Motor Load, Three-Phase Input & Installing VFDs

Q: How does motor load impact the total pump system efficiency? A. Motors, along with pumps, drives, control valves and piping are critical components of pumping systems. It is extremely important to understand the relationship between motor efficiency and load in order to maximize the efficiency of pumping systems and reduce costs. The relationship between motor efficiency and load can be seen in Figure 1, which shows typical motor part-load efficiency as a function of the full load. Generally, at 50 percent load, a given induction motor will operate close to its rated efficiency. Above 50 percent of the full load, the motor will operate at nearly the same percent efficiency. However, reducing the load on a motor below 50 percent causes the motor efficiency to drop more significantly, which therefore reduces the efficiency of the entire pumping system.

The effect of operating a motor at reduced load varies depending on the motor horsepower size, as illustrated in Figure 1. Motors that have higher horsepower (hp), such as the larger 75 to 100 hp motors, experience a less drastic reduction in efficiency when operated below 50 percent of the full load. For smaller motors, such as the 0 to 1 hp motors, the effect of dropping below 50 percent of the full load is much more drastic. For additional information related to pump system efficiency refer to Hydraulic Institute’s online eLearning pump systems assessment certificate training program at pumps.org. Q: How do I calculate the three-phase input power to a motor? To calculate the three-phase input power to a loaded motor, here are three parameters need to be taken into consideration:

• RMS voltage (the mean line-to-line of 3 phases)
• RMS current (mean of 3 phases)
• power factor as a decimal

In the field these parameters can typically be measured directly by electricians using hand-held instruments or determined from the motor characteristic curve (power factor). With these three parameters, the three-phase power input power to a motor can be calculated using Equation 1. Additionally, commercial power meters are available for installation that measure all these parameters, including power factor, to directly measure the three phase power input to an electric motor. For more information related to the calculation of electrical input power in the field refer to HI’s Pump Systems Assessment Body of Knowledge at pumps.org. Q: What should I consider when installing a VFD in a pumping system? Variable frequency drives (VFDs) are electronic devices that control the rotational speed of an alternating current (AC) electric motor by controlling the frequency and voltage of the electrical power supplied to a motor. When properly applied, this reduces the stress on and the energy consumed by the pumping system. As mentioned, VFDs and their controls are crucial elements to the reliability and energy consumption of a pumping system. VFDs can provide controlled start and stop of the pumps and through proper feedback control, change the speed of a pump to match the system requirements. Because a VFD controls the speed of the motor, the system can accommodate varying operating conditions, making the entire system more versatile (see Figure 2). These control functions can include flow control, level control, pressure control, temperature and control. Additionally, VFDs can provide detection of a prevention of upset conditions such as cavitation, pump deadhead and dry running, adding to their benefits. Despite all of the advantages of incorporating a VFD into a pumping system, special consideration should be given when programming and installing these devices. There are many kinds of pumps with different torque requirements, so a VFD must be programmed to provide the correct amount of torque for the given pump system. For example, positive displacement pumps have a constant torque load as speed is reduced, which requires special considerations compared to rotodynamic pumps, which have a variable torque load. Another issue that should be considered is that the speed range increases the chance of a pump-forcing frequency interacting with the natural frequency of the system—a phenomenon known as resonance. These are a few of the considerations in the use of VFDs and variable speed pumping. Specific information on installation considerations for VFDs along with other useful information regarding VFDs and variable speed pumping can be found in the HI guidebooks, “Variable Frequency Drives: Guidelines for Application, Installation, and Troubleshooting” and “Application Guideline for Variable Speed Pumping” at www.pumps.org.