When to Use Low Voltage vs. Medium Voltage VFDs Complete Guide
Correctly sizing a variable frequency drive for a system is complicated. You consider current, application, elevation, temperature, controls, and many more factors. What makes it even more complicated is when you have a situation where either low or medium voltage VFDs might be a good choice.
Low voltage (under 1000V) have several options you might encounter, but medium voltage systems (1000V and over) have even more choices. How do you move a project forward?
A few things tend to limit your options. Your electrical utility will probably only supply you with a few options in medium or low voltage. The fan, pump, crusher, or other system you’re running likely has a motor standard to it that has a specified voltage.
Even with this smaller range of choices, you may be looking at a system and seeing you have both 480V and 4160V available. You see that many manufacturers like ABB, Toshiba, Siemens, TMEIC, and Allen Bradley supply options for both, and other brands like Mitsubishi supply only low voltage. Which way should you go?
In this guide, we’ll break down motor sizing, cost, installation, reliability, safety, and scalability so you’ll know exactly when to choose low vs. medium voltage drives.
What size motor requires a low or medium voltage VFD?
The first and most important factor to determine whether you should be looking at a low or medium voltage VFD is the motor size. Some sizes are easy to determine: a 20HP motor will be low voltage and 5,000HP will be medium voltage. But not everything is so obvious.
In general, what size motor requires a low or medium voltage VFD? Most systems 500HP and below controlled by a VFD should be low voltage, and most that are 1,000HP and above should be medium voltage.
This isn’t a perfect rule. For example, the Nidec Drive Free Standing (DFS) series is an example of a modular drive that can be paralleled to supply 480V control to systems larger than most – we’ve even installed one that ran a 1200HP / 1500A heavy duty overload system. Even with exceptions like this, these general rules hold for most situations for a few reasons.
On the low horsepower side, a medium voltage drive is typically over-engineered and not cost effective. On the high horsepower side, you start to deal with such large current that it makes sense to move to medium voltage. A 1200HP system at 480V will be around 1500 amps, but at 4160V you’ll only have around 150 amps. The reduction in current affects costs and considerations throughout the whole system, many of which we’ll address later in this article.
One thing to also consider with sizing is how manufacturers create VFDs models. Almost every manufacturer uses standard “frame sizes.” In low voltage these may be familiar horsepower increments like 25, 30, 40, 50, 60, 75, etc. Although amperage and overload ratings may vary, this general scaling of frame sizes is common between most low voltage VFD manufacturers.
This same thing happens in medium voltage, but it’s less standardized and the gaps tend to be larger. A full line of 4160V VFDs may only have six standard frame sizes, ranging from 70 all the way up to 750 amps. If you have a medium voltage system that needs 400 amps but the relevant frame sizes are 350 and 500 amps, you’ll be upsizing to something that has much more capacity than you need. This isn’t always a bad thing as we’ll discuss later, but it is a consideration.
What’s the cost difference between low and medium voltage VFDs?
With the general rules out of the way, you need to consider the middle ground, generally in that 500-1,000HP range. If either way could work, cost is usually the first consideration. Which is the most cost-effective way to get the same result? That answer isn’t as simple as a single number. Two main cost concerns are worth considering. First is the upfront cost, and second is long-term value.
When we’re looking at this middle ground, it’s almost always cheaper to go low voltage when considering only the VFD itself. Medium voltage VFDs are a higher initial investment, but they may reduce the cost of other components, especially when looking at the installation.
Unfortunately, it’s not a linear relationship where you can point to the exact crossover. The more options you need to add to a low voltage VFD and the bigger it gets, the more likely that the upfront cost becomes similar to a medium voltage system.
It can be tough for engineers or operations teams to explain to purchasing teams that a higher upfront cost may be best in the long term. It often helps to get your VFD advisor to meet directly and talk through the long-term maintenance and reliability costs to help buyers see the total lifetime value.
How do installation costs compare between low and medium voltage drives?
VFDs rarely operate on their own. In larger systems like those we’re looking at, you should look at the installation cost. Medium voltage VFDs will typically have a larger footprint and be heavier, meaning the space required and the cost of placing the VFD will be higher.
For larger sizes of low voltage VFDs, however, you may be looking at transformers to step down power coming into the VFD or step up power going out to the motor. Because of higher current, you may also be looking at much larger cables. These costs add up, especially with longer lead lengths. Not only do long cable runs cost more, but you also have the labor to install those, and the infrastructure (like cable trays) to support larger cables.
Many of these factors will be influenced by the power distribution you already have on site. If your site only has medium voltage supplied, it’s likely easier to stay that way. Most medium voltage VFDs already have an included incoming isolation transformer. But how exactly does that work?
Many sites are supplied with a voltage like 13.8kV. A medium voltage VFD already has a transformer built in, so it’s a simple (and often less expensive) change to adjust that incoming transformer to bring in 13.8kV and output a common voltage like 4160V. On low voltage, you would need to supply a separate transformer, bringing that 13.8kV down to something like 480V, then possibly another transformer to step it up if your motor is higher voltage.
The reverse is also true. If your facility only has up to 480V supplied, it could be much simpler to keep everything at low voltage rather than paying to step up the voltage.
While you’re looking at your electrical system, consider your backup power. Many sites will have an emergency standby generator, but these are most commonly available as a low voltage unit. Consider what changes might need to be made to make the generator and VFD compatible in an emergency.
How do low and medium voltage VFDs affect power quality and efficiency?
