Autotransformer Wye-Delta wiring doubt.. - Page 1
Forum section: electronics / general

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 17, 2020, 03:47:52 pm
content: So, I'm not completely ignorant as far as power distribution systems go, but the further I travel down the rabbit hole or try to apply it to the real world, the more the doubt rises... So, I need some hand-holding, sanity check, stuff..So, here's the thing, I have a 3-phase unit that runs at 200-208V. So I can't exactly connect it to the nearest 230V outlet. I do, however, have a three-phase system at home. Obviously, a 3-phase transformer is needed. If I go Wye -> Delta, I get 230V where I'm supposed to get 208V. Close, but no cigar. However, as the difference I need to bridge is pretty small, I would think an autotransformer would make the most (economic) sense.So if I get one of these:https://docs.rs-online.com/d22a/0900766b80fa6cb7.pdfI get... erh.. I don't know. Datasheet says 440/460 to 400. Which one is it now? A 1.1 ratio? Or 1.15? I'm confused here. Or will it have addition tabs for these voltages? Either way, I get 208V or 200V which is just fine by me. I hate it that I can't find any reference to the terminals in the datasheet though..My main concern, though, is that I'm not sure that I can use a tri-phase autotransformer like this... An isolated Wye-Delta setup? Sure. Who cares about the phase shift, all phases shift alike so no problem. But an autotransfo does not do isolation. I still think it can be wired up like Wye-Delta but.. doubts. I need somebody to put my mind at easy.Any takers? Comments? Better ways to do things?

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 17, 2020, 03:58:30 pm
content: Quote from: Ice-Tea on January 17, 2020, 03:47:52 pmWhen they say 440/460 there are two taps.

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 18, 2020, 03:50:04 pm
content: I still need that "sure, go ahead, you'll be fine!"-post.Any takers?

author: duak
author_rank: Super Contributor
timestamp: January 18, 2020, 09:56:26 pm
content: I think you will be fine. The datasheet shows the Vector Group is YNaO which is a simple voltage changing connection (see attached figure).One thing to consider: is the load significantly unbalanced and requires a neutral connection? If so, I believe you will have to provide a neutral connection from the distribution system or get a ZNaO transformer.About alternatives. You generally do not want a transformer with a wye primary and a delta secondary. These configurations work, but are often against local electrical codes because the secondary is not referenced to earth. They also tend to cause negative sequence currents in the primary.

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 19, 2020, 08:44:40 am
content: Quote from: duak on January 18, 2020, 09:56:26 pmAFAIK, current is not unbalanced. I wasn't aware this could be against electrical code. I'll dig into that a bit.Also: negative sequence current? Whu?

author: duak
author_rank: Super Contributor
timestamp: January 19, 2020, 07:13:35 pm
content: It seems that nothing in three phase systems is ever simple. A zero sequence current is caused by an unbalanced load and results in neutral current. A negative sequence current is caused by some odd harmonics of the mains frequency vectorially summing together and causing a current to flow that is against that of the positive sequence. These harmonics are usually caused by the non-linearity of the transformer core but can also come from other equipment connected to the mains. These currents can be quite large and usually lead to wasting energy and even damage.For example, a friend who owns a cabinet shop was using a delta-wye transformer in reverse to operate a large saw. One day, one of the 150 A branch line fuses opened. I replaced the fuse and then checked the phase currents. Even with the saw off, there was 50 A flowing. Even worse, there was a large current flowing in the neutral and the transformer itself was hotter than it should have been. I disconnected the neutral and the phase currents dropped to a normal value.

