Running two generators in the same circuit - Page 1
Forum section: electronics / beginners

author: Kjetil
author_rank: Regular Contributor
timestamp: December 30, 2013, 05:57:45 pm
content: At my cottage I'm relying on a generator when I need a lot more power than the solar array and inverter can deliver. At times the 2kW my generator delivers ain't enough (220v AC), but a 4+kW generator is also a lot more expensive. I understand that you can't connect two of them together without something bad happening, is this because the two generators will be delivering different frequencies, or frequencies that are out of sync? If so, is there any way to fix this (that would be cheaper than buying a big generator)?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 30, 2013, 06:03:32 pm
content: It's because they won't be in sync.However as you already have an inverter for the solar, you could potentally rectify the generator output(s) to produce DC to feed into the solar inverter - not trival though, so a bigger gen is probably a better answer, and likely to be more efficient.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 30, 2013, 06:04:04 pm
content: you must either synchronise the 2 generators (and if that fails bad things will happen!) or don't put them in parallel and feed half of your equipment from generator 1 and the other half from generator 2.

author: Kjetil
author_rank: Regular Contributor
timestamp: December 30, 2013, 06:20:24 pm
content: Quote from: mikeselectricstuff on December 30, 2013, 06:03:32 pmOnly got a teensy 150W inverter, so that's not really a good option.Quote from: PA0PBZ on December 30, 2013, 06:04:04 pmThe problem arrives when I use heavy equipment that use 2,5-3kW, so splitting it up wouldn't do much good.I guess this is something that would be available cheaply if it was easy to make as well, but you never know if there's a simpler way

author: ResR
author_rank: Regular Contributor
timestamp: December 30, 2013, 06:23:37 pm
content: Problem with rectifing is that the voltage may be too high for the inverter (solar panels make 30-50VDC as far as I know, but rectified 230VAC is roughly 325VDC), thus you need a voltage down converter. Every conversion stage wastes energy as heat, so the whole efficiency of the circuit is reduced drastically.Use two separate circuits as paopbz suggestedI think it is better to get a more powerful generator, it is hard to syncronize two generators, you need an electronic speed controller for both of the generators and frequency comparator to match the frequencies to 50Hz, use a DPDT or DPCO (on-off-on) switch to avoid the generator AC get into the solar charger inverteroutput terminalsHV side, otherwise it can burn out the circuitry in the inverter's LV side.

author: Kjetil
author_rank: Regular Contributor
timestamp: December 30, 2013, 06:27:00 pm
content: If I wanted to run it off the solar circuit I'd just run it off the 12V output of the generator, it can be fed directly into a spare input port of my regulator. But then I'd need a new 12-220v inverter that could give at least 3kW, which isn't that much cheaper than buying an extra generator.

author: ResR
author_rank: Regular Contributor
timestamp: December 30, 2013, 06:32:20 pm
content: Then it is much wisely to get a bigger generator and sell the smaller ones later. Every conversion stage wastes energy, because the inverters are not 100% efficiency, that's why I didn't recommend that way.

author: Kjetil
author_rank: Regular Contributor
timestamp: December 30, 2013, 06:48:17 pm
content: Ok, thanks for the answers

author: richard.cs
author_rank: Super Contributor
timestamp: December 31, 2013, 05:12:45 pm
content: If it's only an occasional need then it's not that hard to synchronise two generators, although with small 2kw units it gets trickier because their speed varies so quickly and they quite often hunt around their target frequency. Often they hunt worse off-load so you may have to apply a bit of load to each to get this to work.Traditional method:Connect neutral from both together, connect line together via a switch. Connect an ordinary tungsten light bulb across the switch andwith the switch openstart both generators. If their speed is sufficiently similar then the bulb should pulse slowly, if there's a rapid flicker you'll have to tweak the speed of one generator until you get a slow pulsing. At the point of minimum brightness close the switch. It's not all that difficult but there's a risk of damage if you get it wrong, although TBH with small generators you'll probably just open the breakers or collapse the excitation.Alternate method (for horribly out-of sync gensets or those that hunt uncontrollably):Connect as for the traditional method but in place of the light bulb put an ordinary mains socket. Plug a suitable variable resistive load into the socket (a 3-bar electric fire of the kind once common in the UK, with three separately switched 1kW elements is perfect). Start both generators, againwith the switch openand then gradually apply the load - current flowing through the load will tend to pull the generators into synchronisation. Switching 1, 2, 3 and then the shorting switch with a few seconds delay between each will probably work but check there's little or no voltage across the switch before you close it (lamp across the switch again or a voltmeter).Once they're synchronised they should stay that way for as long as you keep them running. I'd suggest you open the switch before you stop either generator or one will probably try to turn the other as a motor (so don't let either run out of fuel :-P). You should be able to supply a 4kW load and they should share nicely unless the voltage regulators or the mechanical speed governors are grossly different. One other thing to note, the load between the two generator outputs will see high voltages until the generators are synchronised, don't use anything that's more complex than a resistive heating element unless you really know what you're doing.

