Device: Dual 208vac circuits combine to 240vac 48a - Page 1
Forum section: electronics / projects

author: magudaman
author_rank: Contributor
timestamp: March 13, 2021, 07:06:17 am
content: I am looking at building a Frankenstein of power conversion wastefulness. I want to take two 208vac circuits of 50a each and provide an output of a single regulated 240vac at 48a.Here is the plan: take 12x 1200w Cisco server power supplies and feed them into a 12 kw inverter. This will evenly divide up the two circuits and boast the voltage.Ps this is to charge an electric car while at the track at the highest rate possible. The onboard charger will provide 11.5kw charging rate at 240vac at 48a. However at the track the 50a outlets are 208vac that can dip to the high 190s vac. In those conditions with single circuit you can get at best 9.6kw. Sometimes the car will trigger to a lower amperage if the voltage drop is to high which would max out 6.4kw.So beside the utter waste of ac to dc to ac to dc , any serious issues with this setup?

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2021, 10:40:26 am
content: Why not simply use a large transformer? I doubt the 240V needs to be regulated.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 13, 2021, 01:26:12 pm
content: A transformer should be easier. For a 208 to 240 V step the transformer would only need to provide the missing 32 V - so some 15% of the power.Chances are one would have to leave some headroom as the line voltage can vary - so not sure if wirth the trouble at all.

author: wraper
author_rank: Unknown
timestamp: March 13, 2021, 01:42:07 pm
content: All you need is 1.5 kVA + transformer with around 32V secondary winding. Connect secondary winding to one of primary terminals and you will get boosted voltage.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 13, 2021, 03:31:07 pm
content: @magudaman lot of losses in conversions .... ask a master electrician for safety reasons and power consuptionsif you have 2 separates inputs voltages maybe an dual input xformer 2x208 to 1x 240vac out could do, you need to be in phase for the inputswould be simpler than adding tons of convertersdont forget when you up a voltage you add current losses in the conversion208*50= 10400 watts240*48= 11520 watts you have a 1120 watts loss who can't be recovered, you drop to around 43-44 amps of output, not 48 ampsare your 2x 208vac 50 amps on the same circuitor totally independant ?? i have doubts if they are going into the same electrical box

author: magudaman
author_rank: Contributor
timestamp: March 13, 2021, 04:05:40 pm
content: Thanks for the responses!I did look at boost transformers for this task but I would then be limited to one circuit. This solution may resolve some of the issue regarding voltage drop down rating the car was doing however I would still be limited to 9.6kw instead of the 11.5kw. I can’t tie into two circuits since I do not know where the other circuit is tied to in regards to phasing. I can’t just parallel two separate plugs together with out making sure they are fed from the same phase or risk a phase to phase short! Maybe an integrated voltage check before closing a contractor, hmmm.I don’t want to use isolation transformer or full size transformer since they a huge and heavy. I would like the have this setup be portable and attempt to keep under 60lbs (~27kg).

author: magudaman
author_rank: Contributor
timestamp: March 13, 2021, 04:25:12 pm
content: I do not know for sure where the circuits are coming from at each location. I should generally have 2x 208vac 50a circuits = ~20kw available but at times it may be 2x 40a or 30a circuits, 16.6kw and 12.5kw respectively. Ideally all three situations would lead me to be able to pull the full 11.5kw to vehicle.The sever power supplies are 80+ platinums so should be around 90% efficiency. Then we have inverter which I am hoping is also around 90%.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 13, 2021, 05:47:07 pm
content: your simplest solution in size would be an auto transformer like the schematic wraper photoyou can have multiples inputs voltages and multiples output voltages ... warning yo wont get any ac isolationAnd your dozen of sisco servers psu's are they supporting load balancing / load share between them ?? no psu's is perfectly identical, the weakest(s) will have problems following the stongest(s) ones in your rig

author: Benta
author_rank: Super Contributor
timestamp: March 13, 2021, 09:30:44 pm
content: Correct me if I'm wrong, but isn't 208 VAC 3-phase phase-to-phase voltage in US systems? Meaning they'll be floating and out of phase?Or have I misunderstood US power distribution?

