# VFD on a PM 949TV  OXYMORON???



## Gersh42 (Mar 20, 2022)

I ordered a PM 949TV single phase on Friday. Being a head case and letting my anxious personality get the better of me hah, I am second guessing the single phase decision. I have done some initial searches but cant find info for my exact scenario.

I do not want to go down the road of a rotary phase converter, that leaves me with the static phase converter or a VFD. Right off the bat I don't like losing 1/3 of my HP to a static converter, so I have been leaning towards the VFD option. I guess my actual question is what is the TRUE benefit of possible changing my order to a 3 phase version? Also like the title says is it a oxymoron to run a VFD on a 949TV machine? any problems you foresee? 

A few things that don't come into play for me, being a TV model I'm not looking at the VFD for speed control only to convert to 3 phase power. Also I'm not  worried about the lack of efficiency of single phase motors.

The benefits I am interested in, is a 3 phase motor is said to be more durable and longer service life. Also increased performance out of the machine itself i.e. better surface finishes. Just wondering how true that is and worth the trouble of installing a VFD

Thank you in advance!


----------



## markba633csi (Mar 20, 2022)

I think the surface finish issue is not nearly as big a deal on a milling machine as on a lathe.  Single phase is fine, and easier to hook up
The variable speed is nice to have but not essential.  Again, more useful on a lathe IMO

The main concern with a vfd is that the motor must be permanently connected to the vfd output with no switching or contactors in between.
Any existing 3-phase motor switching controls on the machine would be bypassed,  and control panels may need to be rewired or re-built, in order to have variable speed and/or reversing functions in a convenient manner without having to scroll thru vfd menus.

You can use a vfd as a fixed frequency power source, but it's kind of a waste since they can do so much more.


----------



## Firebrick43 (Mar 20, 2022)

markba633csi said:


> I think the surface finish issue is not nearly as big a deal on a milling machine as on a lathe.  Single phase is fine, and easier to hook up
> The variable speed is nice to have but not essential.  Again, more useful on a lathe IMO
> 
> The main concern with a vfd is that the motor must be permanently connected to the vfd output with no switching or contactors in between.
> ...


Its very easy to rewire existing controls to make the VFD do forward reverse start stop E stop ect.


----------



## graham-xrf (Mar 20, 2022)

You are doing the same stuff as me.
In principle, VFDs need not much care about the motor type, They accept whatever power is available, rectify AC to get a DC bus, and then make artificial AC out of that, but of a special high frequency pulsed sort, to allow control of the energy in the output. Most motor drivers I have seen allow choice of single or three phase inputs, and all had a U, V, W output, good for three phase, but the kit could be configured to use only two of them to connect single phase motors. Basically, they can be very versatile, and highly efficient, but you do have to check carefully what are the options available on the VFD you might be considering.

Three phase compared to single phase? Three phase has three poles spaced 120°, so the torque tugs are even, and regular at three per rev. Modern motors can have more poles, like 4, 6 or 8, but I am thinking here of using VFDs on old style motors. Single phase motors have two poles, set at 90° to each other. The AC is traditionally directly connected to one pole set, and a nearly 90° phase-shifted version of the AC is contrived by putting the AC through a capacitor, to use on the other pole set. One finds various other capacitor arrangements for START to get it turning in the right direction, or reversible.

The nice thing about VFDs is that you can throw away the capacitors. VFDs are able to make very satisfactory versions of 90° phase-shifted, energy adjustable, artificial outputs to directly drive the pole coils of a single phase. Three phase motor is better, in my view.

*About the motor voltages.*
In principle, a VFD can accept energy at some voltage, and make outputs for any style motor. This is where you find those that use 235VAC or 240VAC inputs are lower cost and perform better. An AC 3-phase motor, wired in "star" connection, in a place where the single phase is 240V would expect 415 volts AC between phases. In USA, 3-phase voltages are lower, but the math is always the same.
Line Voltage = √3 × Phase Voltage

Changing the connections on an old 3-phase motor to "delta" recovers the 1.73x power, and allows it to run on the lower available bus voltage the VFD can get out of a 240V input. It's useful to know if you are trying to use an older 3-phase motor.

