AC/DC and Back: For Things About to Rock, We Condition
Updated: Nov 17, 2020
In a perfect world, whenever any device is powered on, it would receive ideal power free from distortions and interruptions caused by harmonics or other factors. Unfortunately, the world we live in is not ideal and power quality issues can plague any facility, either constantly or periodically. The good news is that we can shape our environment to suit our needs and condition facility power to meet even the strictest of tolerances.
Utility companies are required to deliver voltages of +/- 5% at the point of common coupling (where ownership transitions from the utility to the facility). Although less typical, utilities can deliver power outside of the +/- 5% range, but avoid going beyond the 5% variance due to potential fines they may incur. In regard to power within the facility / building, informational notes for the following National Electrical Code (NEC) articles: Branch Circuits 210.19(A), Feeders 215.2(A)(1), Underground Service Conductors 230.31(C), and Ampacities for Conductors 310.15(A)(1) recommend sizing conductors to limit feeder and branch circuit drop to 5% (maximum drop on feeder or branch circuits should be limited to 3%). It should be noted that NEC informational notes are recommendations, not requirements per article 90.5(C). Therefore, even if best design practice recommendations are followed, voltage drop can amount to a 5% drop within the facility with an additional 5% drop from the utility service delivered to the site. This is important for sensitive equipment, because before power even reaches the plug there is a possibility that power was reduced by almost 10%. Just because you have a Live Wire does not mean that the equipment attached will be happy with it.
If your new medical imaging, manufacturing, or lab equipment is in a facility with existing equipment that is having harmonics issues, or you are considering installing a piece of equipment with strict input power quality requirements, there are many solutions available to you. Uninterruptible Power Supplies (UPS), power conditioners, power/frequency converters, and phase correctors all have benefits that can aid equipment with strict input power quality requirements.
Uninterruptible Power Supply (UPS)
Most people are aware of a UPS’s ability to maintain power despite power outages, but may not know that they also correct power distortions as well, by clamping spikes and limiting sags. UPS’s can condition power so well because they take incoming AC power (illustrated graphically by a sine wave) and use rectifiers to convert AC to DC (turning the sine wave into a straight line on a graph). The DC power from the rectifier then is fed to the batteries to charge them and to an inverter (returning the graph from a straight line to a sine wave) for the AC output.
What if purchasing and maintaining batteries is more of a commitment than you want to make for equipment that may not even require battery back up? Many companies that manufacture UPS’s also offer power conditioners that have the same power conditioning functionality without the need for batteries. Turning a UPS into a power conditioner is as simple as removing the battery portion of the assembly and flashing the firmware to ignore the lack of batteries. The rectifier still converts AC to DC and an inverter shapes its own sine wave converting DC back to AC. The resulting output cuts spikes and lessens minor sags or brownouts that may occur. If a sag or brownout is extreme or if a black out occurs, power conditioners will be unable to offer uninterrupted power since switching to battery power is not an option.
Power conditioners also come in a variety of shapes, sizes, and output capabilities to meet your facility’s needs and offer flexible solutions. Just because the US standard is 480V at 60 Hz does not mean that is what you are locked into using. The beauty of converting AC to DC power and back is that the output voltage waveform including frequency can be controlled. An inverter does not have to output US standard 60 HZ power. So, if you want to use a piece of equipment that will fit your needs but is designed to work 50 Hz, it may be well worth the savings/efficiency/application usage. Fitzemeyer & Tocci Inc. designed a facility that purchased surplus manufacturing equipment from a Korean based company to reduce construction cost. To integrate that equipment into this facility F&T specified a power conditioner that utilized the buildings normal power input at 480V, 60Hz and converted it to 380V, 50Hz that allowed the equipment input it was designed for.
Yet another option applies when a piece of equipment is more concerned with the overall voltage across phases than with how “clean” the power is. There are products on the market that only look at phase voltage and will uniformly distribute voltage across phases. When one phase sags, the other two phases are driven up for the duration of the sag. This allows phase correctors to function like Robin Hood and take from the rich phases to distribute voltage to sagging phase. If phases have the following values; A – 250V, B – 291V, C – 292V, voltage from phases B and C can be distributed to phase A, resulting in A – 278V, B – 277V, C – 278V. The resulting phase voltages may result in a better input for equipment, specifically medical imaging equipment that return less than ideal images when voltage is not consistent across phases.
Maybe you have an MRI that is returning poor images, a piece of manufacturing equipment that is producing items not quite in specification, or a piece of lab equipment that is returning inconsistent or incorrect data, the problem may not lie with the equipment itself. Potential issues caused by harmonics or with spikes/sags in the input power can cause equipment to malfunction or damage the equipment lessening its useful life expectancy. If you are having issues with equipment or want to avoid future issues during the design process, Fitzemeyer & Tocci, Inc. will be happy to work with you to identify the proper solution and implementation to keep your facility from feeling Thunderstruck.
Abdullah Khaliqi, PE, CPQ
Senior Project Manager