author: ajb
author_rank: Super Contributor
timestamp: March 13, 2021, 10:18:30 pm
content: I think you're asking for headaches by paralleling power supplies that weren't designed for load sharing. There are enclosed power supplies from the usual suspects that are designed for this (additional signal connections that are used to balance between parallel supplies), or maybe using fewer larger supplies in series and a 48V inverter would be easier. That may even end up being cheaper if you're looking to buy the PSUs new.A boost transformer solves the voltage problem, but not the problem of total available power at the receptacle. If you're already seeing a nearly 10% drop in voltage at full load, then the additional power required for the transformer is going to result in diminishing returns if it's on the same circuit. A lot of buck-boost transformers have isolated windings so that one transformer can support a number of different applications depending on how it's wired, sohypotheticallywith some interlock logic to verify that the phasing is correct you could run the transformer from a different circuit on the same phase. I'm not sure if that would be code compliant, but there's not atechnicalreason it wouldn't work.Quote from: Benta on March 13, 2021, 09:30:44 pmNot sure what you mean by 'floating', in US 3-phase systems 120V is the phase-to-neutral voltage and 208V is the phase-to-phase voltage, so the 208V circuits are already using two phases 120° apart. If you were to pull from multiple circuits, you would need to ensure that you were matching phases correctly, as well as provide some sort of interlock to prevent output without all inputs connected correctly.

author: Benta
author_rank: Super Contributor
timestamp: March 13, 2021, 10:40:10 pm
content: Quote from: ajb on March 13, 2021, 10:18:30 pmWell, badly phrased, sorry.What I meant was, that the OP plans on usingtwo208 V, 50 A outlets. But who says they're on the same two phases? If not, a transformer solution would result in major fireworks.The AC/DC to DC/AC solution is harmless in this respect, but the load-sharing question of the server supplies remains.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 14, 2021, 02:29:56 am
content: for the server psu'sNormally they share a back plane board with some sort of communication between themAn ac to dc / dc to ac inverter surely exist, but i haven't seen models who can range from 208 to 240vac inputs, i have seen 10kw inverters but their price range are astronomical around 2,5 k$$ usd or moreSince the op want to carry a device(s) under 27kg, it's not practical, even an auto transformer will be heavy at this power range

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 14, 2021, 05:32:31 am
content: Fit some extra chargers to the car? There are relatively cheap Mean Well modules that output up to 400V with adjustable current limiting.

author: eblc1388
author_rank: Frequent Contributor
timestamp: March 14, 2021, 05:55:56 am
content: If you first convert the AC into DC by rectifiers, then you can combine two sources at the DC side, and not have to worry about the phasing problem. However, if there is a voltage difference between the different sources, current sharing will not be the same and you can overload the source with the higher voltage.You then only need to find an inverter of the right size to get back your 240V AC.

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2021, 10:35:06 am
content: Quote from: magudaman on March 13, 2021, 04:05:40 pmNo, it wouldn't.208V is two phases, of a three phase 120V to neutral circuit. If the transformer is connected between the two 208V circuits, it would draw power from them both.https://www.thermalcorporation.com/208-volts-is-a-weird-voltage-where-did-it-come-from/It's also possible to use a transformer to draw power from all three phases to give 240VAC single phase out, but it's not possible to spread it over each phase equally, without some kind of active converter.A transformer will be much more efficient and reliable, than converting it to DC, using server power suppliers and converting it back to AC, with an inverter.

author: fordem
author_rank: Regular Contributor
timestamp: March 14, 2021, 03:43:46 pm
content: Several UPS manufacturers offer single phase UPSs with PPDMs (Power Pass & Distribution Modules containing isolation transformers that allow various combinations of voltages, including 208VAC in and 240VAC out - Eaton Powerware 9125 and 9135 would be examples of such units - the transformers should be available used on ebay (search for Eaton 9125 PPDM) - I have three such units I'm ready to toss, if you were close by, you could have them for free.

author: jmelson
author_rank: Super Contributor
timestamp: March 14, 2021, 04:50:34 pm
content: Quote from: Benta on March 13, 2021, 09:30:44 pmNo, not generally floating. 208 V line-line is what you get in a Wye system with 120 VAC from neutral to any line. This is often used in office and apartment buldings which have many 120 V loads.I don't think there is any safe way to "combine" AC power from multiple outlets. You can't depend on the phases being wired the same way on multiple outlets. A system with several constant-current DC supplies could be powered from multiple outlets and then share the load on the DC side.A buck-boost transformer can be QUITE small, as the iron only needs to be large enough to handle the voltage DIFFERENCE times the current. So, a 208:240 boost transformer only needs to be 15% the size of a transformer that carried the whole power delivered.Jon

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2021, 08:37:25 am
content: Quote from: jmelson on March 14, 2021, 04:50:34 pmIf it's two separate sockets, i.e. two plugs, then no, don't do it! It's very danagerous because one plug will be live, when the other is plugged in!The phases should be hard-wired, or connected to one socket, which is connected to the transformer.

