Delta 3 phase power preferred as generation easy? - Page 1
Forum section: electronics / beginners

author: Faringdon
author_rank: Super Contributor
timestamp: October 27, 2023, 06:58:12 pm
content: Hi,Is it easy to get 3 phase delta , rather than 3 phase star, off a generator?I am just wondering why ever anyone would use delta 3 phase?...if those 3 voltages are not well matched, then huge circulating currents can start to flow in the delta.Just found this out when trying to make a 3 phase delta supply with three 1:1 isolation transformers and a 3 phase star supply...loads of blown fuses.

author: soldar
author_rank: Super Contributor
timestamp: October 27, 2023, 09:44:55 pm
content: Quote from: Faringdon on October 27, 2023, 06:58:12 pmIt is just as easy. It is only a matter of how the windings are connected.Delta has the advantage that if one winding fails all three phases still receive the same voltage, which is not true for wye. Of course, the two windings need to be able to carry the load.And Delta does not have or need neutral.

author: TimFox
author_rank: Super Contributor
timestamp: October 27, 2023, 09:51:25 pm
content: I usually see a delta primary in the utility's transformer with a wye secondary to the customer, so that the wires in the customer's site have a ground reference.

author: Stray Electron
author_rank: Super Contributor
timestamp: October 27, 2023, 10:20:11 pm
content: Quote from: Faringdon on October 27, 2023, 06:58:12 pmIt isifthey bring out all of the connections (aka taps) forbothends of every winding. Look up the description for a 12 pole generator and the instructions for setting one up.The US military MEP generators are capable of it and they are very well documented on line.

author: soldar
author_rank: Super Contributor
timestamp: October 27, 2023, 10:29:46 pm
content: Quote from: TimFox on October 27, 2023, 09:51:25 pmI believe this is the most common arrangement, delta for transportation and wye for distribution.

author: CaptDon
author_rank: Super Contributor
timestamp: October 28, 2023, 02:24:44 pm
content: Open deltas don't need a ground reference but they are found in many strange configurations. There is the 'Corner Grounded Delta', each 'hot' leg measures the same voltage to ground and the two 'hot' legs also measure the same voltage across. Then there is the tapped delta which is often found in industrial settings and is responsible for blowing up a lot of 120VAC equipment. It is also known as a wild leg delta. There is a center tap between two of the legs which is connected to neutral and safety ground. It gives 240VAC across the tapped side or 120VAC on each side of the tap with the exception that the wild leg provides 208VAC referenced to neutral. Hot leg to hot leg anywhere will give 240VAC. The wild leg is often wired with orange to differentiate it from the red or black 120VAC leg. As for generation, either type of delta is common. Many alternators have provisions to do either. One thing about an open delta that is truly floating is that it can build a high static charge and we used balancing resistors to protective ground on each leg. The resistors were chosen to provide less than .005 amps of 'fault current' should any leg become grounded. The resistors looked like big cartridge fuses. This configuration was only found on our 480VAC three phase open delta systems used on huge roof ventilator fans where a service technician may be working in the rain. (Those fans looked more like helicopter rotors!!)

author: djsb
author_rank: Super Contributor
timestamp: October 28, 2023, 03:06:19 pm
content: Another thread that makes me feel queasy and uncomfortable

author: soldar
author_rank: Super Contributor
timestamp: October 28, 2023, 03:12:21 pm
content: Quote from: CaptDon on October 28, 2023, 02:24:44 pmWyes don't "need" a ground "reference" either. As you say, systems are grounded for the main purpose of preventing high static voltages building up. With a wye configuration the grounding point is obviously the center but with a delta, as you explain, you have to decide if you ground one corner or if you want to create an artificial center of the triangle.Delta seems to have more advantages for transportation whereas wye is preferred for local distribution. Most distribution transformers I have seen are delta - wye.One advantage of wye is that you get two different voltages available, due to the neutral, eg: 230/400. This is not possible in delta.On the other hand with Delta having no neutral you save the 4th wire and the voltages will remain good no matter how the phases are loaded or even if one of the three generating voltages or coils fail.

