With some research tonight I have found out a little bit more info on VFDs. Most of the manufacturers stop listing single phase as a source for more than 3 horsepower because mostly motors larger than this have three phase power. The drives that specify single phase as a power source have larger filter capacitors. Most (not all) larger VFDs can still be used on single phase. Sizing can be as easy as doubling the size of the drive to the motor's HP rating. But better yet use the full load amps of the motor to be driven to figure out the number of amps needed from the single phase line. To figure this out multiply the 3 phase Full load amps by the square root of 3 = (1.7320) and add 10-12% for a safety margin. For me a 7.5kw motor with a full load amps rating of 12 amps would yield 20.784 amps and + 12% would be about 25 amps. I would then use a drive with a rated input current of at least 25 amps. From what I have found out anyways both methods are going to be pretty close to the same. I would also need to limit the output current on the drive to the motors actual rating to prevent exceeding the input current. Since a 20HP drive would easily be able to demand from the DC bus more current than the rectifiers now running on only 2 legs instead of three could provide and cause an over-current on the inputs. I think that I would prefer to err. on the safer side to help avoiding damaging the rectifiers. Am I on the right track or am I looking at this wrong?
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