author: magudaman
author_rank: Contributor
timestamp: August 04, 2021, 03:02:18 am
content: Sorry for the lack of posts. This project fell off the table and then I accidently kicked it into a storm drainBut I think I am going give this whole setup a try.Quote from: ajb on March 13, 2021, 10:18:30 pmI agree, I think paralleling power supplies with out the current share setup properly would have led to issues. With that in mind I am going to stick with server power supplies but do series only. I happened to find some beautiful 3000w 12.25v 245a 80 plus platinum units and bought 5 of them, they were only $30 and are new!The plan is to put 5 in series for 61.25v. Actually it appears I will need to adjust the voltage to get voltage to 59.8v. I believe I found the voltage send trace and pin. Photos coming soon.Quote from: fordem on March 14, 2021, 03:43:46 pmI did look at these and they are very interesting. These may be an option. Looks like there are 24a, 30a units and 50a units. Depending on what I get I would need to use 3 or 2 units. Looks like they are about about 45kg (100lb) for the 24/30a and about 85 kg (187 lb)I am also reading about paralleling transformers and it seems a bit sketchy if the input and transformers are not perfectly matched. I need to continue doing a bit more reading on this.Quote from: NiHaoMike on March 14, 2021, 05:32:31 amThis could be done but would likely require the factory chargers to be tied into the can bus. The factory chargers are rather expensive (even used) and may not like seeing additional units on the can bus.Quote from: coromonadalix on March 14, 2021, 02:29:56 amIf I had all the money, I would likely use two ac power sources like the Pacific Power Source 1150ADF to allow me to dial the output voltage to exactly 270vac and parallel them to use two different input sources. They do exceed my weight requirements. Then again if I had that kind of money I might as well bring a race trailer with a 60kw diesel powered chademo fast charger.For the inverter I am looking at taking a serious risk and getting a real "nice" aliexpress unit. 12KW continuous/24kw intermittent, high frequency inverter, 48vdc input, 240vac 60hz. I think I can get it to my door for around $800 USD. Which when I put in writing is sounding really expensive.https://www.aliexpress.com/item/32842555372.html?spm=a2g0o.detail.1000060.3.d7373f9cYpI0DP&gps-id=pcDetailBottomMoreThisSeller&scm=1007.13339.169870.0&scm_id=1007.13339.169870.0&scm-url=1007.13339.169870.0&pvid=b3ff0019-50a2-47fc-89a8-85a4babccdf6&_t=gps-id:pcDetailBottomMoreThisSeller,scm-url:1007.13339.169870.0,pvid:b3ff0019-50a2-47fc-89a8-85a4babccdf6,tpp_buckets:668%230%23131923%2391_668%230%23131923%2391_668%23888%233325%2315_668%23888%233325%2315_668%232846%238114%231999_668%235811%2327175%2325_668%236421%2330830%23897_668%232717%237559%2310_668%231000022185%231000066059%230_668%233468%2315615%23631_668%232846%238114%231999_668%235811%2327175%2325_668%236421%2330830%23897_668%232717%237559%2310_668%233164%239976%23804_668%233468%2315615%23631&&pdp_ext_f=%7B%22scene%22:%223339%22%7D

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 04, 2021, 04:51:24 am
content: Quote from: magudaman on August 04, 2021, 03:02:18 amHow does the BMS react to being supplied with a charging current that's well within the ratings of the pack but significantly more than what the charger says it's putting out? You could put a current sensor on the existing charger to tell the extra PSUs to turn on.

author: Zero999
author_rank: Super Contributor
timestamp: August 04, 2021, 08:40:10 am
content: I still don't understand why you don't want to use an autotransformer. It will be over 99.5% efficent, draw power from two, 208V phases, cost around 200$, weigh 15kg and be highly resistant to damage from overvoltage and overcurrent.An autotransformer only needs to be the same size as an isolating transformer for the difference between the two voltages: (240-208)*48 = 1536VA. You could use a 32V 48A transformer to make a 208V to 240V autotransformer, but I doubt you'll be able to find such an odd voltage. Fortunately that size is just about big enough for a custom wound autotransformer.

author: wraper
author_rank: Unknown
timestamp: August 04, 2021, 08:54:19 am
content: Quote from: Zero999 on August 04, 2021, 08:40:10 amYou can simply use 230/36V transformer, secondary will output 32V when powered from 208V

author: Zero999
author_rank: Super Contributor
timestamp: August 04, 2021, 10:28:27 am
content: Quote from: wraper on August 04, 2021, 08:54:19 amYes that will work. Now show me where you can get a 48A 36V transformer from.

