Variable Frequency Drives

Discussion in 'Other metal working projects' started by Al2O3, Aug 24, 2018.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Considering buying a VFD for my mill and had a couple questions. Electrical controls are not my strong suit.

    My mill is 2hp 220/240 vac 3-phase and the 3-phase source is a rotary phase convertor (RPC) in my shop. I see fairly economical VFDs that use either a single or three phase source.

    Would I be better off using a single or three phase source for the VFD? I ask because the manufactured phase of the RTC is a wild leg that measures higher voltage to ground than the other two legs. I see the cautions about the use of RPCs on CNC machines and making sure to use the non-manufactured legs for magnetic controls. I was wondering how that may affect a VFD. Also, I presume motor performance reduces with speed as well, is that typically the case?

    Best,
    Kelly
     
  2. _Jason

    _Jason Silver

    Is your mill CNC or conventional?

    By and large, you probably won't have a problem using a VFD a motor regardless of whether the source voltage is single or three phase. VFDs electronically convert the input voltage from AC to DC, then back to AC. The output isn't a true AC in the same sense as what you'd get from an AC generator, but it works enough well for motors. And, there's a bunch of garage guys that use them to control the speed of and/or reverse small motors on garage machine tools.

    VFDs come with various modes of operation so your mileage may vary on performance, but generally speaking you should be able to control speed and maintain same torque output over a range of speeds although torque will drop off once the motors slows down enough. That depends on the motor and the settings programmed into the VFD.

    I dealt with them for a few years when I was in electrical distribution so mine is more of a higher level of understanding that a technician in the field installing them. Hopefully someone else can chime with specifics.

    Who's drives are you looking at? Automation Direct or one from a local electrical house?
     
  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Conventional 3-axis Wells Index. I'm guessing 60's vintage. It's an old war-sword but well bought with DRO and does what I need for the most part. 2HP 3-phase induction motor with usual step pulley so VFD would be superimposed over any of the speed combos selected. It sees pretty light duty in my shop. It'd be nice to vary speed with a knob. Also going to make a power draw bar for it to speed up tool changes.

    That would be a bonus.

    So far just the imports. Just saw some posts on a couple of the machining forums that pointed toward eBay auctions and was surprised at the (low) $85-$125 price range but leary.

    https://www.ebay.com/itm/2-2KW-3HP-...135626?hash=item465940e40a:g:278AAOSwrGFZk7t~
    https://www.ebay.com/itm/UPDATED-22...198547&hash=item19f554944b:g:Y2IAAOSw0LlZlUM9

    Best,
    Kelly
     
  4. PatJ

    PatJ Silver

    I would not connect a 3-phase vfd to a rotary converter for several reasons.

    1. There is n0 need to put the wear and tear on the rotary converter when there is nothing to be gained by doing so.
    2. The output of a rotary converter is probably not that good of a waveform as far as 3-phase goes (as you mention).
    3. A single-phase input vfd will produce the same output waveform as a 3-phase input vfd, unlike a rotary converter which produces an approximate 3-phase output.

    I specify large vfd's for industrial/municipal use (so this may not be relevant to small motors), and one of the known problems is that the output of a vfd has spikes in it, thus requiring a "inverter-rated" motor, which has winding insulation that is rated to handle the spikes.
    I have not heard of anyone using vfd's on small motors having problems with the motors, so perhaps the small vfd's have an output filter.

    For large motors, I specify an input reactor (since you don't want to feed harmonics back into your power system) and an output filter. For a small vfd, the harmonics will probably not be significant, but something to be aware of since this can affect electronics in your home.

    The longer the motor leads, the worse the output voltage spike problem and the more necessary the output filter.

    Hope this helps.

    Edit:
    I have also specified using a surge suppressor on the output of vfd's to combat the spike issue.
    The vfd manufacturer will tell you that you should not use a surge suppressor on the output of a vfd, but if you call the manufacturer and get one of their vfd engineers on the phone, you will find that it can work, and in actual experience it does seem to work well in 3-phase applications.
    If you use a surge suppressor on the output of a vfd, be sure to fuse the suppressor in case it fails.
    Another reason I use suppressors on the output of vfd's is because often the motor is located remotely in the field (ie: a remote well pump), and I guess because the motor/pipe is an excellent ground, they tend to attract lightning, which travels back to the vfd via the output wires, and so a suppressor is required on the vfd output (and it works; proven at a water plant that was having hundreds of thousands of dollars of vfd failure every year, and now has none).
     
