Practical Aspects of VFD Cooling Design

Monday, September 15, 2014
Drawing of back-channel cooling

Compliance with ambient temperature specifications

External climatic conditions and ambient conditions have a distinct effect on the cooling of all electrical and electronic components in a control room or cabinet. Minimum and maximum ambient temperature limits are specified for all frequency converters. These limits are usually determined by the electronic components that are used. For example, the ambient temperature of the electrolytic capacitors installed in the DC link must remain within certain limits due to the temperature dependence of their capacitance. Although frequency converters can operate at temperatures down to -10 °C, manufacturers only guarantee proper operation at rated load with temperatures of 0 °C or higher. This means that you should avoid using them in areas subject to frost, such as uninsulated rooms. You should also not exceed the maximum temperature limit. Electronic components are sensitive to heat. According to the Arrhenius equation, the lifetime of an electronic component decreases by 50% for every 10 °C that it is operated above its design temperature. This is not limited to devices that are installed in cabinets. Even devices with IP54, IP55 or IP66 protection ratings can only be used within the ambient temperature ranges specified in the manuals. This sometimes requires air conditioning of installation rooms or cabinets. Avoiding extreme ambient temperatures prolongs the life of frequency converters and thereby the reliability of the overall system.


Frequency converters dissipate power in the form of heat. The amount of power dissipation in watts is stated in the technical data of the frequency converter. Operators should take suitable measures to remove the heat dissipated by the frequency converter from the cabinet, for example by means of cabinet fans. The required airflow is stated in the manufacturer's documentation. Frequency converters must be mounted such that the cooling air can flow unhindered through the device’s cooling fins. Particularly with IP20 devices in cabinets, there is a risk of inadequate air circulation due to the closely spaced mounting of cabinet components, which causes the formation of heat pockets. See the manuals for the correct mounting distances, which must always be observed.

Relative humidity

Although some frequency converters can operate properly at relatively high humidity (Danfoss units up to 95% relative humidity), condensation must always be avoided. There is a specific risk of condensation when the frequency converter or some of its components are colder than moist ambient air. In this situation, the moisture in the air can condense on the electronic components. When the device is switched on again, the water droplets can cause short circuits in the device. This usually occurs only with frequency converters that are disconnected from the mains. For this reason, it is advisable to install a cabinet heater in situations where there is a real possibility of condensation due to ambient conditions. Alternatively, operating the frequency converter in standby mode (with the device constantly connected to the mains) can help reduce the risk of condensation. However, you should check whether the power dissipation is sufficient to keep the circuitry in the frequency converter dry.

Article by Gregers Geilager