Single phase to 3 phase converter, without huge capacitors - Page 1
Forum section: electronics / projects

author: peter-h
author_rank: Super Contributor
timestamp: August 10, 2021, 01:56:10 pm
content: This seems impossible without some "storage" because the input goes through zero, twice per cycle, but two of the three outputs are nonzero at those times, so clearly it is impossible without storage.Now let's say you are trying to build a 100kW one. You are gonna need some massive capacitors... of the order of 0.1 farad, rated at > 500V.I am wondering whether there isn't a way of doing this which uses an inductor, together with a smaller capacitance, to produce a delayed version of the input, so in effect you have a two-phase input which would then remove the "zero input" case, and whether that would be a lot smaller (if not lighter).The input power factor will also be rubbish if full-wave rectifying into a capacitor (and such a product would never get the CE compliance today, although there may be exemptions for industrial use) but perversely a big capacitor enables power factor correction, albeit PFC also needs another switcher stage which needs a big inductor...Here is an inverter of roughly that size - for a mere 33k and that is chinesehttps://www.ato.com/100-hp-single-phase-to-three-phase-converterA 25kW one for a mere 4k:https://www.alibaba.com/product-detail/25KW-Single-phase-to-three-phase_62385669772.html

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2021, 02:05:50 pm
content: A large capacitor is unavoidable. It can be minimised by using a power factor correction stage, consisting of a boost converter, which ensures the input current is sinusoidal and the output voltage is a little over the peak value, of the maximum output voltage.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 10, 2021, 03:53:20 pm
content: As Zero999 implied, capacitor energy storage is related to thevoltage differenceand is linear. So the key of storing energy in a manageable sized bank is to let the voltage change. And for this, a boost converter might be necessary. ... which is likely necessary anyway assuming you convert from 1~ 230V to 3~ 400V, for example. Full-wave rectified 1~ then gives 320V peak, but 560Vminimum at all timesis needed to generate 3~ 400V, for a simple implementation at least.For the same reason, a PFC power brick seems to power the load like forever (seconds for a light load!) once unplugged. This is because the input capacitor is charged to some 320V (in a 230V system) but the PFC boost stage stays functional until the voltage on the cap drops somewhere below 70V. This stores a lot of energy in a relatively small cap compared to old non-PFC supplies with narrow input voltage ranges.

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 10, 2021, 04:37:44 pm
content: Just wonder, 100kW is huge power to deliver over a single phase. Is 3 phase really unavailable?Energy can be stored in the inductor, but it will be huge and cost way more than capacitors for the same amount of energy.

author: Benta
author_rank: Super Contributor
timestamp: August 10, 2021, 04:57:52 pm
content: Yes, it's possible.Do an Internet search for "matrix converter". The electronics and power switches are much more involved, but it's doable.

author: uer166
author_rank: Super Contributor
timestamp: August 10, 2021, 06:01:07 pm
content: Quote from: Benta on August 10, 2021, 04:57:52 pmThat won't help you since it doesn't deal with the fundamental problem where a 1-phase input has discontinuous power and 3-phase output is continuous, you need to fill the gap somewhere..

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 11, 2021, 12:01:53 am
content: Easily done by using a flywheel as the energy storage device, but where do you get 100kW of single phase power?

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2021, 12:37:40 am
content: Even on a small scale I don't think anyone is using capacitors connected to rotary phase converters anymore, solid state VFDs are affordable and offer all sorts of features that make 3 phase motors work better than they ever did before. At one time I was envious of places that had 3 phase residential power available but now I don't care anymore. I can have 3 phase power at any frequency I want at the point of use for a few hundred bucks or less.

author: peter-h
author_rank: Super Contributor
timestamp: August 11, 2021, 10:50:20 am
content: "assuming you convert from 1~ 230V to 3~ 400V"Not quite. "230V" mains is 230V rms, which is 325V peak. "230V 3 phase" is also 230V rms, between any phase and neutral.In switching power supplies, you trade the capacitor size against the lowest voltage at which the output can be maintained. 230V full wave rectified will give you ~320V DC on the capacitor with zero load, and with some load this will drop, before the next cycle comes along to top it off again.Let's say the load is 100W. The switcher will draw current x (about 0.3A) with a 320V DC input, and 2x (about 0.6A) at the lowest point if you let the cap discharge to 50% (160V). Most little PSUs will work at 110V (US supply; 155V DC) and with a margin below that so about 90V rms which is 127V DC. So even common-as-muck switcher designs cope with a pretty wide input voltage range, which allows quite a small cap to be used. And the worst case is a US supply which is below spec"but where do you get 100kW of single phase power?"That's a different questionThe reason for my post was to wonder whether there isn't some clever way to fill in the gaps on the input, apart from the obvious way of having a capacitor.And while a cap charged to a higher voltages makes the job easier (if you use a simple stepdown switcher to generate each phase) the cap also gets physically bigger if it needs to be rated at a higher voltage.

