Due to widely diverse pump torque and speed requirements and new motor technologies entering the market, pump system engineers are increasingly tasked with matching the right motor technology with their particular pumps. Positive displacement (PD) pumps, for example, typically operate at speeds below the capability of conventional alternating current (AC) induction motors. The motors are unable to produce enough torque at low speeds in these applications, necessitating the addition of gears, belts or other speed reduction means. This solution provides the needed speed output, but also adds cost, size, weight and mechanical complexity. The gearbox is a crutch to make up for the motor’s inability to provide the required torque, which not only adds a component to the pump system, it also reduces system reliability. Despite these shortcomings, AC induction motors with gears remained the only realistic economic option for high torque, low speed pump types. Until recently a lack of alternative drive solutions has forced the PD pump industry to accept this added crutch as a necessary burden.
This newer technology excels at the high duty cycle, low speed applications that can cause problems with other motors.
06/26/2018
Image 1. Direct drive motor and pumphead (Images courtesy of Electric Torque Machines)
The good news is that recent advancements in direct drive motor products are now commercially available to address the high torque, low speed pump applications with a compact, cost-effective system. These direct drive motors can provide pump drive systems that are up to 70 percent smaller than incumbents in a lower cost, high efficiency package. Proven direct drive advancements eliminate the need for a gearbox, using the motor alone (no gearbox) to drive the pump. This is accomplished by using a patented direct drive motor design, which drastically changes the size, weight, cost and maintenance requirements of the conventional motor/gearbox combinations. One direct drive motor company has accomplished this through a design that uses 10 times more poles and low resistance coils that drastically reduce losses (see Image 2). This allows much more effective conversion of electrical current into mechanical torque and also produces continuous torque that is 10 times higher than equivalently sized AC induction motors running at nominal speed. The torque improvement is even greater as the motors are operated at lower speeds. These direct drive motors excel in high duty cycle, low speed use, often required for PD pumps.
Image 2. Direct drive motor design vs. conventional motor technology
This direct drive approach also enables much broader turndown ratios and precise low speed operation. Conventionally, variable frequency drives (VFDs) are able to adjust AC induction motor speeds with a 10:1 ratio, which causes pump system engineers to specify a range of fixed gear reductions to fill out the operating speeds needed for their product line. With direct drive, the speed adjustability is essentially infinite from zero to full speed. In one example metering application, speeds have been demonstrated from 0.1 rotations per minute (rpm) to 500 rpm, a turndown ratio of 5,000:1.
Table 1. Legacy gear motor system vs direct drive system