Single phase supply off a three phase diesel generator question

[Chris Wilson]

I am looking to buy a small to medium size diesel powered generator. Here in the UK mains supply is 230V AC at 50Hz. Most medium size (15kVA or thereabouts) generators are three phase. I  have searched but cannot find a simple answer as to how I can draw single phase 230V off one, and at what level? Can you combine the phases and get 15 or so kVA, or is it only one third of the machine's rated output? are there dedicated single phase generators of this sort of size, that are able to supply most everyday and night needs of a medium sized house in a power failure? Ideally I know I would be looking at 30 kVA, but with some circumspection on usage i would think a 15 kVA would allow a comfortable existence? Thanks. I attach a schematic from one such generator I am looking at.

[langwadt]

can't just distribute your loads on the three-phases, like it is done here where everyone gets three phase power?

what do you need all that power for? electric heating?

[Monkeh]

can't just distribute your loads on the three-phases, like it is done here where everyone gets three phase power?

Well, no, because it's all wired for single-phase..

Single-phase standby generators are fairly easily available. This is a job to involve an electrician in.

[bill_c]

Typically a 3 phase 220V generator can supply 220V single phase loads from any two of the 3 phase leads, but at about 50% of the generator rating.  Not sure about grounding requirements in your area though.

[Monkeh]

Typically a 3 phase 220V generator can supply 220V single phase loads from any two of the 3 phase leads, but at about 50% of the generator rating.  Not sure about grounding requirements in your area though.

Please do not confuse things. Our nominal phase-to-phase voltage is not 220V and that would be an explosive mistake.

[langwadt]

can't just distribute your loads on the three-phases, like it is done here where everyone gets three phase power?

Well, no, because it's all wired for single-phase..

Sure, but isn't that done at the main electric panel ?

something like the one shown here by BigClive could very easily put half the load on one phase and the other half on another
 
https://hackaday.com/2019/04/14/a-peek-inside-a-typical-british-residential-power-panel/

This is a job to involve an electrician in.

absolutely

[langwadt]

Typically a 3 phase 220V generator can supply 220V single phase loads from any two of the 3 phase leads, but at about 50% of the generator rating.  Not sure about grounding requirements in your area though.

here 3-phase is 230V phase to neutral and 400V phase to phase, so that would be an expensive experiment  ...

[Monkeh]

can't just distribute your loads on the three-phases, like it is done here where everyone gets three phase power?

Well, no, because it's all wired for single-phase..

Sure, but isn't that done at the main electric panel ?

something like the one shown here by BigClive could very easily put half the load on one phase and the other half on another

You would have to replace it. There's no room, let alone the parts off the shelf, to convert an existing board to 3-phase. In the process you'll end up being forced to find and fix every fault on every circuit.

Once that's out of the way: Not one single large load in the property will be 3-phase. Most of the larger plug-in loads will be clustered in one corner of one room on one circuit.

It's vastly more practical to just get a single-phase generator.

[themadhippy]

Well, no, because it's all wired for single-phase..

just replace the consumer unit with a 3 phase board  fed via a changeover switch with the mains incomer linked to all 3 phases and the genny side wired as 3 phase.Best to do a full test and inspection of the house installation first to avoid any surprises,and don't forget you'll need to use 1000v for your insulation testing.  Or buy  1 of the many single phase units that are available.
ETA. or you could buy a 3 phase 415-240v single phase tranny

[Chris Wilson]

Thanks for the replies so far, my aim was a functional single phase generator for the house, without changing the property's distribution panel in any significant way, but later have the ability to use the generator in three phase mode for workshop equipment currently running off a rotary single to three phase converter that will struggle if i buy any gear with heavier input needs. I had visions of rewiring the generator's alternator from Y to lazy Z or whatever it's called to get a temporarily more efficient single phase generation.

I have a small Honda 13HP single phase generator, but my wife is not to be trusted not to overload it by using the shower and having the clothes dryer on, blah blah. Being out in the sticks power outages are far from unknown, plus we live in troubled times, so taking those into consideration and my desire for a more robust three phase supply for my workshop led me to this post

[langwadt]

Thanks for the replies so far, my aim was a functional single phase generator for the house, without changing the property's distribution panel in any significant way, but later have the ability to use the generator in three phase mode for workshop equipment currently running off a rotary single to three phase converter that will struggle if i buy any gear with heavier input needs. I had visions of rewiring the generator's alternator from Y to lazy Z or whatever it's called to get a temporarily more efficient single phase generation.