author: SeanB
author_rank: Super Contributor
timestamp: August 11, 2021, 12:43:48 pm
content: Simplest method not using capacitors is to get a single phase motor of appropriate power, and then also get a 3 phase synchronous motor of appropriate size, and get a nice shaft coupling to join the 2 shafts. Then add some method to excite the slip rings to get the synchronous motor to generate power, and a voltage control to keep it stable under load, and there you go with no capacitors needed, though the 2 motors will be kind of large in size, and you will max out a single phase motor at 22kVA, as that is the biggest one Alsthom makes and sells, intended for water pumps in rural areas, where you have a feed that is SWER 11kV, with a 30kVA transformer by the pump station to do the voltage conversion, as you only need to have a single overhead aluminium cable installed.With the losses in the system you will get around 15kVA output as 3 phase, though your regulation, both voltage and frequency wise, will be somewhat poor, especially for step loads. But no big capacitors aside from the normal 470uF start capacitor for the single phase motor.

author: artag
author_rank: Super Contributor
timestamp: August 11, 2021, 01:38:23 pm
content: I don't think you need a single-phase motor rated at the system power.3-phase rotary converters are usually built with a single 3-phase motor. They're not motor-generator sets but rotating transformers. The 3-phase output is taken from the motor, but the single-phase input is fed to the same motor. I guess the load motor also plays a part in equalising the currents.The problem with this is that the 3-phase motor needs to be running : it won't start from a single phase supply. This is where the single phase motor comes in - it spins the pair up to speed. However it's also possible to do it with a switched phase-shift network rather like a single-phase AC motor.https://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/rotary-phase-converter-designs-plans-101231/There's still energy storage but it's in the inductance and rotating mechanical mass of the motor, so the capacitors are smaller.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 11, 2021, 03:18:40 pm
content: Rotary converters are simple and something you can basically set up yourself if you have a three phase motor sitting around. But for efficiency, cost, size, and flexibility VFD are better. Capacitors are just cheap and effective energy storage on the 8-10 ms timescale of a half cycle of mains voltage.

author: jonpaul
author_rank: Super Contributor
timestamp: August 11, 2021, 03:43:48 pm
content: Bonjour the technologi for phas conversion is very old and well known.The capacitor is only for small motor loads ~ 1 hP like a lathe.It has many issues.Solutions : 1 ph OR DC to 3 PH > 1 kW in historic orderMotor -GeneratorInverter : Mercury arc ignitronsSCRsIGBTsUse 1 PH >>REC DC >>Inverter r for any number of phases.For PDC use1 ph>>PFC stage>>380V DC bus >>InverterDo a bit of research.Enjoy,Jon

author: Berni
author_rank: Super Contributor
timestamp: August 11, 2021, 03:45:00 pm
content: No capacitors? Rotary converter it is then.All you need is a large 3 phase motor, hook up just one of the phases. Fit a start-run capacitor to another phase to get it going, or since you don't like capacitors, then just give it a spin by hand before plugging it in, direction doesn't matter as long as its spinning. Wait for it to get up to speed and then hook up your 3 phase output across all of its 3 phases, the other 2 missing phases are created by the motor working as a generator. The energy storage that would be otherwise provided by a large pile of capacitors is now provided by the rotational inertia of the rotor. So for this reason the rotor will be speeding up and slowing down 100 times per second as it picks up extra energy at the sine wave peaks and gives it off when the single phase is crossing zero and not providing any power. So for this reason the 3 phase motor will also have that dreaded loud hum of a single phase motor, but the other 3 phase motors you run from this should run more smoothly and quietly as one might expect from the nice continuous power delivery of 3 phase power.So why do all these silly suckers keep using capacitors when you can do without? Well.. to get 100kW you need something along the lines of a 200kW 3 phase motor serving as the rotary converter. Have a bit of a google of just how huge and heavy such a motor is. The raw material cost of such a huge motor likely already covers the cost of a off brand VFD for that power.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: August 11, 2021, 04:39:53 pm
content: I'm of the keep it simple school of thought. To that end I have employed several 2-5KW VFDs for shop motors supplied by 480 VAC single phase. My 480 VAC is transformed 120VAC through a 5KVA transformer. If you need sine wave, the VFD option won't work, but I'm sure there are high power VFDs that can be supplied single phase or at least split phase. One of the nice things about 3-phase is that power delivery is nearly constant and does not need large capacitor filtering.

author: jonpaul
author_rank: Super Contributor
timestamp: August 11, 2021, 05:12:24 pm
content: CORRECTION: No capacitors:Meaning that the use of a single oil cap to shift phase to create a "three phase" motor drive is OK for small loads < 1 kVa but has poor efficiency and does not create a true three phase.Actually it used one main phase and one shifter by the cap ~ 90 deg. The third phase is left open. Thus the motor does rotate but the power less than rated and efficience is low.Of course caps are needed in induction motos start, PFC and electronic converters!Just not the capacitor ONLY phase shift mentioned.EnjoyJon