*VFDs that can hike the voltage.*
These are types that have a built-in front-end inverter. They suck more current from a lower voltage, and deliver normal currents, but at the higher voltage. You can use a separate inverter. Boost-type switched-mode power supplies at machine powers are bound to be expensive, and most VFDs are not. A simple use of delta connection can save money. Some folk like changing out the old motor for a modern multi-pole permanent magnet motor.

*I go for VFD artificial 3-phase, but single phase is OK*
I don't know what the latest available lower power VFD technology does now, but I expect single phase with VFD will do just as well.  I was into inverter-driven servo motors at the 70kW level. I loved that they could deliver full torque forwards, even when stopped, or being driven backwards. These things had a separate small motor strapped onto them just to keep a cooling fan turning. When it came to my own modest vintage lathe stuff, the South Bend 9's had 1/3 HP and 1/2HP single phase. Given their low power, they were big, and heavy, and one had a capacitor _kaput! _ Available VFDs seemed to be much cheaper, simpler things, and one I borrowed for a while had poor torque at low revs, and made huge RF interference.

I am not so intent on "keeping it original" that I won't consider a motor upgrade. Line voltage in UK is 235VAC anyway, so I would replace with a 3-phase driven by artificial, speed-controlled AC . Being cheapskate, I have recovered a used old servo drive, and that will be re-purposed into use as a VFD. I also have a couple of servo motors, but they are too good to be used just for speed control of a lathe motor.


----------



## Gersh42 (Mar 20, 2022)

Firebrick43 said:


> Its very easy to rewire existing controls to make the VFD do forward reverse start stop E stop ect.



Is there any sort kit on the market (VFD + control panel) figured with as common as these style of machines someone would have developed something by now. 

I have to say most of Graham-xrf response went over my head (ill have to read it a few more times) I do commercial refrigeration, mechanical repair and building maintenance for three ice rinks. While I do encounter three phase power and motors very often and have a decent understanding the control portion is a bit of a struggle. 

This is my first milling machine and maybe down the road I can dive deeper into this but for now I would really like to find some "plug and play kit" where I can just bring 240v single phase and wire the motor to the new control panel. In a perfect world I would like to have a panel that can eliminate the direction change when using the backgears.


----------



## mksj (Mar 20, 2022)

A 3 phase motor is more durable, but single phase motors can last a long time mechanically. The problem with single phase motors in this application is that the start capacitor tends to go, in particular with frequent start/stop cycles. The start capacitor will overheat with frequent start/stop cycles. The other concern is that if you try to reverse a single phase motor before it stops, it will continue in the same direction instead of reversing. So for say tapping, you need to wait until motor stops and then reverse. This is less of an issue in a mill vs. say a lathe. You can use a VFD as a fixed frequency source for 3 phase, but the on/off cycle/reverse needs to be controlled by the VFD and NOT a motor switch. The PM-949 has no contactors, the motor is directly wired to a motor switch for FOR-STOP-REV. If using a VFD this would be eliminated. On a mill, I usually recommend 3 wire control, it requires a momentary stop, run and a sustained forward/reverse selector. It is very easy and basic.

When using a Reeves drive, it is recommend to use the VFD more as a fixed frequency source and use the mechanical speed adjustment. This prevents wear in a single position on the drive and also gives you a mechanical advantage of the belting ratio. The down side is they are more expensive, noisier, they wear and can be expensive to repair (although in a hobby environment should last decades). Most individuals buying a new mill with the intent to run it off of a VFD go with a pulley head. You can use a basic VFD for use on the mill, some people install them in the body of the mill, otherwise you need an enclosure and you should have a power disconnect switch at the machine. A simple control box/switch gear is maybe $100, so not much expense if you choose to go that route. There are also mills that come with factory installed VFD's, they run as a single belt speed with a back gear.


