One factor that if over-looked can cause significant equipment issues is proper environmental protection of variable speed drives (VSD’s) and other electronic equipment. We are often requested to install VSD’s and other control components in custom enclosures which will be installed under a wide variety of ambient conditions, including high temperature and/or high humidity environments. The drive electronics are rated for operation within given temperature and humidity ranges in order to ensure longevity and proper current output, so there are real risks and cost impacts associated with ignoring environmental constraints.


Although there is some variability from manufacturer to manufacturer, VSD’s are generally de-rated – that is, their rated amperage output is reduced – for operation between 40°C and 50°C. F0r instance, ABB specifies that output current is to be reduced by 1% for each 1°C above 40°, and the units are not rated at all for operation above 50°. This is to accommodate the rise in resistance under higher temperatures and to protect sensitive electronics from being over-stressed. Sometimes the 50°C maximum rating will be shown in manufacturers’ information as intended for “heavy duty” or “overload” use. This should not be confused with the overload capability required for constant torque applications – the higher load demands under constant torque use should be considered aside from any temperature de-rating required. Manufacturers can provide de-rating curves for their VSD’s which provide the relevant details.

Even with de-rating, additional control is required to maintain temperatures at acceptable levels. This is largely because VSD’s generate significant heat while they are operating. Smaller drives (e.g. below 5 HP (3.7 kW)) are usually equipped with external heat sinks alone, while larger units have one or more internal fans used to draw air over the electronics.  According to data from Emerson/Control Techniques, a 100 HP (75kW) drive may dissipate 5,100 Btu/hr (1.5kW) or more under normal operating conditions. If that heat is contained within a cabinet, the temperature within can easily exceed upper temperature limits and cause premature drive failure. Manufacturers provide very specific requirements for installation clearances and mounting methods in order to ensure their drives are adequately cooled. When drives are wall- or floor-mounted as stand-alone units these methods may be all that are needed, but installation within cabinets often demands supplemental temperature control. This temperature control is typically provided by forced air ventilation or refrigerative cooling.

In cases where the ambient temperature is not excessive, fans can be installed in enclosure walls or doors. The fans are sized to provide air flow per manufacturers’ recommendations, which take into account the drive’s heat dissipation  and assume a rated maximum ambient temperature. Fans are also often equipped with suitable filters to protect the cabinet contents from dust and debris; filter kits can be specified for indoor or outdoor use depending on need.

For larger VSD’s, particularly when the cabinets are installed outdoors in warm climates, refrigerative cooling (i.e. air conditioning) is needed. It is not uncommon to require 8,000 Btu (2.3 kWh) or more cooling capacity within the panel for a 100 HP (75kW) drive and associated control components. Cooling requirements can be affected by installation location as well. For example, manufacturers recommend that drives/drive cabinets not be installed in direct sunlight; if this cannot be avoided, then some type of shelter or sun screen is recommended. Simply siting the drive in a location shaded from the sun during the hotter parts of the day can significantly reduce cooling demands; specific tools for calculating cooling demand can be found on several temperature control manufacturers’ web sites, including among others Pfannenberg, IceQube, and Kooltronic.


Operating ranges for most VSD’s range from 5% – 95% relative humidity (non-condensing), so in all but extreme cases humidity is not a problem. However, drives/drive cabinets subjected to wide temperature swings can be exposed to condensation. For example, a cabinet mounted outdoors in a temperate climate may see winter temperatures of 0°C or lower. This may not be an issue while the drive is operating, but if it is off for an extended period of time, condensation can develop on internal components. This problem is typically addressed by installing one or more space heaters within the enclosure; the heaters are thermostatically controlled and typically interlocked for operation based on drive status.

If you have a specific or unusual drive climate control application, please feel free to share it with our readers in the Comments section. There is a great deal of experience out there which can be brought to bear on your problem. And we are always glad to hear of any suggestions or thoughts you might have.

Proper climate control will help to ensure you get rated output and operating life from your VSD. Should you need additional guidance, please feel free to contact us at And remember to visit us on-line at See you next week!

Jay Baima - Author


Jay Baima
Joliet Technologies