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  1. #16
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    Thanks Chris, I as mentioned I use SEWs and they definitely cannot run any size motor. Of this I am absolutely certain as I just confirmed my memory from the manual. If you are not familiar with these, they are from Germany and considered extremely good drives, some would claim the best available. I use the Movitrac series and as far as I'm aware they are a current model. So it would seem it would depend on the manufacturer. They are also capable of running multiple motors up to the total recommended maximum of the drive, however that is not the situation we are discussing here and is quite a different scenario.

    I must stress that I am not suggesting I'm any sort of VFD expert, far from it. Instead my background was in electronics and so in this area I just do what the manufacturers say. Nevertheless, with respect, I would suggest your comments about the inverter rated motors is not correct. The problem is not with the drive, it is with the motor itself, and is largely to do with the way the motor is wound with regards the amount of voltage seen by the first winding and the potential for it to be wound close to the last winding. The EMF may be such that it can overwhelm the breakdown resistance of the insulation. Unfortunately I'm away at the moment so have limited access to resources, however this is just the first paper I was able to find that partly explains this http://www1.eere.energy.gov/industry...ip_sheet14.pdf

    Yes I would be very keen for you to point to a manufacturer's manual that recommends or in any way suggests using a larger VFD than the recommended size when controlling a single motor. The drives I'm using won't even allow that to happen!! Before I bought the Sews I was reading another manufacturer's manual and that was much the same, but from what you've now said it seems to vary, so I guess "buyer beware" in this regard.

    As far as I'm aware all modern VFDs tune themselves to the motor load, however I'm somewhat confident there is a limit to this. Quite what that limit is I have no idea. It is why I have repeatedly said that it will probably work just fine, but who knows? Will a 22kW drive properly control and protect a 1/2 hp lathe motor? Probably not. So where is the limit? If in doubt I maintain to do what the manufacturer recommends. This is why I'm keen for you to show me where they are recommend over-rating the drive as I've never seen that, only to use external braking resistors if required. I also temper my comments in light of the potential audience. If this was a conversation between the two of us I'd be inclined to say "Different size? yeah go for it!", that is after all precisely what I intend to do, and again I will use the Movitrac drives to do so. However this is a public forum with, with readers of all backgrounds, some of whom believe wires are there to contain the smoke-genie . I'm very reluctant to put into print recommendations that may be picked up by people days, weeks, or even years down the track by way of searches that recommends practices not in keeping with the manufacturers' recommendations. What I may say quietly to a mate is sometimes quite different to what I'd suggest publicly.

    Edit: Just as an afterthought Chris, again I would be also quite cautious about recommending over-rating a drive if the possibility to do so is limited to only very modern drives. Many of the drives seen on ebay are actually used drives that have been pulled out of existing installations so they may not be the latest design technology. I feel it's quite likely these are precisely the type of drives that may be purchased by readers here. From what you've said it seems the most modern variants are somewhat more flexible, however once again it looks like "buyer beware".

    Cheers,
    Pete

  2. #17
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    Pete, the VFD on my mill may serve as example. it is a Siemens micromaster vector 6se32. Very nice to program. It comes with all sort of manuals on a CD, but you can download its main operating manual here: http://www.mawos.com.pl/Files/MMV_6SE32_OI_EN.pdf
    On page 34 is a summary of the motor nameplate parameters you must enter (unless you use predefined motors made by Siemens). These are:

    P080, cosinus phi (the efficiency of the motor)
    P081, base frequency of the motor, usually either 50 or 60Hz
    P082, motor rpm at base frequency (depends if it is a 2, 4, 6 or 8 pole motor)
    P083, nominal current rating
    P083, nominal voltage rating
    P085, nominal power rating

    more details to these on page 58 of above manual.

    Additionally there is P089, the motor stator resistance. This can be measured with an ohmmeter on the cold motor. Alternatively the VFD can measure it using P088 (automatic calibration).

    As you can see, this drive can be optimised to just about any motor from 0.1kW up to its maximum power limits. It can compensate for older European motor voltages of 220V, as well as for Australian 240V by changing its V/Hz characteristic. By knowing the base frequency, the drive knows if the motor power rating is valid at 50 or 60Hz (a motor that is rated say 1kW at 60Hz may only have some 0.85kW at 50 Hz, or in other words a motor can be made smaller for a given power if run at a higher frequency). The proper kW value is important for the overcurrent protection.

