Recently, we covered the three main considerations to weigh when buying a rectifier. Now, let’s dive a little deeper and shed some light a common debate in the world of rectifiers. We’re speaking of course about SCR vs SMPS rectifiers. As you will see, the ultimate differences and similarities may surprise you.
First, let’s run through the characteristics of these two classes of rectifiers. A silicon-controlled rectifier (SCR) is a semiconductor power supply that meters electricity by opening electrical “valves,” working together as a team, to rectify electricity. The longer the “valve” is open, the higher the voltage leaving the rectifier will be.
SCR rectifiers are variable voltage DC power supplies that are low frequency, high ripple systems. These systems are rugged and have a history of durability in the market – a number of our customers have systems running after 40 or 50 years. They regulate and react steadily, carry a lot of power in big copper windings, and are fairly easy to troubleshoot with large, easy-to-identify components.
A switchmode power supply (SMPS) by comparison is the relative new kid on the block. Switchmodes are an electronic power supply that uses a switch from AC to DC, back to AC, then once again back to DC. This is all done at high frequency allowing for the internal parts to be smaller. They utilize a high primary voltage—where 480VAC input units typically see 700VDC—switching voltages inside the transformer.
This makes failure a bit more destructive and violent due to smoke generation. With smaller parts, tighter windings, smaller footprints and an even tinier tolerance, these precision pieces of equipment offer good space efficiency per watt, modern computer interfaces and intuitive controls.
Switchmodes have been around for roughly 40 years. Traditionally, they were used in smaller current applications and it wasn’t until 10 years ago that large-scale switchmodes were successfully deployed. Today, the technology has become much more widely deployed, but as Dynapower’s Repairs Supervisor Remington Schieffer is quick to point out, it wasn’t the increase in technology that spawned switchmodes.
“In the industrial platform, it was the rise of quality, precise manufacturing processes that gave rise to modern day switchmodes, and the globalization of trade,” he states.
SMPS vs SCR: The Basics
Before getting more granular with each type of rectifier, we’d like to dispel a popular myth regarding SMPS vs SCR: Neither type of unit is better than the other. Both SMPS and SCRs perform identically well in electronic regulation, and technology and manufacturing techniques have allowed them to perform to the same level of process performance. SMPS and SCRs can both regularly regulate to the 1000th of a volt and even more, there is absolutely zero difference between them in their ability to withstand exposure to corrosive process chemicals.
What a lot of people don’t factor in is environmental degradation on equipment. According to Schieffer, when you look at total repair costs coupled with chemical exposure over the lifetime of both SMPS and SCR rectifiers, the investments with each type are within 1 percent of each other. This figure is based on customer feedback over the last several years, in the American market, across 13 different rectifier manufacturers.
Regarding the repair scope, the fix is often the same principle whether it’s SCR rectifiers or rectifiers used for welding. “When you have to change a 700 dollar SCR once in 15 years or you have to change ten 70 dollar diodes in batches of five, twice in 10 years, the equipment cost is the same in repairs no matter what you buy,” Schieffer points out.
“What changes is the frequency and the manpower that goes into it. Ultimately, both SMPS and SCR rectifiers are equally susceptible to the environment and misuse by a cost perspective. When you compare the systems financially, they are almost identical in the rate of degradation per dollar to fix it,” Schieffer states.
So, the main questions you should be asking yourself when considering SMPS vs SCR rectifiers are:
- What do you have the real estate for?
- What kind of upkeep do you have the manpower and time to allocate?
- What kind of chemistry will the rectifier(s) be exposed to?
- What is the flexibility in terms of being able to maintain and work on your rectifier(s)?
- What is your design and does it match what the surrounding production facility requires?
SMPS vs SCR: Diving Deeper
Essentially, SMPS units take up less real estate, smaller consumption of cooling resources and tend to—on average—perform the full range of operation at a higher electrical efficiency than SCR rectifiers. The lower you turn a SCR rectifier in voltage, the higher chance the machine will not convert AC to DC as efficiently as the same rated, competing switchmode. When you increase the voltage in an SCR rectifier, the better chance it has of performing more efficiently than the same rated, performing switchmodes.
Furthermore, because it takes more switchmode units to match the regular output amperage of an SCR, you now have more points of potential failure as proven in the Weibull Analysis—which is a methodology used to determine reliability characteristics from field or laboratory test data.
Switchmodes run at higher frequency, allowing a tighter configuration and smaller weight while delivering the same overall power as an SCR rectifier. Those smaller components tend to fail more frequently due to relative size when exposed to chemistry or moisture than the larger SCR componentry.