author: woodchips
author_rank: Frequent Contributor
timestamp: December 31, 2013, 05:44:06 pm
content: The replies assume both generators are synchronous machines, not surprisingly. But what if you modified one to be an induction generator? The beauty of an induction generator is that it will happily generate power with a considerable difference in the generator rotational speed, what happens is that the slip varies.To modify the generator to an induction generator remove all the caps on the rotor and replace with a short circuit, now like a squirrel cage rotor on an induction motor. Then start the unmodified generator. Now connect the modified generator to the unmodified, not started. The modified generator should (note the word should!) now run as an induction motor. Let the engine start. Now by increasing the modified generator engine speed the power flow from the modified generator will reverse, from taking to generating. Provided the modified generator doesn't run at more than twice the speed of the unmodified generator then they will both generate power.The modified generator is taking reactive power from the unmodified generator as magnetising current, so the combined power output will not be the algebraic sum of the two generator outputs.Warning, this is a thought exercise! I haven't tried it, but it is how small wind turbines generate power without requiring to be synchronised to the mains.Comments?

author: C
author_rank: Super Contributor
timestamp: January 01, 2014, 03:15:53 am
content: Quote from: richard.cs on December 31, 2013, 05:12:45 pmI agree with richard,The Traditional method works: but note that if the speed is different which it will be some, that bright / dim light is due to 0 to 2x generator voltage. Better use two light bulbs in series or one rated for 2x the voltage.C

author: woodchips
author_rank: Frequent Contributor
timestamp: January 01, 2014, 09:42:31 am
content: Apologies for the awful grammar and spelling, I hate those on screen touch keyboards. Hopefully all now sorted.One point, I was assuming three phase generators, with single phase then the modified generator won't self start as a single phase motor.

author: richard.cs
author_rank: Super Contributor
timestamp: January 01, 2014, 10:12:20 am
content: Quote from: C on January 01, 2014, 03:15:53 amIt will be 2x generator voltage and a 220V lamp will have a very short life, the advantage of abusing a lower voltage lamp like this is that it allows you to see the voltage minima more clearly.A single 220V lamp might glow faintly on a few tens of volts (30-40 I would guess) and two in series would obviously need twice that meaning you can only identify the minimum to within say 70 Volts. Now that I think about it though 70V isn't all that many degrees, arcsin(70/(2*220))~ 6 degrees, so it'd probably be fine. I'm not aware of any lamps commonly available rated to >250V so it would probably have the be a series pair.

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 01, 2014, 10:13:12 am
content: I was wondering, could you rectify the outputs of the generator, and stick them into the HV DC bus of a reasonably high powered single phase VFD with the frequency set to normal mains freq?

author: axero
author_rank: Regular Contributor
timestamp: January 01, 2014, 04:23:48 pm
content: Maybe I'm grasping for straws here but what if one connected these two generators in series and connected them to a 4+kW rated switching transformer (rated for 240-480V input) that regulated the high voltage down to 240V?I've also heard of so called 'load balancing' circuits but they don't come cheap, especially at those high voltages. They are used for applications where redundancy is needed so I don't know how they would work here...

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 01, 2014, 05:45:40 pm
content: Quote from: axero on January 01, 2014, 04:23:48 pmyou'd still have to sync them.

author: Kjetil
author_rank: Regular Contributor
timestamp: January 01, 2014, 05:56:17 pm
content: Quote from: woodchips on January 01, 2014, 09:42:31 amThe one I already have is a cheap OEM generator that I've seen for sale under many brand names (2.2kW to be precise). I haven't seen any place to modify the frequency of the output either, I would assume that's a bit too much to ask for such a cheap generator.Quote from: axero on January 01, 2014, 04:23:48 pmSo getting two 110V generators (I suppose american generators?) in series should (in theory) work if they were in sync?

author: richard.cs
author_rank: Super Contributor
timestamp: January 01, 2014, 06:04:26 pm
content: Quote from: Kjetil on January 01, 2014, 05:56:17 pmYes, but you'd have to keep them in sync somehow. Parallel connected generators stay in sync because anytime they're not lots of current flows between them producing a torque on the rotor until they are in sync. Series connected generators won't do that so unless they're mechanically linked on the same shaft it'd be near-impossible. In fact series connected generators will tend to go exactly out of sync as this condition provides the minimum load (since there's no output voltage).

author: Kjetil
author_rank: Regular Contributor
timestamp: January 01, 2014, 06:10:23 pm
content: Quote from: richard.cs on January 01, 2014, 06:04:26 pmThat sounds like a rather big mechanical issue to be solved..

author: richard.cs
author_rank: Super Contributor
timestamp: January 01, 2014, 06:18:07 pm
content: Quote from: Kjetil on January 01, 2014, 06:10:23 pmGears, toothed belts, lots of choices :-PMore seriously I'd never attempt to series connect two ac generators like this unless they were mechanically linked and I can see very few applications where it would be useful. Perhaps when you need to build a generator to fit in a specific shape.

author: woodchips
author_rank: Frequent Contributor
timestamp: January 02, 2014, 10:23:58 am
content: The generated frequency is directly controlled by the engine speed. Assuming they are petrol, then the carburettor throttle valve will have a spring pulling it open and a rod going into the cooling cowl. This rod attaches to a vane that is moved by the airflow. There is then a simple force balance between spring and air pressure setting the throttle opening and hence frequency. The spring will have an adjustment so just tweak that to raise or lower the engine speed and generated frequency.