    Further motor depenndant parameters that need be set by experiment are:
    P386, sensorless vector speed control loop gain proportional term
    P387, sensorless vector speed control loop gain proportional term


    On the 260 lathe I use an Omron / Yaskawa Varispeed V7. This drive has an excellent user manual with some detailed explanations. And this drive has more features than any other small VFD I know of. See here for this very informative manual (it is a large file though):
    http://www.yaskawa.com/site/dmdrive....55_1d_11_0.pdf

    Chris

    PS: regarding your afterthought: I would not recommend purchasing used or old stock VFD's, not from eBay nor from anywhere else. VFD's are low cost commodities like TV sets and computers. They have a limited lifespan, say 10 years or so normal use, and repair of low horsepower VFD costs more than to buy new. Especially the large bus capacitors (electrolitic) are prone to loose their formating when not used for a long time, and may simply explode the first time the VFD is turned on. It is very hard to find replacement because of their proprietary sizes. Second, like all electrolitic capacitors, they deteriorate with high operating temperatures, and you cannot know if the used bargain you buy was operated in a small not vented enclore whilst being driven at the top end of its power rating. VFD bargains can be found on eBay, especially from Europe, but make sure you buy only brand new and unused, and make sure it is a current production model that did not spend 5 years unused on a shelf. If you must buy a bargain old stock VFD, use a variable transformer (variac) to progressively increase the mains supply voltage to it during the first hours of power-on time, this will give the capacitors a chance to recreate their thin insulation layer so they can again withstand the full line voltage without bursting. Also the IGBT modules are very special proprietary designs, very hard to find. By the way, there are only few large semiconductor makers for these (Toshiba, Hitachi, Mitsubishi, Siemens....), and they never supply their newest models to the many makers of competition VFD's. These must use the older technology IGBT's. So the newest technology VFD's are always those manufactured by the few well known large semiconductor companies. Chris

  3. #18
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    Hi All,

    Thanks for an interesting discussion, I can support what PeteF is saying, the modern vector drives have a software model of the drive that does some fancy trickery to determine what the motor is doing so far as motor loading, current, shaft rpm and so on, and adjusts the drive to the motor to maintain constant torque and so on. Some have automatic motor adaption algorithms that determine the stator resistance, rotor resistance, stator leakage reactance, main reactance, It's not just kw, voltage and current limits..Sometimes you can have problems where the motor parameters can't be automatically determined because there is an LC filter between the drive and the motor. I wouldn't be surprised to find there are limits on the ranges for some parameters. Most (All) of the systems I've ever been involved commercially with have the drive matched to the motor power.

    That said, there is another type of drive, the lower cost V/F class of drive are just that, voltage varies with frequency, for that type of drive, you could just set the current limit to suit whatever size motor you have. Since they don't do any fancy modelling of what's going on in the motor, it probably doesn't matter if you have a 1.5Kw drive and a 750w motor..

    Regards
    Ray

  4. #19
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    Hi Chris,

    I think I cross posted with you, I'm pretty sure we are saying much the same thing..
    Mostly we use Danfoss drives, but in the past ABB and before that PDL.

    Not so much with Omron or SEW for that matter.. the detail varies a bit between brands

    Regards
    Ray

  5. #20
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    Chris, I definitely don't want to keep going backward and forward with this, as I don't see there's much point in doing so. However can certainly say it's been interesting to see some developments in the latest drives. I appreciate your copying and pasting the programming instructions, however with due respect they are not recommending to run a larger drive than recommended. Once again, if I could trouble you to point to a manufacturer who recommends over-rating a drive I would appreciate seeing that.

    Unfortunately despite my repeated emphasis, it seems that this discussion has zoomed in on driving a motor and completely ignored the other function of the VFD, that is to protect the motor. Just because a drive does one doesn't automatically mean it will necessarily do the other successfully.