----------



## Cletus (Mar 20, 2022)

I went 3-phase and VFD to get away from reliability issues of a single phase motor (cap failures especially with lots of start stop ops) and the mechanical complications associated with a Reeves drive system, and greater control flexibility (speed control, instant reversing, jog, braking, etc.), plus I'm very much at home with VFD systems, having worked with some really huge ones (plastic extrusion) in my career.
I've got my belt set to the second groove down from the top of the pulley system and I've got excellent torque and speed-range for everything I need to do at that setting, so far have never needed to change the belt settings, never even used the back gear to date.  I go from drilling to power-tapping on the machine effortlessly, switching from FWD to REV without even thinking about it. My spindle ramps down to a dead-stop from any speed in 1.3-seconds and ramps up to full speed in 2-sec. I find my jog speed especially useful for power-tapping.


----------



## graham-xrf (Mar 21, 2022)

Gersh42 said:


> Is there any sort kit on the market (VFD + control panel) figured with as common as these style of machines someone would have developed something by now.
> 
> I have to say most of Graham-xrf response went over my head (ill have to read it a few more times) I do commercial refrigeration, mechanical repair and building maintenance for three ice rinks. While I do encounter three phase power and motors very often and have a decent understanding the control portion is a bit of a struggle.
> 
> This is my first milling machine and maybe down the road I can dive deeper into this but for now I would really like to find some "plug and play kit" where I can just bring 240v single phase and wire the motor to the new control panel. In a perfect world I would like to have a panel that can eliminate the direction change when using the backgears.


Sorry to do this to you. I will try and say it simpler.

If you already have a new single phase delivered, you can use it, with VFD, although the machine does seem to have a load of it's own speed options. 

Your mill requires a 220Volt, 20Amp supply for both single and three phase versions, and one of it's features is it normally has a spindle speed range 80-2720RPM, which goes to 40-5000RPM if you use a VFD. This is stated in the specification. If you happen to use a VFD, you can automatically have the 3-phase output option, without any separate converters, rotary or not.
I was looking --> HERE

I am pretty sure you can get a full explanation from your supplier for what you need, if you explain what supply you have. I should think you need the right VFD, 220VAC to feed the VFD it, and have exactly the right 3-phase motor, with correct connection type, to suit. You can also use a 220V single phase motor, though most folk here who have the experience, seem to advise 3-phase, because the inevitable capacitors on single phase kit do not do so well when having many stops and starts.


----------



## Firebrick43 (Mar 22, 2022)

Gersh42 said:


> Is there any sort kit on the market (VFD + control panel) figured with as common as these style of machines someone would have developed something by now.
> 
> I have to say most of Graham-xrf response went over my head (ill have to read it a few more times) I do commercial refrigeration, mechanical repair and building maintenance for three ice rinks. While I do encounter three phase power and motors very often and have a decent understanding the control portion is a bit of a struggle.
> 
> This is my first milling machine and maybe down the road I can dive deeper into this but for now I would really like to find some "plug and play kit" where I can just bring 240v single phase and wire the motor to the new control panel. In a perfect world I would like to have a panel that can eliminate the direction change when using the backgears.


Yes there is but you pay dearly for it.  

Bridgeport Retrofit

I have used this kit before(at work retrofitting Series 2 bridgeports.  

This is no reason other than time that you cant reuse the original buttons/switches.  Its simple wiring directly off the VFD and a parameter setting depending on how you want the switch to act.


----------



## mksj (Mar 22, 2022)

At the price point of that VFD kit, you might as well buy a factory installed VFD with a dedicated vector motor, Yaskawa VFD and Tach. Downside is the run controls are still primitive, the electric motor fan runs continuously unless one adds a timer.


			https://acramachinery.com/?s=lcm-50


----------



## Firebrick43 (Mar 22, 2022)

mksj said:


> At the price point of that VFD kit, you might as well buy a factory installed VFD with a dedicated vector motor, Yaskawa VFD and Tach. Downside is the run controls are still primitive, the electric motor fan runs continuously unless one adds a timer.
> 
> 
> https://acramachinery.com/?s=lcm-50


Don't necessarily disagree.  I did say you pay dearly for it.  It however is an excellent kit.  Few hours and its hooked up, reeves drive is gone, has a switch to know for RPM if in high/low gear and also the Forward/reverse switch automatically swapped over no matter what gear it was in. Much quieter than the reeves drive as well.  

