Why are 3 phase motors with VFD so popular in the UK?

[Michael Cross 4Participant @michaelcross4]

My original point was that changing speeds using a VFD is not a panacea and the view that it's a simple substitute for mechanical gearing is too simplistic, which I'm sure you agree with.

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by Michael Cross 4 on 23/01/2023 17:50:11:

My original point was that changing speeds using a VFD is not a panacea and the view that it's a simple substitute for mechanical gearing is too simplistic, which I'm sure you agree with.

Absolutely.

[Howard LewisParticipant @howardlewis46836]

Replacing a 3 phase motor on a machine, by a single phase does impose a cost, making converting an ex industry machine to single phase by changing the motor does make it economically less attractivee than a machine already single phase equipped

Using a VFD to give a Variable Speed Drive is the main advantage for many of us, I am sure.

The fact that the torque envelope remains pretty much constant has to be a bonus. I doubt if many research the torque characteristics, rather than the speed envelope before buying.

It is a means to an end., an assistance or advantage rather than a universal panacea.

The popularity whilst abnle to run off a single phase supply, must be due to that

Howard

[Robert Atkinson 2Participant @robertatkinson2]

One other disadvantage of single phase motors is poor torque ripple. This is how much the torque varies through a single rotation of the shaft. This gets worse as the load increases. It produces noise, vibration and wear in transmission components. It can also produce poor surface finish on workpieces.

A couple of people have mentioned that torque drops off if speed is increased above nominal. This actually depends on the volatg the drive is able to supply. If the drive has a voltage boost ability it can maintain rated torque as speed and thus back EMF increases. The limitation then is either mechanical (speed limited) or electrical (insulation breakdown).

Used single phase machines only carry a premium in the hobby market because they are easy to use, not because of any performance benefit. This price difference is getting less as VFDs become more commonly known and lower cost. The added benefit of variable speed further tilts the balance in favor of VFDs.

Robert G8RPI.

[HopperParticipant @hopper]

It seems quite likely that VFDs will become the norm in the future, as fractional horsepower three phase motors are already cheaper to buy than the single phase ones, due probably to the large numbers of three phase used in industry so mass market equals lower prices. And like all electronics, the inverter/controller units will continue to come down in price as production numbers rise. So eventually single phase motors might die a quiet death as they become relatively more expensive ..

[John OlsenParticipant @johnolsen79199]

When even your washing machine contains an inverter and a three phase motor there must be some advantage to it, probably lower cost. I suspect the smaller size of the three phase motor is enough to pay for the electronics, at least in a mass production situation.

I wouldn't make too much distinction between three phase induction and brushless DC, both use three phase windings. The magnets are probably cheaper than making the squirrel cage part of the induction motor and the resulting motor will probably have better performance.

John

[HopperParticipant @hopper]

Posted by John Olsen on 23/01/2023 23:31:05:

When even your washing machine contains an inverter and a three phase motor there must be some advantage to it, probably lower cost. I suspect the smaller size of the three phase motor is enough to pay for the electronics, at least in a mass production situation.

I think the big saving in a washing machine is no reduction and reversing gearbox needed to get the back and forth agitator motion. The motor just drives direct and reverses direction at the required intervals. Must save a lot of manufacturing costs on the old gearbox types, which also gave a lot of mechanical problems as they got older.

[Mike PooleParticipant @mikepoole82104]

Although using a VFD to retain the 3 phase motor on a native 3phase machine is attractive it will require attention to the machine control circuit. The quick and dirty solution of screwing the VFD to the wall and using the service/commissioning panel for control is basic and falls far short of a satisfactory installation in my opinion. A machine like a Myford has no original control system unless the industrial stand is used so the freely available control panels are useful. More industrial lathes often have controls integrated into the headstock and a carriage mounted stop start lever. It is nice to retain these devices if possible but will need redesigning and wiring into the VFD. An industrial lathe may just have a simple 400V starter but will often have more sophisticated controls with guard interlocks and reversing capability incorporated. Control circuits can be implemented entirely using the phase supply or a low voltage circuit, typically 110V ac with a transformer supply or 24V dc with a power supply. These will need redesigning to use with a VFD. The often fitted coolant pump will also need a solution. The Steinmetz circuit will often be a cheap and cheerful solution to a coolant pump, a VFD is a bit of overkill for a coolant pump I think. The LoVo light if fitted will also need attention.