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2021, 08:47:04 pm
content: IIRC a rotary converter made using a 3 phase motor still needs capacitors, I helped somebody set one up once a long time ago, I remember them tinkering with the capacitors to balance the phases. They used a small single phase "pony motor" to get the thing spinning, I think once it was up to speed it would sustain itself.

author: jmelson
author_rank: Super Contributor
timestamp: August 11, 2021, 08:53:02 pm
content: Quote from: NiHaoMike on August 11, 2021, 12:01:53 amYup, 833 A, not counting losses and power factor! (Oh, I got the 120 V from the Chinese 100KW converter page.)Jon

author: TimFox
author_rank: Super Contributor
timestamp: August 11, 2021, 10:46:46 pm
content: If you don't want to use a huge capacitor to ride out the dips in the rectified single-phase source, you could use a superconducting inductor instead. There have been proposals to use a large-diameter superconducting coil instead of a flywheel for utility-scale energy storage, since the strength-of-materials factors in keeping the coil or wheel from exploding favor the coil. You need a lot of space, however.

author: jonpaul
author_rank: Super Contributor
timestamp: August 12, 2021, 12:19:54 am
content: The superconductor or flywheel are not needed.A boost PFC stage converts the incoming 1 ph AC to DC 380V.Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 12, 2021, 01:27:50 am
content: Note that capacitors aren't great, because they have impedance.I mean, you need the impedance (reactance more specifically) to get the phase shift and energy storage, but the thing is, it's necessarily "squishy". It varies with load, both magnitude and phase do. You can tune it for a given load, but it will only be right at that load.So it's a good idea to have a big hunk of spinning metal, that can handle some extra reactive current, and the capacitor just serves to keep that spinning. (Or something like that, I don't recall the analysis of the rotary phase converter actually.)Or to the extent that the machine you're running can serve as its own RPC, then that, but obviously that doesn't work for nonrotating things. For which, you might have to settle for the inverter / VFD.Tim

author: Circlotron
author_rank: Super Contributor
timestamp: August 12, 2021, 03:13:10 am
content: One of the more interesting single to 3-phase conversion schemes I've seen uses a 3-phase induction motor connected to active and neutral of the single phase input and a capacitor connected from active to the third motor terminal. No big deal there. But it also had aninductorconnected from neutral to the third motor terminal as well. That is to say, the capacitor and inductor were in series across the single phase input and the junction of L and C connected to the third terminal. Might have been to get the ideal amount of phase shift or maybe better voltage regulation. Don't know.

author: Circlotron
author_rank: Super Contributor
timestamp: August 12, 2021, 03:20:41 am
content: Would be interesting to put a zig-zag transformer across the "output" of the 3-phase motor to pull the voltages equal and phase angles 120/120/120.Edit -> would probably be more effective with a star connected motor, with the star point going to the zig-zag tranny neutral.

author: Berni
author_rank: Super Contributor
timestamp: August 12, 2021, 05:26:43 am
content: Quote from: jonpaul on August 12, 2021, 12:19:54 amExcept that the PFC also consumes the most power when the sine wave is in the peaks, so the single phase input power still drops in the valleys, while the 3 phase output is still outputting full power at that point on the sum of the other 2 phases. The large capacitors that the OP is complaining about fill that gap in power delivery.I suppose you could squeeze more continuous power by getting a PFC stage to do the opposite of what it is meant to do. Make it pull less power in the sinewave peak and pull as much power as it can during the sinewave valleys. This would make power delivery more flat like DC while making the power factor absolutely atrocious (Something you would never be allowed to do at such high power levels).Quote from: Circlotron on August 12, 2021, 03:13:10 amWell sounds like they are just helping make use of the third phase by shifting the phase the other way. The capacitor makes the phase lead versus the single phase input while the inductor makes it lag. Not sure how much it helps since it sounds like it needs a pretty big inductor.The other idea of hooking up a 3 phase transformer primary to the output sounds like a better idea to improve the output power quality. The 3 phases have to sum toward zero otherwise the windings will start acting as secondaries and pushing current around.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2021, 09:06:18 am
content: Quote from: peter-h on August 11, 2021, 10:50:20 amAnd three phase is 400V RMS phase-to-phase, 230V RMS to neutral.Motor inverters convert 230V single phase to 230V three phase, which would be 133V to neutral, which is never used.It's possible to convert 230V single phase to 400V three phase, but it would require a boost converter stage on the input of the inverter.QuoteIf it's just to drive a motor, a large capacitor isn't strictly necessary, as the motor has some inertia. Of course it's not ideal, as there will be some torque ripple, but a 100kW motor is huge and will already store a large amount of energy, especially if it's rated for 100kW at 50Hz.

author: Circlotron
author_rank: Super Contributor
timestamp: August 12, 2021, 11:14:09 am
content: Quote from: Circlotron on August 12, 2021, 03:20:41 amGot my terminology a bit wrong. I think I meant to say a Grounding Transformer.https://en.wikipedia.org/wiki/Grounding_transformer

author: peter-h
author_rank: Super Contributor
timestamp: August 13, 2021, 11:31:10 am
content: "Motor inverters convert 230V single phase to 230V three phase, which would be 133V to neutral, which is never used."That's news to me; the ones I have seen (for e.g. a workshop with a turret mill which has a 3PH motor and you had no choice because a secondhand Bridgeport can be found pretty cheap) output 3 phase which is 230V phase to neutral.That is also exactly the same as standard power distribution down the street. You connect one house to one phase, next house to next phase, etc.Not heard of 133V.

