3 phase to Single Phase transformer

A problem arises in distributing single phase loads across a distribution board that has been supplied with three phases. Habitually, the electrician will start at the top left corner of the board (assuming that [s]he reads a conventional alphabet and is right-handed). The result is that the first phase on the board, which is usually the red in pre-2006 UK installations, has more load attached to it than the others and draws more current than the others.

The solution is usually simple and doesn't involve much in the way of investment.

Re-assess the spread of single phase loads across all the distribution boards on the installation and re-allocate some of them to other phases so that in aggregate they are all about the same. The cost - a couple of hours' inspection and assessment, a couple of hours' reconnection and an hour or so of "as built" recording might be typical for a medium-sized board?

One solution is a motor generator set. This will take up some space and efficiency but if you source a used motor and gen set you could probably get by for a few thousand $'s. You would then be able to have two separate busses or boards that would be isolated from each other and eliminate the shore power hookup problem.

Hi !

The easiest but little expensive way is to install a 3 Phase to 3 Phase Double Conversion Online UPS at the Input of the Transformer which will ensure balanced loading of the line / DG even when the load is unbalanced at the output of transformer.

As a alternative, if the loads are non-switching / continuous duty type, you may distribute the loads on transfomer output in such a way that the current in each incoming conductor of the transformer is within + 10%. Actually the transformer u are using is scott connected type which has inherent characteristic of drawing almost 60% more current in the common phase than the rest of two phases.

Regards

Sanjeev Gupta , sanjeev.gupta@essarups.com

Yes! you can use a three phase to single phase transformer & I have designed and built many of them. However, it is impossible to balance the loading on the three phase input [about 40% imbalance]. This is due to the fact that a single phase load is a pulsating load. Would the use of a 380VAC transformer assist? A three phase to two phase [90 ]degrees phase displacement] transformer will result in balance on the input, but that would not really resolve your problem.

Well, here's my 2 cents:

First choice is to re-distribute the loads as suggested by PWSlack. If it were my vessel, I would do this and forget about everything else until it was time to replace the gen-set.

Second: re-connect the windings or re-wire the generator if re-connection cannot provide a 240V single phase output.

Third: replace the generator with what you need.

Most, if not all, of the other choices require the addition of equipment which will effect efficiency and reliability, take up space, and add weight. All of which are usually frowned upon in a marine environment.

Good luck with your project.

Bobf; you did not say what the output of your generator was, only that the load was about 20 kw. what is your load use from the shore power? what i would do, is ground (L2,S,) & call it neutral, install it to a single phase panel with a 2 pole 63 amp main breaker on phases (L!,R) &(L3,T,), with 2 amp-metres 0-60 or 0-75 amps AC in series, thus you could monitor the load in each leg. then install single pole breakers for your one phase loads & three pole breakers for those loads. for shore loads you could install a double pole breaker, mechanical interconnected with the main breaker to make sure that it is OFF before you turn on the two pole breaker to connect to shore power. buy jumping the two breaker terminals together you have phase LI & L3 tied to gather for your shore power. the other option, if you generator had enough leads/wires in the junction box, you could reconnect for a dog-leg single phase perry

When tied to a shore facility, only 1P is available. This results in very unique switching arrangements, to supply all three electrical panels (taken off each phase of the gen) and supply them from a single phase source. It would seem that it would be easier to have the Generator feed a device that outputs only single phase. Use a single to three phase rotary concerterter when on shore.

guest; bobf does not give enough information, when he's at the dock, does he need 20 KW? your idea would work if shore power was 100 amps at 240 volt, plus a 3 pole transfer switch to disconnect the generator windings. i feel bob has installed 3 sets of 2 wires going to 3 panels with? switches, circuit breakers, or fuses, fuze's. Perry

