Michael Cross 4 | 23/01/2023 12:56:16 |
53 forum posts 1 photos | I can't be the only person who has noticed that machines fitted with single phase motors attract a significant premium over three phase machines so obviously not everyone sees or understands the benefits. It's usually cheaper to fit a used three phase machine with an inverter than to source a single phase machine. It's also usually much easier to equip a three phase machine with a VFD than to replace the motor with a single phase one because single phase motors are much bigger for the same power, so if you go that way you're quickly into new mounts, adjusting pulleys and so on. The benefits of changing speed using a VFD are sometimes exaggerated as if you do this you don't get the increase in torque that you would from mechanical gearing. This is why Variable Speed DC motors (which are actually three phase AC motors) stall so easily when you run them slowly. Any three phase motor will do the same if you turn the frequency down and then push it. Running significantly outside of the designed frequency is also not great for your motor, particularly if this leads to stalling, which robs you of more torque, stalls more easily and the death spiral continues. It's fine as a way of fine tuning your speeds but it's not the ideal way to adjust your speeds across a wide range. That said it is convenient and if you're well within your machine's capability then you might reasonably accept the compromise. Regional differences in the use of VFDs are more likely down to availability of used machines from industry than this or that place being 'backward'. Here in the UK we are blessed with a plentiful supply of both good used industrial machines and VFDs so putting the two together makes perfect sense. I don't know if that's the case in OZ but that sort of reason will surely explain any disparities. |
duncan webster | 23/01/2023 14:04:10 |
5307 forum posts 83 photos | Posted by Howard Lewis on 22/01/2023 15:45:19:
...... Presumably it could be argued that 5 hp 3 phase motor will draw the same average current as a 15 hp single phase machine. to save on capital and power costs ....... Howard 3 phase power is 1.732*V * I, but you have to pay for all 3 phases.
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SillyOldDuffer | 23/01/2023 14:20:50 |
10668 forum posts 2415 photos | Always good to look at stuff from the other way. What do single-phase induction motors bring to the party? Single-phase is conveniently simple for domestic purposes like heating, lighting and electronics, but getting it to turn an electric motor is difficult. To spin, the motor requires the designer to create two magnet fields that oppose and attract in concert as the rotor turns. And it has to do this in a way that allows the motor to start under a reasonable load as well as to stay spinning once it's going. So the only advantage of single-motors is they run off single-phase domestic AC power and do so reasonably well. That was vital back in the day, but the motor is a clever compromise rather than the best of all possible motors. Look closely and they're a bit poo compared with other types:
In comparison a 3-phase motor is remarkably simple, smooth, efficient and reliable - much less to go wrong. Starting torque is good and they self-start without any special arrangements. They're cheaper to make and take up less space. These advantages counted for nothing when homes only had single-phase power available. Not so today, when 3-phase can be produced from single phase. When done electronically, it's easy to control speed by varying frequency, plus the electronics come with host of other opportunities for getting the best out of motors by managing the power feed. For many applications, including machine tools, 'Brushless DC' motors are increasingly challenging 3-phase, even though 3-phase motors are high performers. Brushless DC has been around for years but during this century the availability of affordable super-magnets and power handling electronics has made them much more popular. Single-phase motors used to the obvious first choice. Not so now. I'd say they're 4th choice on a new machine tool. Dave
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John Haine | 23/01/2023 14:37:22 |
5563 forum posts 322 photos | "...if you do this you don't get the increase in torque that you would from mechanical gearing. This is why Variable Speed DC motors (which are actually three phase AC motors) stall so easily when you run them slowly. Any three phase motor will do the same if you turn the frequency down and then push it. " See this website: http://thinkvert.com/variable-frequency-drive-affects-on-torque-horsepower/ As the frequency is reduced below design speed the VFD can drive the motor to generate constant torque down to low speeds. Imagine a motor which is clamped to the rotor can't turn and the stator is driven with current at the slip frequency so that it produced the same field in the air gap as if the motor is running at its maximum torque point. The stator will see exactly the same magnetic conditions and must generate the same torque. The speed of course decreases and therefore the power, but for many jobs torque is what's wanted at low speed - for example tapping. Most of the cheap machine tools use permanent magnet DC motors and thyristor controls (or sometimes MOSFETs). These behave differently. Some DC drives use brushless motors, which I believe generally use permanent magnet rotors. The field windings are 3 phase to generate rotating magnetic fields to drag the rotor round. The torque depends on the stator field and the physical angle between this and the rotor magnets and is independent of speed. |
