Process control through traditional adjustable speed drives (ASDs) with centrifugal pumps has an inherent problem of attempting to use a linear equation to solve a non-linear problem. Common ASD controller software provides an estimated speed to achieve a desired set point. But when the target speed is reached with this method, the set point is not available because the answer is on an unknown non-linear curve. The software must again guess the speed and decrease the error. This process continues until a reasonable deviation from the set point is met. If one aspect of the system curve changes, the whole process must be restarted. Searching for the speed costs energy, even on a single machine. On a system with multiple machines operating either in series or in parallel, a second and more serious problem occurs\'97balancing the load. Even if the machines are built to the same specifications, differences in motor performance, impeller clearances and other mechanical design issues will change a specific pump's speed. Conventional frequency control operates multiple devices at the same speed. In parallel operation, the pump with the greatest pressure capture most of the output and corresponding amperage while the other pumps use a fairly high amount of power and add little to the total output. There is no way to balance a load using frequency directly, because the smallest of physical differences will cause a shift in power to the device with the highest pressure.
Each adjustable speed drive can find its own set point independently without stressing the entire system.
09/25/2015
Figure 1. Determining how to meet amp draw based on virtual linear pumps (VLPs)(Courtesy of Toshiba)
When the frequency is changed in standard ASDs, the output is non-linear. Energy is wasted searching to find the appropriate speed to satisfy the control loop. Because the software produces a virtual linear performance curve, the first calculation from an external controller will most likely be the most accurate calculation. The software changes the power of the motor to produce the desired pressure or flow. No time or energy is wasted accelerating and decelerating.
Each ASD can calculate what speed to run at in order to independently hold the set point. This would be impossible for separate drives running speed control because each drive would fight the other with hunting errors until the whole system became unbalanced. Because the software provides a virtual line of performance, each ASD can select an individual point that represents exactly the same pressure at flow point as every other ASD. So, the overall control system is greatly simplified.