Wire-to-water efficiency is important. A pump and motor’s ability to deliver the water output that aligns with the electrical energy input depends as much on the way the pump, drive and motor are installed as it does on the equipment itself.
The fact is, a motor’s efficiency rating is determined by tests required by the Department of Energy (DOE) to show compliance to motor efficiency regulations. Out in the real world, the output it produces can experience losses at multiple points between the initial connection and the final flow of the pump.
To realize optimal wire-to-water efficiency, steps must be taken at each of those points to mitigate issues that lead to system inefficiency. This requires a systems approach to installation that considers everything from the proximity of the motor and drive, to the filters, grounding and other steps that can be taken to avoid damaging effects on the installed system and loss of efficiency. Below are key factors that affect motor and overall system efficiency, plus how to mitigate these factors.
Power quality issues may be present in any installation. The power quality entering the drive or motor affects not only motor efficiency, but the overall system efficiency. Prior to installation, it is important to assess the quality of the power that will enter the motor, making sure voltage and current are within acceptable tolerances and that harmonics comply with Institute of Electrical and Electronics Engineers (IEEE) 519 requirements. Standard allowable power quality variations can be found in National Electrical Manufacturers Association (NEMA) MG-1. Be sure to discuss any variations found outside the standards with the motor manufacturer. Some problems may have relatively simple fixes. If power is to be supplied by a generator—either continuously or as a backup power source—be sure to consult with the motor manufacturer on any special power quality requirements.
Power Cable Selection
The type of cable used between the variable frequency drive (VFD) and the motor plays a critical role in reducing harmonics and electromagnetic interference (EMI). These cables should be shielded and specifically rated for use with a VFD. Check with the VFD, motor or cable manufacturer to ensure that cables are the recommended size for the system’s voltage and current limits.
Resist the temptation to use smaller or improperly shielded cables to save money. An improperly sized cable can damage a motor. The correct cable will always cost less than a replacement motor and is worth it in the long run.
The distance between the motor and drive matters, too. A drop in voltage between the VFD and motor can cause a drop in motor efficiency. The longer the cable, the larger the voltage drop. While it is common to locate a VFD on one floor and the motor on another, it is wise to locate them as close to each other as possible. There is equipment that can aid in power quality when long cable distances are required. Be sure to discuss these options with your VFD and motor manufacturer if required for the installation.
Motor efficiency can also be impacted if the entire system is not grounded correctly. In addition to the motor being grounded to true ground, it should also be grounded back to the VFD to alleviate high frequency noise that can be caused by the VFD if not installed correctly. The motor should be grounded back to the drive using a braided type of grounding wire that is, at minimum, the same size as a single power lead and runs in the same conduit as the power leads.
Drive Efficiency Over Entire Speed Range
The primary benefit of a variable speed system is its ability to adjust output to fit demand. To deliver this benefit, it must be set up to perform efficiently across its entire speed range.
Every drive has a unique efficiency curve based on its specific drive components and settings. Even drives from the same manufacturer can cause a motor to yield different efficiencies across its speed range. Variable speed makes it possible to move along the system curve to meet demand. When the system needs less power, it uses less power.
Switching frequency, however, can impact efficiency. A higher switching frequency in the VFD can increase motor efficiency, but it can also lower VFD efficiency and induce cable losses. System efficiency depends on multiple components that produce different outputs when they interact.
To achieve optimal motor efficiency, a drive must also be set up to respond correctly to changes in load. A drive should be set up to read system demand and adjust motor speed to meet that demand using a feedback system. Otherwise, it will operate at a constant speed, producing a constant output and creating a system that is less efficient than a regular constant speed system without a VFD. When not used to adjust speed to the pump’s best efficiency point (BEP), a drive only adds losses to the system.
Can Wire-to-Water Efficiency be Guaranteed?
In a perfect world, the wire-to-water efficiency of a pumping system would be simple to guarantee. But no pumping system operates in perfect conditions.
That is why original equipment manufacturers (OEMs) have historically had to compare the efficiency of individual parts—pumps, drives and other system components—one by one. Motor efficiency has traditionally been rated using the specific test criteria of published standards to have something that is repeatable and accurate. Constant speed motors that do not require VFDs use the testing criteria identified in IEEE 112 Test Method B.
Variable speed motors with VFDs must meet the standards in IEC/TS 60034-2-3, which is a new standard released for the European Union.
Values from these tests can be used to directly correlate with a pump’s efficiency at set speeds, without factoring in the efficiency lost due to power quality, harmonic distortion, switching frequency and other factors that can impact overall system performance.
But that is changing. Utilities and others seeking wire-to-water efficiency guarantees can now look to the DOE’s Pump Efficiency Index, which makes it possible to evaluate a pump/motor combination, or a pump, motor and drive control, at multiple points along an efficiency curve.
These added standards make it possible to compare one system’s efficiency directly against another. A system with a high-performance drive or a flatter efficiency curve can receive a higher Pump Efficiency Index rating than one that depends on only an efficient pump.
One limitation: the DOE standards are written around laboratory testing. Factors such as power quality and installed system design will still impact system efficiency.
Utilities may offer rebates for systems that meet these higher standards or penalize those that do not. Because of the variations in the field installations, however, equipment manufacturers charged with penalties for systems that do not meet efficiency standards should have the right to perform verification tests using published standards in controlled settings to get reliable and repeatable readings. It is unreasonable for them to be held to values taken in the field that can fluctuate depending on the installation or the time of day it is and power demand at the time of the measurement.
The bottom line is this: there are many pieces to the pump system efficiency puzzle. Select and assemble them correctly, and output will come far closer to matching input.