If you’ve ever spent time around industrial motors or HVAC systems, chances are you’ve heard of VFDs — short for Variable Frequency Drives. As a VFD supplier, we see these devices everywhere: they’re the workhorses that adjust motor speed to match load demands, cutting energy costs and extending equipment life. But here’s the thing: for all their benefits, VFDs come with a set of harmonic issues that can sneak up on you if you don’t know what to look for. Today, we’re breaking down exactly what those harmonics are, how they cause problems, and what you can do to avoid them — straight from the team that works with VFDs day in and day out. Variable Frequency Drive
First, let’s keep it simple: what even are harmonics in the context of VFDs? Your standard electrical grid sends power as a smooth, sine wave — think of it as a gentle, rolling hill. VFDs don’t just take that sine wave and pass it through; they convert AC power to DC (direct current) and back to AC at whatever frequency the motor needs. That conversion process chops the sine wave into chunks, right? Those chunks aren’t perfect, smooth hills — they’re jagged, squared-off shapes. When you run that squared-off power back into the grid, it creates extra waves called harmonics. These are basically multiples of the original 50 or 60 Hz grid frequency, and they can throw the whole electrical system out of balance.
Not all harmonics are bad, but the ones from VFDs — specifically odd-order harmonics like 3rd, 5th, and 7th — are the ones that cause most headaches. Let’s talk about why that matters. Harmonics don’t just disappear; they travel through your wiring, transformers, and other equipment like tiny, annoying power disruptors. One of the most common issues we see is overheating. If you’ve got a transformer near a VFD, harmonics add extra current to the windings. That extra current turns into extra heat, even if your motor is running at normal speed. Over time, that heat eats away at insulation, shortens equipment life, and can lead to unexpected downtime — which is a big deal for any industrial operation.
Then there’s the problem of power factor. Power factor is a measure of how effectively you’re using electricity. When harmonics roll in, they mess up your power factor, making it harder for your system to do useful work. A bad power factor means you’re paying for more electricity than you actually use — your utility company might even hit you with penalties if it’s bad enough. We’ve had customers come to us saying their utility bills jumped 20% after installing a VFD, and when we checked, harmonics were the culprit. It’s not that the VFD was bad; it was that no one warned them about the power factor hit from unfiltered harmonics.
Another issue is equipment interference. Harmonics can cause all sorts of weird glitches: lights flickering, control systems acting erratically, even sensitive electronics like PLCs (programmable logic controllers) misbehaving. We had a customer in the food processing industry last year who thought their production line was haunted — one day the conveyors would stop for no reason, the next the packaging machines would go haywire. Turned out, their VFDs for the cooling system were pumping out harmonics that were bleeding into the control wiring. It took us a day to track it down, but once we added a simple filter, those glitches were gone. That’s the thing about harmonics: they’re not just a technical problem — they can mess with your whole operation.
Wait, let’s be clear: VFDs aren’t the only source of harmonics. Old fluorescent lights, welding equipment, even computer power supplies can contribute. But because VFDs are so widely used now — more and more facilities are switching to them to save energy — they’re a top source of harmonic pollution in modern grids. The good news is, there are ways to manage it, but first you have to understand where the harmonics are coming from.
So, why do VFDs generate so many harmonics? Let’s get a little deeper into how they work. Most basic VFDs use a 6-pulse rectifier to convert AC to DC. That means six diodes switching on and off each cycle, which creates harmonics at 5th, 7th, 11th, 13th, etc. — the ones we talked about earlier. A 6-pulse rectifier produces a lot of 5th and 7th harmonics, which add up to about 30-35% total harmonic distortion (THD). THD is just the ratio of extra harmonic current to the total current, and most electrical codes (like IEEE 519) limit THD to 5% or less for most commercial and industrial systems. If your VFD is pushing 30% THD, that’s way over the limit — and that’s when problems start.
What happens when you exceed THD limits? Let’s circle back to transformers. Harmonic current causes stray losses in transformer windings and cores, which means the transformer has to work harder to move power. That leads to overheating, which as we said, shortens lifespan. We’ve seen transformers fail early because of VFD harmonics — some as much as 10 years before their expected life. For a factory, replacing a transformer mid-cycle is a costly headache, way more expensive than addressing harmonics upfront.
Motors aren’t immune either. Harmonics cause extra losses in motor stator and rotor windings, leading to higher temperatures. That can reduce motor efficiency by 2-5%, which adds up over time, plus it shortens motor life. We’ve had customers who installed VFDs and thought they were saving energy, but the extra motor losses from harmonics ate up a lot of those savings. It’s a trade-off that you don’t want to miss.
Then there’s neutral conductor overheating. Odd harmonics — especially the 3rd harmonic — add up in the neutral wire of three-phase systems. Three-phase power usually has a neutral that carries very little current, but if there are 3rd harmonics, they don’t cancel out like other harmonics. Instead, they add up, so the neutral current can be equal to or even higher than the phase current. That extra current overheats the neutral wire, which is a fire hazard. We’ve seen cases where a neutral wire melted right through because of unaddressed 3rd harmonics from multiple VFDs. Scary stuff, right?
Now, as a VFD supplier, we don’t just tell you about the problems — we also tell you how to fix them. Let’s run through the most common solutions we recommend, based on what works for our customers.
