HLV-H VFD conversion ten years on
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  1. #1
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    Default HLV-H VFD conversion ten years on

    I've just received a really good PM question from a forum member regarding the HLV-H VFD conversion that I completed ten years ago. I t asks what i would do different now .

    I thought it might be worth posting my answer here for other's to see and comment on.

    Switch or not to switch?

    In 2009, new VFDs were very expensive so , I bought my VFD from eBay (3ph ones then were cheap!) .It was a conventional switched output VFD and so it tends to lack power at lower output frequencies, leaving me little option but to switch motor windings.

    There have been suggestions that there is no need to switch motor speeds, if one used a modern sensor-less vector type VFD. These VFDs can compensate at low frequencies and provide much more torque. I have not tried one , so I can't be certain but: The Hardinge motor has a 3:1 speed change and that is asking a lot of the VFD (in 'low' speed it would be only putting out 17Hz) and motor.

    It is notable that when Hardinge produced an all electric vary speed HLV-H they up graded the motor to 5hp .

    I remain a switcher :-)

    When to switch motor speeds?

    There is at least one forum member that will tell you that VFDs are happy to be switched under load . I strongly disagree, and so ,as it happens, do Siemens and every VFD maker whose manual I have read!

    I played around trying to make the VFD 'catch' the spinning motor, to allow seamless speed changes, after powering down swapping contactors. In the end , that proved unreliable, so the stop /switch/ start logic was used.

    When I started the conversion , I had been building electronics devices for years and wanted to get away from microprocessors and complex logic, which is why I chose to use simple relay logic.

    It turned out to be quite complex relay logic and made the machine somewhat 'clunky', so IF I were to do the SAME conversion, I think I'd now wrap the logic up in a PIC micro-controller and use solid-state output to drive the contactors.

    However , I have been thinking about different ways to do the conversion: Someone on this forum suggested two VFDs ,one for each speed.

    The two windings are separate and, provided the other VFD is disabled, each VFD could be optimised to drive its 'motor'. I have a couple of spare Siemens VFDs (one for the speed jack) and intend to try this at some point. I may be able to get the 'flying motor catch' to work with this setup.

    Bigger VFD and more power ?

    Something that has come up recently, in relation to milling machines , is that the VFD tends to limit the maximum motor power : Motors are very robust and can easily produce far more than their plate rating for short periods if the current is available. A bigger than 1.5hp VFD (and transformer) could be easily used and would provide more power , but I can't say I've ever needed extra power on the lathe.


    Transformer to voltage doubler?

    In most of the conversions (240v 1ph to 415v 3ph) I have used a step-up transformer to get the higher voltage single phase for the VFD. In a couple I have used a capacitor voltage doubler. In one factional Hp device all it required was a simple wire link inside the VFD . For my brother's mill conversion I built a large 4kW unit with a couple of large high voltage caps (from a dead VFD) that is nice and compact and has been working fine for a couple of years.

    There are VFDs available now with a capacitor doubler inside , so perhaps that's the way to go rather than a difficult to find transformer?

    What else?

    One thing I would suggest is a VFD with braking resister option (do not confuse this with dc injection braking); The spinning motor and gubbins generate a lot of volts when the VFD tries to slow them down , this power has to be dumped somewhere and without a braking resister , the bus voltage will rise causing the VFD to give up with an over-voltage fault (requiring a reset) . I managed, to get the braking time down to something usable but would be better with a BR.

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    Nice follow-up Bill.

    BTW I have a couple of spare braking resistors salvaged from a lift motor control if you ever need one. I have one of them fitted in my HLV cabinet for braking with the vfd.

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    Thanks Peter :-)

    My problem is not the resister but the thing to drive it . The VFD I used has no braking transistor, so i'd have to build a bus voltage limiter (I have ideas how to make one but no time to do so ATM)

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    Here's an untested diagram for a twin VFD system
    Attached Thumbnails Attached Thumbnails hlv-h-397-control-diagram-twin-vfd.jpg  

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    As one of, possibly the main "at least one forum member", I should note that my VFD is 23kVA and for that series, the manufacturer (Danfoss) specifically states that downstream switching is not a problem.

    I also looked into the possibility of using a variable speed motor with the HLV (no -H), and also came to the conclusion that a 5hp motor was the minimum that would be needed. Despite what anyone may say about the Reeves drives on Hardinge lathes and vary-speed Bridgeports when they are playing up, they match the motor power to torque requirements of a machine tool better than VFD's generally do.

    PS, On Bill's diagram above, both VFDs need to be powered all the time, or the output stages of one that is 'off' will get fried by the other one.

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    hi Mark, actually you're not the guy i had i mind :-) I'm sure your huge vfd will be fine.

    Re: the twin vfd setup, actually the motor doesn't generate any significant volts on the unused winding, i have measured it . I had hoped it would, so i could use it to power the speed jack and avoid another vfd.

    Certainly both need bus volts but the non-driving one will not see any significant current.

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    Not on a Hardinge but my not quite so huge 10 HP plug'n play Teco box modified to 220 in 440 out by Drives Direct has also notched up over ten years handing the single speed 3 hp motor on my Smart & Brown 1024 and the 3 hp two speed on my P&W B 12 x 30 as well as all the rest of the workshop gubbins. Bridgeport, Pollard 15 AY, Rapidor, Hydrovane, Clarkson et al. Usual stuff. All run via the standard switchgear. Even the P&W is still on the huge and splendidly steam-punk Crabtree oil immersed starter box fitted when it arrived during the war.

    Probably the important issue when using standard control gear is to have enough overload capacity and ability to safely absorb the run down current. Looks like Drive Direct got their sums right for mine but I have output smoothing chokes and wouldn't care to start anything over about 3 hp on a regular basis although DD claim 5 hp would be fine.

    VFDs are hard on their capacitors at the best of times, voltage doubled systems are much worse.

    I reckon you need a two speed belt system to satisfactorily replace a Reeves drive with a simple VFD system. Realistically that means a remote selection system with both belts running all the time. My Bridgeport Varispeed is getting noisy again. As rebuilding once in a lifetime is quite enough for any sane person I'm looking into a two speed belt system. Looks like I'm going to end up either getting creative with ex car air conditioning pump electro magnetic drive clutches or coming up with a shop made spline drive and dog clutch arrangement.

    Clive

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    A couple of addenda:

    I added a three way VFD frequency selector controlled by the threading lever, that allows one to control the speed and hence acceleration when threading :

    Hardinge HLV-H threading-lever speed control

    And , most useful of all, a button in the same box to reverse the spindle , which makes tapping using the tail-stock a joy.

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    Did you leave the original vari-drive in there or do you drive the spindle direct? If you left the varidrive in do you use it at all or leave it set on a certain speed?

    I bought a fully converted Omniturn on a AHC 20+ years ago. it came with a VFD controlled by the computer. They (Omniturn) installed a 5 horse motor as needed for reversing and acceleration. They left the original varidrive in there. The Hardinge varidrive on an AHC had calibrated stops for minimum and max speeds, an air cylinder would have pushed the drive to the upper and lower stops. I set mine with the stop to 1 to 1 on the pulleys and let the VFD do all the speed control.

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    FredC.

    The reeves drive is intact and the main way to vary the speed. It has 8:1 range and torque multiplication which is really necessary with the standard 1.5hp motor.

    I use the VFD speed control mainly for tapping and occasionally for threading , if I have to soften the blow to the dog-clutch.


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