Two of the Series 2 we installed it on were tool room machines that we also retrofitted a 2D anilanm CNC/DRO on.  The other 3 were used for rework

The kit was actually cheaper than repairing the reeves drive on the first one we did.  After that, no one wanted to use the other machines so we converted the rest of them with left over end of year money.


----------



## jmkasunich (Mar 22, 2022)

graham-xrf said:


> ... An AC 3-phase motor, wired in "star" connection, in a place where the single phase is 240V would expect 415 volts AC between phases. In USA, 3-phase voltages are lower, but the math is always the same.
> Line Voltage = √3 × Phase Voltage
> 
> Changing the connections on an old 3-phase motor to "delta" recovers the 1.73x power, and allows it to run on the lower available bus voltage the VFD can get out of a 240V input. It's useful to know if you are trying to use an older 3-phase motor.


Lots of good info in this post, but there is one bit that will just cause confusion for those on the west side of the Atlantic Ocean.

Motors that can be wired "star" (aka "wye") or "delta" are NOT at all common in the States, so this part of the discussion is likely to be confusing.

Graham described the UK/EU style of motors.  Dual voltage motors there have six terminals in the junction box.  There are three windings, and both ends of each winding are brought out to the box.  Each winding expects 230V AC.  To run on 230V line-to-line three-phase power, you connect the windings in delta configuration.  Each winding is between two of the incoming phases and sees the 230V that it wants.  To run on 400V line-to-line three-phase power, the windings are connected in wye configuration.  One end of all three windings are tied together to form a neutral, and power is applied to the other ends of the windings.  Although there is 415V line-to-line, the line-to-neutral voltage is 230V and the windings are happy.  This works because the two commonly available voltages over there are 230 and 400V (or 240 and 415).  The ratio between the available voltages is 1.732, aka the square root of 3.  That comes from the geometry of three-phases.

Dual voltage motors in the US (and other places that use US style power) are _completely_ different.  The most common voltages in the US are 120V, 240V, and 480V, which have ratios of 2:1, not the sqrt(3) ratio found in Europe.  So our dual voltage motors use a different setup.  There are six windings, each rated for 240V.  Nine of the 12 leads are brought out into the junction box, and the remaining three are tied together inside the motor to form a neutral point.  So you have half of the motor preconnected in wye and ready to run on 240V line-to-line, and the other half completely unconnected.  For 240V power, you connect three of the wires from the other half together to make a neutral.  You now have two "half motors", each wired in wye and each rated 240V.  Connect them in parallel to the power source and you are done.  For 480V, you instead connect the three unconnected windings in series with the preconnected wye so the voltages add and you have a 480V motor.  Both voltages are connected in wye, but one has two windings in parallel while the other has two in series.  This gives the 2:1 voltage ratio that we need.

Describing this in words is hard, and I don't think I did a very good job.  So I stole some pictures from the internet.  This is a typical connection diagram for a US style 2:1 voltage ratio 9-lead motor.  The lead numbering is a NEMA standard.  (NEMA is National Electrical Manufacturers Association, a US base trade organization.)






Here is a drawing that shows the series vs parallel arrangement of windings, using the terminal numbers from the above connection diagram.  The 7-8-9 section is the "pre-wired wye" I mentioned earlier, 1 thru 6 are the three unconnected windings.





There is another less common 9-wire NEMA standard as well, where the base connections are delta instead of wye.  And there are 3, 6, 9, and 12 wire arrangements from many countries and manufacturers.  Needless to say, no matter where you are and where your motor came from, you should always always refer to the connection diagram that came with the motor.

Here is another webpage that does a better job of explaining the actual connections than I did.  Since they are a US centered website, they _only_ talk about 9-wire motors and that page would be just as confusing for EU/UK readers as Gerald's info is for US readers...








