How VFD Heat Dissipation Affects Cooling in Electrical Enclosures


How VFD Heat Dissipation Affects Cooling in Electrical EnclosuresWhen installing a variable frequency drive (VFD) into an electrical enclosure, careful consideration needs to be given to the much higher heat losses of VFDs compared to other items of electrical equipment. This will have a direct impact on the total heat load of the electrical enclosure and the capacity and method of cooling to be adopted. Other factors to consider are the position of the VFD inside the enclosure as well as an assessment of the maximum allowed operating temperature of the VFD.

VFD Losses are Proportional to Efficiency

The amount of heat generated by a VFD is much higher than other forms of electrical equipment, and a single VFD can produce more heat than all the other items in an electrical enclosure.

Heat losses are directly related to the efficiency of the VFD. The effect of this can be illustrated by considering a 5 HP VFD with a rated efficiency of 95 percent. In order to produce an output of 5 HP (3.73 kW), the drive actually consumes 5 percent more power, giving a total power consumption of 5.26 HP (3.92 kW). The excess power of 0.26 HP (0.19 kW) represents the energy lost in the drive converting the mains power into variable frequency power and is dispersed as heat.

Depending upon the design of the drive, its efficiency at 100 percent of full speed will normally be between 85 percent and 97 percent. As the motor speed is reduced, the efficiency of the drive falls and at 50 percent speed it may be between 60 percent and 90 percent based on the design of the drive.

Consequently, to correctly calculate the heat load of a VFD, it’s important to evaluate the heat loss over the anticipated range of operating speeds.

In some instances, certain additional equipment may be specified, such as DC reactors and phase shifting transformers, especially on larger drives, and their heat loses also need to be considered.

Recommended Maximum Temperature

It is important to take careful note of the drive’s maximum operating temperature under actual operating conditions. Although some drives can safely operate at temperatures as high as 122 °F, others can only withstand 104 °F.

Bearing in mind that it’s very difficult to be sure of the actual temperature distribution within electrical enclosures, a conservative approach toward selecting the maximum allowed enclosure operating temperature will ensure that VFDs and other equipment operate at temperatures below their rated maximum operating temperature. This will promote extended equipment life and reduce the risk of high temperatures causing localized overheating.

Positioning VFD in Electrical Enclosures

Careful note needs to be taken of manufacturers’ recommendations regarding the stacking of VFDs and recommended clearances between adjacent units. Ideally, VFDs should be placed towards the bottom of electrical enclosures where the enclosure temperature is lower, and as close to the cooling air inlet as possible. Also, never place temperature sensitive equipment above VFDs.

Cooling VFDs

The method of cooling chosen will depend upon several factors. Firstly, the total heat load in the enclosure should be calculated and the maximum ambient temperature established. Note should be taken of other factors such as solar radiation, insolation and the design of the enclosure.

Once these have been established, the cooling options that will work can be calculated using an Enclosure Temperature Management Calculator.

If the ambient temperature is low and the total heat load is moderate, the choices would include filtered fans and air to air heat exchangers. The choice depends upon the location of the electrical enclosure and whether it is necessary to use a sealed NEMA enclosure to prevent the ingress of dust, dirt and pollution.

In higher ambient temperatures or if the electrical enclosure contains several powerful drives that generate a significant amount of heat, a closed loop enclosure air conditioner would be the recommended solution because of its ability to reduce the enclosure temperature below ambient temperature, even at high heat loads.

Getting Help with VFD Cooling

The selection of the best enclosure cooling solution for VFDs in electrical enclosures needs to take cognizance of the number, size, design and efficiency of the variable speed drives that have been specified. The layout of drives within the enclosure must take into account the need to ensure an adequate supply of cool air to the drives as well as to ensure that hot air from the drives does not cause inadvertent localized overheating. For help with selecting the best VFD cooling solution for your application, speak to our Technical Support Team.