    Last edited: Aug 24, 2018
  5. ESC

    ESC Silver Banner Member

  6. The main difference between a three phase and single phase VFD is the bridge rectifier unit, in both cases the mains gets rectified into DC and then a reservoir capacitor bank. It's possible to run a three phase VFD off single phase if you derate it by at least half i.e. a 5Hp three phase becomes a 2.5Hp single phase and you match the output voltage. This is simply because one set of diodes pass all the current in the three phase bridge rectifier when running off single phase. My older Allen Bradley 5Hp three phase inverter actually will run off single phase 415 VAC (240 to 415 step up transformer), three phase 415 VAC and 600 and something DC Volts and has dedicated DC input terminals.

    There are more expensive VFDs that will even step up the voltage, so a 240V input will be stepped up to three phase 415V. Most common units just convert 240V single phase to 240V three phase which requires your three phase motor's voltage to be changed (fairly straightforward).
     
    Last edited: Aug 24, 2018
  7. Petee716

    Petee716 Gold Banner Member

    I bought 2 of these a couple years ago for a table saw (2hp) and a Clausing 8520 mill (3/4hp). Both machines are 3 phase. I have another potential deal coming up on a bandsaw which I suspect is 3ph as well so I may be in the market for another VFD.
    https://www.ebay.com/itm/US-2-2KW-2...448615&hash=item3b193a29a0:g:iFAAAOSwf~9ZaYN7
    Once I figured out how to program them they seem to work as advertised although I may be having a power issue with the saw. I was ripping some rough cut oak at full depth last winter and was having some stalling issues but I really don't know if that was because of the motor or the VFD. Perhaps I should revisit my programming one more time, but I'm reasonably confident in the settings.
    I don't have any trouble with the mill. Although I use a potentiometer on the mill to vary speed through the VFD, I still do my major speed changes with the pulleys.

    Pete
     
  8. Melterskelter

    Melterskelter Gold Banner Member

    "Electrical controls are not my strong suit."

    Sort of repeating myself from another thread, but I do like the service that goes along with Automation Direct VFD's. They do have a USA distribution center and help desk that picks up the phone and can patiently and clearly answer questions you may have. I have two in my shop and set up a third for a friend. The documentation and design of the VFD's made that easy. One time I did get a defective drive from them and they immediately sent out a replacement on my word that I would send in the unit I had. Can't do much better than that.

    Denis
     
  9. Mister ED

    Mister ED Silver

    Kelly, I cannot comment on a VFD and phase converter ... but ... get the VFD! How many motors are on your Index? Separate 3ph motors for X and/or Y axis? That could complicate things.

    My first experience (at home) was when I bought my "new" lathe a couple years back. 3ph 1.5hp motor form 1965, runs flawlessly. Of course it gets light use, I leave it at 60hz (lathe has a reeve's drive). Easy to program and convert existing reversing drum switch to low voltage to run the VFD. This is a TECO 1ph to 3ph VFD.

    I like the VFD so well, I decided to convert my Jet import mill to 3ph. Finally picked up a motor of same metric frame, bought a surplussed Allen Bradley 1ph to 3ph VFD, and a bunch of push button controls wiring and hardware. This thing is slicker than monkey snot. I have hooked up controls for on, off, for/rev, jog, and a pot. The AB vfd has a small built in brake, so I have it set for a rapid deceleration time. All simple to hook up.

    Although I really have not had a chance to work it hard, I think it will do the trick. My biggest issue was I couldn't get slow enough with available belt settings for some operations (gear cutting).

    Best advice on choosing a VFD ... look up the manuals on line and see which you can understand. I have dealt both with Automaton Direct and Wolfe Automation ... both have been good.
     
  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Thanks for comments so far guys.

    IMG_2950.JPG

    Two motors, spindle and feed. This is why I posted the question because my Index has power feed on both X & Y and a separate fractional horse motor (no name plate but I suspect 3ph) that drives the table feed. It's not the typical add on system on the lead screw but a motor driven gear box with drive shafts and mechanical engagement dogs for each axis. There is also a reversing barrel switch for the table feed.

    s-l160010.jpg

    So.....I'm thinking keeping the input power 3-phase and just introducing the VFD at the spindle motor would be the simplest thing to do for the conversion but thus the question about how the VFD is going to cope with the manufactured phase from my RFC.

    Best,
    Kelly
     
  11. Mister ED

    Mister ED Silver

    Interesting, hard to tell but looks like a model 747 but with the earlier Super 55 feed setup?

    I think I would call Automation Direct and talk to one of their VFD techs about the RPC and VFD.
     
  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Agree.

