Derating Guidelines
Temperature and altitude derating specifications outline the maximum operating temperature and altitude at which a VSD/Soft Starter can function safely and efficiently.
Generally, as the ambient temperature increases, the unit’s output power must be reduced to prevent damage or failure, this reduction is known as “derating”. It is important to consult the manufacturer’s documentation for the specific unit in question to understand its temperature derating factor. Similarly, as altitude increases, the air density decreases which can affect the cooling of the unit, thus it may need to be de-rated, or its output power reduced, to prevent damage or failure.
EMHEATER Environmental requirements state that units should be de-rated based on Ambient Temperature and Altitude. These are specified as follows:
- Altitude: Lower than 1000m
- Ambient Temperature: -10°C~ +40°C (de-rated if the ambient temperature is between 40°C and 50°C)
For conditions where the Altitude or Ambient Temperature exceeds the abovementioned specifications, the following applies:
- Altitude: For every 1000m above the original 1000m limit, please select a kW option larger than normal.
- Ambient Temperature: For ambient temperature between 40°C and 50°C, please select a kW option larger than normal – This is to accommodate for the rise in resistance under higher temperatures and to protect sensitive electronics from being over-stressed. Using units at Ambient Temperatures above 50 degrees is not advised. For all circumstances with Ambient Temperatures above 40 degrees, please consider making use of forced cooling to reduce the temperature.
As a general (conservative) rule of thumb AC Motors/VSDs/Soft Starters are all rated for up to 1000m (~3300 feet) in altitude and 40 degrees Celsius (and is not rated at all for operation above 50°) and both generally de-rate at the same rate. Both then de-rate at about 1% for every 100m (~328 feet) above 1000m and then de-rate at about 1% per degree above 40 degrees Celsius.
Temperature Control:
Even with de-rating, additional control might be required to maintain temperatures at acceptable levels. This is largely because VSDs/Soft Starters generate significant heat while operating. When the units dissipate heat and the heat is contained within a cabinet, the temperature within can easily exceed upper temperature limits and cause premature drive failure. EMHEATER do provide very specific requirements for installation clearances and mounting methods in order to ensure the units are adequately cooled. When the units are wall- or floor-mounted as stand-alone units these methods may be all that are needed, but installation within cabinets often demands additional temperature control. This temperature control is typically provided by Passive Cooling (fan cooling / forced air ventilation) or Active Cooling (refrigerated / air conditioned and water cooling).
In cases where the ambient temperature is not excessive, Passive Cooling (fan cooling) might be required for unit installation in enclosures. Fans for Passive Cooling should be sized to provide air flow which take into account the unit’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 typically be specified for indoor or outdoor use). For larger units, particularly when the cabinets are installed outdoors in warm climates, Active Cooling (air condition and water cooling) might be required.
Cooling requirements can be affected by installation location as well. For example, it is not recommended for cabinets to be installed in direct sunlight (if this cannot be avoided, then some type of shelter or sun screen is recommended). Installing a unit in a location shaded from the sun during the hotter parts of the day can significantly reduce cooling demands.
Suggested Cooling as follows:
Active Cooling (air condition and water cooling) Rule of Thumb = 75 BTU/h is required for every 1 HP
Passive Cooling (fan cooling) Rule of Thumb = 4 CFM is required for every 1 HP to maintain 10°C above ambient in the enclosure
Humidity/Condensation Control:
EMHEATER units are rated for up to 95% (RH) relative humidity (non-condensing) for VSDs and 90% for Soft Starters, so in all but extreme cases humidity is not a problem. However, 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 (heaters are typically thermostatically controlled and interlocked for operation based on unit status).