Mike

[Clive IndiaParticipant @cliveindia]

Posted by Tony Jeffree on 23/01/2023 17:25:4
My view is that with a VFD you really do get the best of both worlds. Use the VFD to change speeds where that is convenient/useful/appropriate; where it isn't, you still have the option of running the motor at "normal" speed (where it operates at max power and max torque) and increasing the available torque by using the pulleys/gears.

Yes, sums up my experience nicely. Very rarely do I have to revert to changing gear but you might do more difficult jobs – in which case nothing is lost in changing gear if you have to. Most of the time, considerable gain in having a VFD.
We all know torque diminishes at low speeds and other downsides – it's where the theory meets the practicality.
SOD makes a good comparison between 1 and 3 phase motors.

Edited By Clive India on 24/01/2023 09:40:28

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Olsen on 23/01/2023 23:31:05:

When even your washing machine contains an inverter and a three phase motor there must be some advantage to it, probably lower cost. I suspect the smaller size of the three phase motor is enough to pay for the electronics, at least in a mass production situation.

I wouldn't make too much distinction between three phase induction and brushless DC, both use three phase windings. The magnets are probably cheaper than making the squirrel cage part of the induction motor and the resulting motor will probably have better performance.

John

Doing it cheaper is behind all engineering, or should be! 

Three phase motors are cheaper to make than single-phase and more reliable.  What's making  the difference is the electronics.  

Unlike mechanical engineering where the basics haven't changed much for a century, electronics are still developing rapidly, with costs dropping continually. 

The theory of VFDs was understood when I was a boy, probably earlier, but the technology of the day couldn't make a VFD for less than the cost of a small country.  And the result was a bit unreliable.  60 years later, VFDs are consumer items. 

Once integrated circuits have been debugged, their manufacture becomes progressively cheaper.  Stampings rather than skilled work.  Although the contents of a VFD are high-tech, there's not much material inside, and they can be mass-produced very cheaply.   Hobbyists might think they're pricey, but a VFD can be bought for well under £100 retail; the wholesale cost is much lower, and bare boards even cheaper.   There's a price point where 3-phase + VFD is both cheaper and better than single-phase, even in a domestic washing machine.

In my simple mind, the reason Brushless DC motors outperform conventional 3-phase is by substituting powerful permanent magnets for the squirrel cage.  Squirrel cages are delightfully simple, cheap to make and robust, but they exist to induce a magnetic field, which is a bit lossy, wasting energy as heat, effectively reducing the power to weight ratio. 

Modern super-magnets don't lose magnetism as did older magnetic materials, and are heat resistant.  Their prices too have dropped, making it possible to sell a version of the 3-phase motor that's usefully better than the original.   The theory is as old as the hills, what makes them practical and affordable is cheap modern magnets and electronics.  I think they're more expensive to make than squirrel cage motors, but their higher efficiency pays off in the long run, and in any application – like EVs – where efficiency is vital.

LBSC back from the grave would have no trouble using my lathe and milling machine, but the way they are powered is science fiction to him.  And then I show him a smart phone…

Dave

 

Edited By SillyOldDuffer on 24/01/2023 10:22:53

[John HaineParticipant @johnhaine32865]

As I pointed out before, Tesla cars use induction motors.

[HopperParticipant @hopper]

Posted by SillyOldDuffer on 24/01/2023 09:51:41:…

…

The theory of VFDs was understood when I was a boy, probably earlier, but the technology of the day couldn't make a VFD for less than the cost of a small country. And the result was a bit unreliable.

And it would have been full of glass tubes or "valves" and taken up a room of your house.

[JellyParticipant @jelly]

Posted by Mike Poole on 24/01/2023 09:20:55:

Although using a VFD to retain the 3 phase motor on a native 3phase machine is attractive it will require attention to the machine control circuit.

This is one of the reasons that I have chosen to avoid doing any unnecessary VFD conversions to machines and to stick with a convertor running the whole workshop.