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 19, 2020, 07:49:10 pm
content: So, I'm not getting a "sure, no prob" but more a 'should probably be fine, but is your life insurance up to date and do you have a fire extinguisher handy". Got it.I get how you can have a neutral current with unbalanced loads, not sure how how to picture 'against the positive sequence' though. Which is my problem. I get more or less how 3-phase works, but the're a world of difference between knowing how it works and*knowing*how it works...

author: duak
author_rank: Super Contributor
timestamp: January 19, 2020, 10:56:40 pm
content: I mostly get the concept of sequence current but I'm old and retired and don't have the time to go deeper. AFAIK, the problem is that certain things were done in three phase distribution systems to either economize, increase safety or increase reliability. If the load is unbalanced then you require a neutral line right back to the generator to carry the zero sequence currents. It would be easier to understand but it is more expensive and there aren't usually large neutral currents anyway. A neutral can be a problem if there is a voltage difference between the generator and the load caused by something external to the system like a lightning strike or a geomagnetic storm. It's better to isolate the load end with a delta-wye transformer and tie the secondary neutral (edit: was load neutral) to earth for safety. The unbalanced currents are then handled by the delta-wye transformer that spreads the imbalance around the three phases. This is a bonus because now the generator doesn't have to handle all the imbalance itself as the local transformer has already handled most of it.It's actually an amazing system that took a number of years to completely work out.What sort of load do you have? A three phase motor should be no problem as it is inherently balanced. Some large AC-DC power supplies as in CNC machine tools will induce harmonics in the phase lines but these are usually handled nicely by the local distribution transformer. Besides, I've never seen a CNC machine that had a neutral connection. I have seen transformers that were used to operate European machine tools on our local power. I believe they are Autotransformers.Cheers,

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 20, 2020, 07:01:32 am
content: Quote from: duak on January 19, 2020, 10:56:40 pmYep, but that wouldn't get me the right voltage. In addition:Never easy, is itIt's 10kW DC PSU.

author: Gregg
author_rank: Super Contributor
timestamp: January 20, 2020, 08:02:01 pm
content: Another option would be to use three individual transformers with 24 volt secondaries and wire them as individual auto transformers; one for each phase. They won't be magnetically coupled like a single three phase transformer and the efficiency may be a little less, but the initial cost may be much less as 24 volt transformers are widely used and available from many sources. Since you are using them as auto transformers, the secondary amperage should be sized for the total amperage needed.

author: richard.cs
author_rank: Super Contributor
timestamp: January 21, 2020, 03:01:12 pm
content: Quote from: Ice-Tea on January 20, 2020, 07:01:32 amThat's basically saying don't use it from an unbalanced "corner grounded delta" three phase supply, a delta-wye (i.e. delta primary Y secondary) transformer would be fine here. You want a transformer with a Y secondary, you don't really care if primaries are Y or delta. If you're starting from a 230 V German supply that will be Y and therefore you could get away with an autotransformer (which would need to be Y).Separately you want a voltage ratio of around 1.1 to 1. it doesn't matter to you whether that it accomplished internally as a Y:Y connection (isolated or not) with a 1.1:1 turns ratio or a delta:Y connection (isolated) with a 1.9:1 turns ratio. The lowest cost approach is almost certainly an autotransformer, whether three single phase ones or one transformer.One of these of the appropriate power rating would work:https://airlinktransformers.com/category/3p-n-auto-star-star?input_voltage=415&output_voltage=380&va=Note these are listed as 415 V to 380 V, in both cases these are phase to phase voltages so these correspond to phase-neutral voltage of 240 V and 220 V. They will therefore convert 240 P-N to 220 P-N or just as effectively 230 P-N to 208 P-N (an identical ratio).

author: duak
author_rank: Super Contributor
timestamp: January 22, 2020, 08:08:22 pm
content: Whoops, I should have been clearer: "It's better to isolate the load end with a delta-wye transformer and tie the secondary neutral (edit: was load neutral) to earth for safety. "This is only for the case of an isolating transformer, not for an autotransformer. One reason why the secondary should not float is in case the transformer (or something else) fails with a connection to a high voltage will not cause the secondary circuit to have an additional voltage added to it and cause a hazardous condition. Another reason is that if one of the secondary phase lines is earthed, then a 1.73X higher voltage is not applied to the two other phase lines, equivalent to the prohibited corner grounded delta. Applicable electrical codes should have something to say about this. These codes may also have something to say about protective devices such as fuses or circuit breakers.However, an autotransformer should be just fine and I think it solves the above problem. Note that the RS unit referred to above is undersized for a 10 KW load.