I have a small Honda 13HP single phase generator, but my wife is not to be trusted not to overload it by using the shower and having the clothes dryer on, blah blah. Being out in the sticks power outages are far from unknown, plus we live in troubled times, so taking those into consideration and my desire for a more robust three phase supply for my workshop led me to this post

turn the problem around and instead make a system with some remote controlled switches prevents by loads being on at the same time, overloading the generator

[trobbins]

Chris Wilson, you may want to do an energy/power audit of your existing 1ph loads, including any anticipated future loads (as you just indicated).  Other options often become clearer when you fully appreciate what you do with your power, and when, and whether that default useage profile can be not too inconveniently modified during an extended power outage period.  Also any thought of deploying a genset option also requires a very good understanding of your load profile.

Obviously 3-ph gensets prefer balanced ph loads, and there are PQ issues for imbalance and short duration high power loads, and diesel maintenance implications for imbalance and for low load operation (ie. overnight operation).  And if the system is only used for power outages then you still have a cost/inconvenience to maintain the genset and auxiliary systems for what may be long durations without an outage (given most gensets have a minimum runtime recommendation with a minimum load and you may want to do monthly testing).  You may also have an adverse environment that could require cold starting, which has an adverse affect on genset maintenance and start-battery capability, and perhaps fuel type.  A can-of-worms .....

[Jackster]

Chris Wilson, you may want to do an energy/power audit of your existing 1ph loads, including any anticipated future loads (as you just indicated).  Other options often become clearer when you fully appreciate what you do with your power, and when, and whether that default useage profile can be not too inconveniently modified during an extended power outage period.  Also any thought of deploying a genset option also requires a very good understanding of your load profile.

Obviously 3-ph gensets prefer balanced ph loads, and there are PQ issues for imbalance and short duration high power loads, and diesel maintenance implications for imbalance and for low load operation (ie. overnight operation).  And if the system is only used for power outages then you still have a cost/inconvenience to maintain the genset and auxiliary systems for what may be long durations without an outage (given most gensets have a minimum runtime recommendation with a minimum load and you may want to do monthly testing).  You may also have an adverse environment that could require cold starting, which has an adverse affect on genset maintenance and start-battery capability, and perhaps fuel type.  A can-of-worms .....

100% this.

I use to run small diesel generators on small film sets and use to own one also for film work.
The advice is always to run a 3ph generator as balanced as possible. I have talked to MEMS about it and they say you should always do your best to balance the load but they run generators the size of shipping containers and they are connected together on larger operations...

That being said, does it really damage the generator? Probably not that much if any.
3ph generators always sound better than single-phase when balanced as (and correct me if I am wrong);
A, they are typically larger
B, they have more current phases throughout the rotation of the stater/camshaft which results in a much smoother running engine.
If B is correct, you will probably find that a 3ph under a balanced load will last a lot longer than a single-phase under a typical load because of the lower vibrations, smoother operation etc.

You should also see a much nicer voltage waveform on 3ph compared to single.
If you have ever looked at one on a single-phase generator, you will see why people recommend not to plugin IT equipment into them and that you need an inverter generator for those instead.


A big mistake people make when using a diesel generator is not having a load test check every few months.
In mission-critical deployments such as data centres, government builds and hospitals etc, tests are carried out monthly under load for 30+ minutes.
Backup generators for offices blocks, industrial sites, food factories, maybe 2-4 times a year depending on their policy.
Some people/firms don't do it and then wonder why the backup power does not work when they need it most...

If you are serious about plumbing a backup generator into your house and don't want to over-spec your 3ph generator to suit 100% house load on a single phase, consider splitting your consumer unit (breaker board for the yanks) in two.
Mission-critical and everything else.
Mission-critical having a switchover to either run on mains or backup. These should be things like lights, networking, certain sockets, a shower if you must etc.
As others have suggested, a 3ph board is an option. I would counter that it is actually not a great idea unless if the house is big enough to warrant it or can balance the load well enough to make it worth doing.
If you put the heating on ph1, kitchen on ph2 and everything else on ph3. Well, it could be summer and you are not using the heating so ph1 is no longer being used, though during winter it is 100%.
The kitchen is only used 1-2 hours a day and spikes a few times a day for tea. etc etc

You might actually do better on a larger single-phase generator that can ramp to meet demand unless vs doing all the work to split into 3ph or load a single phase on a 3ph generator?


