Modifying genset voltage for 120v RV

Ian, tku v much... you point to a potentially frightening path (for me), but it is the path that has promise for the elegant solution I prefer.

I will try for dwgs before digging into this.

Ian, tku. I apologise to you & to all others for the very long time no reply.

I was able to locate a manual for this genset. Despite some legibility problems, & with some good assistance from an ex-military tech, the critical control wiring was id'd in the control box & the configuration was able to be changed to deliver all power at 120v.

(The set ran well most of this year, until the carb decided to become bunged up. In that area I am competent, but I've removed, dismantled & cleaned this carb twice now, & it's still behaving bunged-up. I will seek the most aggressive cleaner that will not corrode aluminum, brass or steel & will go at it one more time. If no cigar, a new carb will do the trick. No need to go around or through the mountain if you just replace it.)

Awesome. I'm glad you were able to configure it as you needed. So often, we never seem to get a follow up. It is very satisfying to know that we could help. :)

Unless you have 4 wires coming out of the windings that allows you to wire the two winding in parallel you are better off to simply use a step down transformer to give 12V from the 240V. Granted you have slight eddy current losses in the transformer but it is the least complicated way to get your 120V at the required power levels. Air conditioners typically surge at 3X or sometimes even 4X the running amps.

Have you actually tried starting the roof top A/C and measuring the actual surge peak. If you cannot change either the genset or the air conditioner maybe a soft start module is required. These soft start modules are often used for deep well submersible pumps in off-grid homes with limited capacity power sources.

elnav, tku v much. Your advice appears to be most practical. When/if I find wiring dwgs, I will open up the genset & seek clarifications as needed, like exactly how to wire 2 windings in parallel...

I'd guess measuring actual surge requires good instruments in experienced hands. That's neither my tools nor me. But this genset is 4.5 Kw, the AC is stock & presumably did not start-stress the 4 Kw OEM-spec'd unit, so I am confident of the genset capability to handle start surges. If a stepdown transformer turns out to be the way to go, I would seek conservative rule-of-thumb info for sizing it appropriately.

In the meantime, I am grateful for the encouraging & practical feedback. This group constitutes an excellent & impressive resource. You've moved my preconceptions from, "It's a long shot..." all the way to, "Wow! With a touch of luck, elegant & practical both might be in the same solution!"

My thanks to each of Wareagle, IanR and Elnav.

As far as step down transformers goes use the same rating as the genset can deliver - 4.5kVA -which is almost the same as 4.5kW This is the approach I use in my designs and even on a typical North American boat design I spec 240V high demand equipment then use a 3.6KVA step down transformer for the 120V utility outlet circuits.

Doing so avoids the problem of unbalanced neutral current, which is inherent with normal 120/240 V style designs and why you were told this genset cannot deliver maximum power except at 240V.

I had an Onan 6 kW genset in my RV and they are so basic there is maybe a chance of converting it but not likely. Onan had their gensets made special for them to get the price reduced as much as possible.

If you intend to use this RV a lot you might want to consider going the inverter route. It's actually a better approach but Until recently the technology was not there to support it properly. I recently did a design for a 50 foot boat where we only installed a 4 kVA 240V genset used strictly for battery charging. All the AC power is derived from inverters fed from the battery bank including the air conditioning.

I like this particular answer the best. It uses the existing generator to it's best abilities, and then it's off. The inverter is quiet, and with a big battery bank, the gen doesn't have to be run too much.

Can you run the gen while you are going down the road? Then the noise wouldn't be a bother, hopefully.

This also allows for solar charging in the future. I don't know beans about generators, but I'll bet one at half the usual RPM will have wiring that you can't change too much.

I'll bet one at half the usual RPM will have wiring that you can't change too much.

REPLY

I have worked in both RV and marine. Cap blanc is correct as far as marine Onan goes but the RV air cooled models used in North America are more cheaply produced and do not always allow 120 or 240 wiring except as a centertapped neutral which is not a configuration used elsewhere in the world.

4 kW is tiny by North American standards where massive air conditioning is the norm. Its not unusual to see 2X 16,000 BTU air conditioning units which has a huge start surge.

Most big charger or inverter/chargers only deliver 100 to 130 Amps at 12V so that only amounts to 1500 watts loading at the most. Yet 100 amps charging is considered quite hefty.

Large gensets are not required except where the market demands large air conditioning capacity.

You should be able to wire the genset into a panel with 240/120 power available. the key is to run all of your appliances you can on the 240v and balance the rest of the load on the two legs. this should be fine.