author: bdunham7
author_rank: Super Contributor
timestamp: January 23, 2020, 12:22:42 am
content: I think with your plans you will blow something up and maybe burn your house down.We don't have enough info and I'm not quite understanding what you have. Keep in mind I'm US-based and not a supreme 3-phase power expert, but I've solved some of my own problems with industrial three phase and I can tell you it can get tricky fast. There is a well known historical case where data centers misused 3-phase and caused some real headaches. You need to thoroughly understand both the supply and load characteristics.First, at the end you mention that your load is a 10KW DC PSU that requires 200-208 volts input. Can you supply more information on this unit? It is critically important to understand how it draws its power. This sounds like a North American power spec and we would need to know whether how the primary input and rectification draws power--does it use 3 120V inputs merged into a single set of parallel filter capacitors? 3 individual 120V inputs? 3 individual 208V inputs? Is it power-factor corrected? In some cases it could be set up to have a neutral current of 3X the individual phase currents. So--can you supply make/model/data sheet/other info on the PSU?Second, the transformer you specify seems to be too small by a factor of at least six. Although you might run a transformer at half its rated voltage, it can't handle any additional current as a result. So you have about half of 3500VA vs 10KW. The configuration, YNaO, cannot supply much neutral current, so the configuration of your PSU is again critical.Third, I'm really confused by your reference to 230 volt supply. I'm not up on every variation of European power supply, but what I thought was fairly standard was a system that had a Wye configured distribution transformer with 380 volts between each leg (L-L) and 230 volts to neutral/ground (L-N). Our US systems commonly have 208/120 or 480/277 in a similar setup. Less commonly (much less--but I've run into it) are delta systems at 220-240 volts with either one leg grounded (corner ground) giving you two single phases at full voltage, plus the three phase or high-leg delta, with one pair center-tapped so that you have two 120 volt split-phases, one 208-volt phase (the "high leg") and regular three phase. If you truly have 230 volts L-L three phase, I'd suspect it was corner grounded. Correct me if I'm wrong. If you have that, then things can get really messy due to unbalanced loads, especially if you have motors of any size starting up. Is this in a commercial building or home?

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 23, 2020, 08:02:27 am
content: Quote from: richard.cs on January 21, 2020, 03:01:12 pmWell, that doesn't work. It's a US 3-phase, 200/208 supply. It's wired up for delta (don't think you'd get 200/208 3-phase in the US otherwise?). There's actually a connection for a Neutral at the back, but it's simply not wired up internally.I get that it would remove some questions and doubts, to wire Y:Y, but I don't think it would like it much...Quote from: bdunham7 on January 23, 2020, 12:22:42 amI'll dig up some pictures and paste relevant manual data, but delta and delta only 3x208. Doesn't even have N hooked up inside, which excludes anything 120V.QuoteIt was my understanding that it's a 10kVA unit, with 3.5kVA per phase. But I had not considered the voltage rating. All in all, I'm not overly worried. I need to test these things, figure out how they work, I don't neccesaraly (sp?) have to run them at full load.QuoteSorry if I explained badly. I have 3-phase-with-neutral, 230V phase to N, 400V L-L. Which is why I'd run primary in Wye and secundary in delta to get the right voltage.

