In a typical plant, pumps often are controlled by semiconductors such as microprocessors, power electronics and variable frequency drives (VFDs) to digitally link assets and provide better process control. By using “smart” centrifugal pumps with VFD controls, plant operators can vary the pump and impeller speed, allowing the motor output to match the system requirement for improved system performance. A VFD operates by converting alternating-current (AC) power to direct-current (DC) power, which is then changed back to a pulse width modulated AC waveform in the form of variable frequency and voltage to the induction motor. Feedback from the VFD provides operators with valuable data to determine pump performance. In essence, VFDs give plant operators control of the pump’s speed and output by changing the voltage and frequency to a pump’s motor. Modern VFDs can produce accurate and reliable speed and torque feedback using sensorless control algorithms—offering improved pump reliability, extended product life and reduced energy and processing costs.
VFDs and sensorless technologies are often the best solution for controlling and protecting a centrifugal pump system against failure.
10/22/2014
Figure 1. A typical VFD schematic (Courtesy of ITT PRO Services)
Figure 2. Speed control (Courtesy of ITT PRO Services)
Figure 3. Torque control (Courtesy of ITT PRO Services)
By measuring the torque and speed data that VFDs provide, operations can achieve sensorless flow control. With this information, each pump can be calibrated for optimal performance. Modern VFDs offer advanced methods of torque control for more accurate pump flow management through the induction motor.
Figure 4 is an example of how torque control can be used for a parallel multi-pump system. To achieve a combined flow rate of 1,500 gallons per minute (gpm), each pump shaft must be synchronized for a torque output. This output is related to the pump’s flow rate. In this example, one pump has wear of 5 percent and must, therefore, run at a faster speed. However, the torque output remains balanced between the two pumps. The logic that controls the VFD’s output uses the torque accuracy to control the pump’s output in flow rate.
Figure 4. Torque control for a multi-pump system (Courtesy of ITT PRO Services)
Figure 5. The design (green) and typical performance (red) curves of the pump system (Courtesy of Paul Barringer)
By incorporating the sensorless flow control, the pumps maintained operation at or near their BEP, providing lower energy costs and higher reliability. Since the addition of the smart VFD, the pumps have consistently achieved a flow accuracy of plus or minus 3 percent, and the mining operation has significantly increased meantime between failures. Development of smart pump logic for the VFD based on applications similar to this one made these results possible.
Figure 6. Total cost of pump ownership (Source: Pump costs from a top-10 global chemical manufacturer, FY 2006)