Boosting Your Bottom Line: Three Cautions Regarding Drives

One common cause of wasted energy—and wasted money—around industrial facilities is the running of motor-driven equipment at full speed, regardless of the load. Installing an adjustable speed drive (ASD) can help by matching motor speed to application requirements. There are, however, some important exceptions. This article explores several situations where installing an ASD may warrant a second look.

Where Can Drives Save Energy?
ASDs can save energy if they are used in appropriate applications, are installed properly and if potential harmonics issues are identified and addressed. Generally, ASDs are recommended for centrifugal loads such as fans, pumps and blowers that operate at least 2000 hours per year; in systems where flow varies over time; and where valves, throttles or dampers are used to regulate the flow and pressure.

Where Are Drives Unlikely To Save Energy?
While ASD energy efficiency is typically high (approximately 97% at full load), they are less likely to save energy in applications where motor speed remains relatively constant over time. Below are a few more examples where ASDs are unlikely to save energy.

1.High-static-pressure installations: A system that is static head-dominated (open loop) is one where the pump is working to overcome gravity or elevation. Examples of these applications include boiler-feed water pumps, submersible pumps, above-ground pumps that operate with a high static-dominated pressure level and pumps that lift water to fill a reservoir. In these applications, ASDs may not achieve overall energy savings as a control option; however, they may make sense where the ASD is used to address water-supply demand that modulates continuously.

2.Poor sequencing: Some motor-driven equipment is designed and installed with sequenc-ing in mind. For example, cooling towers or evaporator fans are often set up in lead-lag fashion, a good practice where each fan immediately turns on and off based upon demand. Adding an ASD to the existing lead-lag configuration may consume more energy because the drive programming algorithm could activate multiple fans to start earlier and operate longer at a higher energy-consumption level. 

3.ASD functioning as soft starter: An ASD used solely to eliminate equipment failure at startup, or to reduce demand charges by soft-starting motors does not necessarily save energy. Soft starters can provide this functionality.

So What’s The Bottom Line?
In the right applications, drives can offer solid benefits. Be sure to ask your utility representative or a motor and drive expert about other application-specific considerations.

More information on motors and drives is available through the MDM campaign at www.motorsmatter.org/resources/asds.html. LMT

The Motor Decisions Matter (MDM) campaign is managed by the Consortium for Energy Efficiency (CEE), a North American nonprofit organization that promotes energy-saving products, equipment and technologies. For further information, contact MDM staff at mdminfo@cee1.org or (617) 589-3949.