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2021, 02:37:53 pm
content: Quote from: peter-h on August 13, 2021, 11:31:10 amIf the inverter doesn't have a boost converter, then if the input voltage is 230V phase to neutral, the output voltage will be 230V phase-to-phase, which is 133V phase to neutral. Three phase motors can be wired in star, for 400V operation, or delta, for 230V operation.QuoteHouses are generally wired phase to neutral though, except in some parts of Europe which are 230V, phase-to-phase, or 133V phase to neutral.

author: peter-h
author_rank: Super Contributor
timestamp: August 13, 2021, 03:49:12 pm
content: "If the inverter doesn't have a boost converter, then if the input voltage is 230V phase to neutral, the output voltage will be 230V phase-to-phase"I haven't looked at the topology of real products but if you rectify the incoming mains, you end up with 320V DC, and a buck converter can switch between 0V and 320V, producing 320V P-P which is 113V RMS. But that is no use to anybody (well it could drive an American "110V" 3-PH load) so my assumption was that there was anegative320V rail produced also from the incoming mains, and for the negative-going output you would switch over to that, to generate the 0V to -320V portion of the output cycle. Then you needtwocapacitorsThen you have also preserved where the neutral level is. That is how I would design such a thing. In fact the neutral wire could run all the way from input to output.Or if you can get semiconductors rated at 640V (plus a nice margin) then the input stage could be a boost converter producing 640V and your outputs can swing 640V P-P, but the neutral potential will be 320V above the incoming mains, which is OK if the input stage is isolated (which a boost converter normally could be). But you don't get a neutral wire coming out of such an arrangement. You just get a delta 3-PH.

author: Berni
author_rank: Super Contributor
timestamp: August 13, 2021, 06:27:25 pm
content: Neutral is not really important for motors since they don't need one, even in Y configuration as they produce there own "neutral" in the middle.This is what a VFD typicaly does:So it does store the peak to peak voltage in the capacitor since when one phase is near the top another phase will be near the bottom, those are the ones that get to pass trough the diodes. This is why IGBTs are common for the switching since the voltages are pretty darn high.This is pretty similar as if you ware to feed a biphase 220V into a rectifier (You get a bit more voltage but still pretty close) however since biphase is only a thing used to let the 110V americans get 220V for the heavy loads, so this is not an option. But using two single wave rectifiers to separately rectify into +300 and -300V DC on two separate capacitors is the same thing. Two capacitors in series is actually commonly done anyway because the electrolytic caps tend to not have enough voltage rating to handle it. So in the end the only difference is that single phase provides a neutral connection to the capacitor midpoint while 3 phase doesn't need to.As for the 3 phase with 110V phase voltage, this is a thing. I have some huge 5kW lab power supplies that i scored cheep on ebay from the USA and they have 3 phase input. At first i was worried about how to run these, but it turns out they run on 190-250 V AC. It came with the weirdest 3 phase plug i have ever seen (It's what id imagine aliens to come up with when told to make a 3 phase plug) turns out the 190V is actually the between phase voltage of 110V. So that must mean Americans do have a 110V three phase somewhere. Luckily running this on our European 230V is just a matter of connecting live to one phase and neutral to another phase to get the 230V "between phase" that it can run on. The rectification is also all diodes inside so it doesn't care about the "missing 3rd phase"

author: TimFox
author_rank: Super Contributor
timestamp: August 13, 2021, 07:15:38 pm
content: One normal standard in US is 208 V phase-to-phase, or 120 V line to neutral. 240 V phase-to-phase is used in industrial power, but is not directly compatible with 120 V single-phase loads, unless the neutral is connected at the midpoint of a delta winding between two phases (watch out for the higher voltage on the third phase!)An advantage of three-phase rectification is that the unfiltered rectifier output does not go to zero periodically, so the ripple voltage is much lower than FW rectified single phase.

author: jpanhalt
author_rank: Super Contributor
timestamp: August 13, 2021, 07:50:40 pm
content: Quote from: james_s on August 11, 2021, 08:47:04 pmNot really. The capacitors are needed if you want self starting. There are designs where one uses a foot wheel to start. The one in my shop, is self starting, and I don't consider the capacitors particularly large. You do need a voltage controlled relay to cut the start capacitors out once up to RPM.