While considering component pricing, it’s important to look at power quality. Motor systems of this size tend to have a large effect on the harmonics and power factor of a facility, so that needs to be addressed.
Although medium voltage VFDs seem more complex, these issues are simpler in this case. Medium voltage drives almost always meet the IEEE 519 standard based on their topology. In low voltage, you would need to add a filter like a passive harmonic filter or active harmonic filter, or else get a more advanced topology like an 18-pulse VFD. These all add cost, space, and lead time.
VFDs typically improve the power factor of a system, raising it from the motor’s lower power factor (in the 0.7-0.8 range) to somewhere around 0.96. This will make most utilities happy with that specific system, but you may have the chance to save even more money.
If you’re looking at an active front end (AFE) drive, which is much more common in medium voltage, you can get all the way to a unity (1.0) power factor for your motor system, but you also may also be able to correct for poor power factor in the rest of your facility. This is where the extra capacity we talked about earlier comes into play. If you have spare capacity, you can use that for active VAR compensation, reducing power factor penalties and lowering the power consumption you pay for.
You also want to protect your motor from the drive. In most cases, you’re considering the reflective wave/dVdt from the VFD. Medium voltage VFDs usually have an output waveform clean enough for longer distances than low voltage VFDs. On a low voltage system, you’re often looking at adding output filtration like reactors or sinewave filters, again adding more cost, space, and complexity.
Energy efficiency is another huge factor to consider. Both low and medium voltage VFDs can be a big factor in saving energy on a motor system, but they’re not always the same. You should compare the efficiency of the two products but also look beyond the VFD. On larger motors, losses in low voltage cabling and transformers can add up to be significant costs over the long run.
Are low or medium voltage drives more reliable?
As you get deeper into this analysis, the costs become harder to quantify. For example, how much does downtime cost? You have to factor in lost production, standby labor, emergency parts, expedited freight, and other costs. Having a reliable drive can help avoid these costs. Choosing incorrectly, especially in critical applications, can lead to entire plant outages and other issues that quickly justify higher up-front costs.
Look at the mean time between failure (MTBF) and other reliability metrics on all options. We often see that medium voltage VFDs have a higher MTBF because of higher quality parts and tighter controls on production.
Also consider parts availability. Medium voltage components will be more expensive and harder to get. At the same time, relatively few facilities stock additional low voltage parts, where most who buy a medium voltage VFD have a plan and stock spare parts. An expensive part that’s already on site and saves days of downtime may suddenly become the cheaper option.
Also consider technician availability and time to site. Technicians who are certified in medium voltage are less common and typically have a higher hourly cost. At the same time, they likely are more experienced and may be able to save time in the diagnostic and repair based on their experience.
For facilities where downtime costs $100k+ per day, the upfront cost of a medium voltage system could be solved with one avoided emergency shutdown. For those where downtime costs are lower, it’s not unheard of to avoid a major issue every year or two, which still doesn’t take long to justify a more reliable system, especially if you have a team of medium voltage technicians available that you trust, whether on-site or through a third party.
Which is safer: low or medium voltage VFDs?
Although safety is toward the end of this list, it should always be the first consideration. Medium voltage is dangerous if you’re not trained and knowledgeable. Surprisingly, low voltage can be even more dangerous.
The issue often comes from a mix of two things: higher current and familiarity. We’ve already discussed that a similar low voltage system will have higher amperage than a medium voltage system, but the fact that it seems more common can be dangerous.
Too often we hear about technicians who open live 480V panels because they see them every day and forget about the dangers. Much more rarely do we hear about someone opening a 13.8kV panel on a whim. The fact that one is more familiar can be deceiving, making technicians believe it won’t be as dangerous. Whichever way you go, make sure that your system discourages unauthorized use and keeps your team safe.
Are low or medium voltage VFDs more scalable for the future?
Few facilities stay the same forever. Future-proofing your electrical system is essential to reduce long-term costs and headaches. What are the plans for your facility? Is it on its last legs and you just need something to last a few years? Is it in the beginning stages and growth is just around the corner? Do you expect future systems to be larger, smaller, or the same? Preparing your facility for the future means installing systems that are similar and compatible to what you’ll have in the future.
If you anticipate that future systems will be so large they have to be medium voltage, it may be worth it to make your current project medium voltage as well and gain the benefits of the more advanced system.
If your facility only needs to survive two more years and then will be decommissioned, your timeline to recoup higher upfront costs is short enough that low voltage is probably a better option.
Installing systems that are similar to what you anticipate in the future will help you have a common set of spare parts, adding reliability without increasing cost. It also means that your technicians can get familiar with the equipment. We often talk to operations teams who struggle to quickly fix issues because they have a dozen brands of VFDs across both low and medium voltage. The more you can standardize and plan, the easier it becomes to maintain, troubleshoot, and repair.
You may even want to consider functionality like synchronous transfer, which allows one VFD to control multiple motors and sync them to the line as needed. This may mean investment in one VFD could run multiple motors as you expand in the future, but may also require oversizing the initial drive to handle larger motors in the future. This functionality is more common in medium voltage VFDs.
Ask an Expert
Even knowing the factors you should consider, there’s not always an obvious choice. The wrong choice can lock you into higher costs, more downtime, and increased safety risks for years. Our experts deal with these exact situations every day and can help you weigh the options, giving a clear recommendation for how to move forward. Reach out and talk through your system with one of the VFDs.com experts to determine whether low or medium voltage VFDs are right for you.