    With regards you opinion on new versus second hand drives, I appreciate that is your opinion and you are perfectly entitled to it. I do not agree with some of your conclusions, however see little point in taking that further. In addition there are many very good brands of drives available not made by the companies you mentioned. Personally I would far prefer to have a quality drive from a quality manufacturer, albeit used, than a piece of unsupported junk from China complete with its Chinglish "instructions". Some members here have already discovered how using these cheap drives may not be quite as simple as it may have been. Perhaps our definition of "cheap" is different, however I would hardly call a quality drive from a quality manufacturer a "low cost commodity". For members here who may be looking to convert, say a used Hercus 3 phase lathe to VFD drive, if they took your recommendation and over-rated the drive to 1.5 kW I would expect them to pay about the same amount for a new Sew or equivalent quality drive as the cost of the lathe they have just bought!

    As far as "exploding electrolytic capacitors", can you point to some examples where this has actually happened in used VFDs? Not here-say or rumours, but actual documented examples. Buying used is always a risk, however I personally feel that VFDs are no more risky than any other used purchase. Anyway, all of this is a somewhat long-winded way to say that I would NEVER, EVER recommend running a VFD through a variac. That type of thing "testing" is old-school and should have gone out along with the tube tester. Modern electronics, ESPECIALLY microprocessor controlled, should be protected against gross input voltage variations. However there are a lot of "shoulds" in the world and so they aren't always successful. Under-voltage of appliances kills far more electronics than over-voltage (with the exception of lightning strikes), and I would never suggest anyone run them through a variac. That is of course just my opinion, however I can honestly say in all the years I worked in this field I didn't even personally own a variac!

    Thanks again for going to the trouble of reproducing the programming instructions, it's interesting to see how other drives tackle the situation.

    Pete

  6. #21
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    Quote Originally Posted by Pete F View Post
    I appreciate your copying and pasting the programming instructions, however with due respect they are not recommending to run a larger drive than recommended.
    Yes Pete, they ARE recommending to use a drive rated larger than the motor!!! They just say it in another way than what you are probably expecting.

    They say that you must derate the drive by 10 or 20% if you want to use the two highest chopper frequencies. What other is this, than to say you must use the next size up VFD if you want to use this feature. To drive a 1HP motor without this nasty annoying singing noise, you must therefore buy a 1.5HP VFD.

    Same if you want to run your VFD at higher than room temperature. For example in a non vented enclosure.... like maybe a lathe cabinet. They ask you to derate your drive by 10% for every 10C above room temperature. That is easily 20% dearting if your VFD is in sommer in a hot garage workshop. Again you must either buy a 1.5HP VFD to drive a 1HP motor, or else accept that the VFD will throttle back into limp mode when warm and you won't have 1HP at the motor shaft but maybe only 0.8HP.

    Repeated start and stopping also requires derating. A VFD used to drive a fan or pump may be started only once every hour. But on a manual machine tool, you may start and stop in short succession to take measurements etc. Accellerating that heavy chuck takes a lot of energy. The VFD gets hot and must be derated.

    Take my word for it, in industry it is common practice to oversize the VFD. It improves reliability as it lowers VSD operating temperature, it improves overall drive performance, especially if often accellerating/braking as the VFD won't go on "limp mode" all the time, nor will it trip out as often. It let's you be more casual about how you use and setup your drive, because you need not always worry if you should or should not derate your drive..

    Your argument about motor protection is valid. But I assure you it is 100% taken care by the VFD, as long as you correctly enter the nameplate figures of your motor.

    Chris

    PS: I think the reason the drive makers do overrate VFD's, is because of the way these are marketed like commodities. It is a VERY price competitive business. VFD's are priced by their power rating. Consumers compare Price with HP rating, without always understanding why a larger drive would be better for their particular use. It is a litte like when you buy an air compressor. The motor may well read 3HP, but it has the size and pumping power of a 1 HP industrial compressor motor......

  7. #22
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    Hi Pete, Chris,

    Just a few comments and clarifications...

    I see no problem using a low cost 1.5Kw V/F drive to drive a 750w motor, just set the current limit accordingly.

    However, using a vector drive which sometimes depends on complex software for electrical and thermal modelling of the motor, to drive a smaller motor than the drive is rated for, might cause problems, it probably depends on the brand of drive. I've generally used drives rated the same as the motor rating, but that's a cost issue..

    Certainly it's preferable to use a drive rated higher than the motor rather than one with a lower rating...

    Just to clarify a few other things that have cropped up during the discussion...

