Backup generator single phase - Page 1
Forum section: electronics / projects

author: helobin
author_rank: Newbie
timestamp: March 07, 2016, 02:45:03 am
content: I have pretty unreliable power so I've got a 12 kVa single phase generator. The house is single phase, but the sheds have three phase. At the moment the generator is hooked up through a transfer switch that connects it just to the single phase house sub-board and a couple of single phase circuits in the main shed (water pumps etc)The problem is that most of the shed circuits come from a 3-phase distribution and remain connected to the mains all the time (don't appear to go through the transfer switch).My question is what would happen if the single phase generator was connected to all three phases in common?Presumably all the three phase loads (just DOL delta motors and VFDs) will see the same potential on both sides of the load and draw zero current? (except for minor distortion of the wave). All the single phase loads would then be on the same phase but shouldn't otherwise be disrupted.It's theoretical at the moment, but I'm curious if anyone sees any obvious problems with such a setup?I'll ask the electrician next time they're around, but they haven't done many generator installs. Needless to say it'd definitely be an electrician job - the mains comes in straight off a massive 11kV transformer through 100A service fuses... a lot of energy there!

author: uncle_bob
author_rank: Unknown
timestamp: March 07, 2016, 02:53:06 am
content: Quote from: helobin on March 07, 2016, 02:45:03 amHiFeeding a three phase circuit with anything other than a proper three phases is asking for trouble. Even dropping one of three phases can cause problems. Exactly what goes wrong depends on exactly what you have hooked up and how it's wired. I've replaced a *lot* of three phase motors over the years before people got religion about cutouts.Simple answer:Hook up a transfer system that *properly* isolates things. Feed 240V to the stuff that needs it. Feed 120 off of *balanced* sides of the 240. Take the 208 three phase off line when you just have a 240V feed.(There's also the minor problem of that three phase 208V / 40A (per phase) compressor in the shed taking out the 12KW generator when it tries to start up ...).Bob

author: Ian.M
author_rank: Super Contributor
timestamp: March 07, 2016, 03:33:27 am
content: Errr Australia was historically 240V phase to neutral,315V415Vphase to phase, and is now nominally 230V phase to neutral, 400V phase to phase. Therefore your experience with the eccentricities of North American 120V/240V split distribution systems does not apply.However I agree the three phase loads need to be isolated when the generator is in use, as feeding the same 230V to all phases would lead to an unsafe condition if any equipment uses the neutral conductor.Edit: Thanks, Richard for spotting my typo.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 07, 2016, 05:58:22 am
content: How do you get 315V phase-phase when the phase-neutral voltage is 240VAC?To get phase to neutral voltage it's root 3 times the phase to phase voltage and that's fixed.So 230* sqrt3 = 380V and 240*sqrt3 = 400V

author: peter.mitchell
author_rank: Super Contributor
timestamp: March 07, 2016, 07:35:04 am
content: he means 415v phase to phase

author: Ian.M
author_rank: Super Contributor
timestamp: March 07, 2016, 10:23:48 am
content: You do need to know what thee phase system you are dealing with. e.g. North America uses oddities like Wild leg Delta for low power three phase installations.https://en.wikipedia.org/wiki/High-leg_deltaAlthough its derived from a normal (120 deg between phases) three phase HV distribution system and its three phases power balanced loads with no neutral normally, the wild leg, phase to neutral is actually 208V, at a phase angle of 90 deg relative to the L1,L2 120V/240V split supply.Needless to say, it is absolutely critical to avoid wiring 120V single phase loads to the wild leg!

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 07, 2016, 12:10:50 pm
content: Ok, so I also made a mistake! 240*sqrt3 = 415V

author: BradC
author_rank: Super Contributor
timestamp: March 07, 2016, 12:18:32 pm
content: Quote from: Richard Head on March 07, 2016, 12:10:50 pmAnd it hurts a hell of a lot more than 240V.Back on topic, yes if all your 3 phase gear is either star or delta wired, then having the same voltage on each leg will see a net zero current flow and should be harmless. I've seen a lot of 3 phase equipment that also has 240V control gear that just goes between a random phase and neutral, so that would all go live. I have a a VSD here that uses a neutral reference, and I could not predict what that might do in your proposed configuration, but then that's not your issue

author: uncle_bob
author_rank: Unknown
timestamp: March 07, 2016, 12:34:51 pm
content: Quote from: Ian.M on March 07, 2016, 03:33:27 amHiSorry, missed the flag icon.My main point in going into the voltages is that over here in the US there is a further "gotcha" in having both 240 and 120 as "other than three phase" voltages. That leads to it's own set of tangles.Bob

author: Ian.M
author_rank: Super Contributor
timestamp: March 07, 2016, 12:41:33 pm
content: What's the issue here anyway? Obviously its desirable to keep the single phase loads reasonably well balanced over the three supply phases, but that just means you need another transfer switch at the outbuildings panel to feed the maintained single phase loads off a different phase to the house and transfer to the generator (e.g. the aforementioned pumps + a certain proportion of the outbuildings lighting. Other single phase loads that will always be shed to stay within the generator capacity would connect to the remaining phase with no transfer switch.Any decent 'leckky' with industrial experience can specify and install such a system to local code, and the cost of doing it right is probably less than the cost of bodging it via a three phase heavy duty transfer switch.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 08, 2016, 06:06:12 am
content: I have a three pole transfer switch that shorts all phases together in "Generator"mode. That means when running on the genset I don't have a 3-phase supply, but luckily it's not a problem as I don't have any 3-phase equipment in the house. It does mean that I can use a single phase genset on my 3-phase wiring. Just got to watch the neutral current as it doesn't sum to zero.I was shocked to see how crappy the sinewave from the 10kVA was though. Much worse than I expected. So crappy that the washing machine electronic speed control won't work properly.

author: Harks11
author_rank: Newbie
timestamp: June 08, 2017, 12:04:16 am
content: Richard, been a while since you posted that you shorted out the transfer switch to feed single phase in to three phase house ( no 3 phase loads). Has there been any problems with this approach. Also does it break any codes ( apart from neutral sizing)

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 23, 2017, 08:45:53 am
content: Harks11Apologies for the tardy response. Eevblog only gets a visit occasionally lately.I've been running the house on the genset periodically with all 3 phases connected together for several years now with no issues. I even run it through the earth leakage unit. I'm not aware of any violation of wiring code but it may well be technically illegal. However, when you have no power you are prepared to compromise and give the local bylaws the middle finger occasionally! Just don't forget the neutral current issue. In a balanced 3-phase system the neutral current will sum to zero and in practice is normally quite low. In a paralleled single phase system the neutral current is the same as the live current and the neutral return has to be sized for this, including the neutral busbar. Also ensure that the genset ground (chassis) is connected to the distribution board ground.