author: langwadt
author_rank: Super Contributor
timestamp: August 13, 2021, 08:05:25 pm
content: Quote from: TimFox on August 13, 2021, 07:15:38 pmif it is 208V phase to phase and 120V phase to neutral it is not split phase but two out of three phases and you could with a transformer generate the third phase

author: TimFox
author_rank: Super Contributor
timestamp: August 13, 2021, 08:32:16 pm
content: Quote from: langwadt on August 13, 2021, 08:05:25 pmSorry--I was imprecise. 208 V and 240 V in my reply were both three-phase supplies. I was replying to Berni about his power supply input voltage.In US, 208 V is a common supply in commercial buildings, allowing 120 V single-phase feed on standard outlets and 208 V three-phase for HVAC and similar high-power loads.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2021, 08:44:46 pm
content: To clarify:US residential neighborhoods are predominantly (almost entirely??) single phase; MV distribution lines are split by phase (out of three), and pole or pad transformers feed a few houses at a time; their secondaries are 240VCT ("split phase"). Thus when there's a distribution fault (e.g. wind-whipped lines, fallen tree branches, etc.), typically a neighborhood at a time goes out. (And yes, overhead lines are very common. Some neighborhoods use buried cables, too.)Which is, in part, why it's essentially impossible to get 3ph in the house. Commercial sites typically have it though.3ph is mostly 208, 240 or 480V. I'm not exactly sure when 208 vs. 240 is used, but 208 as mentioned above gives 120V line-to-neutral circuits so you can do the same kind of phase sharing for office, lighting, etc. circuits. 208V lights are also common.There's also a really old version,https://en.wikipedia.org/wiki/High-leg_deltathat's still around in some old buildings but I think prohibited by code (i.e. the old stuff is merely grandfathered in).Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2021, 08:45:54 pm
content: Quote from: Berni on August 13, 2021, 06:27:25 pmA single phase input VFD looks like this.Yes, you're right, there's no neutral. It was a silly thing for me to have said.The phase-to-phase output voltage is normally slightly less than the phase to neutral, input voltage, although power factor correction is now mandatory, on all but the smallest drives, so the boost converter will enable it to output a slightly higher output voltage, than the input, iff needed.

author: TimFox
author_rank: Super Contributor
timestamp: August 13, 2021, 08:51:50 pm
content: Quote from: T3sl4co1l on August 13, 2021, 08:44:46 pmWhen designing high-power equipment for remote installation, I checked with Commonwealth Edison's website and those three voltages are standards for industrial/commercial 3-phase installation. Other choices require engineering at the utility end. I only know about "high-leg delta", where the neutral is a center-tap between phases, due to a horror story from one of my colleagues who encountered it in a surprise when helping with the power at his church.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 13, 2021, 09:05:19 pm
content: Quote from: T3sl4co1l on August 13, 2021, 08:44:46 pmI think 240 V three phase is (mostly) only used with high-leg delta, or possibly in applications where old equipment expecting that configuration is used. Otherwise 208 V three phase is most common power distribution in commercial and light industrial applications.

author: TimFox
author_rank: Super Contributor
timestamp: August 13, 2021, 09:18:48 pm
content: Before I retired, my workplace (US) had 208 V 3-phase, which also fed the 120 V single-phase outlets.When the building was expanded, the new transformer also had 480 V 3-phase, which we rarely used except for HVAC.The feed from the utility was unchanged, but there were two local transformers in the final configuration.(When the industrial park was under construction, there were occasional accidents elsewhere in the neighborhood that would drop one of the three phases: this was a problem for the HVAC system, which was fuse protected.)

author: EPAIII
author_rank: Super Contributor
timestamp: August 14, 2021, 02:29:48 am
content: You would be surprised about what some people are using. Especially hobbyists, non-electronic ones that is.Quote from: james_s on August 11, 2021, 12:37:40 am

author: EPAIII
author_rank: Super Contributor
timestamp: August 14, 2021, 02:34:39 am
content: 435 Amps at 230 Volts! I can hear the electric company laughing from here.Quote from: NiHaoMike on August 11, 2021, 12:01:53 am

