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Thread: Using VFD on single phase source

  1. #11
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    Also many states in "private clubs" smoking may be permitted.

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  2. #12
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    Quote Originally Posted by zbang View Post
    Wouldn't it be a lot easier, and more maintainable, to replace the motor (if possible) and let it run full-speed? Seems like a lot of work to re-use the fan & drive.
    That was my thought too. A 1.5HP fan seems like a lot for a vent fan.
    Si hoc legere scis nimium eruditionis habes.

  3. #13
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    Quote Originally Posted by Jraef View Post
    There are two issues: the current flowing through the diodes in the bridge rectifier will pull 1.732x more current
    All other things bein equal, and they rarely are, the three phase diodes will conduct all of the current for a third of the time. For single phase it is all of the current for half the time. So I'm not sure where yoi get your sqrt(3) factor from. Trivia I suppose. I just think a vent fan is the wrong application for a three phase drive. But I readily admit that it's not my area of expertise.
    Si hoc legere scis nimium eruditionis habes.

  4. #14
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    Quote Originally Posted by Besoeker View Post
    All other things bein equal, and they rarely are, the three phase diodes will conduct all of the current for a third of the time. For single phase it is all of the current for half the time. So I'm not sure where yoi get your sqrt(3) factor from. Trivia I suppose. I just think a vent fan is the wrong application for a three phase drive. But I readily admit that it's not my area of expertise.
    They did want it to maintain a certain level of negative pressure when doors are opened.

  5. #15
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    Quote Originally Posted by Besoeker View Post
    All other things bein equal, and they rarely are, the three phase diodes will conduct all of the current for a third of the time. For single phase it is all of the current for half the time. So I'm not sure where yoi get your sqrt(3) factor from. Trivia I suppose. I just think a vent fan is the wrong application for a three phase drive. But I readily admit that it's not my area of expertise.
    The motor draws 3 phase current from the drive, the drive draws single phase current from the line. The difference is the sq. rt. of 3, 1.732.
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  6. #16
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    Thank you for the feedback everyone.

    Just following up to a couple of the reply's. It is a 480V 1.5HP 3phase in line fan with a 480V 1.5HP drive. The drive was not ordered with any "fudge" room. And my thought for the fan and drive was to 1) Meet minimum ventilation codes 2) keep the smoking area negative with differential pressure sensors by giving analog signals to the drive to ramp up and down the drive output.

    JRaef, thanks for the 50% reply, not sure why I didn't think of doing that...... I'll program it in the parameters for a 50% max output. Without doing the calculations, I'm guessing this fan ducted properly will move 3-4K CFM, so I couldn't see us needing to move that much anyway.

    As for the smoking part, it's one of only a few bars anywhere around here that I know of that meet the qualifications. The smoking room is only connected to the bar area with common hallway, and the smoking area is more than 75% windows, which meets the guidelines for a smoking area.

  7. #17
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    Getting into the nitty gritty details of exactly how the current is drawn by the individual diodes is not actually helpful to the OP, who was simply asking for advice on feeding single phase power into a VFD that is driving a 3 phase motor. I've let it go this far because I apparently made a semantic error and was concerned that the direction that some responders had gone was causing confusion, but I really think this needs to be done with now. I don't think the OP is needing any more clarification and certainly would not really care about the semantic accuracy of what I said. I have removed the superfluous postings as best I could, it was getting too far afield and was not germane to the OPs question.

    The "1.732" issue was only mentioned because frequently, I see people purport that this is the ONLY issue when over sizing a VFD for a single phase input, when in fact it is not, the DC bus capacitance is the bigger issue and requires a minimum of 2x over sizing, 1.732 is inadequate and risks compromising the VFD. Let's leave it at that now.
    Last edited by Jraef; 05-15-17 at 03:29 PM.
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  8. #18
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    Jraef:

    The 1.732 factor should not be suggested in this analysis, it is not correct. The propagation of incorrect ideas occurs far too often and creates problems when the next person repeats the same error.

    Besoeker provided the correct analysis that if 4 diodes are used for a single phase rectifier compared to 6 diodes in the three phase rectifier that a diode in the single phase circuit will pass 1.5 times the average current of a diode for the same output power in the three phase rectifier.

    This also means that total diode power dissipation in the single phase circuit only goes up by a factor of about 1.5 because diode voltage drop is approximately constant.

    You are correct that capacitance in relation to ripple is probably the more important factor.

    .

  9. #19
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    So Gar you are good with 1.5 factor?

    Try this: 10a x100v 3ph produces 10x100x1.73=1730watts. Agree?

    How many amps 1ph for same 1730 watts?

    Still want to stick with 1.5?


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  10. #20
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    Quote Originally Posted by Jraef View Post
    The motor draws 3 phase current from the drive, the drive draws single phase current from the line. The difference is the sq. rt. of 3, 1.732.
    We've been there, done that, and had sound technical posts being unreasonably deleted.
    At the risk of the same action being taken again I will reitterate that you simply cannot apply a simple fixed multiplier. For reasons already given - but banished to the ether.
    Last edited by Besoeker; 05-15-17 at 11:18 PM.
    Si hoc legere scis nimium eruditionis habes.

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