    The Index's control box looks like something someone put together for it....5lbs of crap in a 1lb box. It has a reversing contactor, transformer for control voltage, and main power contactor with thermals, fuses, and a few other components. Inserting the VFD at the spindle motor would be easy enough. I wasn't looking to reengineer the whole electrical control system, as I certainly don't need to add another project at the moment. I have too many started already. But obviously, wouldn't want the feed system to be affected by the VFD and although it wouldn't be the end of the world, would rather not have both a 3ph and single phase connection.

    Best,
    Kelly
     
  13. Mister ED

    Mister ED Silver

    Also, maybe it is an early 747 and that feed setup was carried over from the Super 55/555. If you have serial # plate, a call to Wells may shed some light (still in business not too far from me). They may even be able to provide original wiring diagram (although no telling if it has been changed over the decades).
     
  14. crazybillybob

    crazybillybob Silver Banner Member

    Kelly,
    I have an old Clasuing 8540 Horizontal Mill happily running on a VFD. The trick is to wire up the drum control switch on the machine to the VFD controls. VFD's don't like the loads switching off, it will blow them out. So you just attach the contacts in the control switch to the control input pins on the VFD instead of the line power to the motor.

    CBB
     
  15. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    CBB, as mentioned, not my strong suit, but as near as I can tell from reading a couple VFD manuals, it doesn't seem like the input phasing has any affect on their output. But it also seems that a reversing feature on the VFD output is pretty standard......so if I'm understanding your advice, I'd need to remove the downstream barrel switch on the spindle motor and use the reversing feature on the VFD. That's probably slightly less convenient but not a big deal as I don't do much tapping on the mill and if I did, I'd probably buy a tapping head. I need to take a closer look at the controls and the table feed and drive system.

    Best,
    Kelly
     
  16. Melterskelter

    Melterskelter Gold Banner Member

    Actually, I find my remote keypad considerably handier than a drum switch. Mine is mounted more conveniently than the drum. Tapping is great as reversing is done by just hitting the reverse button. The motor ramps down, reverses, and ramps up. Ramp rate is easily selected by the user.

    Denis
     
  17. _Jason

    _Jason Silver

    Kelly,

    Reversing 3 phase motors is pretty easy - the phases are labeled A, B, C - reversing usually just swaps A for C. Your drum switch may do this now and a VFD can definitely do it with the correct programming.

    Were I set up a VFD on a motor, depending on the application, I would likely wire it with these buttons:

    • Start/Stop or Start/Jog/Stop
    • Forward/Reverse
    • Speed (rheostat)
    • E-Stop
    Depending on how the VFD is configured, there'll be a terminal block on it somewhere that's used for input wires coming from switches/buttons/knobs. This is one of those things where you'll have to sit down and think about exactly what you want to do it so you can configure all the controls etc. appropriately.

    Here are a couple photos I found online that kind of demonstrate what I'm referring to about setting up the machine with pushbuttons:

    [​IMG]

    [​IMG]
     
  18. Melterskelter

    Melterskelter Gold Banner Member

    Yup, swapping any two leads on a 3-phase reverses it. So, if you hook up the VFD and it defaults to reverse rotation, just switch any two leads. From then on it will default to forward rotation.

    Denis
     
    Last edited: Aug 27, 2018
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Though not my strong suit I do have a decent grip on the basics of motor controls both 3-ph and 1-ph. The two VFD manuals I have downloaded and browsed indicated the output phasing of the VFD is insensitive to the input power phasing. Just means any rotation phasing adjustments is done on VFD output not input. Didn't know about CBBs caution about their sensitivity to downstream load switching. No reason to do that since all inputs for all the switching functions of interest seem to be standard on most all VFDs.....and man, there are a lot of them out there. I sent off a question to a couple manufactures about how well their VFDs would cope with the input power from the manufactured phase from my RPC where the voltage to ground differs from the other two legs. We'll see what they say.

    Best,
    Kelly
     
  20. crazybillybob

    crazybillybob Silver Banner Member

    Kelly,
    You understood what I said completely correct. The drum switch gets disconnected from the motor. So the Line voltage goes to the VFD. The L1,L2,L3 go to the motor. To keep the convenience of the drum switch I ran some 18AWG wire from the drum to the proper terminals on the control bus of the VFD (these very by manufacture and model so my AB VFD is different from my automation direct VFD hookups) as melter noted you can set the ramp up and ramp down when changing directions. I have palns to add a remote speed knob so that I can adjust the speed of the mill more easily (my VFD is mounted on the wall behind the mill so a box by the drum switch with a knob would be much better) .

    CBB
     

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