But interestingly, the rate at which both SCR and SMPS rectifiers break down is much more dependent on environmental factors rather than in how the rectifier is used or the time that the machine is exposed to the environment. In fact, 93% of rectifier repairs are based on environmental abuse instead of equipment misuse—such as rack crashes, improper setpoints, or chemistry exposure.
The Importance of Quality Rectifier Cabinets
The real value in long-term care of your equipment starts with a good cabinet. If you can control where the chemistry goes—if you can control the environment exposure—while still cooling the equipment, you are good. For instance, as Schieffer points out, “A NEMA 1 cabinet is just door panels and when air cooling it’ll pull air from everywhere, which allows chemistry to invade the equipment from every angle.”
When evaluating any type of rectifier, it’s critical to realize that not all cabinets are created equal. Ideally, your cabinet will use gasketed air paths and pull the environment’s ambient air used through a designated heat sink channel for cooling.
If the unit has all its components mounted on the opposite side of that heat sink, then it won’t be exposed to outside ambient air. In such a case, your rectifier will keep running for as long as the heat sink is the barrier between your rectifier components and the potentially hazardous ambient environment.
“You’re choosing the heat sink to be the sacrificial component of your rectifier over the actual electrical components. It is designed for protection and controlled exposure,” Schieffer adds.
One thing that customers should be aware of when it comes to SMPS is the learning curve involved with these systems. The equipment is much smaller and has tighter arc paths with larger voltage potentials inside. The opportunity for a breakdown is different, and the equipment follows a different preventative maintenance path to keep it running. It is similar in how the maintenance on your sportscar is different from the maintenance on your trusty diesel pickup truck.
“People don’t understand it’s a different animal and if they are misrepresented in the sale of the rectifier, they will have nothing but problems down the road,” Schieffer warns.
Chemistry, Real Estate & Maintenance
It’s important to ask yourself what chemistry is being used in your process. What environmental hazards does your facility have? For instance, if you use hydrochloric, sulfuric, nitric, or hydrofluoric acids, which is a chemical family typically used in acid etching and surface prep for plating, painting, or power coating—you need to consider that acids destroy aluminum and copper.
So, if you install a new air-cooled rectifier the heat sink and bussing is going to get chewed up in 6 to 8 months. In that case, a water-cooled NEMA 4X or IP56 fully sealed rectifier may be your best bet.
SCR parts are large, heavy, and costly. The parts used in SCR rectifiers are subject to long lead times and market trends. SMPS parts are smaller, easier to store due to their size, but also easier to misplace. It is typically easier to have spare switchmode parts on your shelves due to their overall lower price and material cost.
If your rectifier manufacturer forces you to carry a whole module rather than just the parts needed to fix a bad module, then this trend may not be true.
“It has nothing to do with one being better than the other,” reaffirms Schieffer. “They are just a different tool to consider in your arsenal.”
SMPS vs SCR rectifiers have very different resource requirements. SCRs are more resource hungry both in footprint and real estate. SCR rectifiers require strong weight supporting platforms and wide installation spaces. Air cooled SCR rectifiers require room for forced air cooling to escape to. Water cooling SCR rectifiers require discharge to be waste treated. Sometimes the production real estate becomes expensive to support.
“There is no single point of access, so you need to be able to get three different angles to perform maintenance work on an SCR unit. If you can’t get into it, you can’t fix it if it breaks,” Schieffer states. You will need at least two feet around the unit so there is enough space for a technician to perform maintenance checks and complete repairs.
Planning for maintenance is very important when searching for a new or refurbished SMPS or SCR. “Don’t buy new without considering maintenance,” Schieffer says. “At Dynapower, we understand because we are aftermarket service providers. We know some other sales people avoid talking about life cycle of the equipment because it’s a tough subject. We embrace the need and try to educate our customer base – whether you have our equipment or not.”
Dynapower SMPS and SCR Rectifiers
At Dynapower, our SCR and SMPS units are designed to be easily accessible and serviceable for total cost of ownership. We offer rectifier training classes with hands on repair application, so you become comfortable with working on each of the topographies of rectifiers offered in the market.
“For SCR components, our documentation and record keeping goes back 40 years and is second to none in the industry,” Schieffer states. “Most of our SCR equipment is designed to be serviceable without lifts or cranes for general repairs and maintenance.”
Dynapower switchmodes offer some of the best long-term value in the industry. 100 percent of the power electronics can be changed and replaced, and a trained technician with a dedicated repair area can perform these repairs in the comfort of their own facility.
“With our competitor’s equipment, when it’s done you have to throw out all of the frame and useable materials,” Schieffer illustrates. “It’s not meant to be repairable. We can change out the internal components in our switchmodes and you can have brand new power electronics without scrapping the case, interface, or controls—which makes us unique in the marketplace. It’s a quality unique to Dynapower’s equipment that saves our customers money each year they have it in operation.”