						Common Motor Windings and Wiring for Three-Phase Motors - Technical Articles
					

This article looks at some common windings and wirings for three-phase motors, including internal Wye windings and low and high voltage wirings.




					control.com
				












						Common Motor Windings and Wiring for Three-Phase Motors - Technical Articles
					

This article looks at some common windings and wirings for three-phase motors, including internal Wye windings and low and high voltage wirings.




					control.com


----------



## graham-xrf (Mar 22, 2022)

@jmkasunich 
Thank you for your helpful contribution, particularly how US common motor windings are split to give more choices in adapting to voltages using series and parallel connections. I knew we were rapidly getting into complicated territory, so I tried to simplify, and yet my post #4 proved too much for folk.

I tried again in post #8 to give the OP some way to get to a good decision. I guess there is no substitute for laying out the whole deal. Also, I may not be much good at this! 

It is pure physics truth that the power relationship between line voltage and phase voltage is always *√3*, even for a motor in USA. The subtle point here is that if a VFD has to live with a single phase input, the highest bus voltage that can be wrung out of a 240VAC input without a boost inverter is 240 x *√2* =339.4V, or in practice, a bit less than that. To start with a single phase 110VAC, it ends up as little as 155.5V, which is probably why using both 110V phases in a USA house to get the 220V is recommended. If the VFD used such voltages to make it's version of 3-phase, on a motor that started out connected in series (ie. "star" connection), there would be a dramatic loss of power. Changing to the delta connection recovers much of that

We have a potential confusion about terms we use.  It's about what is a "parallel" connection. Yes - the low voltage connection in your second diagram does make a motor into a lower voltage, higher current type, but both connections are "star", with a central neutral point. For me, the delta connection _is_ the parallel form of connection, regardless that windings within it may have been further locally paralleled.

The real series connection, compared to the parallel form (delta) is shown here..






Changing to the delta connection is the immediate convenient way to make a old style induction motor better suit a VFD offering 3-phase at about the (artificial) 330VAC line-to-line. One can, of course, take it further by splitting the  windings, and connecting them in parallel. This is why we find motors with multiples of 3 connections, like 6, or 12.

Regarding USA common voltages, I know of 110V, and some places it's 120V. These come from a pole transformer with a centre-tapped winding, which is how you can have a 220V or 240V supply. It's not actually 3-phase, but a VFD can easily make it's own version. Much as I do love the old vintage squirrel-cage induction motors  on old kit, this is one place where I shrug, and I am happy to put a modern permanent magnet multi-pole brushless AC motor in there, and connect up a VFD. I am just seduced by electronic tricks that can deliver near full torque, even when hardly moving. Not all VFDs can do that. Many cheaper ones run out of power puff at low revs.


----------



## jmkasunich (Mar 23, 2022)

graham-xrf said:


> @jmkasunich
> 
> 
> It is pure physics truth that the power relationship between line voltage and phase voltage is always *√3*, even for a motor in USA.


Agreed.  The US uses different terms: "line-to-line" and "line-to-neutral" voltage.  I would claim that the US terminology is clearer, it spells out exactly where both terminals of the voltmeter are connected.

This is turning into a dissertation on power systems and motor wiring that most people won't care about, but I'm going to put the info out for the few that might find it informative.  Source:  I'm an electrical engineer, and I spent about 30 years of my career designing VFDs.  Most of my experience is with larger drives.  The smallest thing I've personally designed was rated at over 100 amps at 480V, or about 80kW/100HP.  The largest was 1.5MW/2000HP.  I've worked with (but not designed) "medium voltage" drives rated to 7500 volts and over 25 megawatts, as well as tiny ones rated at a fraction of a kW.



graham-xrf said:


> The subtle point here is that if a VFD has to live with a single phase input, the highest bus voltage that can be wrung out of a 240VAC input without a boost inverter is 240 x *√2* =339.4V, or in practice, a bit less than that.


The maximum DC bus voltage of the VFD is equivalent to the peak voltage of the supplied input.  For either single phase 240V _or_ three phase 240V, the DC bus voltage is 240 * sqrt(2) = about 340V.  The three-phase version has smaller dips between the peaks and a slightly higher average DC voltage compared to the single phase, but with suitably sized bus capacitors both can produce almost the same AC output voltage - just under 240V RMS.



graham-xrf said:


> To start with a single phase 110VAC, it ends up as little as 155.5V, which is probably why using both 110V phases in a USA house to get the 220V is recommended.



