# US style power converters, a couple of questions.



## simonmd (Dec 20, 2013)

I have a 1995 US C Class and although its been a UK registered vehicle for some years, the 110-220vac conversion hasn't really been done. 

So far, all previous owners have done is use a yellow building site style transformer to convert incoming mains down to 110v (so the fridge, AC and charger work) and then another transformer inside that steps the power back up to 220 for a couple of UK sockets. Obviously this isn't the way to do it so i'll be converting it properly in due course. I know I can buy a simple transformer that can be used just for the AC and fridge but my question is about the power converter.

My question is two fold, 1. The US system seems to convert mains AC power to 12v for when hooked up to shore power as well as charging batteries, can this simply be replaced with a normal intelligent batter charger?

2. My previous experience is with caravans and boats and have not seen this setup before, my boat simply runs all its 12v systems off the batteries which are charged by a mains charger, engines or solar. Do Euro motorhomes use this system as well or do they have a converter like the American system?


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## cabby (May 14, 2005)

Uk Motorhomes have a mains charger built in for hook up which charges the leisure batteries and supplies 230v to the sockets of that voltage fitted. Also the engine charges the batteries and a lot of us have solar panels for the same use. All equipment runs off 12 volts unless it is wired up for mains running as well, such as the fridge.
hope this helps. Oh yes nearly forgot, when not on hook up many use an inverter to run 230 volt items.

cabby


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## raynipper (Aug 4, 2008)

Just a point Simon.
I have converted several RVs in my time to 240 volt. You are correct about the yellow power tool transformers not being suitable for extended use. They melt.!!
I have a variety of 110 to 240v transformers from 1KVA to 5.5KVA. But beware of using one too large as the initial switch on surge will often trip or blow any protection devices.

The ideal situation is a 3KVA transformer built inside the van but NOT under the bed (noise) with the van 110 connected to it. You don't need another step down or up as the one will work both ways.
Then the 240 volt power outlets taken from the primary side to wherever you decide.
If you are anticipating powering the 240 v outlets from the genny, this can be done with a sophisticated heavy duty changeover relay or just having a plu and socket from the genny.

Ray.


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## Pudsey_Bear (Sep 25, 2008)

Excuse me if I have this wrong, but if you have incoming 230v why does the transformer have to take it up to 230v, wouldn't it be better to just bring the 230v in, and power the sockets as would be normal for us, and simply plug in and continue to use the the 110v transformer for whatever needs it, That seems the most logical and cheapest method to me, and if the 110v tranny goes down, another can easily be bought from most builders merchants, rather than paying silly RV dealer prices.

Sorry just read Rays post, so the 110v builders tranny may not suit this job.


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## raynipper (Aug 4, 2008)

Transformers work both ways Kev.
So you keep your 110v side and either trans down from 240 or up from 110 genny.

But with transfrmers you get what you pay for. Then there is square wave and true sine wave. But we don't need to go into that yet.

Ray.


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## simonmd (Dec 20, 2013)

Kev- At the moment, there are three main appliances that require 110v or they may be damaged, the fridge, rooftop AC and battery charger. That's why the 110v step down transformer is needed at the moment. 

I will in due course be fitting the fridge (although as that's just a heating element I believe I'm able to convert that with a 220v one) and the AC with their own smaller step down transformers and then intend to replace the converter/charger with a 220v one. My research on YouTube and American forums has told me that the old converter/charger setup is not very efficient anyway and they all hum as well, which can be annoying. Apparently they also have a tendency to overcharge the batteries if left on for long periods as well, not good!

I plan to fit a smart charger similar to one I have on my boat (a 20 amp Sterling unit), simply to charge the batteries and then the 12v would run exclusively off them all the time, hence my original question. ironically, most marine smart chargers can automatically run on either 110 or 220vac so I could fit this first without worry.


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## listerdiesel (Aug 3, 2012)

Unless you need isolation, a 110V auto-transformer would do the job for fridge & aircon.

The 'Christie' shore supplies from that era that I have had dealings with are basically a constant voltage transformer with a rectifier, no proper voltage control and noisy. The design of the transformer is peculiar to its function, and is known as a Ferro-Resonant Transformer:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Constant-voltage transformer

The ferroresonant transformer, ferroresonant regulator or constant-voltage transformer is a type of saturating transformer used as a voltage regulator. These transformers use a tank circuit composed of a high-voltage resonant winding and a capacitor to produce a nearly constant average output voltage with a varying input current or varying load. The circuit has a primary on one side of a magnet shunt and the tuned circuit coil and secondary on the other side. The regulation is due to magnetic saturation in the section around the secondary.

