It’s a harsh world… part 5 – Drives vs High altitudes

Wednesday, April 13, 2016
Skiers sitting in a chairlift

Those of us living at or near sea level put very little thought toward the day-to-day struggles taking place at high altitudes. However, if you’ve ever traveled to higher climbs for a skiing vacation or to go hiking through the incredible mountain scenery, you’ve no doubt felt its impacts first hand.

What would typically seem like a normal distance to walk or run now seems to leave you far more tired than usual. The higher up you are, there is less of the Earth’s atmosphere pushing down on everything. What this means is that there are fewer air molecules in a given area. So, for each breath you take, your body doesn’t receive as much oxygen.

That much is pretty straightforward, but what does it have to do with your AC drives? Well, that’s a bit of a different story, but this thin air creates some challenges for our installations.


Finding the right cooling solution

The first major challenge comes from the ability of the air to remove heat. At ground level, we have certain expectations for how well our drives will be cooled given certain ambient conditions, as we investigated in Part 4. At higher altitudes, the amount of cooling capacity the air has decreases as the altitude increases because of the reduction in air density. The only way to adjust for this is to decrease the output current of your AC drive or decrease the ambient temperature far enough to ensure 100% output current is available.

Although the cooling capacity of air at high temperatures is reduced, there’s one benefit to high altitudes that we can, in part, rely on for outdoor installations. That benefit is the ambient temperature has a tendency to decrease as the altitude increases. On average, the temperature decreases by about 6.5°C/11.7°F for every 1000 m/3281 ft increase in altitude. As this is an average, it’s important to understand the more specific temperature data throughout the year in the location where your AC drive is being installed to ensure proper sizing. Of course, as we looked at in our Drives vs. Low temperatures topic, as the temperature drops, there are additional environmental impacts, such as snow and ice, on the operation of your AC drive.

For indoor installations, we can’t necessarily count on the outdoor temperatures to help us out as much. For these installations, there are various methods to help increase cooling capacity as we discussed in Drives vs. High temperatures. On the one hand, you can always increase the amount of airflow over the cooling fins of your AC drive. On the other, you can use air conditioning to bring down the ambient temperature of the enclosure to account for the limited cooling capacity of the thin air. Lastly, using some of the back-channel cooling options that can be found, for example, in the VLT® AutomationDrive, you can bring in the lower-temperature outdoor air through the back cooling channel which will keep the AC drive cool in a way that doesn’t require a larger investment in air conditioning solutions.

When neither option is ideal, there’s a unique solution that allows one to work in higher altitudes without derating due to cooling capacity. This solution, the VACON® NXP Liquid Cooled, doesn’t rely on air cooling but, as the name implies, liquid cooling methods to remove the heat from the AC drive. Because this cooling relies on water or a water/glycol solution to remove the heat from the AC drive, the cooling capacity isn’t really impacted at high altitudes in the way air is. Of course, it’s important then to ensure that your heat exchanger solution is properly sized in the case that it’s a Liquid-to-Air heat exchanger to account for the cooling capacity reduction of the air at high altitudes.

A good rule of thumb

I’m sure you’ve noticed so far that we haven’t really defined exactly what high altitude means to AC drives. Most drives require derating to start at 1000 m/3281 ft. But, since I don’t speak for all drive manufacturers out there, the specifications section of the product manuals should definitely specify the maximum altitude without derating. In our case, we will also specify how much to derate a VACON® or a VLT® drive above 1000 m/3281 ft. If you open up the manuals for any of the Danfoss Drives products, you’ll see that the rule of thumb is a reduction of 1% of the output current from the AC drive for every 100 m/328 ft above 1000 m/3281 ft. Depending on the AC drive product being used, the maximum allowable altitude is typically 2000 m/6562 ft or 3000 m/9843 ft.

At this point, you may be thinking that if there’s a good rule of thumb system in place to help derate your AC drive for operation in high altitudes, why are maximum allowable altitudes specified in the first place? Well, the very short answer ties back to how thin the air is at very high altitudes, and how this thin air affects electronic devices. The slightly longer version is that air itself is a pretty good insulator. You get confirmation of this any time you look up at power transmission lines carrying high voltage from power plants to substations. These lines are not insulated; so, if not for the air, there would be constant arcing between these wires.

In electronics, this property is always a consideration for component construction and circuit board layout. The reason your computers and smartphones work in ski resorts and mountain retreats is that, while there is very little space between the copper traces on the printed circuit board, the voltages are quite small. So, even in thinner air, arcing will not occur. This is not the case with AC drives, however, due to the voltages in the power circuitry. When working with an input of 690 VAC three phase, the voltage seen on the DC bus is typically expected to be around 930 VDC. These voltages can be high enough to impact the isolation of low-voltage circuits (such as communications and I/O for instance). And, as a result, can reduce both the reliability and personal safety of your AC drive. Obviously, this is something you will want to avoid which is why, if the manual of the AC drive doesn’t specify the maximum operating altitude, you should always confirm with the AC drive manufacturer.

It’s amazing when you really dig into the topic that something as seemingly passive as thin air can greatly impact your AC-drive installation. Making smart, well-informed decisions in sizing, selecting and installing your AC drive will help ensure that your investment operates worry-free for several years. At Danfoss Drives, we have worked on installations in many high-altitude applications throughout the world. So you can count on us to help make sure you have the most optimal solution for your application.

Jake Roeder, Global Product Marketing Manager, Danfoss Drives
Abraham González Ponce, Application Knowledge Manager, Application & Service Products, Danfoss Drives