There are probably a few VFDs designed for 120V single-phase in and 120V three-phase out, but they are uncommon - mostly because 120V (line-to-line) three-phase motors are vanishingly rare.

By far the most common VFD rated for 120V input will have a voltage doubling input rectifier which gives it the same ~340V DC bus voltage as a 240V input VFD, albeit with even more ripple voltage and even bigger bus capacitors needed.  That's how you get VFDs that can run on 120V single phase and deliver almost 240V three phase output.  For example: https://www.amazon.com/Single-Output-Variable-Frequency-Control/dp/B0899T3QQ4

The voltage doubling rectifier doesn't scale well to high power, so you'll rarely see them rated more than a few HP/kW.  (Also, 120V single phase is a terrible source for any load requiring more than a couple kW.)




graham-xrf said:


> If the VFD used such voltages to make it's version of 3-phase, on a motor that started out connected in series (ie. "star" connection), there would be a dramatic loss of power. Changing to the delta connection recovers much of that
> 
> We have a potential confusion about terms we use.  It's about what is a "parallel" connection. Yes - the low voltage connection in your second diagram does make a motor into a lower voltage, higher current type, but both connections are "star", with a central neutral point. For me, the delta connection _is_ the parallel form of connection, regardless that windings within it may have been further locally paralleled.
> 
> ...


There is certainly confusion here.

Series and parallel are one thing, start and delta are a completely different thing, and they should never be confused.  That is why I posted in the first place.  Whenever a person on one side of the Atlantic tries to answer a motor question from someone on the other side, this issue almost inevitably arises because each person is making assumptions based on what they have seen, without realizing that things are totally different on the other side of the pond.

Your EU/UK motor has three windings and six terminals.  Series and parallel connections are NOT available.  Your ONLY options are star and delta, and the required voltages vary by a factor of sqrt(3).

Our US motor has six windings and nine terminals.  Star and delta connections are NOT available.  Our ONLY options are series and parallel, and the required voltages vary by a factor of 2.



graham-xrf said:


> Regarding USA common voltages, I know of 110V, and some places it's 120V.



110, 115, 117, and 120V have all appeared on the nameplates of US appliances, light fixtures, etc. over the years.  The current standard is 120V distribution voltage (meaning that is what the utility aims to give you) and 115V utilization voltage (meaning that your motor or whatever is expected to be able to deliver full performance at 115V).  The difference is to allow for voltage drops in wiring and other such effects.

This difference between distribution and utilization ripples into the higher voltages as well.  You'll see breaker panels labeled 240V (distribution) but most motor nameplates say 230V (utilization).  Likewise with 480V panels and 460V motors.  Older gear may be labeled 220V or 440V.



graham-xrf said:


> These come from a pole transformer with a centre-tapped winding, which is how you can have a 220V or 240V supply. It's not actually 3-phase



Correct.  It is single-phase.  Sometimes called split-phase or 240/120V.

US power distribution is a mess compared to EU/UK.  Normal residential service in the US is from a center tapped single-phase transformer fed from two of the three phases of the high voltage distribution grid.  The tap is grounded (or earthed in UK speak ;-)  ) and called "neutral".  We have 120V line-to-neutral for light loads and 240V line-to-line for larger loads like stoves and air conditioners.  But both are single phase.

In modest commercial and office buildings where most of the load is lights and computers and such, the distribution transformer is three-phase, with a star (we usually say "Y" spelled wye) connected secondary.  The center point is grounded and again called "neutral".  Each winding produces 120V, so you have three individual 120V line-to-neutral phases, each of which supports 120V single phase loads.  The line-to-line voltage is 208V (120V times sqrt(3) ).  This is commonly referred to as 208Y/120V meaning 208V line-to-line from a wye (star) connected transformer with 120V from each of the three lines to neutral.  The limited three phase loads in such buildings are almost always motors, and they are either designed specifically for 208V line-to-line, or they are dual-rated motors that can run on 230V or 208V without changing any connections at all.

