Pumps & Systems, July 2008
In the June 2008 issue we investigated the importance of identifying the actual operating point of a pump in an “as built” system or one that has changed over time. This month, we explore the electrical side of the system and its effect on motor life. To read part one, click here.
Most of us have seen the graph shown in Figure 1, which shows how a motor's various operating characteristics change with a corresponding increase or decrease in supply voltage. NEMA motors are designed to accommodate a voltage variation of +/-10 percent of their nameplate voltage.
Will a variation of this magnitude affect motor life? It depends on the motor loading. For example, the graph shows that a 10 percent decrease in supply voltage will result in a 10 percent increase in current draw. As long as this increase does not exceed the nameplate current, no damage will occur. In fact, a small voltage drop on lightly loaded motors can actually improve efficiency (the Nola effect). However, if the motor is already loaded to its nameplate amperage, a 10 percent voltage drop will definitely adversely affect motor life due to the additional heat that will be generated.
Figure 1
Figure 1 shows a similar increase in starting current when voltage increases by 10 percent. Some stator designs also undergo a much larger increase in full load amps than shown on the right side of the graph. This increase is due to magnetic saturation and can be even greater on lightly loaded (<50 percent) motors. In the end, the motor's service factor (SF) defines its ability to handle the additional current due to voltage variation (or varying loads). SF is intended to provide protection against short periods of rises and sags that occur in many circuits. If low or high supply voltage is continuous, it should be corrected at the source. The closer to nameplate voltage a motor is operated, the longer its life.
Voltage Unbalance
A rise or sag in supply voltage may or may not have an adverse effect, but another type of voltage variation will almost always shorten the life of a fully loaded motor - -: voltage unbalance. Voltage unbalance occurs when the individual phase voltages in a three-phase circuit are not equal. The major effect of this unbalance is an increase in stator and rotor I2R losses, which results in an even larger phase current unbalance. A voltage unbalance among the phases of just 1 percent can result in a current unbalance of 6 to 10 percent (even higher when a VFD is involved).
Figure 2 is a close approximation of the percent current unbalance generated by unbalanced phase voltage at various motor loads. The ratio of current to voltage unbalance increases dramatically as motor loading decreases. In the case of a fully loaded motor, a 2 percent voltage unbalance will typically result in a current unbalance of about 15 percent. NEMA motors are designed to accommodate a maximum phase voltage unbalance of 1 percent.
Figure 2
The net effect of voltage and current unbalance is a substantial increase in motor operating temperature. This increase can be estimated by the following equation: % Increase = 2(% Voltage Unbalance)2. For example, if unbalance is 2 percent, the expected temperature increase would be 8 percent. If a motor normally operates at 130-deg C, a 2 percent unbalance would raise the operating temperature to 140-deg C. This may not seem like a huge increase, but insulation life is reduced by 50 percent for each 10-deg rise in operating temperature. A worst case scenario occurs when there is a combination of significant supply voltage variation and voltage unbalance.
Sources
Unbalanced voltage can originate with the utility or within your own distribution system. (The "Fixes" section will identify a number of the potential causes.) Another source of current unbalance is voltage distortion. This condition, due to the harmonics produced by non-linear loads, is characteristic of solid state devices. These harmonics may not lead to measurable voltage unbalance but can cause a significant current unbalance.
Current unbalance can also be due to problems on the motor side of the circuit that usually do not result in voltage unbalance. Since current unbalance can exist without an accompanying voltage unbalance, an accurate system analysis will require measurement of both voltage and current in the circuit.
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.
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.