Worldwide, pumping systems account for nearly 20 percent of the energy used by electric motors and 25 to 50 percent of the total electrical energy used by certain industrial facilities. Significant opportunities to reduce pumping-system energy consumption exist, the most popular being through the application of variable-frequency drives (VFDs).1
This article will explain why VFDs and their cousins—electronically commutated motors (ECMs)—are a valid and cost-effective means of varying pump speed and identify many downstream benefits of their use.
With variable-speed pumping, savings go well beyond energy and include enhanced performance, improved reliability, and reduced life-cycle cost. For new projects, capital cost is reduced through the elimination of valves, starters, wiring, pneumatic lines, and smaller-diameter piping with bypass lines. Additionally, the use of VFDs permits the use of smaller pumps with lower-horsepower motors, ensuring a tangible bottom-line benefit for building owners.
There are several types of variable-speed drives (VSDs). For applications requiring flow or pressure control—particularly systems with high friction loss—the most energy-efficient option is the VFD. The most common form of VFD is the voltage-source, pulse-width-modulated frequency converter (often incorrectly referred to as an inverter). In its simplest form, a converter develops voltage directly proportional to frequency; its job is to control alternating-current-motor speed and torque. For instance, when controlling pump operation for a closed-loop system, a converter compensates for changes in process by changing the power and frequency supplied to the motor and, thus, motor speed.
VFD control of a pump often results in operation at 50 to 75 percent of full speed. As a result, horsepower and current draw are reduced significantly. For example, a 300-hp motor at 60 percent of full speed requires about 20 percent of full load power.
Inherent in VFDs is a soft-start capability that prevents overcurrent and overtorque conditions on startup and avoids pressure spikes (water hammer), reducing the risk of damaged pipes and pumping-system components.
Today, manufacturers offer a wide variety of pumps with on-board VFDs and ECMs. Both technologies offer the intelligence for optimal operation, communication between smart devices, process control, and diagnostics.
“Proportional pressure control is prevalent in sensorless ECM smart pumps,” Steve Thompson, lead project manager for Taco Inc.’s commercial pump line, said.
According to Thompson: “Energy savings and noise reduction are achieved with proportional head regulation that also compensates for varying pressure drops within the system as loads change (Figure 1). In this mode, the pump head changes continuously as the demand for water changes, reducing as the water demand declines and increasing as the water demand rises.”