A small industrial facility like a machine shop needs mostly three-phase for the machines, with just a little bit of 120V single phase for lights and maybe an office computer or something.  They sometimes use an arrangement referred to as "high leg delta".  This uses a 240V line-to-line delta transformer but grounds the middle of one side of the transformer.  (Wikipedia has details and a drawing.)  So you have 120V from two of the three phases to neutral.  Those two phases and the neutral can be connected to an ordinary "split-phase" breaker panel and used just like residential power for the office computer and lights.  The third phase is about 208 volts from neutral - it is the "high leg" in the name "high leg delta".  No line-to-neutral loads are connected to this phase.  All three phases can be sent to a three-phase panel to run 240V three-phase motors and other three-phase loads.

As buildings get bigger and need more power the distribution switches to pure three phase at either 240V or 480V line-to-line.  Usually the distribution transformer is star (wye) connected with the star point grounded, but ungrounded delta is not uncommon.  On a 480V system, the line-to-neutral voltage is 277V, and some lighting is designed to run on 277V to avoid the need for separate lighting panels.  But in most buildings, a smaller transformer (or several, scattered through the facility) is used to step the 240V or 480V down to either 208Y/120V (using all three phases) or 240/120V split phase (using only two of the three phases).  The resulting low-voltage power goes to "lighting panels" which supply lights and receptacles, while machinery is served from power distribution panels at either 240 or 480V line-to-line.

This mess is mostly the result of a desire for backwards compatibly and the hap-hazard growth of the original systems from the days of Edison and Tesla.

My understanding (please correct me if wrong) is that in the UK and EU, the majority of installations are fed by star connected transformers with 230V from line to neutral/ground and 400V from line-to-line.  Residences get one phase from a utility owned transformer, and all loads are single phase 230V.  Commercial and industrial buildings get all three phases.  Lights and computers and such are connected line-to-neutral (and evenly distributed between the phases for balance reasons) while motors run on 400V line-to-line.  The motors are star connected for this service, so the individual windings see 230V.  This allows you to reconnect them in delta so they want 230V line-to-line.  A VFD running on 230V single phase has a 325V DC bus and can create 230V line-to-line three phase, so the motors are happy.  For really large loads, there is also 690V line-to-line which works out to 400V line-to-neutral.  So a large motor with 400V windings can be connected in delta and run on the "normal" 400V, or reconnected in star to run on 690V with lower line currents to allow for smaller wire, breakers, etc.  Overall it seems like a much better system.




graham-xrf said:


> Much as I do love the old vintage squirrel-cage induction motors  on old kit, this is one place where I shrug, and I am happy to put a modern permanent magnet multi-pole brushless AC motor in there, and connect up a VFD. I am just seduced by electronic tricks that can deliver near full torque, even when hardly moving. Not all VFDs can do that. Many cheaper ones run out of power puff at low revs.



Full torque at zero speed comes from a vector drive, either with an encoder or "sensorless vector" (which doesn't usually work quite as well as true vector).  However, we retrofitting an older machine, I like to keep any mechanical speed selection in place.  Full torque from the motor at low speed does not mean full power.  The gear or pulley mechanical reduction actually increases torque at low speeds, and no VFD can do that.

Sorry for the wall of words.  Hopefully the info is useful to someone.


----------



## graham-xrf (Mar 23, 2022)

@jmkasunich  You are a man after my own heart!

I do agree that we have gone well beyond what the OP was after, although (rightly), he should know the useful stuff he was getting us into. @Gersh42 's option is reasonably clear. I think it is the VFD. I know some folk love rotary phase converters, but now we can have stuff with high efficiency and no moving parts. We can distribute the little VFDs onto any machine that needs one, instead of a big fat expensive central 3-phase supply, even a non-rotating one.