Tony Jeffree | 23/01/2023 15:32:11 |
![]() 569 forum posts 20 photos | As John's link above shows, a VFD/3-phase motor setup will generate broadly constant torque below its "normal" operating frequency (60Hz in the example shown) and broadly constant power above that frequency. If you are wanting to maximise the amount of metal that you can remove, then you need to maximise power, so you want to be operating in the middle/right hand half of that curve, but for some other operations (as John observes) like tapping where torque is needed but relatively little power, then operating in the left half is just fine, and particularly for operations such as tapping, the speed control is very helpful indeed. I often tap at low speed under power in my ML-7 - haven't yet broken a tap that way. Having had one of the first Newton Tesla kits fitted to my lathe for a good many years now, what I find in practice is that the variable speed allows me to use the middle speed range of the lathe for most of the work that I do, and that it is relatively rare that I need to shift pulleys up or doen (or engage back gear, for that matter). Fitting a VFD to an existing 3-phase setup is significantly cheaper than other options for providing a 3-phase supply from single phase, so even if you plan not to use the variable speed option (always run at 50 Hz and change speeds via the pulleys) then it is still a no brainer if your mains supply is single phase only. |
Michael Cross 4 | 23/01/2023 17:08:32 |
53 forum posts 1 photos | My point is that: you don't get the increase in torque that you would from mechanical gearing Constant torque is one thing, but mechanical gearing increases your toque and adjusting the frequency doesn't, so for a given speed you end up with less torque if you use your VFD to reduce speeds than if you shift the pulleys / gears. |
Tony Jeffree | 23/01/2023 17:25:49 |
![]() 569 forum posts 20 photos | Posted by Michael Cross 4 on 23/01/2023 17:08:32:
My point is that: you don't get the increase in torque that you would from mechanical gearing Constant torque is one thing, but mechanical gearing increases your toque and adjusting the frequency doesn't, so for a given speed you end up with less torque if you use your VFD to reduce speeds than if you shift the pulleys / gears. ...all of which is consistent with what I wrote. 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. |
old mart | 23/01/2023 17:28:12 |
4655 forum posts 304 photos | A lot of people who want the convenience of variable speed which a VFD gives are simply too lazy to bother changing gears or belts. My view is that the VFD complements the mechanical advantage provided by manual gearing. |
Michael Cross 4 | 23/01/2023 17:47:56 |
53 forum posts 1 photos | Yes Tony I completely agree with you I think our views on this are very close together. I was addressing John. |
Michael Cross 4 | 23/01/2023 17:50:11 |
53 forum posts 1 photos | 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 Jeffree | 23/01/2023 17:55:41 |
![]() 569 forum posts 20 photos | 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 Lewis | 23/01/2023 20:32:28 |
7227 forum posts 21 photos | 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 2 | 23/01/2023 22:00:49 |
![]() 1891 forum posts 37 photos | 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.
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Hopper | 23/01/2023 23:10:30 |
![]() 7881 forum posts 397 photos | 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 Olsen | 23/01/2023 23:31:05 |
1294 forum posts 108 photos 1 articles | 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 |
Hopper | 24/01/2023 05:33:46 |
![]() 7881 forum posts 397 photos | 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 Poole | 24/01/2023 09:20:55 |
![]() 3676 forum posts 82 photos | 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 India | 24/01/2023 09:38:47 |
![]() 277 forum posts | 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. Edited By Clive India on 24/01/2023 09:40:28 |
SillyOldDuffer | 24/01/2023 09:51:41 |
10668 forum posts 2415 photos | 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 Haine | 24/01/2023 10:33:06 |
5563 forum posts 322 photos | As I pointed out before, Tesla cars use induction motors. |
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