Trouble is most RVs are not designed to have 120/240V panels. I have only seen such on high end Prevost and Foretravel models with big 12kW gensets. Even on the coaches we built as bus conversions, we did not use that approach because the available RV equipment was only 120V VAC and 12VDC. Only the Double Eagle and MCI had 24V DC but still only had 120V air conditioning.

Many RV parks only provide the old style 3 wire outlets rated 30A, so if the RV was set up for 120/240AC there would not be many places where it could plug in. Some of the new RV parks catering primarily to high end class A RV now have dual plug sets so both common 3 pin triangle style plug and the newer circular twist lock style can be accomodate.

Industry compatibility needs to be addressed when doing a refit to an existing RV.

TKU, MoldDoc.

None of the load is 240v. Most of the time, shore power would be used, & genset power is used where shore power is not available. Shore power for the RV is typically a 30A or 50A 120V feed. So for seamless change between shore power & "dry camping" genset power, I'm constrained to deliver all power on a single feed to maintain a clean "ambidextrous" configuration where no transfer switch is needed & there is still no possibility of shore power & genset power simultaneously online.

But thanks for making me think that through.

Going back to your original post it sounds like you are refurbishing an existing RV and the genset might not have been part of the original equipment. In most cases the 120/240V gensets were intended for use where the RV used an electric stove needing 240V for the elements. Propane stoves and appliances have enough regulatory compliance issues that they are often considered too expensive for installation in a cost controlled entry level production RV model. Low cost domestic electric appliances were chosen but these then needed household style voltage.

It is an unfortunate reality that gensets must be chosen for their ability to handle peak demand such as start surges of motor loads (air conditioning) and so becomes over sized for average runnig loads. Running a genset for prolonged periods of time at light loads has its own service issues. Hence the growing popularity of inverters. Inverters can handle surge demand of 2X and sometimes even 3X their normal full load continuous rating. When only light AC loads constitute the majority of use and only occasional peak demands are present an inverter makes a lot of sense because of lower overall maintenance cost and silent operation. RV and marine style inverters come in a dual mode design so that when plugged in, the inverter actually functions as a charger while passing the surplus shore power on to the remaining circuits.

Much depends on where you travel with the RV and how ofen you 'dry camp' in locations having little or no utility power access.

It all depends on how much you use it and where. I have been doing this sort of system design work since 1995 and recently have concluded that gasoline fuelled, air cooled engines actually make a lot more sense for occasional use than heavier (also much more expensive) water cooled diesel sets. I had even gone so far as to build a diesel genset for my own use but have now concluded it should have been a gasoline unit given how I actually use it.

Since you are looking at a prewired system, all finished it makes more sense to get a 4.0 kVA stepdown transformer from ACME or whomever is closest to you. Just be sure to mount it in a locationm with adequate cooing or else install a fan to assist.Dry transformers do tend to get hot in use.

High capacity 12V electric fans can be had from auto wreckers for not much money. These are used on the radiator on front wheel drive cars so they can be used directly fron the RV battery supply. Use 12V DC instead of 120V AC so you can still run the cooling fan after genset has stopped running. Yopu do not want the transformer sitting ther heat soaking after shut down. If you did not get a transfer switch as part of the AC pre-wiring send me a direct message and I can give you details on how to set that ip in the ;least cost method. You do not need that expensive rotary selctor switch most RV come with.

Hi Fixx,

firstly, Onan have been very good with supplying diagrams, even for ancient models.

2ndly, Ian Rs post is pretty well on the button. Every single phase Onan I have seen (hundreds) has dual windings and so it usually a very simple reconnection job. Do it the way Ian describes.

regards

Chas

Capblanc (Chas),

TKU v much. It is the elegant solution that I like. The Onan elec dwgs are close by & I will collect 'em soon. I hope they are clear & complete enuff so I can figure 'em out sufficiently to ask intelligent questions!

Great site, many people offering! I hope I can reciprocate in my tech areas.

I almost forgot to mention. Check these guys out. They really know generators, both old and new, military and civvie. As a matter of fact they even have an area dedicated to onan gensets. I happen to be a member and go there often. I can almost guarantee someone there can help you. http://www.smokstak.com/forum/forumdisplay.php?f=6

Drop back in and let us know how you fare.

Ian

Hi Fixx,

take a pic of the AC terminals. it will usually be clear from this what the situation is regarding rewireability.

As Elnav says, I have no experience on RV sets, although I see quite a few "building site" gens which almost invariably still have 3 terminals (with the centre one having two wires) even though, here in Europe, only the 230V pair are connected externally.

If the joint is buried in the windings I would suggest you take it to someone with experience.

Chas