author: jonpaul
author_rank: Super Contributor
timestamp: October 28, 2023, 06:01:48 pm
content: Transmission: Distribution 13kV>>360 kV DeltaTrsf sec user side eg 480/240/120V can be either wye or delta.j

author: Faringdon
author_rank: Super Contributor
timestamp: October 28, 2023, 06:51:51 pm
content: Thanks, the concensus appears to be that Delta is mostly for the supply side, distribution side. Here they can put in highly expensive control systems to ensure that the 3 coils remains equally matched, and that the voltage on each coil remains the same.......because if the voltages just stray apart a little bit, then those huge circulating currents start to flow in the delta. This is why customer equipment simply cannot be delta......the customer wouldnt be bothered to pay for the circuitry needed to ensure the coil voltages are always balanced, and mayhem would result.We are trying to build a simple delta three phase supply to supply a test three phase delta voltage to some kit that will only take <100w....but it is no easy thing to be sure that the matching is tight enough to avoid the horror currents of delta.

author: soldar
author_rank: Super Contributor
timestamp: October 28, 2023, 07:08:51 pm
content: Quote from: Faringdon on October 28, 2023, 06:51:51 pmI do not think that is correct at all. Very much the contrary. On a Delta configuration the currents tend to balance. If you load only one winding (i.e.: two corners) then the three windings will balance to supply the current. Precisely one large benefit of delta configuration is that it balances generation windings with loads. You can have unbalanced loads and even unbalanced generation windings and they tend towards equilibrium on their own. If you are having problems it is because you are doing something wrong. Like connecting one winding in reverse.

author: CaptDon
author_rank: Super Contributor
timestamp: October 28, 2023, 07:11:36 pm
content: I mis-stated the fault current of .005 amps in my previous post. That current created by the static bleeder resistors is limited to .005 amps current flow between ground and any ONE leg as long as the other legs are not grounded. So the theory is you could grab any point of the 'floating' delta while standing in water and not get killed. In the hospital where I worked for a few years our 120VAC circuits in the operating rooms were fully floating and there was a monitor circuit between ground and the center point to detect ground fault issues. They were made by Square D and called LEM's. Each room had its own 5KW isolation transformer and LEM circuit. As for the unbalanced circulating current, if all three transformers are identical and all three primary voltages are very close you should be able to tolerate load imbalance on the delta secondary. One interesting thing is that a true 3 phase transformer in many cases are on an E E core with a primary and secondary on each of the legs of the E so flux is shared throughout. All of our high power transmitters had these type of cores up to around the 50KVA size.

author: johansen
author_rank: Super Contributor
timestamp: October 28, 2023, 10:52:34 pm
content: Quote from: soldar on October 28, 2023, 07:08:51 pmNoHowever i agree with the last part. He has one winding backwards.

author: soldar
author_rank: Super Contributor
timestamp: October 29, 2023, 06:40:03 am
content: Quote from: johansen on October 28, 2023, 10:52:34 pmYes.

author: soldar
author_rank: Super Contributor
timestamp: October 29, 2023, 12:53:56 pm
content: I have a feeling this question is probably related to this thread:https://www.eevblog.com/forum/renewable-energy/favourite-connection-method-for-two-phase-mains/msg5139354and it makes me think the OP needs to learn the basics of three phase power.

author: Faringdon
author_rank: Super Contributor
timestamp: October 29, 2023, 03:24:49 pm
content: The attached (LTspice and jpeg)is what I mean by the horror currents of delta 3 phase…as you see, the phases of the star that get used to make the delta are slightly different (139Vpk vs 141vpk)…a tiny difference, and yet look at the “Horror over current of delta” that result from this. The huge overcurrent that results in the coils of the delta connected coils.This is why Delta is strictly only for 3 phase systems with loads of expensive circuitry to be sure the coil voltages of the delta are all very finely and closely matched.

author: Faringdon
author_rank: Super Contributor
timestamp: October 29, 2023, 03:39:26 pm
content: ..And again, the attached LTspice sim and jpeg, this time show 3 phase delta, but this time with the 3 transformers having slight differences in primary to secondary coupling...as you see, the result is very destructive...the currents in each delta coil are very different...nothing like the well matched delta system on the left hand schem......these kind of issues do not occur with 3 phase star systems.This is why Delta 3 phase is so difficult to produce cheaply. We just need a Delta 3 phase supply to give 100W as a quick test sourse, but we are up against these factors, which make the whole exercise not a cheap and easy one at all.

