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  1. #1
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    Default Assistance with a flexible VFD installation

    The time has come to bring power to my hembrug lathe and I need some advice about selecting the right vfd, there is quite a lot I am grey on and the ebay listings for VFD's havent really cleared much up. What i hope to have is as follows.

    It needs to drive a 3hp star/delta motor sourced off Joe H here.

    I have full 415v three phase available in the factory currently, but in future the machine may come home with me, it would be nice to not need to buy another VFD. But it doesnt seem like many VFDs are designed for both cases but someone may know of a special snowflake.

    I would like to incorporate the machines remote fwd / off / rev switch and a braking resistor but im having a hard time sizing and finding the right braking resistors. Also should reversing be handled at a logic level or by flipping the polarity after the VFD as per it would have originally. Im guessing the back EMF would be nasty.

    The other thing is it would be nice to replicate the original 3 speed ranges, can this be done with fixed value resistors in addition to the traditional pot? or do they generally only have one input for speed? I guess i could garry it so that the pot is switched out and the fixed value resistors in? im assuming the speed circuit is run at something sane like 3.3 or 5v?

    I am absolutely not an expert, when I do come to do this I will have someone experienced over my shoulder but i would like to have all the right lego first.

    Regards - Ralph

  2. #2
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    Default

    Quote Originally Posted by caskwarrior View Post
    I have full 415v three phase available in the factory currently, but in future the machine may come home with me, it would be nice to not need to buy another VFD. But it doesnt seem like many VFDs are designed for both cases but someone may know of a special snowflake.
    seeing as its only 3HP I would just use a single to 3P VFD.

    I would like to incorporate the machines remote fwd / off / rev switch and a braking resistor but im having a hard time sizing and finding the right braking resistors.
    At a pinch you could use an electric heater element cut to size.

    Also should reversing be handled at a logic level or by flipping the polarity after the VFD as per it would have originally. Im guessing the back EMF would be nasty.
    Logic makes more senses - if you flip the logic rev-fwd switch the VFD looks after everything by ignoring the reverse direction until it has fully decelerated the motor.

    The other thing is it would be nice to replicate the original 3 speed ranges, can this be done with fixed value resistors in addition to the traditional pot? or do they generally only have one input for speed? I guess i could garry it so that the pot is switched out and the fixed value resistors in? im assuming the speed circuit is run at something sane like 3.3 or 5v?
    The resistor idea won't change the speed range.

    You won't get the same full power speed range with a VFD as you would with the gears so if you use the full range of speeds at full power the gears provide you will still have to change gears. Even with a Vector drive VFD you can only get a full power speed range of about 5:1 .

  3. #3
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    Default

    I'm not sure if I understand the speed issue correctly: do you want to have just 3 fixed speeds or do you want variable speed?
    Most (if not all) VFDs have the facility to set up 5 to 14 fixed speeds. So you can have a rotary switch with 3 steps, or three push buttons (one for each speed) etc. As far as I can make out on all the VFDs I've worked with, you can't have variable speed as well as fixed speeds.
    For a lathe, I would ALWAYS choose variable speed, irrespective of the original drive arrangement.
    When you say "the original 3 speeds" do you mean the original motor was a three speed motor or are you referring to a gearbox or pulley steps? In either case, I would definitely keep those, since they change the torque of the drive. As BobL said above, VFDs don't do that well.
    Cheers, Joe
    almost completely retired - more time to contemplate projects and spend more shed time....

  4. #4
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    Default

    Sorry i wasnt super clear in the original post, the original motor that i never had was 3 speed, 920 1440 and 2880 rpm, i was hoping for three fixed speeds as well as variable. I had assumed i was going to need to spoof the pot but thats great information there Joe!

    The original speed range is from 40 - 4000 rpm.

  5. #5
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    Quote Originally Posted by caskwarrior View Post
    Sorry i wasnt super clear in the original post, the original motor that i never had was 3 speed, 920 1440 and 2880 rpm, i was hoping for three fixed speeds as well as variable. I had assumed i was going to need to spoof the pot but thats great information there Joe!
    It's not really clear what your new motor is but I assume Joe would not have given you something that is unsuitable.

    I assume the new motor from Joe has a single speed @ 50Hz and is suitable to run on 240V 3P under a VFD?

    The original 3 speed motor would have been very difficult to convert all the speeds to delta. I have only ever tackled 2 speed motors and the the way the coils are interconnected I could not find a way of converting both speeds to delta. It's been a while since I converted one of these motors but I seem to recall that converting the original 1440 RPM speed resulted in 2880 rpm. I could not find a way to convert the other (920 rpm) speed but someone with more experience might be able to do this. Even if all 3 speeds could be converted the next problem is that the switches used to change motor speeds would need to be between the VFD and the motor. This goes against the requirement that the motor be hard wired to the VFD. It is possible to set up an switch box with an interlocking cover to stop the VFD before switching is performed.

    I guess this is immaterial because Joe probably supplied you with a single speed motor. It would be useful to know what the speed @50Hz is.

    This means you will have to toss the 3 speed switch and hard wire the motor to the VFD (no switches between the VFD and motor) and as Joe says run on only on variable speed.


    The original speed range is from 40 - 4000 rpm.
    So, 100:1 speed range - you won't get that at full power with any VFD and you would need a DC powered motor to get that speed range at full power.
    You will have to resign yourself to occasionally changing gears, This will of course give you an even wider speed range than 100:1

    With a VFD on my lathe I change the gears at least 10 times less than when I had no VFD.

  6. #6
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    The new motor is a 1440 rpm GMF, there is then a 4 speed pulley and the backgear to give you the rest of the speed control, all i need from the VFD is 920 - 2880, i suspect the high end may be unachievable but im not goong to be using it like i would the low. Thanks so much for the info Bob.

  7. #7
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    Quote Originally Posted by caskwarrior View Post
    The new motor is a 1440 rpm GMF, there is then a 4 speed pulley and the backgear to give you the rest of the speed control, all i need from the VFD is 920 - 2880, i suspect the high end may be unachievable but im not goong to be using it like i would the low. Thanks so much for the info Bob.
    That sounds like the hercus.
    In that case the high end is easy.

    If the pulley positions are numbered 1(lo)-2-3-4(hi) - might be able to go without gear changes from 1440 to 3500 rpm at full power in gear number 2 and only need to engage the back gear to cover the low range.

  8. #8
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    but im not goong to be using it like i would the low.
    I should hope not.

    That sounds like the hercus.
    In that car the high end is easy.
    A Hercus car no less. Sounds confusing to drive.

    Dean

  9. #9
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    That motor will spin at 2880 (100Hz). I tried it. However, it was not driving anything. My guess would be that it won;t have all that much torque above 75 or 85 Hz. So I wouldn't count on your 4000 rpm (which in my humble opinion is way too fast to spin a decent sized chuck anyway - and only mildly useful for collet work).
    Cheers, Joe
    almost completely retired - more time to contemplate projects and spend more shed time....

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