author: richard.cs
author_rank: Super Contributor
timestamp: January 23, 2020, 01:00:49 pm
content: Quote from: Ice-Tea on January 23, 2020, 08:02:27 amOK so what is needed is 208 phase to phase and 120 phase to neutral, I don't think that was ever explicitly stated though in hindsight it should obvious as that's the only place 208 is ever used. My answer was for making 208 phase:neutral from 230 V phase:neutral which wasn't useful, sorry.Quote from: Ice-Tea on January 23, 2020, 08:02:27 amYou only care about the voltage ratios here. You have 230 V P-N and 400 V P-P, you want 120 V P-N and 208 V P-P. You're saying your load is wired delta so doesn't need a neutral. Any transformer topology you choose can be used here, YY, Y-delta, delta-y, delta-delta so long as you get the voltage ratio you want. The actual turns ratio you require varies but if you get the right P-N and P-P voltages at the end that's not important. You want a voltage ratio of 1.91:1, and it's not important whether that's all from the turns ratio or a combination of 1.73:1 from a delta-star and 1.10 from turns ratio.The most easy and cost effective thing to do with standard off the shelf transformers is probably three single phase 230:120 autotransformers, effectively creating a YY autotransformer. Using a Y secondary to feed a delta load is fine, you just don't connect the star point. Autotransformers have the advantage of needing a smaller and therefore cheaper transformer for the same power output because they are essentially only handling the "difference power" but are inherently non-isolated so don't work for delta-Y or Y-delta topologies. A delta-delta autotransformer is possible but needs 400 and 208 V windings rather than 230 and 120 which is less common and therefore probably more expensive.Whatever you do it will be a large and expensive transformer, are you sure this equipment is worth it?You could buy one of these:https://airlinktransformers.com/product/3p-n-auto-star-star-3pas0107Or three of these:https://airlinktransformers.com/product/america-europe-voltage-converter-eu-us4000hwwhich is basically what's inside the first one.Ifyou determined that the equipment in question would work OK on 230 V phase-phase, i.e. 133 V phase-neutral then you could use an isolating Delta-star with as 1:1 turns ratio, whichmightbe cheaper than a YY auto transformer. This is unlikely though, especially as you have to account for supply tolerances.

author: bdunham7
author_rank: Super Contributor
timestamp: January 23, 2020, 01:47:56 pm
content: Quote from: Ice-Tea on January 23, 2020, 08:02:27 amOK, so it is using 3 208V inputs. It would be interesting to see how it is internally configured. If it is just six diodes feeding a single filter cap bank, then it will be "balanced" to a degree--but don't think you can stop worrying! As for US 3-phase, every 208V system I've seen has been Wye. The whole purpose for the existence of such a low voltage 3-phase system is so that one service could feed all types of circuits--120V L-N, 208L-N (single phase), 208V Wye or 208 Delta. Medium-size single phase motors are commonly rated for a range of voltages from 196 to 240 to allow them to be used on either a split-phase 120/240 or 3-phase 208 system.QuoteYour delta power supply would work fine on wye circuit. The unused N connector would be to give the electricians somewhere to put the extra wire so they didn't ask silly questions like "will this work?"QuoteI'd check those specs! According to the data sheet, the unit weighs 6kg. I have home audio amplifiers with transformers that weigh more than that so I seriously doubt it is rated that high. Even if it is, you still have to consider the power factor of the SMPS in addition to the voltage issue. A non-PFC SMPS will have harmonic currents that would typically be handled by a large distribution transformer but may overwhelm a local buck transformer operating nearer its full capacity. I'm saying all this not because I think it is marginal, but because I think it is severely undersized for your application and will likely melt.QuoteI'm unclear as to why you think switching from wye to delta is going to change the voltage here. If you connect the 1.15:1 transformer you have listed to your 400V Wye system, you will have 347 volts (L-L) at the output terminals. Your SMPS is expecting 208 volts (L-L). I think that may blow up!

author: richard.cs
author_rank: Super Contributor
timestamp: January 23, 2020, 02:14:20 pm
content: Quote from: bdunham7 on January 23, 2020, 01:47:56 pmIf he had a isolating transformer with a 1.15:1 ratio then he could connect the primary in Y so it sees the 230 V P-N voltage. Its secondaries would produce 200 V and if wired in delta he'd have 200 V P-P and everything would work (well, 8 V low but whatever). The RS datasheet from the first post however is for an autotransformer which cannot be wired star-delta. I interpret that datasheet as 3500 VA total rather than per phase, it's light even for that because it's an autotransformer.

