Whether they’re employed for specific energy saving reasons or to optimize the life of the motors they’re connected to, AC drives have increasingly become a regular fixture in all industries. But given that most AC drives have chassis rated as NEMA 1 or IP21—meaning they have minimal protection against anything encountered beyond a climate-controlled indoor location—the need for a proper drive enclosure is not uncommon.
Even if you’re not installing the AC drive in a potentially hazardous location, if it will be placed in a cabinet alongside other electronic automation devices, heat issues arise that call for the enclosing cabinet to have the proper environmental capabilities. After all, AC drives are comprised of very sensitive electronic components that must be cared for in order to get the maximum life from them.
To learn more about how to properly enclose AC drives, I spoke with Dennis Murphy, product marketing manager, enclosed drives, at Vacon.
The first point Murphy brought up was power loss, which is directly connected to heat issues when a drive is housed in a cabinet. “AC drives, like all electronic equipment, are not 100 percent efficient and will lose some power, which is an issue that must be dealt with when designing an enclosure for an AC drive,” he says. “The typical power loss for modern drives is about 2-3 percent of the driven motor load, expressed in kilowatts. For example, if the load being driven by the AC drive is 22kW, the resulting power loss from the drive is 22,000*0.02=440W. Even if the drive is rated for a higher load, such as 30kW, the power loss will be the same because 22kW is the actual power being delivered to the motor. Also, when designing the enclosure, care must be taken to sum the power loss of every component that will be installed, and not just the loss from the AC drive.”
Starting with the power loss factor as a base point, Murphy said the next thing to examine is the environment where the finished enclosure will be installed. “The enclosure type should be determined using governing standards such as NEMA, UL, and CSA to define the requirements of a particular installation,” he says.
As an example, here are some common ratings taken from “UL50, Enclosures for Electrical Equipment”:
- Type 1 – Indoor use primarily to provide a degree of protection against contact with the enclosed equipment and against a limited amount of falling dirt.
- Type 3R – Outdoor use to provide a degree of protection against falling rain; undamaged by the formation of ice on the enclosure.
- Type 4 – Indoor or outdoor use to provide a degree of protection against falling rain, splashing water, and hose-directed water; undamaged by the formation of ice on the enclosure.
- Type 4X – Same as Type 4 with corrosion resistance.
- Type 12 - Indoor use to provide a degree of protection against dust, dirt, fiber flyings, dripping water, and external condensation of non-corrosive liquids.
After the installation environment is determined, the cooling means can be defined. “There are at least five primary ways to cool an enclosure that contains an AC drive,” Murphy says, “however, not all of these methods are practical, depending on the environmental rating of the enclosure required for a particular application. Also, for very large drives, the enormity of the heat loss may dictate a particular cooling method.”
Here are the primary cooling methods Murphy recommends based on the application:
Convection cooling. The AC drive is installed in an enclosure, with intake vents toward the lower part of the enclosure and exhaust vents near the top. This is one of the more cost-effective means of cooling—and the taller the cabinet is, the more effective it becomes in transferring heat as a “chimney” effect is created. This method is mainly suitable for NEMA 1 and 3R installations, but can also be applied very creatively to meet other requirements.
Forced convection cooling. A variation on convection cooling, this method relies on fans to push air through the cabinet—using the intake and exhaust vents. It’s also possible to use this configuration by relying on the AC drive’s fans to move air through the enclosure with careful mechanical engineering and the use of baffles to steer the air flow. This method is most often used for NEMA 1, 3R, and NEMA 12 installations when using properly rated filters and grills or air-to-air heat exchangers.
Radiation cooling. With this method, the surface area of the enclosure is sufficient to radiate the heat lost inside to other surfaces around the enclosure. This is the easiest and most cost-effective means of cooling, but is extremely limited in its ability to cool, especially with high-powered AC drives equipment. This method is inherently rated whatever the enclosure is rated up to and including NEMA 4X.
Refrigerant cooling. This is most often accomplished using an air conditioning unit that is specifically designed for cooling electrical enclosures. The important advantage with this method is that the interior of the cabinet can be cooled to a temperature below the surrounding air temperature, while the other methods can only reduce the interior temperature to some level above the ambient air. Suitable for NEMA 3R, 12, 4, 4X.
Liquid cooling. It is becoming more common to cool electrical enclosures and drive modules using liquid, especially when very high-power loads are being driven. The most popular ways include the use of a liquid-to-air heat exchanger mounted to an enclosure or by using specially designed liquid-cooled AC drives. For higher power levels, liquid-cooled AC drives become a more feasible option, as most of the heat loss is generated into the coolant with very little loss into the enclosure. These drives typically use a liquid-to-liquid heat exchanger or chiller to create two separate closed-loop cooling circuits: One circuit for the connected AC drives, and another circuit for the installation site’s cooling loop which uses a liquid-to-air heat exchanger for final removal of the heat into the air in a remote location. This option is suitable for the most extreme NEMA ratings, since the enclosure can be completely sealed if necessary by adding an internal liquid-to-air heat exchanger for removal of internal enclosure heat into the liquid circuit.
Murphy adds that many standard cabinet manufacturers supply online calculation programs to assist with sizing cooling components, including fans, heat exchangers, and air conditioners, “making it easier than ever to properly design the enclosure.”