The effort required to power the ancillaries on, and replicate the relay logic of a well appointed manual machine tool can be substantial, and I have seen more than one mechanically sound machine end up inoperable and being sold as a project/for parts after someone got in over their head trying to convert it.

It's not usually hard to do, so much as it is awkward, time consuming and head scratching.

There's a reason that you rarely if ever see manual machine tools in industry retro-fitted with a VFD for speed control of the motor, even if they have had to have upgraded control circuits for PUWER98 or just because the old circuits failed after 50+ years service.

[not done it yetParticipant @notdoneityet]

Posted by Mike Poole on 24/01/2023 09:20:55:

Although using a VFD to retain the 3 phase motor on a native 3phase machine is attractive it will require attention to the machine control circuit.

…..

Mike

Not always.🙂 I simply bypassed any control circuitry (the starter) and connected my lathe motor to the VFD.

The most likely alteration is that of converting to delta (simple, or motor disassembly required). Or purchase a more expensive VFD.

Still rather better than an expensive converter, IMO.

 

Edited By not done it yet on 24/01/2023 11:16:34

[JellyParticipant @jelly]

Posted by not done it yet on 24/01/2023 11:15:43:

Still rather better than an expensive converter, IMO.

IMO there's no "better" or "worse", just appropriate for the use case…

People should select the approach which suits their budget, aspirations and equipment best, which will be different for each user.

​

For you, the VFD's are the best option.

For my investment in a single converter I can run the Lathe, Mill, Drill and Grinders which would all have required one or more VFD's, and I didn't have to re-wire the Lathe to get it to work properly (because the electrical and mechanical controls are closely intermeshed, and some functions would be lost if the switches buried in the mechanism weren't connected to the VFD)…

I can also run my Migatronic 500 Mig Welder, Oxford Welder (doesn't need 3-phase but the arc is smoother when running on it) and the recently acquired Plasma Cutter;

Running welders is something which can only be done on a Rotary Phase Convertor (and then only with a specific option fitted).

​

So for about £400 outlay, I'm running 7 machines or about £57 per machine, before you consider that 3 of those machines can't be VFD converted at all so I just wouldn't be able to have them at home.

Had I bought the RPC brand new, the price would be closer to £200 per machine, which given that for two of the machines I wouldn't have seen much change from £300 per machine buying VFD's for them still makes it look economic in the long run.

If I was running smaller machines, or machines which had motors which were more easily adapted to running on 240V 3-phase, then the economics would stack up for VFD's instead, but I wouldn't be able to run such nice welding gear then.

[Andrew TinsleyParticipant @andrewtinsley63637]

If a machine, with a single phase motor, works to your satisfaction, then leave well alone, If it does not meet your requirements and a VFD and 3 phase motor will, Then do the conversion. It is as simple as that

Buying a new 3 phase motor and a good brand of VFD is not cheap. Some of the cheap Chinese VFDs are not user friendly and have a dubious reputation, although many on this forum are very satisfied with them. New 3 phase motors are cheaper than equivalent single phase ones, but prices are not bargain basement. Only other alternative is a second hand item, but then you need to have some experience in bearing replacement and rewiring the input to match the VFD

I have both 3 phase VFD powered machines and single phase powered ones. Horses for courses.

Andrew

[Russell EberhardtParticipant @russelleberhardt48058]

I have an old (about 80 years) lathe and a Chinese mill that I've converted from single phase to VFD/3 phase, a smaller mill with brushless DC motor and a bench drill with single phase motor. I certainly don't regret doing the conversions, the biggest improvement being the in finish I can achieve with the mill. The only reason I haven't converted the bench drill is that if I need high speed low vibration for small drills I can use the small mill.

Russell

[VicParticipant @vic]

“Why are 3 phase motors with VFD so popular in the UK?”

I’m not sure they are, I have five or six machines in my shed and only one is 3 phase with an inverter.

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Jelly on 24/01/2023 10:45:19:

There's a reason that you rarely if ever see manual machine tools in industry retro-fitted with a VFD for speed control of the motor, even if they have had to have upgraded control circuits for PUWER98 or just because the old circuits failed after 50+ years service.