    VFD rated motors, the main issue here is the breakdown voltage of the insulation, NOT the way the motor is wound. It's common practice to install dv/dt filtering where long cables are involved, i've only ever seen dv/dt ( high voltage spikes) cause insulation breakdown on submersible pumps, which involve long cable runs. The cable capacitance can cause a ringing effect.. I doubt that you would ever see voltage spikes great enough to cause insulation breakdown running from a 240V vfd. The place to look with a cro if you want to experiment with this is on the transitions.

    Please note LC filtering is not the same as dv/dt filtering...

    Chris, mentioned that pumps and fans have limitations on starts per hour.. partially true, but that has nothing whatever to do with the drive loading. The problem is usually mechanical stress on bearings, and for example grundfos specify a limit of 10 starts per hour for some DOL pumps, but no limits on number of starts per hour if operated on VFD, provided that the accelerations are kept with certain limits, no more than 3 seconds from 0 to 30Hz if I remember correctly. Fans, I usually configure for spin start, so that if the fan is auto-rotating, I can catch it without overcurrent trips

    Lastly exploding capacitors and variacs.. what a great topic... first, you only use the variac to power the drive and slowly charge the caps, to reform the dielectric, you wouldn't run the motor from the variac powered vfd...

    As to how long a drive would have to sit before the electrolytics deteriorate sufficiently to cause a problem, I don't know, but it would be quite a few years... maybe 10 I guess. So, yes it's a real problem, but I suspect not very common.

    Regards
    Ray

  8. #23
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    Ray, as I said to Chris I'm pretty much done with this but just to clarify a couple of points. Yes it is the way the motors were wound AND the insulation. Unfortunately I just broke my laptop screen so can't get the links to the relevant papers, so I'll have to leave you to do the reading yourself. Essentially however the voltage peaks during the ringing that occurs are far greater than the motor operating voltage. Studies have placed measuring points of each winding of the motor and found the first winding in particular receives a disproportionately higher voltage. If this winding happens to pass close by the last winding the voltage potential is at it's greatest and may be above the breakdown resistance of the insulation. It is often just pot luck as to whether that will occur and part of the reason why one motor will fail when run on an inverter, while an otherwise identical motor will be just fine. Inverter rated motors have both better insulation and are wound such that this coincidence won't occur.

    I think our discussion has now well established that not all VFDs can run any size motor. The ones Chris use apparently all can, the ones I use cannot (at least by using factory settings). So I would suggest blanket statements about "upsizing" the VFD are misleading and the user would need to confirm that the VFD is capable of doing this. As Chris has pointed out, with the latest VFDs they should be able to accept the motor size but that may not be the case with older used drives.

    Pete

  9. #24
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    Hi Pete,

    Yes, I think it's all been said already, no point repeating ad nauseum.

    No argument from me about comparing breakdown voltages of lap wound versus concentric wound, it's just not relevant for this case...

    I can recommend a good reference work in this field, get yourself a copy of "Electric Motor Repair by Michael Rosenberg". (I have a copy I can borrow, but it's not here at present)

    The big variable for dv/dt spikes is the length of cable (the cable capacitance) between the drive and the motor..

    You would have to have cables hundreds of feet long to get into trouble with insulation breakdown on a "normal" induction motor. Especially when we are talking primarily about 240V operation.

    There is another (as yet unmentioned) consideration for vfd operation, and that is the cooling fan, some "vfd rated" motors have separate power for the cooling fan, might be a factor if the lathe is going to be operated for long periods at low Hz..

    Regards
    Ray

  10. #25
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    Quote Originally Posted by RayG View Post
    snip....VFD rated motors, the main issue here is the breakdown voltage of the insulation,
    Regards.... snip
    Ray
    Ray, VFD rated motor or not, everyone contemplating a VFD upgrade comes across this question. In short, for a home shop a normal motor will do perfectly well. For serious industrial use, a VFD rated motor is the better choice.


    Here some hopefully useful thoughts in the context of snall HP motors as used on lathes and mills in a home shop.


    1) Transient spikes

    old VFD's (say 20+ years ago) used square wave inverters. These produced sharp high voltage spikes that did cause the insulation of ordinary motors to fail. That is where the term "VFD rated motor was born". These used higher insulation class wire.