I never did get into fractional megawatt powers as you did. The biggest I handled was 110kW, and I mean getting the motor + inverter set up and working in response to software, not designing the inverter(s). Also a pair of 70kW servo motors with encoders etc. designed to emulate RATS, meaning those 60HP little propellers on fuel-pumping tanker aircraft, and a way to return energy to the grid. I suppose I did have some adventures with 22kV at 20mA (electron beam kit), and a whole bunch of instrumentation stuff in a Faraday box at 500kV, in a vacuum chamber about the length of a tennis court. Now I am retired, the most exciting it will get is perhaps with a TIG welder, and that's OK by me!

You do have it right about the distribution in the UK, and many factories do run 3-phase motors without the neutral connected. The norm used to be 415V, but it was changed to about 407V. That corresponds to reducing the voltage to neutral from 240V to 235V. It was part of a bit of a dodge to bring the whole grid into conformance with EU grids, by altering the tolerance from +/- 5% to "+5% / -10%" (I think).  It meant nobody had to actually change anything!

There is one thing done here that is special. The source has the big true earth connection to the star point. In theory, one can put rods into the ground, for a local earth, and it will work just fine, but also, they have here the concept of "protective multiple earth" (PME), where as many local ground points as possible are added in. The neutral is directly connected to this earth on the supply side at the entrance to a building, and then immediately goes through a RCD (residual current device safety breaker. If there is any live leak beyound 30mA, or even if the neutral touches anything as a fault, it trips out. I have both. PME and also two deep rods, but I happen to be backed onto a farm.

There is an obvious hazard to this if the neutral line to the local estate ever gets broken, and to this day, a developer has "to seek permission from the secretary of state" to install it. They have to go to great lengths to ensure a neutral fault will not happen, and put in stuff to remove the supply if it does. In practice, country wide, the use of RCDs is the norm. Also, UK houses, there are ring mains, each fed from a magnetic+thermal trip switch, usually at 20A or 32A, and every appliance has a plug with a fuse in it.

Thanks for explaining the US energy distribution system. It does look like a bit of a tangle. We now face a future with much more micro-generation, use of inverters, PVs on the roofs, battery storage (like Tesla walls ), and much else. There is now so much PV electricity in Germany that if a cloud front rolls away, and the sun comes out, the grid faces some instability.

Most definitely, if the lathe or mill has belt or gears or both, then choose to run it from VFD with the motor spinning quite fast. It's the best way to have the torque in a sweet way.


----------



## Firebrick43 (Mar 25, 2022)

jmkasunich said:


> Lots of good info in this post, but there is one bit that will just cause confusion for those on the west side of the Atlantic Ocean.
> 
> Motors that can be wired "star" (aka "wye") or "delta" are NOT at all common in the States, so this part of the discussion is likely to be confusing.


Having worked for 2 large companies as a CNC machine tech, star/delta motors are pretty common in the states.  Its been a long time since I wired a motor in series or parrallel.  But a few times a week for delta normally.  Since normally 480v motors, wye is rare except the odd ball motor designed for 640 volts.  

Had fun a couple of days ago trying to replace a rewound motor on a star/delta starter.  &#$^&* germans


----------



## Cletus (Mar 26, 2022)

_"Had fun a couple of days ago trying to replace a rewound motor on a star/delta starter.  &#$^&* germans"  _


----------



## Rangeraleg (Apr 9, 2022)

Gersh42 said:


> I ordered a PM 949TV single phase on Friday. Being a head case and letting my anxious personality get the better of me hah, I am second guessing the single phase decision. I have done some initial searches but cant find info for my exact scenario.
> 
> I do not want to go down the road of a rotary phase converter, that leaves me with the static phase converter or a VFD. Right off the bat I don't like losing 1/3 of my HP to a static converter, so I have been leaning towards the VFD option. I guess my actual question is what is the TRUE benefit of possible changing my order to a 3 phase version? Also like the title says is it a oxymoron to run a VFD on a 949TV machine? any problems you foresee?
> 
> ...


Realize that with TV model the VFD is built in and the motor is already 3 phase. If you check last page of the manual wiring diagram you can see that. Nothing more required than just machine. Enjoy I'm thinking of getting same machine, soon.


----------