You need to ask yourself this. In a power outage. Do you really need to have a backup power system in place that can handle 100% capacity or can you live with the essentials?
Don't get me wrong, I love my tumble dryer but during a power outage my priorities shift to either fixing the issue or getting the DNO to fix it  I can live without soft towels after my monthly shower but not my computer or internet or fridge or air fryer...

15kVA is something like 10kW? On that, you can run a 3-bed house with all the LED lights on, a TV, Computer and even a 2kW plugin oven or kettle with ease.
You probably won't be running a decent electric shower though depending on the shower's rating obviously. Electric heating or all the 7.2kW induction hobs? Forget it!


TL;DR get an expert in who specialises in backup generators and get them to work out a plan for you.
My personal option for things like this. Just get a portable inverter generator that can power the essentials in your house unless if power outages are common in your area.
If you need a 3ph for the workshop, consider a static phase converter? They hold their value and running costs would be lower surely?

[PaulAm]

In a word, it depends on your generator.

I have a Vietnam era 3ph genset that can be configured for either delta or wye and 120/208  or  240/416 output.  In the US with a center tapped 240 supply, I'd use 2 of the 120 phases and get 208 instead of 240.  However, most equipment is rated to run at 208, so it works out.  I have it to run a CNC lathe that requires a 100A 208 3ph supply and you're just not going to get that out of a phase converter.

It probably would be happier if the phases were balanced, but as long as you're not exceeding the generator capabilities, it should be fine.  Check with the manufacturer and see what they say it can handle with a single phase load.

[IDEngineer]

This scenario is exactly why I configured my house with NatGas appliances everywhere possible (heating, water heater, oven, cooktop, dryer). If you want heat, burn something.

By deriving heat from actual fuel, a reasonable inverter generator can run your entire house as if nothing happened. When we lose power (several times a year, sometimes for 24+ hours and occasionally a couple of days) we shut off the main breaker, fire up the 8KW inverter generator, and use the house like nothing happened. Zero thought given to electrical loads that can't be run because they'd overload the generator. We invite neighbors over for meals, showers, etc. Everything works and life goes on.

[Jeroen3]

The advice is always to run a 3ph generator as balanced as possible. I have talked to MEMS about it and they say you should always do your best to balance the load but they run generators the size of shipping containers and they are connected together on larger operations...

That being said, does it really damage the generator? Probably not that much if any.

Not electrically, but mechanically. The additional vibrations will shake it to bits faster over time.

There are single phase generators available. kohler-sdmo has some for example. Probably based on a meccalte alternator.

[richard.cs]

You can definitely get single phase generators that size in the UK, though 15 kVA is pretty large for backup usage in a domestic house. Dual purpose ones also exist where one phase is wound in heavier wire and can be used for full output capability at single phase, and the other two phases are only adequate for 1/3 power each - this seems to be more common on PTO generators for some reason. You should definitely review which loads you actually want to use during a power cut, and how much manual load management you feel is reasonable. How is your house heated?

Using an oversized generator will clearly end up costing more (purchase price plus fuel) and most diesel engines are not very happy running for extended periods of time at light load. If you have gas heating and cooking you would probably be fine with 1-3 kVA, sufficient to run circulation pumps, lighting, fridge, etc. Even if you're electric heat would you really want to heat the whole house from a diesel generator during a power cut rather than keeping a couple of rooms comfortable and the rest above pipe-freezing temperatures?

For what it's worth I can live comfortably in my house from a 1 kW inverter with a little load management.

[jonpaul]

Simple....3 ph motor><1 ph generator
Rectification 3ph >>DC 380V>>1 ph  inverter
Jon

[NiHaoMike]

Even if you're electric heat would you really want to heat the whole house from a diesel generator during a power cut rather than keeping a couple of rooms comfortable and the rest above pipe-freezing temperatures?

Add some heat exchangers to the generator so it also acts as a heat source, a 3kW generator would also be a 9kW heater.

[trobbins]

I wouldn't want a 30kVA genset near my bedroom.  I hope there are no audible noise limits applicable, especially at night.  Maybe you are in a rural zone and not subject to domestic suburban regulations.

