If you’re running a business that relies on AC frequency changers (also called variable frequency drives, or VFDs, for short), you’ve probably dealt with the headache of equipment overheating at some point. I’ve been supplying AC frequency changers to factories, small manufacturing shops, and even commercial facilities for over 10 years now, and one of the most common questions I get from customers is: “How much cooling do I actually need for this thing?” It’s not just a throwaway question—skip the right heat dissipation, and you’re looking at premature part failures, unplanned downtime, and a way bigger repair bill than if you’d planned for cooling up front. Let’s break this down in plain terms, no engineering jargon overload, and tie in what I’ve learned from working with every type of motor application out there.
First, let’s start with the basics: What is an AC frequency changer, and why does it generate heat in the first place? If you’re new to this, an AC frequency changer adjusts the speed of an electric motor by modifying the frequency and voltage of the power going to it. This is way more efficient than running a motor at full speed all the time for tasks that don’t need it—like a conveyor belt that only needs half speed for light loads, or a fan that slows down on mild days. But here’s the catch: No electrical device is 100% efficient. Even the best AC frequency changers lose about 2-5% of the power they draw as heat. For context, if you have a 10-horsepower (HP) motor running at full speed, the frequency changer will pull around 7,457 watts of power (since 1 HP = ~745.7 watts), and waste roughly 150-370 watts as heat. If you scale that up to a 100 HP system, that’s 1,500-3,700 watts of heat just from the drive itself. Multiply that by how many hours a day the drive runs, and that heat adds up fast.
That’s why getting the heat dissipation requirement right isn’t just about “keeping the drive cool”—it’s about protecting the whole system. I’ve seen customers cut corners here before: A small bakery I worked with a couple years back installed a cheap VFD on their dough mixer, stuck it in a cramped electrical closet with no extra ventilation, and within 6 months, the drive’s IGBT (insulated-gate bipolar transistor, the main power component) fried because of overheating. They ended up having to replace not just the drive, but the mixer motor too, which cost them way more than if they’d added a small fan or heat sink to the closet when they installed it.
Now, how do you calculate the exact heat dissipation you need for your specific AC frequency changer? It’s not a one-size-fits-all number, which is why I always tell customers to start with two key factors: the drive’s power rating and its operating efficiency. Most modern AC frequency changers have an efficiency rating between 95% and 98%, so the formula for heat loss is pretty simple: Heat Loss = (Drive Input Power) x (1 - Efficiency). Let’s walk through that with a real example. Suppose you have a 5 HP AC frequency changer powering a pump (you can check out our small motor-focused model at /vector-frequency-converter/economic-frequency-converter/vfd-for-small-motors.html for reference on low-power units). The input power for a 5 HP drive is around 3,728 watts. If it’s 96% efficient, that means it loses 4% of that power as heat: 3,728 x 0.04 = ~149 watts. That’s the amount of heat the drive itself puts out. But wait—we’re not done. You also have to account for the heat from the motor and any other nearby electrical components in the same enclosure. A 5 HP induction motor, for example, loses about 1-3% of its power as heat when running at full load, so add another 37-111 watts there, plus any heat from wiring or other controls in the closet. All told, your total heat to dissipate is around 200-260 watts for that small system.
For larger systems—say, a 100 HP AC frequency changer powering a conveyor system in a warehouse—the math changes, but the logic stays the same. A 100 HP drive has an input power of ~74,570 watts. At 97% efficiency, that’s 2,237 watts of heat from the drive alone. Add in a 100 HP motor’s 745-2,237 watts of heat loss, and you’re looking at 3,000+ watts total to dissipate. That’s a big number, which is why large drives usually need dedicated cooling rather than just a box fan in the same room.
But here’s a mistake a lot of people make: They use the drive’s rated power at full speed to calculate heat loss, but most applications don’t run at full speed 100% of the time. If your VFD is only running at 50% speed, its efficiency goes up slightly (to around 97-98%), and its input power drops, so heat loss drops too. For example, if that 5 HP drive is running at 50% speed, its input power is ~1,864 watts, efficiency is 97%, so heat loss is only ~56 watts. That’s why I always tell customers to use their actual average operating load, not the maximum rating, when figuring out cooling needs. It’s easy to oversize cooling, but that wastes energy and money—undersize it, and you get the same overheating issues we talked about earlier.