    Since then VFD's have improved. Makers use inverters that try as good as possible to reproduce sine waves - they do this with dozens of small sqare steps for each sine period. The result is no more sharp spikes that could damage motor insulation. And much less RF interference as a side effect. Nowdays there is cerrtainly no more need to use a VFD rated motor because of transient voltage spikes.


    2) Maximum motor rpm

    truly VFD rated motors have better balanced rotors that vibrate less at high rpm, as well as better bearings. A VFD rated rotor in a 2 pole motor should survive being spun at 150Hz, that is about 9,000rpm. But check the specs before trying....

    Ordinary motors below some 5HP all use the same rotors, wether the stator is 2, 4, 6 or 8 poles. This means the rotor is safe to spin at 3600rpm (that is a 2 pole motor at 60Hz, modern motors are made for both 50 and 60 Hz markets the same). It is safe to run a 4 or 6 or 8 pole motor at 120Hz VFD frequency, but NOT a 2 pole motor, the rotor could potentially disintegrate. Fortunately most lathe/mill motors are 4 pole types.


    3) Efficiency

    If you run an ordinary TEFC motor at 120Hz, it will just about produce enough shaft power to overcome its own bearing friction and its own fan drag. Nothing left for cutting metal or wood. At 50 Hz the typical efficiency of a 1HP motor is 75%, but beyond 120 or 130HZ it is, well, zero pecent. In other words, 100% of the power delivered by the VFD is converted in heat. This is so, because the iron used in the stampings for the stator and rotor are causing increasing magnetic losses at higher frquencies. This is only a problem if used with a VFD, because these motors are designed to normally run at 50 or 60Hz.

    True VFD rated motors use thinner iron stamping to reduce losses at the higher frequencies that a VFD can generate. They may also use diffent iron materials. A VFD rated motor should still provide some useful shaft power at 150Hz. As more extreme example, I work with special motors that run at 500 and 1000Hz (as used in turbomolecular vacuum pumps, these spim at 60,000rpm and have magnetic bearings). Aircraft motors have since WW2 been running at 400Hz (to save weight, 3-phase motors can be made smaller and lighter if run at higher frequencies). It is not rocket science. But if talking VFD rated motors, you get what you pay for, and it is expensive. So forget about if it is only for a home shop lathe or mill.


    4) Noise

    A VFD rated motor is usually quieter, because it is not only optimesed to avoid resonance vibrations at 50 or 60 Hz, but across the whole range of speeds from 0 to 150Hz or more. This means the stator stampings and windings are vacuum impregnated in resin, and possibly strip welded together. Also the motor housing is more massive, rather cast iron than aluminium.

    There is possibly also some sort of slip ring between rotor shaft and housing to ground eddy currents that would otherwise pass through the ball bearings and shorten their life.


    5) Cooling

    The fan of a normal TEFC motor is useless for cooling at rotor speeds below 25Hz. Such motor overheats if run continuously at low rpm. And at very high speeds, the fan of a normal TEFC motor eats away too much motor power and makes noise.

    Truly VFD rated motors do not use a fan at the other end of the shaft, but a completely separate fan with its own motor. Sometimes available as optional kit. Also, VFD motors use a higher temperature rated wire insulation - not because of transient voltages, but simply because these motors are designed to run hotter!!


    6) Shielded cable between VFD and motor
    See also 1) why this is nowdays less a problem than 20 years ago. For an average home worksop, shielded wire is not anymore necessary. A Modern VFD will not anymore cause your DRO to display crap from RF interference. A machine manufacturer still has to use it though, because the equimnet he sells muss pass RFI testing to comply with electrical standards. In essence, this is about the VFD should not disturb your neighbour's radio and TV reception. If you do not live in a densely populated area no need to worry about. The same applies to optional chokes to be installed between VFD and motor. If you do not have to care about RF interference, and the line is not tens of metrs long, forget about, there are better ways to spend money.

    Enough today. Chris

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    Quote Originally Posted by dle147 View Post
    Hi folks, I'm a newbie to this forum so hope you can help.

    I've just bought an ATM260 - ex training school machine, currently powered with a 1/2 HP 3 phase ASEA motor. I've been wresting with what to do about the 3 phase motor; ie either convert to a 240 volt 3/4 or 1HP CMG motor (I have both) which will require a fair amount of modification, or look at installing a converter; ie 240 volt to 415 volt 3 phase.