[CaptDon]

I would continue to look for a single phase unit in your desired 15kw (not unreasonable) power range. We had Onan 120/N/120 240vac generators at 50kw as backup power at both our AM and FM transmitter sights. They had Ford 260 cu in engines and one was dual fuel propane or gasoline. I think the little Onan at the studio sight was 18kw and also 120/N/120 240vac single phase. I think it was a 4cyl Wisconsin engine and ran on propane. Most people have sort of the opposite problem as you, they own single phase backup power and need three phase. A very sad statement is how many different sizes of 'PhaseMaster' rotary converters I have seen laying in the scrap yard. They are expensive and once very popular mainly in rural workshops fed by single phase. Now they are getting scrapped as power companies upgrade their rural service. I have seen so many very expensive desirable usable items going for scrap it would make you cry. Most of the items also carry a mandate "Must be destroyed or made unusable" by the companies who surplussed the items out. One company I worked for paid over $50,000 to Cornell to have their '5S' or Six-Sigma flunkies come in and TELL US what to throw away!! Probably trashed $250,000 in parts, equipment, test gear and so forth and within a year the company probably spent $250,000 in 'current pricing' to replace about half the items they threw out!!!   

[trobbins]

There are also domestic gas single phase gensets for domestic application (eg. 8kVA Generac style with an acoustic-weather shroud) that could be worth considering.  They can be integrated with Li-Ion battery/bi-directional inverter and PV grid-connect inverter for stand-alone domestic where the two inverter power sources sync to the genset, and the battery inverter is bi-directional so the battery can be charged from genset and/or PV, and the genset can be stopped and the battery inverter then supplies the local grid to allow PV grid-connect.  An integrated solution is starting to get costly due to all the parts, and not really aimed at a simple back-up application, but indicates what can be overlaid on to a typical domestic single phase abode.

[IDEngineer]

There are much larger NatGas backup gensets for residential use. We looked at 20KW+ units with auto start, auto self test on a programmable schedule, inverter output for low THD, etc. But by the time you get the upgraded gas meter/regulator, pour the concrete pad, and hire the mandatory licensed electrician (because the state wants to insure the auto start won't be stupid enough to power the grid and kill linemen) it's $10-20K.

For under $1300 we got our 8KW wheeled inverter unit and we have a dedicated two phase outlet on the back patio for it which I installed at our last remodel. The additional convenience wasn't worth the delta cost to us. It's quiet, reliable, and very efficient. Plus with all heat sources being NatGas, 8KW was more than enough to run the entire house as noted earlier.

BTW, more and more things are starting to specify THD in their power requirements. We just replaced our furnace, and it very clearly states that incoming AC must be less than 3% THD. That kinda surprised me, but I will say that when the whole-house blower ran from our previous, "construction" style generator the motor made some very unhappy noises. I didn't expect an AC motor to be sensitive to THD but I was already upgrading due to constant complaints from all the UPS's in the lab and on the telecom equipment. Since the inverter, everything has been happy.

[Monkeh]

BTW, more and more things are starting to specify THD in their power requirements. We just replaced our furnace, and it very clearly states that incoming AC must be less than 3% THD. That kinda surprised me, but I will say that when the whole-house blower ran from our previous, "construction" style generator the motor made some very unhappy noises. I didn't expect an AC motor to be sensitive to THD but I was already upgrading due to constant complaints from all the UPS's in the lab and on the telecom equipment. Since the inverter, everything has been happy.

Single-phase motors are a bit ugly to begin with, feeding them a distorted sinewave doesn't help matters. And portable generators are generally big, nasty, ugly pieces of crap to save a buck.

The upshot of those gas (actual gas, not gasoline) generators is the low maintenance. You've no worry about fuel going bad and so forth with them, and generally if you go that route you already have a very substantial propane tank to feed them. Change out the oil once a year and put a quality cranking battery in, and they're solid. Just don't burn yourself on the exhaust getting down to the drain plug.

[IDEngineer]

Those are definitely advantages. I just couldn't justify the 15X cost increase. Our generator lives indoors so it's warm, dry, and easy to start. Roll it out the back door, either push the button to use the battery or pull the recoil rope, and away it goes. I run it around the first of every month to keep the fuel system happy, and change the oil once a year.