Machine Design

Techniques to Build Energy-Efficient and Sustainable Machines

A few basic practices can help meet goals for machines that must demonstrate energy efficiency.

Authored by:
Robert Repas
Associate Editor
[email protected]

Key points:
• Energy costs rise an average of 6 to 8% each year, so it often makes sense to design for energy efficiency.
• Direct-drive motors can, on average, improve operating efficiency from 60 to 90% when replacing worm gearboxes.

Rockwell Automation,

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Energy costs rise an average of 6 to 8% each year. It’s no wonder manufacturers now specify that the machines they purchase from OEMs be as efficient as possible. One Rockwell Automation customer admits that two years ago it never bothered to track energy usage in their factory. Now, it looks for every opportunity to improve efficiency ratings. Those savings drop directly to the bottom line, so it’s crucial that every new piece of equipment run as efficiently as possible.

Stephen Stokes of AMR Research has pointed out that energy efficiency is also a key element in promoting environmental best practices. “A reduction in energy usage is a reduction in greenhousegas (GHG) emissions,” he says.

To help machinery run more efficiently and support sustainability goals, machine builders and manufacturers should use a few critical motion control best practices. That list includes mechatronic design, servo technology, and direct-drive technology.

Servo technology
Steadily dropping prices make servomotors one of the easiest places for manufacturers to start saving energy. Permanent-magnet (PM) servomotors can work with 25% less power than induction motors when operating over a wide speed range. They also typically use less energy to accelerate rapidly.

Everyone understands the need for more-efficient motors to meet preset goals. Less well understood is how the power factor of the motor figures into the calculation. Simply stated, power factor is a calculated ratio of the difference in degrees between voltage and current peaks in an ac circuit. The ideal power factor occurs when both peaks occur at the same instant, described as unity power factor or 100%. Power factors less than unity lead to higher currents that do not supply any additional power to the load, but do boost the I R losses in the circuit. Companies that have low power factors are generally penalized with higher electric rates or added surcharges by power companies. To keep power factors high, some companies install expensive power-factor correction equipment into their power systems.

Power factors for typical induction motors vary with load and can range from 40 to 80%. On the other hand, the amplifiers used with servomotors typically exhibit a constant power factor of 96% or greater. This high power factor combined with greater efficiencies means servomotors waste less energy, need less power-factor correction equipment, and help lower the amount of power spent cooling a plant.

Servomotors are generally smaller than their less-efficient counterparts comprising up to 30% less raw materials. If the applications uses positioning in any form, servomotors can improve position accuracy and help cut potential scrap.

Direct-drive technology
Direct-drive technology is another route to better efficiency. Worm gearboxes are generally an inexpensive investment compared to other motor speed-control techniques. But they exhibit typical power losses of 30% or more. Motors driving gearboxes must be larger than the application would seem to call for because they must take these losses into account.

Simply replacing gearboxes with direct-drive motors can improve operating efficiency from 60 to 90% on average. This approach may permit use of smaller motors that cost less and draw less energy.

Similarly, direct-drive technology coupled with mechatronic analysis can help eliminate mechanical line shafts once used to power machines. Experience shows that replacing a line shaft with several individual servomotors electronically geared together can typically help reduce energy loss by 20%. And there may be unexpected side benefits as well. For example, a Rockwell Automation customer that makes OEM packaging equipment not only saved energy by switching from a single line shaft to individual drives, but also tightened manufacturing tolerances that reduced accumulated error and associated scrap.

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