author: wraper
author_rank: Unknown
timestamp: August 04, 2021, 11:19:59 am
content: Quote from: Zero999 on August 04, 2021, 10:28:27 amhttps://www.ebay.com/itm/173871215468?hash=item287b88336c:g:tuQAAOSwzpFa0yVI

author: magudaman
author_rank: Contributor
timestamp: August 04, 2021, 12:07:39 pm
content: Quote from: Zero999 on August 04, 2021, 08:40:10 amEach circuit is only able to provide 50a (50a breaker is really 40a continuously). That means I can only get 9360w per circuit (7800w @40a). The onboard charger will actually go to 12kw if I can feed 250vac-270vac. So I wanted to combine two 50a circuits of unknown sources (any multiples of different 3 phase power, 195-208vac) into a single output of ~250vac 48a.

author: Zero999
author_rank: Super Contributor
timestamp: August 04, 2021, 12:40:36 pm
content: Quote from: magudaman on August 04, 2021, 12:07:39 pmWhere did you get 9360W from?208V at 50A is 10.4kW.The problem with using DC:DC converters and inverters is the losses will be huge, compared to a simple transformer.A transformer could be used to draw power from all three phases.http://carroll-meynell.com/technical-3phaseIf isolation is not required, a small transformer can be used.Contact a transformer manufacturer.

author: ajb
author_rank: Super Contributor
timestamp: August 04, 2021, 06:36:38 pm
content: Quote from: Zero999 on August 04, 2021, 10:28:27 amI mentioned this way upthread, this is the territory of buck-boost transformers, 120/240V to 16/32V is a standard configuration and would provide 236V when powered from 208V, which at least in theory is almost certainly close enough for this task. Fully enclosed UL listed buck-boost units at ~2kV are readily available for $200-300 new. However, wiring one of those up in the conventional way has a big downside, because the additional current required to power the transformer will increase the voltage drop the OP is already seeing on the available receptacles. Based on the OP's statement that the voltage at the receptacles drops from 208V to the "high 190s" at 48A indicates a supply wiring impedance of about 0.2R, so an additional 2kVA for a boost transformer results in a couple of additional volts drop, something like a 5% loss before even adding any additional conversion efficiency. This is all a little fuzzy because it depends on how the power consumption of the load changes with line voltage. But it's all moot because adding 10A from a 2kVA transformer to the existing load means exceeding the rating of the 50A circuit.You could run the buck-boost transformer from a different circuit (in order to support a wide range of configurations the windings are isolated by default), but this again requires some sort of interlock to ensure that the phasing of the two receptacles is correct before the output will be activated. And if you're going to the trouble of using more than one circuit, it would be better to split the load more evenly between them. The method the OP has put forth may or may not be substantially worse in efficiency than a transformer depending on the components selected, and has an advantage in not being dependent on correct phasing of the input circuits and being able to easily scale to an arbitrary number of supply circuits depending on how it's implemented. The 'better' solution will depend on the OP's specific need and priorities (cost/efficiency/flexibility/simplicity).So far this ignores another big wrinkle, which is *where* the impedance that's causing the voltage drop the OP is seeing is coming from. If it's mostly in the wiring from the receptacle back to the local panel, then spreading the load to additional circuits in that panel will help. But if it's mostly in the wiring upstream of that panel (like, from a subpanel back up to a primary panel), then there's not nearly as much benefit. The only thing that would help in the latter case would be spreading the load more evenly across phases, which could be done with one of those wacky three-to-one-phase transformer arrangements, or with something like the OP described initially.

author: najrao
author_rank: Regular Contributor
timestamp: October 12, 2021, 03:54:24 pm
content: To maximize power draw from an ac source, it is desirable to use PFC in some form; this removes wasted current.If choosing the dc bus route, you would do well to picktelecom rectifiers. These are extremely well engineered, compact, and efficiencies you can only dream of, 96+ % offered. 48V output is standard, which means 54.5V in practice because they are made to charge lead acid batteries. PFC input stage guarantees 0.99 power factor as standard. A telecom rectifier rated 4kW is 360×80×80, and can be picked up by one hand, (I have four). Even higher power may be available, please check. They aredesignedto be parallelled, up to six and more. Back planes and rack cabinets are offered. Totally flexible control and monitor units are on offer. Hot plug is standard. The dc output is isolated from the input to over 2kV ac, so thephasingof inputs is irrelevant. The dc is alsofloating,, i.e., not grounded. Series operation for higher dc voltage should also be possible, but not usually offered.Two of three, or all three, of a three phase source can be tapped to provide a maximum of single phase power using zig zag connection. At least part of the power has to come through isolating transformer/s. The source phases will have severely unbalanced load. Using all three phases, you gettwice, not three times the maximum loading of one phase.Well, with the dc link method, the most unreliable part may be the inverter. I do not know whether the charger can, or can be made to, accept dc directly. If 200 to 400 V dc will work, telecom rectifiers can be arranged in series/ parallel.