How to Calculate Heat Dissipation for VFDs


When you’re building a panel containing Variable Frequency Drives (VFDs), it’s easy to underestimate their contribution toward the electrical enclosure’s heat load. Part of the problem arises from the paucity of information supplied by manufacturers who don’t always publish power dissipation or efficiency information in brochures.

VFDs generate a significant amount of heat and unless the heat is removed through the use of enclosure cooling, the drives can overheat and trip, causing plant outages. Here is a simple guide to calculate the heat dissipation requirements for VFDs.

Drive Efficiency

The efficiency of most VFDs is between 93 to 98 percent and the balance of the energy is lost as heat. The power dissipated is calculated by subtracting the efficiency from 100 percent and multiplying the result by the power consumption of the drive. The heat loss of a 95 percent efficient, 100 horsepower drive can be estimated as 5 percent of 100 horsepower which equals 5 horsepower or 3729 watts.

In order to make this calculation, it’s essential to obtain the VFD drive efficiency at the design load from the equipment supplier.

Allow For Other Losses

Take into account the thermal losses of ancillary equipment such as DC reactors, phase shifting transformers, power supplies and switch gear. Unless these are insignificant, their losses must be added to the total heat load. The losses from a transformer could be another 4 percent of power consumption.

Again, obtain these figures from the equipment suppliers.

If the drive uses braking resistors and is frequently stopped and started, the power dissipated by the braking resistor should also be taken into account.

Maximum Operating Temperature

Drive manufacturers specify the maximum allowed operating temperature of their devices. In some cases, this is relatively low as it allows for the heating effect of power electronics on the circuit boards inside the drives. Additionally, some manufacturers derate their drives above temperatures of 104 ºF.

It’s good engineering practice to design the enclosure cooling for a temperature that’s 20 ºF below the equipment’s maximum temperature; this will promote long life and minimize the possibility of overheating.

Total Heat Load

The enclosure heat load is established by adding the total heat dissipation of all items of equipment. Additionally, the calculation needs to take into account heat transfer through the enclosure walls as a result of the ambient temperature, local heat sources, and solar radiation. Although there are various ways this can be calculated, the most effective method is to use an online heat load calculator to establish the enclosure’s total heat load and associated cooling capacity requirement.

Cooling Solutions

Provided the ambient temperature is lower than the required enclosure temperature, a filtered fan package may provide sufficient heat removal capacity to adequately cool the enclosure. Alternatively, should there be a need to use a sealed enclosure, an air-to-air heat exchanger solution will work well.

However, if the ambient air temperature is close to or above the required enclosure temperature, then an enclosure air conditioner, air-to-water heat exchanger or air-to-water heat exchanger will be required. This has several advantages. First, because the enclosure must be sealed, no dust, dirt, or pollutants are able to contaminate surfaces. Second, an air conditioner dries the air, which ensures that the maximum humidity requirements of VFD drives are met.

Don’t Underestimate VFD Heat Load

It’s easy to overlook the cumulative heat load of VFDs. This could have serious ramifications in terms of drives tripping, poor control, and plant outages caused by high temperature. It could also cause premature drive failure. Although the calculation of enclosure heat load is simple, the effect of external influences is more difficult to assess: if you need help sizing your enclosure cooling system, contact our experts at Thermal Edge.