Variable-frequency drives (VFDs) are a hot topic. Advancements in VFD technology and reductions in price are driving rapid market adoption. Dramatic energy savings can deliver a payback period measured in months, and VFDs enable precise motor control in many industrial process applications.

But, VFDs are also hot in the literal sense: Advanced electronics pack more semiconductor components into a smaller form factor, resulting in more intense heat generation. Elevated temperatures degrade performance, impair operational reliability, and shorten service life.

A variety of cooling methods have proven effective, including passive air cooling with fans and heat exchangers as well as active cooling with air conditioning and water cooling.

Unfortunately, determining the cooling load can be a bit confusing. Calculations are needlessly complicated by a mismatch of systems of measurement — imperial units (hp, Btu, cfm) mixed with metric units (watts) — and the conversion gets lost in translation.

Protective enclosures cause overheating

The basic challenge of VFD cooling comes from the fact that VFDs usually need to be placed in an enclosure to protect them from the immediate environment, and, paradoxically, these enclosures trap heat, which necessitates protection from overheating.

Basic NEMA 12-type enclosures are often specified to protect against common hazards, such as settling dust, dripping water, and condensation of noncorrosive liquids. Increasingly, advanced technologies in new VFDs, such as fiber optics, require enclosures with more enhanced levels of protection.

And, with the wide-scale adoption of VFD technology, many applications require enclosures specially designed for challenging environments — from weather- and impact-resistant outdoor enclosures to tightly sealed stainless steel enclosures. As an enclosure becomes more sealed, it naturally starts to hold more heat due to the decrease in passive dissipation, thus creating a cooling challenge.

The size of the enclosure also matters a great deal. Typical enclosure dimensions have been dramatically scaled down in recent years to fit in tighter spaces and economize on the cost of the enclosure. In a large box — imagine a space the size of a room — the difference in temperature between the floor and ceiling causes natural convection. The smaller the space, the less objects are able to benefit from this cooling effect. Without adequate airflow, a phenomenon known as “hot spots” is more likely to develop on the surface and in the interior of VFDs, wreaking havoc on sensitive electronics.

The smaller form factor of VFDs and their enclosures contribute to overheating in another way: Less surface area on the exterior is available to transmit heat to the surrounding air. All of these factors necessitate effective and reliable cooling solutions.

VFD adoption

The energy efficiency of VFDs is not just good for individual businesses, it is also key to addressing climate change.

Worldwide, about a quarter of all electrical energy is used to supply motors in industrial applications. In the U.S., an estimated 40 million motors consume 60% to 65% of all electrical energy. Three-quarters of these motors are variable-torque fan, pump, and compressor loads — applications ripe for the energy efficiency afforded by VFDs.

Today, only about 3% of AC motors are currently controlled by VFDs, but about 30% to 40% of new motors installed each year have a VFD. According to a 2021 report by Research Dive, the global VFD market is estimated to grow at nearly 5% annually to $25 billion in 2027.

VFDs reduce energy consumption by enabling electric motors to operate at less than full speed. Basic AC induction motors are designed to run at a constant speed, but, in actual use, speed requirements fluctuate, with full speed typically employed only about 10% of the time. The inherent inefficiency is obvious, analogous to running a car engine with the tachometer showing the engine constantly at its maximum speed.

The energy savings can be calculated using the laws of affinity: The electrical power drawn is proportional to the cube of the rotational speed. Therefore, slowing a pump or fan to 75% or 50% speed reduces energy consumption by nearly 60% and 90%, respectively.

From these efficiency gains, it is necessary to subtract the relatively minimal energy waste of about 3% due to heat loss from the VFD. This heat loss from the VFD is important to quantify, not for its financial impact, which is minimal compared to the overall efficiency gains of utilizing the technology, but rather for the danger that overheating poses to the VFD electronics if the heat trapped in the enclosure is allowed to exceed acceptable temperature limits.

Passive and active cooling

There are two different types of cooling: passive and active. Both utilize the second law of thermodynamics: Energy goes from a higher source to a lower source. Passive cooling utilizes the natural path of heat transfer, with the heat going from the higher temperature source to the lower temperature source. A good example of this is filter fans, which move colder ambient air into and through an enclosure where it absorbs heat until it's exhausted and the heat dissipates into the environment.

Active cooling requires a source of energy to be put into the system in order to create a path for heat to transfer. This is commonly done with the use of a vapor compression cycle, which has four major parts: a compressor, condenser, throttling device, and evaporator. The cycle starts when refrigerant enters the compressor under low pressure and low temperature and then gets compressed, which causes the refrigerant to leave the compressor under high pressure and high temperature. Next, the refrigerant travels through the condenser, where heat is removed, causing the refrigerant to become a saturated or subcooled liquid. Then, the refrigerant passes through a throttling device, where its pressure and temperature drop. Finally, the refrigerant passes through the evaporator, where heat is absorbed, turning it into low-pressure and low-temperature gas so the cycle can repeat.

Deciding when to use passive and when to use active is fairly simple. If the ambient temperature is lower than the target enclosure temperature or there's a source of passively chilled water, then a passive cooling unit can be used, which is desirable for energy savings. Passive cooling uses significantly less energy than active cooling, as the passive cooling does not require energy to be put into the system to allow a path for heat transfer. If the ambient temperature is higher than the target enclosure temperature or there's no source of passively chilled water, then an active unit has to be used.

Calculate cooling requirements

Here is a simple way to calculate cooling requirements for both active cooling and passive cooling methods.

Active Cooling Rule of Thumb

75 Btuh is required for every 1 hp

In other words, for a 100-hp VFD drive, 7,500 Btuh of cooling is required.

And, here's how that rule of thumb came to be.

• In VFDs, 3% of the electrical energy is converted to heat.
• 1 hp = 746 W.
• 746 W x 3% heat loss = 22 W of heat loss per 1 hp.
• 1 W = 3.4 Btuh.
• 22 W x 3.4 Btuh = 75 Btuh per 1 hp.

Passive Cooling Rule of Thumb

4 cfm is required for every 1 hp to maintain 10°C above ambient in the enclosure

In other words, for a 100-hp drive, 100 cfm is required. This rule of thumb is derived from the following equation:

1 cfm = 1.82 x watts of heat loss / Δ T (°C)

These rules of thumb provide a general guide for selecting a cooling method and sizing the cooling load requirements. For more precise calculations that account for ambient temperature and humidity and other critical considerations, sizing software is available.