author: EPAIII
author_rank: Super Contributor
timestamp: August 14, 2021, 05:18:15 am
content: I see a lot of comments on this, that, and the other thing. But there is one clear fact that your post brings up and that is, in any scheme which converts single phase (or US split phase) into three phase must have some form of energy storage. That is a basic, physical fact and there is no way around it.And you also, correctly state that capacitors are not the only way to construct that energy storage. I don't know if I can be correct in saying that all forms of energy storage have been tried, but I don't know if anyone would want a water column in their shop.Mechanical storage, like flywheels is certainly possible and has been done. Then there is the motor-generator set. That works and I have even seen one. Etc.But lets talk about electrical and electronic methods and why capacitors are so popular.You mention a delay line. Great idea! At 50 or 60 HZ a delay line may be possible but just look at how. First there is a straight wire. One that can handle the current. And the energy will not quite travel down it at the speed of light, but probably around 75% of it. The copper mining people will have to open some new mines.OK, so wind that wire into an inductor. With just stray capacitance in that coil, you may get down to 50% of the speed of light so those additional copper mines can only operate on two shifts a day, not all three. We may be getting there, but the progress is slow.So, start adding actual capacitors to that delay line/coil. I have seen actual delay lines made with inductors and capacitors. The capacitors are between every adjacent inductors and the physical size of the capacitors was about the same as that of the inductors. Not a real engineering principle, but just a ball park observation. So, the ideal ratio of C vs. L does seem to be somewhere around equal weight for the two sets of components. If you get far from that in the direction of more L and less C, then the amount of copper in the Ls is going to go up a lot faster than the reduction in the sizes of the Cs. Really.Actually, if you go the other way, then the reduction in the amount of copper in the Ls will not be met with an equal increase in the mass of the Cs. And things like insulators and conducting films on those insulators are a LOT lower in mass than the copper that they replace in the Ls.AND, things like insulators and conducting films on those insulators are a LOT lower in COST than the copper that they replace.And there, is why you see large capacitors being used with no or SMALL inductors. It is a simple matter of COST!$$$$$$$$$ or whatever your local or favorite form of currency may be.And cost is ENGINEERING 101.So go on all you want about this or that scheme. When you get down to it, the real reason is the cost.Quote from: peter-h on August 10, 2021, 01:56:10 pm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2021, 06:10:09 am
content: Quote from: EPAIII on August 14, 2021, 05:18:15 amNote that a transmission line is a reactive network, and like any other, it's only valid for an impedance-matched load.So for general purposes, it won't help you any, and you need something with more energy storage, like a rotary.If your load already has a lot of rotating mass (like a motor), it can work out alright. Hence we have many designs with start or run capacitors (or both!).There is the possibility of nonlinear elements to help out: one can construct a voltage-regulating transformer using a saturable core. That is, a ferroresonant transformer. (It's actually regulating voltage proportional to frequency; since line frequency is pretty stable, it works out alright.)I wonder if anyone's made one that has enough energy storage and/or stages, to reconstruct three phases from only one? Hmm...QuoteAnother reason you don't see inductors, is because they are integrated -- for a motor-run cap, the companion inductor is the stator's magnetizing or leakage inductance, as the case may be. The phase shift might vary with load (which affects power factor, torque curve, and I suppose torque ripple as well), but the main thing is that it starts up at all, into whatever the load is.For another example, a lot of induction motors have a start winding and no capacitor whatsoever; the RL time constant of the much smaller winding provides the phase shift. Needless to say, such a winding can't be operated continuously, hence the centrifugal switch on these types. Others use a phase-shift ("motor start") cap to improve starting torque.The various types are of course for cost reasons, as you say, and power density, efficiency, and suitability to various applications. For instance, the pitiful starting torque and efficiency of a shaded-pole motor (the "start winding" is always on: the "shading" windings are copper links welded around part of the core!), is adequate for small fans and other light rotating loads (turntables, etc.).For heavier loads, more efficiency is demanded, and shaded poles can be morphed into a proper winding (again at an angle to the main winding). It's then powered by a run capacitor, giving the split phase motor. (You can indeed manually start a split-phase in either direction, if you disconnect the aux winding and give it an initial spin. Or let it growl and overheat at a stall!..) Also the resistive (LR phase shift) type, with starting switch, for higher torque at expense of much higher starting current; or with a cap as well for the best of everything (start and run caps), but it costs more.Or if synchronous operation is required, the rotor can be magnetized, permanently or with an electromagnet; or by cutting grooves on it, a hybrid induction reluctance motor is had. Apparently such a design was used for teletype machines (back in the 30s-50s). Synchronous machines are still equipped with shorting bars like the induction machine has (just not a full squirrel cage), to provide starting torque and to dampen oscillation as it settles to synchronous speed.Tim

author: woodchips
author_rank: Frequent Contributor
timestamp: August 14, 2021, 09:41:10 am
content: Yes, single to three phase must have some form of energy storage, see Steinmetz.Energy storage can be capacitive, inductive or inertia, see SteinmetzThis problem was solved a century ago, electric trains use single to three phase converters, see Steinmetz.What is new are the static inverters. My very limited experience of these suggests that the professional, 400V input, type actually have direct connection to the smoothing capacitor terminals. There isn't actually any need for the three phase input if you can generate 500V or so DC and feed to these terminals.By the time you have built one of these converters, 100kW, you will have spent a lot of money. Why not just buy a 200kVA diesel generator and use that?Oh, and the boring 230V to 400V transformer with a few caps will run a motor at 100%, just get the voltages balanced. What you do need to think about is the synchronous reactance of the converter and how that reduces available power. Practical experience, but only a 6kW motor.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 14, 2021, 02:26:32 pm
content: You dont like big sexy caps?Back to the 60's, (flux capacitor fully charged). How about a nice big MG set - no caps needed.There are penty of VFD phase converters that convert single to three phase. They all have DC bus with a massive cap to smooth it out, just like the MG has a massive flywheel.Its hard to imagine another way to do it. If you get something low cap running, I'm sure to hear one of its harmonics on the home service and third and light...