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2023, 03:51:39 pm
content: Delta has a higher current capacity and star a higher output voltage.A transformer, consisting of just three windings, connected in star, can be used to provide a neutral on a generator, or another transformer, connected in delta. This can be useful, when the neutral current is low. as the transformer can be close to the consumer.

author: soldar
author_rank: Super Contributor
timestamp: October 29, 2023, 04:50:48 pm
content: Quote from: Faringdon on October 29, 2023, 03:24:49 pmWhat expensive circuitry? I do not know where you get this from. There is no "expensive circuitry". You just do not understand how it works. The combined voltage around the delta triangle is always zero. The sum of the voltages on all three windings is always zero. If there is a slight imbalance due to non-identical construction then all it means is one winding will supply a tiny bit more or less current.The delta configuration is self-balancing. You can load between two vertices and all three windings will work. You could even interrupt one winding and the other two will maintain the voltage on all three vertices (although they can be overloaded).In a certain way this is similar to connecting two transformers in parallel. They will share the load with a small difference due to their slight differences in characteristics.This company makes very small delta transformers. Scroll down.https://temcoindustrial.com/product-guides/transformers/small-kva-euro-transformer-selection-guideHave a look athttps://ctlsys.com/support/four_wire_delta_circuits/

author: johansen
author_rank: Super Contributor
timestamp: October 29, 2023, 05:13:12 pm
content: Quote from: Faringdon on October 29, 2023, 03:24:49 pmBut no one in real life has zero impedance 3 isolated voltage sources. They have one voltage source, a magnet spinning inside a 3 phase winding. Any minor 1% variation in voltage due to loads being different, just passes through to the load without causing circulating currents (not talking about harmonics). The voltage variations pass through the transformer btw.Also no one in real life parallels two different 3 phase ac systems without regard to balancing. You can parallel large 3 phase generators btw. The intrinsic impedance of a large 3 phase synchronous generator is around 20 to 30%. A 2% difference in voltage only causes at most a 10% difference in current.Most of the time, its a top down generator to transformer distribution to load.I have a rotary phase converter on my lathe that nominally produces 240v 3 phase, dropping to 240-215-210v under heavy load.. if i were to put a A star delta transformer on the output, i would not get 240-216-210 underload at the lathe. I would get a 30% phase shift and the voltages under load would be different, however the percentage of imbalance would be precisely the same. Neglecting the resistance of the transformer, i would have the same starting torque.. If i put a scott t transformer after my star delta transformer, i would have the same voltages as i do at the beginning, at my two phase rpc, made from a 5 ton scroll compressor. There would be no circulating current in the delta secondary either.Transformers store no energy, neglecting the 1% leakage inductance for sub 200kva transformers. (Leakage inductance increases as the transformer gets larger. Gigawatt transformers have about a 20% impedance due to leakage inductance, but they are 99.8% efficient)

author: Faringdon
author_rank: Super Contributor
timestamp: October 29, 2023, 06:45:32 pm
content: QuoteThanks, we coudl use the below but is $800 and several weeks lead time....we only need a small low power 3 ph delta supply to test some kit with.https://temcoindustrial.com/temco-control-transformer-tt1261/I recognise what you are saying, that the circuit impedances will prevent the delta currents from getting overly large when there are mismatches etc. I think we will just use three 240-12VAC mains transformers in reverse and put the output of an audio power amplifier into each of them, shifted by 120degs, and at 60Hz...and then just ring_delta connect them, and use that. There are no cheap audio power amps for sale offtheshelf, and so we will use TDAxxxx chip by st.com to get the power sine to feed to the transformer.TDAxxxx sounds the quickest and cheapest way to do this, unless others know a cheaper /quicker chip?We will get the reference sines from an arduino, putting a square wave into an LC circuit.

author: soldar
author_rank: Super Contributor
timestamp: October 29, 2023, 07:01:11 pm
content: It seems to me you are making this more complicated than it needs to be. If all you need is three phase power than you can borrow, rent or buy a rotary converter or a variable frequency drive.https://en.wikipedia.org/wiki/Variable-frequency_driveJust to be clear, if you do not need neutral then whether the power supplied is from a delta or wye source is irrelevant. I am not sure why you want delta because it does not matter to the receiver.