The main reason is the machines tend to be dedicated to a particular type of work and or material. They are chosen for that work so the original speed(s) are appropriate to the task so there is no need or advantage to having variable speed. Unlike hobbists they are not trying to do everthing with one or two machines, often on the edge of their capabilities. The single phase supply issue is moot.
One thing you DO see in industrial settings is replacement of mechanical variable speed drives on older specialist machines with VFD set ups.

Not related to machining is the widespread retrofitting of VFDs on pumps, fans, conveyors and the like. There are big energy savings in running these at just the right speed. The savings will soon pay for a VFD. There can also be government incentives for improved efficency. Running at optimum speed can bring process performance improvements too.

Robert G8RPI.

Edited By Robert Atkinson 2 on 24/01/2023 19:49:34

[samuel heywoodParticipant @samuelheywood23031]

Posted by Michael Gilligan on 21/01/2023 04:21:37:

Posted by samuel heywood on 21/01/2023 01:34:31:

Posted by Michael Gilligan on 20/01/2023 21:20:19:

I think it must be “or what”

The advantages are very obvious, and the cost is realistic in the U.K.

[…]

The advantages are only obvious if you have' the knowledge' Michael.

Always keen to learn.

Please could you or someone else briefly explain the advantages of three phase?

.

Sorry, this will have to be brief … I’ve just had my sleep disturbed at 04:00 and need to get back to the land of nod.

The fundamental advantage [on which all the others depend] is that 3-phase is smoother

… like comparing a single cylinder motorcycle with a Triumph Triple

MichaelG..

Thanks~ an analogy i can relate to.

[Peter SansomParticipant @petersansom44767]

If anyone in Australia is looking for a VFD, I have just got my hands on 3, 2nd hand Bosch Rexroth 1.5kw 3610 EFC units. On will be used to convert my S7 to vfd and 2nd will replace a cheap Chinese one currently on the drill press.

If anyone is interested on the 3rd unit PM me.

Peter

[john fletcher 1Participant @johnfletcher1]

Regarding VFD in here in UK , if and when industry upgrades, if ever, at all, out goes the original drive motor and all, lots find there way to ebay and other auction site, auto jumble sale, etc. Some folk buy a VFD kit and take the easy, but costly way out, other do a DIY job. I've found it easy and cheap to buy a second hand motor and a VFD via ebay. I have read on 'here' about those VFD from PRC and which are not user friendly and have a dubious reputation, but I've not actually met a person who has had any problems whatso ever. I'm no expert on computing and I have managed to program for my mates. Simply have a look on youtube plenty of instruction there, maybe I'm lucky. John

[John HaineParticipant @johnhaine32865]

The overwhelming reason why 3 phase is preferred overall is, as has been understood since the late 19th century when Tesla amongst others invented much of the underlying technology, the substantial saving in iron and copper in generators, transformers, and distribution for a given amount of power. Added to that, given a 3-phase alternator, the 3-phase motor is a "natural" complement to re-generate mechanical motion. In many countries 3-phase is connected domestically as well but in the UK and others money was saved in the distribution network by only connecting most houses to one phase. As electric heating and vehicles become more important in this respect as in so many others this short-sightedness is catching up with us.

[SillyOldDufferModerator @sillyoldduffer]

Posted by john fletcher 1 on 30/01/2023 16:20:42:

… I have read on 'here' about those VFD from PRC and which are not user friendly and have a dubious reputation, but I've not actually met a person who has had any problems whatso ever.

My take is rather the other way round! Recent inexpensive VFDs tend to be rather simpler than older models, and come with sensible defaults. The Manuals may be a little Chinglish but I find them comprehensible, and the VFD has a good chance of powering a motor out of the box.

In contrast, the manual of an ancient VFD is likely to be aimed at a professional installer who has done a course! Full featured VFDs of the older type often require the owner to configure many details. Multiple configuration options allow the installer to match the VFD to a particular motor in sophisticated ways, but the level of complexity they provide is over the top for driving a home workshop machine.

Buying second-hand or new, I'd make sure a manual was available and that I understood it. If not, better to go with a new unit of basic type, PRC or not, because lots of the people on the forum can advise. If baffled by manuals of any sort, buying from one of the firms who sell bundles is a good option. They make sure the VFD and motor are compatible, and provide wiring, controls, instructions and a help line!

Dave

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