author: Zero999
author_rank: Super Contributor
timestamp: August 14, 2021, 05:27:18 pm
content: Proof, using LTSpcie that single to three phase power conversion requires energy storage. The power in each resistor, run off a 3 phase supply is plotted individually (red, yellow and blue) and the the sum (green). The total power doesn't vary, so some energy storage would be required, to dump power continuously into three resistors, from a source which only supplies power 100 times per second.

author: james_s
author_rank: Super Contributor
timestamp: August 14, 2021, 07:38:25 pm
content: Quote from: woodchips on August 14, 2021, 09:41:10 amThe modern ones probably do, but they used to be DC, and used similar large rotary converters or mercury arc rectifiers to get DC from the AC feed.

author: EPAIII
author_rank: Super Contributor
timestamp: August 14, 2021, 08:46:42 pm
content: And when those "...large rotary converters or mercury arc rectifiers..." produce DC, just how does that DC retain it's value when the SINGLE PHASE rotary converter or mercury arc rectifier output goes to a zero value due to that SINGLE PHASE input current undergoing a zero Voltage crossing between positive and negative swings? That is exactly the zero voltage problem that the OP is talking about and, without some form of energy storage, there is no way that the output can be anything but zero at those points.The "...large rotary converter..." uses mechanical energy storage or momentum as another poster said. But it is energy storage non the less.A SINGLE PHASE "...mercury arc rectifier..." will also suffer from that zero Volt condition at the zero axis crossings and must use CAPACITORS to store the energy in order to have an output. This is correct for any type of rectifier and not limited to mercury arc ones.That is at the heart of the OP's question. Some form of energy storage is needed when SINGLE PHASE power is the source. And my answer above simply states that capacitors are probably the least expensive form for that energy storage. That is why many solutions to this problem do employ capacitors. They are CHEAP or, at least, cheaper than the other devices that can be used. It is simply a matter of dollars and cents.Quote from: james_s on August 14, 2021, 07:38:25 pm

author: Berni
author_rank: Super Contributor
timestamp: August 15, 2021, 09:50:41 am
content: Quote from: woodchips on August 14, 2021, 09:41:10 amTrains tend to take a different approach and just simply use DC motors.The main problem there is that they need to vary the RPM of the motor as the locomotive picks up speed and they need lots of starting torque in order to get things moving. This was difficult to do with asynchronous motors back when we didn't have VFDs. So motors that needed more flexibility and fine control tended to all be brushed DC motors back in the day (this is why all old elevators run DC motors). You can simply control a DC motor by varying the amount of current you let into it, you can also control there speed by varying the power trough externally exited field windings(much lower power than the rotor).For this reason lots of rail electrification was DC. Once they needed more power they needed more voltage, but it was a problem making a DC motor that can run at 25kV input. So they switched to AC so that they can have a transformer on the locomotive to change the nice efficient high voltage into low voltage that the motor can run on. You can turn this into DC locally, or now in more modern times have a VFD on board.

author: Zero999
author_rank: Super Contributor
timestamp: August 15, 2021, 01:47:11 pm
content: Quote from: Berni on August 15, 2021, 09:50:41 amI think you'll find that modern locomotives use AC motors, irrespective of whether the supply is DC or AC. DC is probably easier, as another inverter isn't required for regenerative braking.

author: TimFox
author_rank: Super Contributor
timestamp: August 15, 2021, 02:01:12 pm
content: Another common railway electrification used relatively low AC frequency at high voltage, 25 Hz in US (Pennsylvania Railroad) or 16-2/3 Hz in Switzerland (1/3 commercial frequency).This was a compromise between using "DC motors" (which needed low frequency) and the weight of the on-board transformers (for speed control).

author: Berni
author_rank: Super Contributor
timestamp: August 16, 2021, 07:02:49 am
content: Quote from: Zero999 on August 15, 2021, 01:47:11 pmYep hence why i said in the end of my post "or now in more modern times have a VFD on board" hence why they can finally ditch the DC motor (possibly even the transformer too, now that we also have high voltage power electronics)Railways with AC power installed date back to the 1930s and its only about in the 1990s that solid state electronics got far enough to make these big ass VFDs a commercially viable solution. For those 60 years in between they had to make it work without it, hence the good ol brushed DC motor (Tho yes more accurately a universal motor. Having the permanent magnets replaced with field windings come in useful anyway since it lets you have easier speed control)Motor control was such a hard problem back those days that most elevators used a AC motor to spin a DC generator to run a DC motor. They basically needed 2 extra motors to make 1 motor spin. All this just because it made control easier, since you could apply a tiny amount of power to the field windings of the DC generator to create a huge DC current that would move the big DC motor and that would move the elevator with a nice smooth start and stop. All this is now replaced by a box of semiconductors the size of a shoe box and a simple async or bldc motor with 1 moving part.