author: johansen
author_rank: Super Contributor
timestamp: October 29, 2023, 11:58:29 pm
content: A delta to Y transforme would be good for vfd experiments because you can ground the neutral of the secondary and you get a true neutral, which vfds dont produce.Capacitors across the secondary can utilize the leakage of the transformer as an inductor, and filter the pwm into a ground referanced sinewave.Good luck with the audio amplifiers. At least they are cheap.1kw car audio bass amps are 50$ on ebay. You can use the 8volt or maybe 13 volt to 120v transformers out of UPS's..Technically ypou only need one 2 channel amp, and 2 transformers to get open delta 3 phase

author: Faringdon
author_rank: Super Contributor
timestamp: October 30, 2023, 07:20:13 am
content: QuoteThanks, the kit we are doing some basic testing on needs a delta three phase input.Doesnt have to be much power, say a few watts...but it has a 220uF after the 3 diode bridge, so maybe if it is too weak it will flat top some...but we can maybe live with that.QuoteThanks, yes we do not need neutral, we just need a delta 3 phase supply at 60Hz 100VAC....i dont see how we can get that from a Y source without three isolation transformers to give us delta?..but i guess thats not what you meant anyway.

author: soldar
author_rank: Super Contributor
timestamp: October 30, 2023, 08:19:17 am
content: Quote from: Faringdon on October 30, 2023, 07:20:13 amYou say your device needs a three phase delta power supply but you do not explain why.The device does not know, cannot know, if the source is wye, delta or any other possible configuration. If the three wires carry voltages spaced 120 degrees apart the receiver has no way of knowing how they were generated. They are exactly the same.Why do you say it needs delta? I think you are most probably mistaken.

author: Faringdon
author_rank: Super Contributor
timestamp: October 30, 2023, 09:29:35 am
content: The PCB has only L1, L2 and L3 connectors.They go to a 6 diode bridge.The PCB is the control PCB.Its connected to the bigger system, which has a 3 phase delta supply at the installation.Why does the customer use Delta?, sorry i dont know.Thanks for the VFD suggestion...we found some cheap VFD's on the web, but we dont think we can easily use them to generate our 3 phase delta supply?The VFD is for supply to a motor as you know.....the loading therefore looks mostly resistive, as you know, the loading can be just capacitive for a mains supply on eg no load....so i dont see how a VFD can be used?I can produce sinusoidal PWM and filter it, and then give a sine into a pure resistive load, but doing it for the mains supply, to any load, is not possible i am finding.

author: Zero999
author_rank: Super Contributor
timestamp: October 30, 2023, 10:25:48 am
content: Quote from: Faringdon on October 30, 2023, 09:29:35 amYour device just sees a voltage between each phase. It doesn't know or care, whether the supply powering is is configured as star or delta.Why do you need a VFD?Going from your other threads you mentioned the need for 60Hz, but it 50Hz will be fine in this case. If it works off 50Hz, then it certainly will off 60. This is because the capacitors on the rectifier don't need to be as big, with 60Hz vs 50Hz and if it's designed properly, there should be enough headroom for 50Hz operation.All you need is a transformer to get the right voltage. If you can't get a suitable three phase transformer, three smaller transformers can be connected together.

author: soldar
author_rank: Super Contributor
timestamp: October 30, 2023, 10:55:34 am
content: Quote from: Faringdon on October 30, 2023, 09:29:35 amIt seems we are not getting through to you. I do not know how many times we have to repeat it. Your device does not know and cannot know whether the power it receives came from a star, a triangle, a zigzag or any other configuration. All it perceives is the voltage it receives. Do you understand that? Do you get that? You need to understand and absorb that. There is no difference.I do not know what you are designing and building or for what type of application but it seems to me you might be a bit over your depth here.

author: Circlotron
author_rank: Super Contributor
timestamp: October 30, 2023, 12:17:16 pm
content: Quote from: johansen on October 29, 2023, 11:58:29 pmIf you go the 2 amplifier, 2 transformer route then they need to be 60 degrees apart, not 120 deg. Surprising perhaps, but true. I found this out when driving a twenty-four volt 3-phase induction motor from a pair of class D audio amplifiers a while back.

author: soldar
author_rank: Super Contributor
timestamp: October 30, 2023, 03:31:54 pm
content: Frankly, given the tenor of this thread, I doubt going the audio amplifier route is going to produce any useful results. The thread just looks to me like the OP is quite confused. That is the root of the problem.It is a circuit fed by a three phase rectifier bridge. I would just test it feeding it with a single phase. You will have a bit more ripple but if it is acceptable then by connecting the third phase you are only improving the conditions and it should work fine.I really do not see the problem. Maybe I am missing something...

