Pump applications account for a significant portion of worldwide motor-driven electricity usage with estimates as high as 20 percent. However, conventional alternate current (AC) and direct current (DC) motors currently in use are unable to meet the speed ranges (especially low speeds) of many positive displacement pumps that require the use of a gearbox. Pump users must live with the high cost, bulk and complexity of the gearbox to achieve lower pump rotational speeds at the torque required. Substantial time is dedicated to optimizing the motor and speed-reducing gearbox combination to be more effective and efficient for pumps. While this combination provides an overall "Band-Aid" solution for broad pump ranges, the gearbox masks the basic underlying issue that today’s motors are unable to meet the speed and torque requirements on their own. Nonetheless, it has been the best practical solution to date — until now. A major new advancement in direct-drive motor technology enables the motor alone to meet the low-speed/high-torque requirements for many positive displacement pump applications, providing a huge step forward for the pump industry.
These advanced, direct-drive motors provide turndown ratios that are far greater than AC-induction motors. While variable frequency drives are typically limited to a 10-to-1 speed range, forcing pump system designers to use several gear ratios to provide the range of pump speeds shown in their catalogs, these direct-drive motors achieve turndown ratios greater than 100:1 (5 to 500 rpm), enabling a single direct-drive motor to replace several AC gear motors. The broad speed range capability not only allows a single, affordable motor to address a wide range of products, but it also reduces inventory costs.
This new motor design has been used in other industrial applications from large fans to conveyors over the past five years, validating performance in harsh environments. These direct-drive advancements were only recently applied to pumps, which have similar high-torque/low-speed requirements.
The key technology lies in the electromagnetic design of the motors. These direct-drive motors use high pole count and a unique coil design to deliver five to 10 times more continuous torque by mass compared to conventional AC or DC motors. Compared to conventional motors, the pole count in the new motors is typically 10 times higher, enabling efficient, low-speed torque production, an area in which the performance of conventional motors suffers. The new motors also use a simple, low-loss coil design, which minimizes heat and converts more of the incoming electrical power to mechanical output power (see Figure 2).
These design advancements provide a direct-drive alternative for pump types that require high-duty cycle operation at low rotation speeds such as rotary and reciprocating positive displacement pumps. The direct-drive solution is also significantly smaller and lighter (up to 80 percent lighter than AC gear motors), and is more cost-effective than the incumbent. Furthermore, eliminating the gearbox reduces noise and eliminates maintenance and reliability issues related to gears.
While the technology is relatively new to the pump industry, various positive displacement pump manufacturers have already started implementing these affordable direct drives in their pumps. As shown in Figure 1, a peristaltic pump manufacturer is able to replace a 45-pound (lb.) (20-kilogram [kg]) AC-induction motor, gearbox and controller with a 10-lb. (4.5 kg) direct-drive system. This results in a 78-percent weight reduction and a more compact system, occupying less than half the volume than the AC-geared system. In turn, it provides increased portability and flexible packaging for the system integrator.
As an added bonus, the electromagnetic design and reduced system losses provide higher efficiency throughout the speed range and especially at lower speeds. AC-induction motors produce their nameplate efficiency within a narrow speed and load range. As load or speed moves away from nominal, the motor’s efficiency drops dramatically. Gearboxes add further losses and, multiplied together, the system efficiency often drops below 50 percent, meaning most of the incoming electrical power is wasted as heat. Direct-drive motors operate efficiently over a broad speed range, often over 92 percent, which reduces waste heat by up to 80 percent. Figure 4 shows the energy consumption of the new direct-drive solution versus the AC-geared incumbent over a range
With drastically different requirements across the industry, it is impractical to have a single optimal motor solution across all pump applications, and many parameters need to be considered when choosing a motor. However, these direct drive advancements fill a long-standing gap by meeting high-torque demands while running at low speeds. These direct-drive motors not only provide a solution under these conditions, they thrive in them.
- EERE 2004. https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/variable_speed_pumping.pdf
Scott Reynolds is director of engineering for Electric Torque Machines (ETM). He has 30 years of experience in engineering, technology development and product commercialization in the motors, electronics and medical device markets. Reynolds is co-inventor on several patents in the field of motors, microelectronics and a mechanical engineering graduate of the University of Arizona. He served as a design engineer and lead engineer at Texas Instruments — Defense Systems and Electronics Group in Lewisville, Texas, and as a process engineer, platform technologist and technology leader at W.L. Gore and Associates Inc., Electronics Products Division (Phoenix, Arizona), Medical Products Division (Flagstaff, Arizona). As director of engineering at ETM, Reynolds manages ETM’s product commercialization; intellectual property; and leads technical sales activities of ETM’s proprietary direct drive motors. Visit etmpower.com to learn more.