What is in this article?:
- Understanding Variable Speed Drives (Part 2)
- Sizing VFDs for the Load
- Key VFD Specifications
- Applications to Watch Out For
A thorough understanding of how to match the VFD to the driven load is the key to a successful application.When applied properly, the variable frequency drive (VFD) is the most effective motor controller in the industry today. Modern VFDs are affordable and reliable, have flexibility of control, and offer significant electrical energy savings through greatly reduced electric bills.
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Key VFD Specifications
While there are many specifications associated with drives, the following are the most important.
- Continuous run current rating. This is the maximum rms current the VFD can safely handle under all operating conditions at a fixed ambient temperature (usually 40 [degrees] C). Motor ball load sine wave currents must be equal to or less than this rating.
- Overload current rating. This is an inverse time/current rating that is the maximum current the VFD can produce for a given time frame. Typical ratings are 110% to 150% overcurrent for 1 min, depending on the manufacturer. Higher current ratings can be obtained by oversizing the VFD. This rating is very important when sizing the VFD for the currents needed by the motor for break-away torque.
- Line voltage. As with any motor controller, an operating voltage must be specified. VFDs are designed to operate at some nominal voltage such as 240VAC or 480VAC, with an allowable voltage variation of plus or minus 10%. Most motor starters will operate beyond this 10% variation, but VFDs will not and will go into a protective trip. A recorded voltage reading of line power deviations is highly recommended for each application.
