Two Steps to Longer Pump Life (Part Two): Voltage Unbalance

Written by Joe Evans, Ph.D.

Page 2 of 2

Measurement and Analysis

For a standalone pump station, perform all of the voltage and current measurements at the control panel. If the pump is installed in a treatment plant or other multi-load/motor facility, measurement should also be performed at the supply's point of entry, major circuit branches and each pump installation.

Perform the following steps with the pump off. On the line side of the contactor, measure and record each phase-to-phase voltage (L1/L2, L2/L3, L3/L1). Calculate the average and identify the voltage with the largest deviation from that average, and then calculate the voltage unbalance. For example, if the measured voltages are 468, 458 and 469, the average is 465 and the largest deviation is 7 (465 - 458). The percent voltage unbalance = 100 X (deviation / average) = 1.5 percent.

Repeat these steps with the pump running at its maximum load (typically the pump on level in a pump down application) and measure and record each phase-to-phase current while the pump is running. Use the same method to calculate the percent current unbalance. Once you have all of this information, begin your analysis.

If the voltage unbalance in a standalone pump station is greater than 1 percent with the pump off, contact the utility as their system is probably the source of that unbalance. If it is 1 percent or less with the pump off but increases to more than 1 percent with the pump running, evaluate your current measurements. Even if it remains at 1 percent or less, evaluate the current readings because current unbalance may exist even if voltage unbalance does not.

If the calculated current unbalance is 2 percent or less, you should see a normal stator life. Current unbalance between 2 and 5 percent can be acceptable as long as the highest current leg does not exceed the nameplate current. In this situation it will be worthwhile to roll the leads (described below) as there are instances when a different lead connection sequence will reduce current unbalance. A current unbalance greater than 5 percent will shorten the life of a fully loaded motor.

If current unbalance is greater than 5 percent or one or more of the legs are above nameplate amperage, you will need to locate the source of the unbalance, which can be accomplished via a technique known as rolling the motor leads. Figure 3 shows a magnetic contactor with incoming power connected at the top (L1, L2, L3) and the pump motor leads connected at the bottom.

july08 pumped fig 3.jpg

Figure 3

In a three phase installation, there are always three possible motor lead connection sequences that will allow rotation in the same direction. Motor lead sequence M1, M2, M3 is the "as installed" connection that was used for the original measurements. Lead sequences M3, M1, M2 and M2, M3, M1 are the new connections that need to be measured.

In each case, all three leads are "rolled" to the next terminal to maintain proper motor rotation. Record the voltage and current for each of the two new connections while the motor is running. If one of these connections reduces current unbalance to 2 percent or less, use that connection. If one does not, use the deviation data to determine the source of the unbalance. If, on the three lead sequences, the leg with the greatest deviation from average remains with the same incoming power lead, the unbalance is a product of the distribution system. If it moves with the same motor lead, the unbalance is on the motor side of the contactor.

Fixes

If the utility is the suspected source of voltage unbalance, there can be many causes. Faulty power factor correction banks, open wye or delta transformers, harmonic distortion and uneven distribution of single-phase customers are just a few. If voltage unbalance is greater than 1 percent with the pump off, involve the utility.

If the voltage unbalance is traced to your distribution system, check all of the system connections since vibration and corrosion can reduce their connectivity over time. A common control panel problem is magnetic contactor wear or corrosion. If there are a number of single-phase loads (fans, welders, heaters, etc.) connected to the system, make sure that they are evenly distributed across all three phases. Also check for runs of unsymmetrical phase wiring. If your facility has a capacitor bank for power factor correction, check the fuses. If harmonic distortion is present, locate its source and install harmonic filters. Existing variable frequency drives may require line reactors.

If the current unbalance is on the motor side, check for poor splices or J-Box connections. If none are found, the problem is probably due to a faulty winding or internal motor connection and is usually not correctable.

Joe Evans is the western regional manager for Hydromatic Engineered Waste Water Systems, a division of Pentair Water, 740 East 9th Street Ashland, OH 44805. He can be reached at via his website at http://www.pumped101.com/. If there are topics that you would like to see discussed in future columns, drop him an email.

Comments (0) Add Comment