Balancing 240 volt generator with step down transformer

[lovetofix]

So I have been reading this forum for months and trying to absorb as much of your experience and wisdom as I can.
I am working on finishing a partial conversion on my new to me 1981 MCI9 and want to ask a couple questions before I do something I have to change.

I am wrapping up the mechanical maintenance and starting to gather components for the electrical systems, A/C system, and interior.

My main question is: Can a 240V to 120V transformer be used to balance the varying loads on the generator windings?

I understand the importance of having the loads balanced on a 240V generator. Mine is a 7.5KW diesel with the capability of being configured for 120/240V or straight 120V but I want to run a 30,000 btu A/C. (which can only be had in 240V)
So with that in mind, has anyone ever used a 240V to 120V step-down transformer to power the 120V inverter and loads? I would have a 240V panel box fed from an ATS which would select either shore or generator power. That 240V panel box would supply the A/C, a plug for my welder, and the step-down transformer which would feed my 120 volt loads. This should keep the generator windings balanced at all times with minimal power loss.
I would have another transfer switch between the transformer and 120V panel box so I could run all my 120V loads from a 30amp shore connection if that is all I have available. The big A/C could only be run on Generator or 50amp shore power which I am totally okay with. From what I read generators are usually underused and the one I have only burns 0.4gal/hr at full rated load.

I am putting in a 120V A/C in the dash air system for the driver and windshield so I will have a little bit of A/C even on a 30 amp connection.

Let me know what you all think!
Thanks, Michael.

[richard5933]

Michael - Welcome to the forum!

I've seen lots of buses with 120/240 generators, and they all fed the 120v side of life with the two legs, just like a residential box. No need for special transformers to do this.

My first bus had some 240v appliances, but for the most part things were 120v. The panel was balanced the best that could be done, and the generator lasted for many years before I got it running that way. Our current bus has a 12,500 KW Kohler which is also wired the same. Each of the two legs powers one side of the panel, just like a house would have. I have no 240v loads, but they would be fed from both legs if I did.

There are other guys with Custom Coach buses with similar setups, some with 240v appliances. They are all set up similarly.

No need to over think this or make it more complicated than necessary.

[dtcerrato]

We've been running a 6.5 kw Onan on 120/240 for over 30 years - no issues. We've got the distribution panel fairly balanced not perfect and no issues ever...

[buswarrior]

I appreciate your concern about balancing the generator.

Did you consider going the other way?

Wire the generator for 120 and use a step up transformer for the AC?

Percentage wise, it is typically very hard to balance loads in an RV. Whether the generator withstands that imbalance, well, a busnut gets to wonder, when it fails, what contributed to it...

Happy coaching!
Buswarrior

[lovetofix]

I am not an electrician by any means but know enough to make me dangerous. I have been using and working with micro-grids for the past ten years in Haiti where there isn’t grid power. I have experienced two stator failures in the last three years that in speaking to a generator technician and doing the troubleshooting could not attribute to any other cause than imbalance. The one had around 15,000 hrs on it the other around 3,000. From what the technician told me, the smaller the generator, the more critical balancing becomes.

Buswarrior: I considered using a step-up but the A/C will be by far and away the lions share of electrical load and transformers generate heat which means power loss. The higher the amperage the greater the heat/power loss so my thought was use the direct voltage to power the largest load but I am Very open to being taught where I am in error.

[sledhead]

in the past I was going to use a 240 volt 3 head split unit a/c and got a great deal on a outback 120 to 240 step up transformer used . if you are going this route the a/c units are very low in amp rating if you spend the money on a high seer unit . this is the easiest way as per my off grid buddy as he does it all the time .

as for equalizing the load what I have is 2 of these units
 ( 1 for each leg )  and reworked the ac breaker panel to try to make the reg. loads equal

   https://www.amazon.ca/DROK-Frequency-Multimeter-Voltmeter-Transformer/dp/B07KXG3LFG/ref=sr_1_2?dchild=1&keywords=120+volt+ac+amp+gauge&qid=1594559370&sr=8-2

dave

[David Anderson]

Get you two ammeters and two current transformers. Put the CT,s on your 120 volt legs and keep your balance on the phases by watching your loads on the ammeters. I’ve done mine like this for 18 years.