First, if you’re installing new VFDs, go for a higher-pulse rectifier. Instead of 6-pulse, you can get 12-pulse or even 18-pulse VFDs. These reduce THD to around 10% or less, which is way closer to the IEEE 519 limit. 12-pulse VFDs work by using two sets of diodes, offset so their harmonics cancel out. It’s a simple upgrade that can cut harmonic distortion by half or more. We’ve seen customers who switched from 6-pulse to 12-pulse see their THD drop from 32% to 8% — that’s a huge difference.
Second, add harmonic filters. There are two main types: passive and active. Passive filters are basically LC (inductor-capacitor) circuits tuned to target specific harmonics, like the 5th or 7th. They’re cheaper and work well if your harmonic load is predictable. Active filters are more high-tech — they monitor the harmonic current in real time and inject a canceling current back into the system. They’re better for variable loads or multiple VFDs, but they cost more upfront. As a supplier, we help customers pick which filter makes sense for their setup, not just push the most expensive one.
Another option is line reactors. These are simple inductors you install between the grid and the VFD input. Line reactors limit the rate of current change in the VFD, which reduces harmonic distortion and also protects the VFD from voltage spikes from the grid. They’re cheap, easy to install, and can cut THD by 5-10% right off the bat. A lot of our customers start with line reactors because they’re a low-cost fix, then add other solutions if needed.
Then there’s proper installation and wiring. A lot of harmonic issues come from messy wiring. If your VFD cables are too long, or you run power and control wiring in the same conduit, you can get harmonic interference. We always advise our customers to keep VFD power wiring separate from control wiring, use shielded cables, and keep cable runs as short as possible. Even small changes here can make a big difference in how harmonics behave.
Wait, but let’s be real — you can’t eliminate harmonics entirely, right? They’re a side effect of how VFDs work. The goal is to keep them within code limits so they don’t cause problems. That’s where THD testing comes in. We recommend having a harmonic test done before installing VFDs, and then periodically after, to make sure everything is within specs. A lot of customers skip this, and then wonder why they’re having issues down the line.
Let’s talk about a recent example to make this concrete. A metal stamping plant we work with installed six new VFDs on their press lines last year. They went with basic 6-pulse VFDs, didn’t add any filters or reactors. Within three months, their control PLCs started glitching, and one of their transformers was running 15 degrees hotter than normal. We did a THD test and found their total harmonic distortion was at 28% — way over the 5% limit. We recommended adding three active filters (one for every two VFDs) and line reactors. After installing those, their THD dropped to 4%, the transformer temperature went back to normal, and the PLC glitches stopped. They ended up saving money on repairs and avoided a potential transformer failure — that’s the kind of outcome we’re all about.
Now, I know some of you might be thinking: “I just want to install a VFD and not have to deal with all this.” We get it. VFDs are supposed to make your life easier, not more complicated. That’s why our team at the VFD supplier side doesn’t just sell you a drive — we work with you to figure out the right setup for your specific system. We’ll ask about your load size, your existing equipment, your utility’s THD requirements, and recommend the right combination of VFD, reactors, filters, and wiring to keep harmonics in check.
One common mistake we see is customers buying the cheapest VFD they can find, without considering harmonic performance. Budget VFDs often use basic 6-pulse rectifiers with no built-in harmonic mitigation, so they’re more likely to cause issues. Spending a little more upfront on a VFD with better harmonic specs, or adding mitigation devices, saves you a lot in downtime and repairs later. We’ve had customers switch from cheap VFDs to our mid-range models with built-in line reactors, and they haven’t had any harmonic issues since.
Another point: harmonics can affect other utilities on your grid too. If you’re in a commercial building, your VFDs might be causing problems for the office next door — flickering lights, noisy computers, etc. That’s a quick way to end up in a dispute with your building manager or utility. Addressing harmonics ensures you’re not a nuisance to everyone else on the grid.
Let’s wrap this up. Variable Frequency Drives are an incredible tool for saving energy and improving equipment efficiency, but their harmonic output is something you can’t ignore. The key takeaways are: harmonics from VFDs can cause overheating, equipment damage, higher utility bills, and operational glitches; you can’t eliminate them, but you can manage them with higher-pulse drives, filters, line reactors, and proper installation; and it pays to plan for harmonics upfront, instead of fixing them after problems arise.
If you’re installing new VFDs, or you’re having issues with existing ones that you think might be harmonic-related, we’re here to help. Our team can run a quick assessment of your system, recommend the right solutions, and make sure your VFD setup is running smoothly without those annoying harmonic headaches. Reach out to our team to discuss your specific needs and find the right VFD and harmonic mitigation plan for your operation.
Three Phase Wall Mount Energy Meter References:
- IEEE Standard 519-2014, Recommended Practice and Requirements for Harmonic Control in Electric Power Systems
- NEMA Standard MG 1-2021, Motors and Generators, Section 12: Adjustable-Speed Drive Systems
- Electrical Construction & Maintenance, “Harmonics: Causes, Effects, and Solutions for Industrial Systems” (2022)
- VFD Basics: Harmonic Distortion and Mitigation, Power Electronics Technology (2021)
Zhejiang Liuyi Electrical Technology Co., Ltd.
Zhejiang Liuyi Electrical Technology Co., Ltd. is one of the most professional variable frequency drive manufacturers and suppliers in China. We have world-leading production equipment and strong manufacturing capabilities. Please rest assured to wholesale high quality variable frequency drive made in China here from our factory. Customized orders are welcome.
Address: No. 16 Zhengshun East Road, Xiangyang Industrial Zone, Liushi Town, Leqing City, Wenzhou City, Zhejiang Province
E-mail: hazel@chliuyiele.com
WebSite: https://www.chliuyiele.com/
