Thyristors and other solid state devices in variable frequency drives generate a significant amount of heat. These devices are mounted on heat sinks and use a chimney effect to draw cooler air from the bottom of the enclosure and discharge the hot air from the top. The amount of heat is directly related to the efficiency of the drive. This varies between 94 and 98 percent and, based on a 50 hp motor, generates between 0.75 and 2.25 kW of waste heat.
The removal of this heat necessitates cooling the electrical enclosure and in many circumstances the only viable method is the use of enclosure air conditioners. Here are 6 applications when enclosure air conditioners are recommended for variable speed drives.
Cabinet Temperature Exceeds 105 ºF
The internal air temperature of an electrical enclosure should not be too high. The allowable temperature depends upon the temperature rating of the equipment, but ideally should not exceed 105 ºF (40 ºC) especially as localized temperatures could be noticeably higher. It’s important to remember the service life of electronic equipment is halved for every 18 ºF temperature rise above normal ambient temperature.
The temperature inside electrical enclosures located in direct sunlight will significantly increase due to the effects of solar radiation. An enclosure facing the sun could receive as much as 100 watts of heat per square foot of surface depending upon the sun’s angle. Although the heating effect can be reduced by reflective paint, in most cases the temperature inside the enclosure will exceed 105 ºF even without the additional heat load from variable frequency drives.
Near External Heat Sources
It is often convenient to locate electrical drives as close to the motor as possible. However, a nearby heat source such as a furnace or oven could increase the heat load in the drive enclosure, especially if the ambient temperature is elevated. In these locations, an enclosure air conditioner is the most effective method for cooling an electrical enclosure.
The ambient temperature on a mezzanine floor in naturally ventilated industrial buildings is generally elevated because hot air is trapped under the building’s roof. The temperature could easily be 10 to 20 ºF hotter than on the ground floor. This often overlooked effect increases the heat load in electrical drive cabinets and could cause premature failure or incorrect operation of the drive unless enclosure air conditioners are used to keep the drive cabinet temperature below 105 ºF.
Most electrical drives have a requirement that they are not used in locations where the humidity is high and especially when condensation can occur. In coastal, hot, or humid environments, condensation will readily occur at temperatures close to ambient. An enclosure air conditioner will dry the air inside the cabinet, lowering the dew point and avoiding condensation. In such situations it’s also wise to install a heater to ensure condensation does not occur overnight or during cold periods.
Electronic equipment is susceptible to corrosion and should be protected from airborne corrosive substances. These include salt spray in marine environments, corrosive chemicals, and dusts containing chemicals. Drive enclosures should have a dust and moisture proof NEMA 4 rating and be provided with a closed circuit enclosure cooling system.
Enclosure Air Conditioning Best for Variable Speed Drives
The high heat dissipation requirements of variable speed drives means that enclosure ventilation systems are generally insufficient to maintain safe internal enclosure temperatures in harsh conditions once all other heat sources are taken into account. However, closed loop enclosure air conditioners will maintain enclosure temperatures within manufacturer’s specifications, avoid condensation, and keep contaminants out.
If you have an enclosure cooling need for a variable frequency drive, contact the sales team at Thermal Edge today.