author: peter-h
author_rank: Super Contributor
timestamp: August 19, 2021, 02:15:41 pm
content: Certainly if just driving a motor you don't need the neutral connection. In fact if just driving a motor you don't need a 3 phase supply; you just buy a 3 phase motor and buy a variable speed drive from any of the many mfgs of these.The need for a neutral connection is if you want to generate a generic 3 phase supply, with different phases potentially used for different things.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2021, 03:28:46 pm
content: A typical 3-phase AC motor has the windings connected in a delta configuration, with no neutral wire.Feeding that load from a wye-configured transformer secondary gives a neutral connection that, while unneeded for motor current, is a convenient place to connect to ground, ensuring the usual safety benefits of a protective earth against insulation failure to the metal case of the motor.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2021, 05:23:01 pm
content: Quote from: TimFox on August 19, 2021, 03:28:46 pmThe motors I've seen can be connected either as star, or delta, depending on the supply voltage. The most common is 400V (star) or 230V (delta). This is useful, as the same motor can be connected directly to 400V three phase mains, or run off 230V, via an inverter.

author: Berni
author_rank: Super Contributor
timestamp: August 19, 2021, 06:05:16 pm
content: Quote from: Zero999 on August 19, 2021, 05:23:01 pmYeah these are very common as soft start circuits because those big motors can pull some serous amps on startup.While you would usually connect a Y load with the neutral connected since it behaves more nicely if a phase disappears. However actual Y-Delta switches for motors tend to skip the neutral connection for the Y position. This does save them one extra switch contact pair, but i would assume more of the reason is that you can also have 1 less wire in the fat power cord leading up to the motor. Not like the motor needs it anyway.But yeah the main point is that the vast majority of 3 phase motors actually have 6 terminals (Well 7 if you include case earth). There are 2 terminals for the ends of each of the 3 windings. That way the person installing the motor can decide if the want to run it in Y or delta by connecting the terminals appropriately (Or connect the before mentioned softstart switch that can reconfigure the motor with a simple turn of a knob). The Delta configuration is indeed the preferred one to run in, because extra voltage means more power with less current (and so less copper crosssection), but you might have reasons for running in Y instead. Perhaps you have a 3 phase motor from Norway where 230V between phase is standard and you bring it to Germany where 400V between phases is standard, obviously you can't just plug it in. However if you configure it Y mode than that Norwegian motor will run perfectly fine on the more energetic German electrons.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2021, 06:15:28 pm
content: Quote from: Berni on August 19, 2021, 06:05:16 pmI don't know. It's just most of the motors I've seen are designed for 400V star, but I tend to work with relatively small motors <5kW. I just thought most motors are this way because they can be run off a 230V inverter.I just did a quick Google for 3 phase motor and here's a link to one of the motors in the adverts.https://alphaelectrics.com/product/0-55kw-3-4hp-2p-3000rpm-b3-ls-71l-230vd-400vy-50hz-new-leroy-somer-ac-motor/?utm_source=Google%20Shopping&utm_campaign=Alpha%20Google%20Shopping%20v1&utm_medium=cpc&utm_term=7893&gclid=EAIaIQobChMI_v7tsde98gIVw7TtCh3TXwygEAQYBSABEgLiMfD_BwEI suppose a 400V delta motor could be run off 690V, another common industrial voltage, in star.

author: Berni
author_rank: Super Contributor
timestamp: August 19, 2021, 06:36:58 pm
content: Quote from: Zero999 on August 19, 2021, 06:15:28 pmIt probably depends on what area of the world you are in. There are lots of old motor manufacturing companies still producing motors locally, so they produce whatever clients want to buy.Here in Slovenia we have the 400V between phase power and most motors are made to run on 400V in delta. So yes these motors could run on 690V between phase if wired in star configuration. I don't think i seen 690V power even in industry here, we just use this to feed it 400V when it is expecting 690V so the motor is being undervolted, hence it works as a soft start.The motor in your link is that kind of motor that when wired correctly will work on both Norways 230V between phase and most of Europes 400V between phase power. Id imagine a large equipment manufacturer might like using such a motor in there machines since it makes sure can run almost anywhere in the world. Tho only people like Norwegians can use the Y as a undervolted softstart.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2021, 10:33:48 am
content: Quote from: Berni on August 19, 2021, 06:36:58 pmIt's 400V three phase here, but single phase, with an inverter is often used to provide 230V three phase, especially for smaller motors, which explains why 230VS / 400VD motors are quite common. I haven't looked but I imagine 400V D is more common, for larger motors.

author: elekorsi
author_rank: Regular Contributor
timestamp: August 20, 2021, 06:42:37 pm
content: Only small motors (usually up to 3kW) are 230/400, all larger motors are usually 400/690. As Berni said, this is widely used for star/delta starting combination to reduce the inrush current.Speaking for most of Europe which is 400V, if you connect 230/400 motor in delta, it will release the magic smoke (due to too high voltage on windings, hence it draws too much current). If you connect 400/690 motor into star for permanent operation, it will usually also smell bad, because it cannot deliver enough mechanical power, resulting in increased current per winding

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2021, 09:59:47 pm
content: Quote from: elekorsi on August 20, 2021, 06:42:37 pmThat makes sense, although you can run a motor wired for a higher voltage, at a lower voltage, so long as the load is proportionally lighter.