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  1. #1
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    Default Hypothetical Question

    I have a bit of a hypothetical question. Which would make for a better dust extractor, a 2hp motor with a 350mm impeller or a 1hp motor with a 400mm impeller?
    I ask because a lot of what i read talks about minimum 2hp but the more the better but also the size of the impeller makes a difference too.

    Cheers Peter

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  3. #2
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    The size of the impeller comes first but the motor must be able to drive it for long periods of time. A 15" impeller needs a bit more than 3hp so in the US they use 5hp motors but here in Oz we use 3kw which falls somewhere between the two.
    CHRIS

  4. #3
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    Default

    Quote Originally Posted by woodtryer View Post
    I have a bit of a hypothetical question. Which would make for a better dust extractor, a 2hp motor with a 350mm impeller or a 1hp motor with a 400mm impeller?
    I ask because a lot of what i read talks about minimum 2hp but the more the better but also the size of the impeller makes a difference too.

    Cheers Peter
    Motors need to be matched to impeller sizes / capacity otherwise there is a good chance you will burn out the motors.

    A 2HP is typically matched to a 12" impeller.
    If you try and drive a 13" impeller with it the 2HP will still try to run at full speed and to do that it has to produce more than 2Hp (7A) and in doing so will burn out the motor.
    Throttling down air flow through an impeller by using filters/cyclones/smaller/longer ducting means the motor does less work but also reduces dust collection which makes it a bit of a pointless exercise.

    For a 3HP motor the biggest safest impeller is 13". A 14" impeller can be used although this again assume the impeller is throttled down either with a cyclone or restricted (eg 4") ducting
    It's probably safer to use a 4HP motor with a 14" impeller

    For a 15-16" impeller you will need at least a 4HP motor.

    Bill Pentz recommends any swaps between motors and impellers be accompanies by close monitoring of the motor current. If the motor draws more current than that stated on the motor name plate you run a risk of burning out the motor.

    The same applies to using a 3phase motor and VFD to get the impeller to spin faster. If your motor just meets the current spec at 50Hz driving it faster will draw more current and may burn out the motor.

  5. #4
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    Thanks for ghat explanation, now it all make sense. Sometimes it just takes a little while for it to sink in. Really appreciate the input.

    Cheers Peter

  6. #5
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    One must love this board, Peter.

    I have never asked a question that did not get considered responses from folk that knew what they are talking about. Of course, there are differences of opinion sometimes, but we expect that and I have always found myself wiser after reading the responses to my question.

  7. #6
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    Quote Originally Posted by woodtryer View Post
    Thanks for ghat explanation, now it all make sense. Sometimes it just takes a little while for it to sink in. Really appreciate the input.

    Cheers Peter
    To clarify it a bit more for you and all being equal with ducting etc, a 12" impeller powered by a 2hp motor will not move any more air than the same impeller powered by a 10hp motor because the speed remains the same in both instances. A 3 phase motor driving any impeller will if using a Variable Frequency Drive aka a VFD can flow more or less air because the VFD will allow speed changes. I have come to the conclusion that if it is possible to buy a 3 phase dust extractor do it because of the advantages it provides. When shopping always ask the size of the impeller first and ignore everything they tell you or see written about the flow figures they reckon it achieves because they are ALWAYS optimistic by a huge amount.
    CHRIS

  8. #7
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    I get that but surely there must be other factors at play, eg height of impeller blades design ie straight , curved swept back etc and the design efficiency of the impeller housing ?

  9. #8
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    When comparing motor and impeller sizes one has to assume other factors are constant.

  10. #9
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    I've only had one extractor motor burn out and that was when I lost one phase of three when the fixed wiring (by a sparky) behind the 3ph wall outlet came adrift.

    Having seen a few impellers it's my observation that the manufacturers trim the blades and outlets to ensure that the attached motors supplied with an extractor units don't burn out... at least during the warranty period...
    As a result most units come with under specked impellers to ensure a safety margin for the supplied motors.




    The cost increase for a slightly larger motor is not so great, but the issue for us here in Australia is the step up above 2hp that requires a 15amp circuit, which creates a fault line in the home workshop market. But, why 12" impellers are found in units with 3-5hp motors is puzzling.

    Centrifugal impeller design is a branch of engineering in itself that I took a peek at some years ago when I thought I might have to have an impeller manufactured here in Australia. See a brief overview of the different blade configurations, pros and cons, in the first article in the following link...

    Centrifugal Fan - an overview | ScienceDirect Topics

    Fortunately larger impellers can be bought here now for a reasonable cost compared to importing a one-off from overseas.

    Besides cost, I think that the impeller designs being sold here for use in the home workshop have as much to do with their noise level characteristics as other performance features.

    If anyone is thinking of making or modifying an impeller have a look at static and dynamic balancing, which will be an essential final step.
    Stay sharp and stay safe!

    Neil



  11. #10
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    Quote Originally Posted by NeilS View Post
    I . . . . . . . .The cost increase for a slightly larger motor is not so great, but the issue for us here in Australia is the step up above 2hp that requires a 15amp circuit, which creates a fault line in the home workshop market. But, why 12" impellers are found in units with 3-5hp motors is puzzling.
    2HP motor and a 12" generic impeller on a single bag DC sees the motor slightly overspeced for that setup so opening up the impeller to accommodate 6" ducting is possible.

    However, a twin bag DC will have much less restriction to flow so using a 12" impeller and 2HP motor on such a set up will draw too much current so the easiest way around this is to put a 3HP motor on it. Fortunately the 3HP will be overspeced so the impeller can also be modified to take 6" ducting.

    The same applied to 5HP DCs which usually have even bigger filters.

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