author: Faringdon
author_rank: Super Contributor
timestamp: November 11, 2023, 01:43:59 am
content: QuoteThanks, yes,Its just as well, because our 3 phase delta supply, which was made as in the following attached, does not work....we just "ringed" three isolation transformers, (and they were the right way round) each connected to a phase of a 3 phase star supply.I believe that the fact that only two of the isolations transformers were matched was the problem...this thing just blows a fuse as soon as is powered up...even when we bring up slowly with three phase variac giving the star.I believe one of the isolation transformers just saturates. Would you agree?(LTspice and jpeg as attached)

author: Zero999
author_rank: Super Contributor
timestamp: November 11, 2023, 05:47:22 pm
content: Are you sure the secondary sides of those isolation transformers are phased the same? It's possible they're not controlled, as it's not something that normally matters.You appear to not have taken on board any of your replies. For the final time: your device doesn't know whether the supply is star or delta. Those isolation transformers won't do anything, even if connected correctly.

author: CaptDon
author_rank: Super Contributor
timestamp: November 12, 2023, 04:57:36 pm
content: Yes, to balance the voltage and achieve low circulating current the transformers must be balanced. Yours aren't. No wonder it is giving you fits. You state that your load doesn't draw much current yet you have two 3KVA transformers?? Just go buy three matched 100VA dual primary / dual secondary 240/480 transformers or even three matched 100VA 120/240 dual primary / dual secondary and create your Wye to Delta lashup. It should work fine. It would help to know if your customer has an open delta with no ground reference or a tapped delta with the 'wild leg'. A tapped delta is one that shows 120/120 and 208 on the wild leg or possibly 240/240 and 416 on the wild leg when measured to ground/neutral. You will want to account for the neutral connection if it is a tapped delta. The tap would be neutral / safety ground / chassis reference. It is no wonder you are killing fuses with that unmatched transformer grouping you have in place.

author: johansen
author_rank: Super Contributor
timestamp: November 12, 2023, 07:31:09 pm
content: A couple turns miss match on that 750va vs the 3 kva, would probably not blow a fuse.I think one of them is connected backwards. The voltage should sum to nearly zero.

author: CaptDon
author_rank: Super Contributor
timestamp: November 12, 2023, 08:04:38 pm
content: I tend to agree that to blow fuses one of the transformers is phased backwards and it wouldn't be the first time I have seen a transformer itself incorrectly marked where X1/X2 - X3/X4 were incorrectly labeled causing a shower of sparks when hooked up as a parallel primary where X1/X2 seemed to be reversed compared to all the other windings. However, the O.P. doesn't even seem capable of determining how three phase works or how to tell if one of the transformers is incorrectly labeled and in fact mis-wired. I always hated those tapped Delta wild leg circuits!!! I have worked on a few where over the years the Red or Black got hooked to the wild leg and not the Orange. 208 wired to an outlet where 120 was expected. KABOOM was the last word the appliance spoke!! We had 240 D.C. on Iceland for heating circuits, It killed more people than 480 here in the states.

author: Faringdon
author_rank: Super Contributor
timestamp: November 12, 2023, 09:34:46 pm
content: Thanks, it definitely wasnt polarity error , i had , at one point, 82R in series with the output of each secondary...and then connected them in delta, and then raised the voltage up on the primary side with the 3 phase variac.....and did this with only some 10-15VAC so it wouldnt give much current...and then switched the connections of each secondary in turn, and then back again...but nothing worked......with them on open cct i had the typical 3 phase voltage appearance.....soon as you delta them...one always goes much smaller and the phases dont even look 3 phase 120 degrees.Thanks, i am looking into the "no difference star or delta" situation

author: Zero999
author_rank: Super Contributor
timestamp: November 12, 2023, 09:54:17 pm
content: Mismatch could explain it. I imagine the 750VA transformer will have a higher unloaded secondary voltage, than 3kVA units.There's still no point in using transformers to get delta. Fair enough, if the voltage needs changing, but it's otherwise pointless. Here's an example showing three 230V sources connected as star and 400V as delta. Note the meters display 400V either way. The difference is, given the same load, the voltage sources supply √3 more current in star, but are a lower voltage, so the total power is the same.