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2018-07-06   Control Engineering

Variable frequency drives (VFDs) have many benefits and it’s critical to explore strategies to achieve maximum efficiency.

By: Scott Garner, Devin Pellicone, and Richard Bonner, Advanced Cooling Technologies

Variable frequency drives (VFDs) provide power and control to large and small commercial and industrial motors, and they must be thermally protected appropriate to their designs and environments. The key benefits of VFDs are the flexible control, smooth start and stop performance, and significant energy savings for centrifugal fans and pumps operating at variable loads. Most high-power VFDs and their complimentary suite of electronics are integrated into electronic enclosures as shown in Figure 1.

VFDs improve system efficiencies and also are very efficient devices themselves with losses of 2 to 4%. However, even these inefficiencies create kilowatts to tens of kilowatts of waste heat that must be dissipated as a result of the high amount of electrical power that is converted in high-power drives.

Dissipating this heat in an open air-cooled cabinet is straightforward. However, in harsh environments, which preclude filter fan-cooling or direct flow through air-cooling, thermal management for the enclosures becomes an important part of the design process. It's critical to explore a strategy for efficiently, passively, and cost-effectively cooling medium and high-power sealed enclosure VFDs in harsh environments to achieve maximum efficiency.

Flow through systems versus sealed enclosures

As shown in Figure 2, open airflow cabinets allow ambient air to flow through the cabinet and efficiently cool high-power modules directly. This efficient cooling may allow external contaminants into the enclosure, which are often minimized by using fan filter systems that filter the air flowing into the cabinet. The filters help reduce dust and debris, but they require maintenance cycles to clean or replace the filters.

Sealed enclosures do not allow external air to flow through the cabinet, they use the air inside the cabinet to cool the electronics and exchange heat to external air through a heat exchanger. Sealed enclosures prevent dirt, dust, humidity, salt fog, and other airborne corrosive agents from entering the cabinet and impacting the life of the electronic components.

Either system works well for low-power cabinets. However, for many high-power VFD cabinets, the power dissipations levels are higher than what can be achieved with air cooling. Lower power components are often cooled by direct airflow and the higher power components are cooled directly or indirectly by facility cooling water, vapor compression systems, or pumped liquid systems. In these systems, the high-power components (insulated-gate bipolar transistors (IGBTs), integrated gate commutated thyristors (IGCTs), silicon-controlled rectifiers (SCRs) etc.) are typically attached to a fluid-cooled cold plate. The fluid then dissipates the heat to ambient air using a vapor compression system or through a liquid-to-air heat exchanger.

In either case, the required ambient air-heat exchangers can be located inside or outside of the facility. The main drawback to these systems are the challenges associated with introducing fluid into the cabinets and hard plumbing coolant lines to and from the cabinets.

Loop thermosyphons as an alternative to active liquid cooling

Loop thermosyphons (LTS) are gravity-driven, two-phase cooling devices. They operate in a similar manner to a heat pipe, in so far as a working fluid is evaporated and condensed in a closed loop to transfer heat over a given distance. The main benefit of LTS over heat pipes is their capability to efficiently transfer high power over long distances using dielectric working fluids. Compared to active liquid cooling, vapor compression, or pumped two-phase cooling systems, loop thermosyphons have no moving parts, and higher reliability. LTS are well-suited for transferring high power waste heat from the power electronics in a cabinet to a heat sink external to the cabinet, as shown in Figure 3.

The cabinet-level benefits of LTS cooling systems are significant. The cabinet, electronics, and cooling system can be installed into a sealed, standalone enclosure at the panel shop. Each cabinet is self-contained and can be delivered and easily installed at the end customer. The simplest integration of this concept is an air-cooled LTS condenser located on top of the cabinet, as shown in Figure 3. This allows the cabinet to be standalone and only needs electrical connections at the final installation.

The LTS condenser also can be connected to a facility or chilled water system. This allows for the waste heat to be dissipated further from the cabinet and multiple cabinets to operate on one loop. With an LTS and chilled-water condenser, plumbing and water connections are external to the cabinet, separating the coolant and electronics. 

LTS and sealed enclosure heat exchangers

An LTS is an excellent way to remove high powers directly from the higher dissipation components. Residual heat loads from secondary devices still need to be cooled. These secondary components consist of many lower-powered devices dispersed throughout the cabinet, which makes direct contact cooling difficult. For these lower power, lower heat flux components, direct air cooling is the most practical approach. Lower power components can be cooled easily by an air-to-air heat exchanger while maintaining the sealed integrity of the enclosure.

In a combined LTS/sealed air-to-air heat exchanger combination, the high power IGBTs or IGCTs mount to the LTS cold plate and their 10 kW plus heat loads are dissipated via the LTS to the external cabinet air (Figure 4). All of the secondary electronics are cooled via a sealed air-to-air heat exchanger that can dissipate 1 kW or so of waste heat.

The LTS and the sealed air-to-air heat exchanger maintain the original NEMA cabinet rating. The combination of the LTS and sealed enclosure heat exchanger combination allows high-power cabinets to remain sealed against external airflow and does not require liquid coolant flow within the cabinet.

LTS and sealed enclosure coolers provide many benefits to power electronics cooling applications. An LTS passively provides the removal of the large amounts of heat dissipated by high power electronics components, using dielectric working fluids that are well suited for medium- and high-voltage applications. Sealed enclosure coolers can remove the heat from the lower power, distributed components found in power electronics cabinets, while preventing the contaminants found in outside air from interacting with these components. The combination of the two cooling solutions provides, reliable, passive cooling of high-power motor controllers in sealed enclosures required for harsh operating environments.

Scott Garner is the vice president of strategic product development; Devin Pellicone is a lead engineer; and Richard Bonner is the manager of the custom products group at Advanced Cooling Technologies. Edited by Emily Guenther, associate content manager, Control Engineering, CFE Media,


KEYWORD: Enclosure cooling, VFD cooling

  • VFD cooling options
  • Cooling with loop thermosyphons (LTSs) and sealed enclosures.

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