    Would like to gauge what others in the forum have done. Must admit, I'd rather not change the existing motor, but I'm prepared to do it if the option of power conversion is too expensive.

    Would really appreciate your input and advice.

    Just a bit of background as to why I chose the Hercus - I had a technical education when I was young, attending a Technical school for my secondary education. As part of the trade training, we were trained in woodwork, sheetmetal work and turning and fitting. The Hercus was what we used for the latter, so after all these years, the wheel has turned a complete 360 degrees. Can't wait to fire this little bloke up and make the first cut ...
    Hi newbie,
    I will defer to people of greater knowledge in the area of VFD's but if you already have the motors, some wire and a new for/rev switch is cheap (30-$40 for a push button type). The wiring is simple, I have done many of them without problems. There are some modifications in the casting but they're not hard.
    Drop me a line if you want any help.
    Mal

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    Quote Originally Posted by cba_melbourne View Post

    1) Transient spikes

    old VFD's (say 20+ years ago) used square wave inverters. These produced sharp high voltage spikes that did cause the insulation of ordinary motors to fail. That is where the term "VFD rated motor was born". These used higher insulation class wire.

    Since then VFD's have improved. Makers use inverters that try as good as possible to reproduce sine waves - they do this with dozens of small sqare steps for each sine period. The result is no more sharp spikes that could damage motor insulation. And much less RF interference as a side effect. Nowdays there is cerrtainly no more need to use a VFD rated motor because of transient voltage spikes.
    Hi Chris,

    I think that was an excellent summary of the features of a VFD rated motor, but I'd like to clarify a little on the issue of voltage transients

    Modern IGBT based drive output stages don't actually produce sine waves, they produce a PWM signal at some carrier frequency, usually something like 5-20 kHz or so, the pulse widths are modulated to approximate a sine wave. which if followed by appropriate LC filtering gives you something that looks like a spiky sine wave.

    So if used without LC filtering (which is often the case) the transient spikes are actually worse than a square wave. There a a number of good papers on-line that can provide more detail, but it all comes back to cable length, if you have short cables (say less than 25ft) you won't have any problems.

    I'll attach an ABB technical note, which covers a lot of the issues we've been discussing.

    Regards
    Ray

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    Quote Originally Posted by allterrain50 View Post
    Hi newbie,
    I will defer to people of greater knowledge in the area of VFD's but if you already have the motors, some wire and a new for/rev switch is cheap (30-$40 for a push button type). The wiring is simple, I have done many of them without problems. There are some modifications in the casting but they're not hard.
    Drop me a line if you want any help.
    Mal
    Many thanks for the offer Mal, but I've managed to get hold of a 1HP 3 phase ASEA motor, which is quite compact - the only issues are the attachment bolt pattern, which I can adapt, and the diameter of the output shaft (19mm) which means opening up the hole and keyway in the drive pulley. Other than that, it looks like it may be a neat installation with no mod to the headstock casting. The motor cost $20 and I guess I'll be up for about $150 for a matched VFD.

  14. #29
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    One great thing about the VFD fitted to my lathe is I have no need for the back gears at all.. Put the belts in teh slowest direct position and you can run it as slow as the back gears and still have enough touque to do useful cutting..
    Gold, the colour of choice for the discerning person.

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    Quote Originally Posted by RayG View Post
    I'll attach an ABB technical note, which covers a lot of the issues we've been discussing.

    Regards
    Ray
    That was a very good paper Ray. I hadn't seen it before, but it explained things clearly in plain English and I think should be read by anyone with an interest in this area. Page 10 was a good explanation of what I was referring to in regards the motor winding and how inverter rated motors are typically wound differently (for these operational voltages).

    Despite having a VFD on my lathe (and mill for that matter), I still sometimes find myself using the backgear when I need a low speed but still plenty of torque. For example I will typically thread large threads using the backgear. Also I sometimes won't use a runout groove so taper the thread out at the end. By using backgear not only do I still have plenty of torque but can slow the lathe right down approaching the end of the thread. Every time I use the backgear I'm reminded that I need to repair a broken tooth however

    Pete

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