Patrick Hogg is application engineering manager of General Industry & Integral Horsepower Pumping for Nidec Motor Corporation. He may be reached at firstname.lastname@example.org. For more information, visit nidec-motor.com.
Power quality and grounding are a couple of things to consider for optimal motor and system operation.
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.