the oversizing of a motor. A key issue is that the additional operating costs of oversized motors continue indefinitely.

According to Automation World magazine, of the engineers changing their practice to more accurately size their motors, 20% cite a better understanding of the overall system requirements as the reason. More flexible systems design (14%) is another major driver of change, as is tighter cost control (9%).

Then of course is the energy efficiency question. Given energy costs can account for over 97% of the lifecycle costs of a motor, getting this right delivers considerable value over time.

Oversizing a motor and using a VSD in a situation where the motor runs at a fixed speed has a major impact on energy efficiency (covered in detail here). The VSD alone will reduce energy efficiency by several percentage points, made worse by the overall inefficiency of such a system.

For a constant speed application, soft start provides a lower cost, more energy efficient option.

Deciding on the appropriate motor size for an application, as well as the correct motor starting technology, needs to be based on a system-level analysis that considers upfront and ongoing costs, particularly in relation to energy consumption.

Want to get to grips with the basics of applying soft starters to your motor control application? Download our free guide “Where do I start?”

Topics: System Design, Energy Efficiency

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