[lovetofix]

in the past I was going to use a 240 volt 3 head split unit a/c and got a great deal on a outback 120 to 240 step up transformer used . if you are going this route the a/c units are very low in amp rating if you spend the money on a high seer unit . this is the easiest way as per my off grid buddy as he does it all the time .

dave

That is what I am thinking of doing. An LG 30,000 BTU with 18K,9K,7K interior ceiling cassette units with a SEER Rating of 21. I found a link to a transformer in another thread, I think it might have been yours. That transformer can do up or down, might be handy.

[lovetofix]

My reason for wanting to “automatically“ balance the loads is that I will not be the only person using the coach. Not many people I know will care enough to look at a gauge let alone do anything about an imbalance.
Most people just don’t care about taking care of things to make them LAST. I have money to do this right ONCE, not to be replacing stuff from a lack of care, so it needs to be somewhat idiot proof.

Also, per the use of our S&S travel trailer we “boondock” a lot at Walmart’s, Cabela’s, etc. so the generator use will be substantial. 
I worry most about things like the 120V dash A/C and kitchen loads like a crockpot or instapot that can pull 1kw+ for extended periods of time. Short run items like a hair dryer or microwave are not as big of a deal but still matter.
When a single 120V appliance can draw 20% of the generator capacity and be greater than the Total of all the other 120V loads I don’t see a good way to balance the load between legs without constantly switching circuits around. That would be to much bother, even for me.

[richard5933]

I hear what you're saying, but I can't say I've ever heard of anyone installing a system like you're describing to accomplish a balanced load.

Have you talked to the tech support team at the manufacturer of your generator? They could tell you if it's necessary to go through all these steps.

[Jim Blackwood]

You might see if you can find a transformer with a 240 primary and a 240 center tapped secondary. I'm pretty sure they make those for industrial applications. That way you could feed a standard 240 breaker panel and pull the 120 off any way you please.

Jim

[jap42]

Balancing a split phase generator is not so much about making the loads the same as not over loading one half of the phase. An 8000W 240V generator can only nominally provide 4000W on one half of the phase. If you have a 4000W load on the 240 then you have 2000W left on each Half.

The goal here is to setup your breakers to split heavy loads evenly. So take your 240V AC away from the capacity. Now make sure your fridge and microwave are on separate sides. (In 99% of breaker boxes the breakers alternate, so 1,3,5 are on L1 and 2,4,6 are on L2. ) If you have tandem breakers each pair is on the same phase half.

Another trick is put user controlled high power loads on the same side. Like Microwave, Coffee Maker, toaster so you can actively make sure only one gets used at a time, this would be if you wanted to use a smaller generator.

[jap42]

PS Step up would probably not work. 110V would step up to 230V single phase(European). Not 240 split phase like your system may require. I believe most heat pumps use the center tap to provide 110V to the indoor unit.
Plus it would require a much higher amperage to run. The same thing happens the other way. 110V needs to be half phase, but using a transformer would leave you with 110V full phase. (not sure if that would work still or not. It might.)

[Jim Blackwood]

It depends on how the transformer is wound and wired. Here are a couple of examples: A 230v isolation transformer with a center tapped secondary can be wired with 230v on the primary, the center tap of the secondary grounded, and each outside leg of the secondary will give you 115v, generally in opposite phase but that does depend on how the windings are laid down. The same transformer with a center tap on the primary can be used with a 115v input from one leg to the center tap and still give a 230v output across the secondary. The input amperage capacity will be half of the above and the output amperage will be half of the input, minus any losses (double the voltage, half the amperage). Or it can be wound with a 115v full amperage primary and still give 230v on the secondary as two 115v legs relative to the centertap. Isolation transformers are generally pretty efficient.

Transformers like sine wave power. That sets up a magnetic resonance in the core which induces sine wave power into the secondary in proportion to the number and direction of the windings, so for the most part you can use anything as a source and get anything out, within the limits of the wire and insulation. You can step up or step down. As long as the transformer is designed for the job you are good to go.

As a general thing you might not want to use a transformer after an inverter or PWM device as the results might be unpredictable. Inverter output is often square wave and the transformer would try to convert it to sine wave, making excess heat in the process.

Widely used by industry, as well as power companies everywhere.

Jim

 

[TomC]

I have my 120v straight wired. Hence that means only 50amps at 120v at the pole, but is never has been a problem. It eliminates balancing the load. And if one leg goes out, I can just switch to the other leg. I like it so much, I wired my truck the same way. I have 3-13,500 roof top airs-which I can run all three on 50 amps. Good Luck, TomC