Key factors to size VFDs

Optimize a VFD’s efficiency and cooling-air requirements by following these key points.


Optimize a variable frequency drive's (VFD) efficiency and cooling-air requirements by following these key points.

  • The methodology is developed to specify required volume of the cooling-air for the VFD enclosures, derived from the critical cooling-air velocity criteria. As long as the actual VFD enclosure's cooling-air velocity is maintained equal or above the critical velocity, no moisture exchange will occur in the enclosure. Under these conditions, the enclosure's total cooling load will be equal to its sensible load only. 
  • The VFD enclosure's dry-bulb operating temperature and relative humidity of the entering and exiting cooling-air should be employed as the system monitoring and control parameters. A VFD enclosure's design cooling-air volume should be determined by considering maximum anticipated surrounding air dry-bulb temperature and manufacturers' allowable operating temperature. The VFD enclosure's relative design airflow could be controlled either by increasing operating air temperature or reducing cooling-air temperature entering the enclosure.
  • Failure to provide adequate cooling-airflow through the enclosure during design and off-design conditions might lead to unwanted moisture deposits in the enclosure, due to the moisture separating from cooling-air and negatively impacting VFD service life and reliability, which might cause costly premature VFD replacement.
  • We suggest using variable cooling-airflow rate and operating temperature concepts to prevent damaging moisture exchange from taking place in the enclosure. This could be done by adjusting the cooling-airflow rate to maintain equal humidity ratios for air exiting and entering the enclosure, and by limiting the enclosure's exiting-air temperature to its predetermined allowable level.
  • Cooling-air fans flow rate selection should be customized to satisfy system design requirements for the most demanding conditions, so the cooling-airflow rate could be adjusted to satisfy a particular application during design and off-design modes to conserve energy.

Alexander L. Burd is president and Galina S. Burd is a project manager and vice president at Advanced Research Technology. Alexander Burd has 35 years of experience in the design, research, and optimization of HVAC and district energy systems with verified monitored electrical and thermal energy savings for large facilities and energy utilities. Galina Burd has more than 25 years of design and research experience in the HVAC and architectural engineering field and has co-authored many technical and research papers both in the U.S. and Europe.

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