Hello to everyone who has taken time to reply. Perhaps I should add here exactly how I have wired the vessel: 1.) When underway, the 3P generator is used. Since each leg, (U-V, V-W, W-U) is 220V, I take each "leg" and feed a separate panel board for that leg, with a main breaker and several sub-breakers, all double-poled. The Gen is 13KVA for those who need to know the exact power source. As you can imagine, it is hard to balance the total load on the Gen because not all circuits are used simultaneously. For example, maybe Aircon 1 and the Electric Stove is used one moment, and at another time, Aircon2 might be needed along with the battery charger and fridge. Yes, some sort of balance can be achieved if ALL circuits (up to the capacity of the gen) are being used, since the panels were wired to account for the proposed balance. However, this rarely occurs because of power conservation, need, etc. When connected to shore power, I have wired up a 100 amp rotary switch, which is essentially a 5-pole, double throw connection. With all power off (gen and shore), this 5PDT switch is turned to the Shore supply. With jumpers on the switch, this allows me to send 1P to each of the separate 1P panel boards. Thrown the other way, the U-V leg goes to one panel, V-W to the other, and so on. I cannot rewire the gen to single phase because there are some 3P consumers that are essential: The Windlass, the Capstan, the Watermaker, the Welder, and the Dive Compressor. None of these are needed when using Shore Power. Perhaps the need for a balanced output from the Gen isn't as critical as I imagine? How far "out of balance" can each set of windings take? Or, maybe asked another way, is it REALLY out of balance within the windings themselves? Can, for example, each "set" take a 13KVA load, meaning all the load is on one set for a while, when there are no other loads present on the other windings? This is a theoretical question only, for my own understanding of just what is going on inside this type of 3P gen. I look forward to additional comments on this question!

hi is it bobf or guest; 13KVA would equal 35 amps per leg (3) why not put a 40 amp 3p breaker on the generator out put, why worry about balance? put a couple 50 amp ac amp meters in series with 2 of the lines "U","W", then you will know what your load is! so what if you taking 10 amps off of U-v and/of 20 amps of off W-U or?
I don't know if you are running the dive compressor, the capstan, welder at the same time the amp meters or circuit breaker would indicate the amps in use, it is possible if you are using the welder & and the frig comes on you may have a overload on a phase the amp meters would let you know. usually 2 horsepower per kilowatt is used on i.c. gas engines perry

Hi. This is Bobf in reply to Perry and others who are participating. I would appreciate knowing the math behind the 35amps per leg, since I was looking at 13KVA times Power Factor of 0.8 or 10.4KW total. Divide this by 220 V, and I get a TOTAL of only 47.27 Amps to be spread over the three legs! Where did I go wrong? To me, having 35Amps per leg would be a blessing, but wouldn't that theoretically equal a total 105 potential amps of supply? Here, I must confess, is where my confusion sets in when using a 3P generator. I agree with having an amp meter on each leg, and that is what I have planned to do for visual confirmation of my calculations. As to usage, both the Capstan and the Windlass (3hp each) could be used at the same time. However, if using the Watermaker, I would be anchored, or underway, and thus not needing either the capstan or the windlass OR the welder! (well, at least I hope not..that would indicate I need to patch something in a hell of a hurry!) Domestic appliances could be on when using the windlass or capstan, which by the way would normally be used sequentially, not simultaneously. The water maker, dive compressor plus some domestic 1P items could be all on at the same time, pending upon, as you indicated, the total load. I use load management as a manner of routine, mainly to keep the gen LOADED enough, and rarely to keep it from being OVER LOADED. Hope this all helps everyone understand my use of the 3P gen and running mostly 1P consumers. So, now to the conclusion..do I worry about trying to keep the individual windings balanced, or not? What percentage of the total output of the gen be carried on each leg? Can one leg carry the whole 47 amps, if there is no load at all on the other two legs? Because of the phasing, can each leg put out 47 amps, since they are out of phase (not meaning out of balance)? This fascinates me, and I look forward to all comments. Thanks in advance. Bob F

bobf; it was late, a re correction, a 10 kw generator @ .8 power factor would be rated 12.5 KVA. 10000 divided by 220=45.45, divided by square root of 3 (1.73)= 26.3 amps per phase (leg) no you can not add up the phases for 79 amps per phase. watts relates to the mechanical mover of the generator, (746 watts=1H.P.) V.A. relates to the generator windings, this is the load on the generator, has nothing to do with the H.P. with .8 power factor you would have 33 amps in the copper winding instead 26.3 at 1.00 pf . you could run 33 amps ,1 phase all day with no problem, but if a 3 phase load came on line, you could be overloading the generator if the generator is self excited the voltage would more then likely collapse. the phases are 120 degrees apart. the amp meters would tell you a lot, if you had every thing in one panel board it should be a lot easier to monitor. perry

I have been following this discussion with interest. Earlier you mentioned that the generator is delta wound, therefore, the line current (i.e. current consumed by the load) is square root of 3 more than the phase current of the generator windings. VA (apparent power) = 3 x V x I 13000 VA= 3 x 220 V x I A Phase current = 13000/3 x 220 = 19.7 A And Line current = √3 x 19.7 A = 34 A

Epsilon; I don't disagree, i used 13000 divided 220,= 59 divided by 1.73=34 amps. perry

Thanks Perry, it`s correct. Only I used a basic approach.