Next, let’s talk about the different ways to dissipate that heat, because the method you use depends on your setup. The most basic is passive cooling: a heat sink attached to the drive’s internal components, which lets heat radiate into the surrounding air. This works great for small, low-power drives (under 5 HP) that are installed in open, well-ventilated spaces. But if you have a small motor drive tucked into a tight control panel with other equipment, passive cooling isn’t enough. That’s where forced air cooling comes in: a small fan mounted on or near the drive that blows cool air over the heat sink, or pulls hot air out of the enclosure. For medium-sized drives (5-50 HP), forced air is the go-to option—it’s affordable, easy to install, and works for most commercial and light industrial setups.
For larger drives (over 50 HP), or drives installed in enclosed spaces with high ambient temperatures (like a furnace room or a warehouse in Arizona during summer), you’ll need liquid cooling. This uses a coolant loop to pull heat away from the drive’s internal components, then runs that coolant through a heat exchanger to release the heat into the outside air. Liquid cooling is more expensive upfront, but it’s way more efficient for high-power systems, and it keeps the drive at a consistent temperature no matter how hot it gets outside. I’ve seen customers with 100+ HP drives in unconditioned warehouses save thousands on repair costs by switching from forced air to liquid cooling.
Another thing to consider is ambient temperature. The heat dissipation requirement doubles if your control room’s average temperature is 10°F higher than standard (77°F, or 25°C, which is the industry standard for drive operation). If your space runs hot for most of the year, you’ll need extra cooling capacity to make up for that. For example, if you’re in a factory where control rooms stay around 90°F (32°C) year-round, that 5 HP drive I mentioned earlier that only needed 150 watts of heat loss at 77°F will now need around 200 watts of cooling. Small adjustments like that add up, especially for systems that run 24/7.
Now, let’s circle back to why this matters for you as a buyer. Whether you’re replacing an old AC frequency changer or outfitting a new machine, getting the heat dissipation right will extend the life of your drive and avoid unplanned downtime. I always tell customers to ask themselves three questions before they finalize their setup: 1) What’s the total heat output from the drive, motor, and any other components in the same enclosure? 2) What’s the ambient temperature where the drive will be installed? 3) How often does the drive run at full load, versus partial load? If you can answer those, you’ll know exactly what cooling you need.
And if you’re not sure where to start, that’s what I’m here for. I’ve worked with every type of application, from small bakery mixers to 200 HP conveyor systems, and I can help you size your drive and calculate your cooling needs without the headache. Our team specializes in AC frequency changers for all motor sizes, including the small motor-focused units linked at /vector-frequency-converter/economic-frequency-converter/ac-frequency-changer.html, so you know you’re getting a drive built to handle real-world heat conditions.
Wait, a quick note to avoid one more common mistake: Don’t forget to factor in the space around the drive. Even if you have the right amount of cooling, if your VFD is crammed into a panel with no room for air to circulate, heat will build up. I usually recommend leaving at least 6 inches of space on all sides of the drive for airflow. That 6 inches might not seem like a big deal, but it can make the difference between a drive that runs for 10 years and one that dies in 2.
At the end of the day, heat is the biggest enemy of AC frequency changers. I’ve seen so many customers skimp on cooling to save a few bucks upfront, only to end up paying for a new drive and downtime down the line. If you’re in the market for a new AC frequency changer or need help figuring out your heat dissipation needs, reach out to our team to talk through your project—no sales pitch, just straight advice based on years of working with equipment just like yours. Don’t let poor heat management derail your operations; let’s get it right the first time.
REFERENCES
- Motor Drives: Understanding AC Variable Frequency Drive Fundamentals, National Electrical Manufacturers Association (NEMA)
- Heat Dissipation Guidelines for Industrial Variable Frequency Drives, International Electrotechnical Commission (IEC) Standard 61800-5-1
- Small Motor VFD Application Guide, Industrial Motor and Control Association (IMCA)