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 23, 2020, 02:38:08 pm
content: Quote from: richard.cs on January 23, 2020, 02:14:20 pmYep, that was the plan ;-QuoteWhat now? Arggh... Do you have any kind of reference material for that? Would like to learn/understand. The whole point of using an autotransformer was that they are light and cheap for small ratios.QuoteWell, not terribly important. Don't have anything to load it down anyways. But it's listed by RS as:Block 3.5 kVA, 10 kVA Open Frame Autotransformer, 460 V ac, 3 x 440 V ac Primary, 400V ac Secondary, 3 UI 75/41,5 CoreNot that the RS discription is gospel or anything

author: bdunham7
author_rank: Super Contributor
timestamp: January 23, 2020, 02:58:17 pm
content: Quote from: richard.cs on January 23, 2020, 02:14:20 pmIf the isolating transformer had the appropriate primary topology (such as 3 completely separate single phase transformers that just happen to be in one box) or if you actually use three separate transformers as you suggest, then yes it would all work out. However, if those conditions aren't met, you can get into trouble fairly quickly. I would think his best options would be finding an appropriate autotransformer with the appropriate derating or possibly to modify the input of his SMPS to work with the existing available power. I realize it is labelled 200-208V, but it may well be set up to work fine with 230 or 240 volts input.Something else I'll add for discussion is that the benefits of an autotransformer--lower cost and size, greater efficiency--are much diminished when the difference between the primary and secondary voltage is large. For a 10-15% buck or boost, they are great. For a 2:1 or greater ratio, the benefits fade quickly.

author: richard.cs
author_rank: Super Contributor
timestamp: January 23, 2020, 03:49:20 pm
content: Quote from: Ice-Tea on January 23, 2020, 02:38:08 pmI don't have a reference to hand, but if you think of an autotransformer as an isolation transformer with one end of each winding common, then some section of the two windings is common (sees the same induced EMF) between the two halves and can be replaced with one piece of wire. You can't make it star-delta because you don't have an independent primary and secondary. Draw it out and ask yourself how you would connect it.QuoteThe datasheet is ambiguous, the RS description says 10 kVA and whilst it's fairly common for them to be wrong it is probably plausible for a 6 kg autotransformer with a small voltage ratio.Quote from: bdunham7 on January 23, 2020, 02:58:17 pmIt works with three-phase transformers with a shared core as well. The only prerequisites are 1) that there is sufficient insulation voltage rating for the now-higher voltages seen between parts of the two windings, and 2) the windings see a voltage which is no more than their maximum value or the core will saturate.QuoteIt may, but providing 230V P-P three-phase in Europe would still need a transformer and running on 230 V single phase might need significant modifications - it probably doesn't have enough DC bus capacitance to run on single phase at the moment.QuoteAgreed.Making a 10 kW 208 V three phase supply from a typical EU supply will need an sizeable transformer which will be heavy and expensive, Ice-Tea are you sure this piece of equipment is worth it? How does the transformer cost (my best guess would be about €700) compare with selling this one and buying a 400 V or 230 V variant?

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 23, 2020, 04:39:30 pm
content: Quote from: duak on January 19, 2020, 07:13:35 pmThe fifth harmonic in particular, is specially damaging.It rotates in reverse with respect to the sequence of the fundamental current. If there are three phase motors connected nearby, a negative torque is produced.Being close to the fundamental, its amplitude may be a significant ratio.The motor will suffer an overload that will increase the energy consumption in the best of cases, or premature failure in the worst cases.

author: richard.cs
author_rank: Super Contributor
timestamp: January 23, 2020, 04:47:10 pm
content: Quote from: schmitt trigger on January 23, 2020, 04:39:30 pmThat's interestingThe power distribution people I work with seem mostly worried about the third which sums in the neutral (especially bad on legacy cables where the neutral is undersized, often half the phase cross section) and circulates in the delta winding of the distribution transformers making them run hotter.

author: f4eru
author_rank: Super Contributor
timestamp: January 24, 2020, 08:25:58 pm
content: Not sure what your "unit" is, but perhaps you have the option to :a)rewire the motor from delta to starorb) exchange teh motor, or just the stator.may be cheaper than the transformer option.