The ferroresonant approach is attractive due to its lack of active components, relying on the square loop saturation characteristics of the tank circuit to absorb variations in average input voltage. Saturating transformers provide a simple rugged method to stabilize an AC power supply.

Older designs of ferroresonant transformers had an output with high harmonic content, leading to a distorted output waveform. Modern devices are used to construct a perfect sine wave. The ferroresonant action is a flux limiter rather than a voltage regulator, but with a fixed supply frequency it can maintain an almost constant average output voltage even as the input voltage varies widely.

The ferroresonant transformers, which are also known as Constant Voltage Transformers (CVTs) or ferros, are also good surge suppressors, as they provide high isolation and inherent short-circuit protection.

A ferroresonant transformer can operate with an input voltage range ±40% or more of the nominal voltage.

Output power factor remains in the range of 0.96 or higher from half to full load.

Because it regenerates an output voltage waveform, output distortion, which is typically less than 4%, is independent of any input voltage distortion, including notching.

Efficiency at full load is typically in the range of 89% to 93%. However, at low loads, efficiency can drop below 60%. The current-limiting capability also becomes a handicap when a CVT is used in an application with moderate to high inrush current like motors, transformers or magnets. In this case, the CVT has to be sized to accommodate the peak current, thus forcing it to run at low loads and poor efficiency.

Minimum maintenance is required, as transformers and capacitors can be very reliable. Some units have included redundant capacitors to allow several capacitors to fail between inspections without any noticeable effect on the device's performance.

Output voltage varies about 1.2% for every 1% change in supply frequency. For example, a 2 Hz change in generator frequency, which is very large, results in an output voltage change of only 4%, which has little effect for most loads.

It accepts 100% single-phase switch-mode power supply loading without any requirement for derating, including all neutral components.

Input current distortion remains less than 8% THD even when supplying nonlinear loads with more than 100% current THD.

*Drawbacks of CVTs are their larger size, audible humming sound, and the high heat generation caused by saturation.
*

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Were it me, I'd find a boat to anchor with the shore supply, get a half-decent switch mode PSU or two:

http://www.ebay.co.uk/itm/130568043688

that will give you plenty of charge capability, and work as a shore supply when you have 240V available.

Wire the internal sockets to the incoming 240V via an RCD and MCB, and run the Fridge and Aircon from an auto-transformer.

Job done.

Peter

PS: I gave the info on the Ferro-Resonant Transformer as it is an interesting device, although in Europe it isn't popular due to size and weight, and the noise.


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## raynipper (Aug 4, 2008)

Well that told him Peter.
Mine ended up trial and error till I got it right.

Ray.


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## listerdiesel (Aug 3, 2012)

We've had a few come in for repair over the years, they have very tinny cases, all self tappers and generally about as cheaply made as possible.

We had a large access platform company come to us that was using the Christie units, and wanted to source them in the UK. Their buying in price was about half of our cost price. We didn't pursue it and they had all sorts of problems, mainly the units disintegrating on rough sites.

Peter


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## simonmd (Dec 20, 2013)

Peter, you are a mine of info, thanks very much! 8)


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## simonmd (Dec 20, 2013)

If I may ask for your advice again, will this be man enough for the AC and fridge?

http://www.ebay.co.uk/itm/VOLTAGE-C...al_Components_Supplies_ET&hash=item417a0162fb

I know the fridge element is going to be around 100w but no clue what the AC draws. Best I can find online so far is that an average RV AC can be run on a 2000w generator so I'm thinking 3kw should be ok?


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## raynipper (Aug 4, 2008)

I would assume it would be OK personally but I'm sure Peter will know the product.

Peter, as I said I have several heavy duty transformers from 1kva to 5.5 kva if your ever likely to pass my way and make a bid?
I can list their exact details later if your interested.

Ray.


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## listerdiesel (Aug 3, 2012)

Doesn't look like 3kVA to me, too tinny!

I think we have a 'proper 240/110V isolation tranny at the facctory with a broken cheek on the bobbin, I'll see what rating that is.

We rate all of our stuff continuously at 30 deg C, so they might be larger than most, but we never have failures!

Ramon: thanks for the offer, nearly all of our stuff is custom made, and we wind our own up to about 3kVA, after that we use Carroll & Meynell in Stockton on Tees for the big stuff up to 100kVA.

Peter


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## simonmd (Dec 20, 2013)

Well if anyone has a suitable one for sale, let me know.


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## listerdiesel (Aug 3, 2012)

simonmd said:


> Well if anyone has a suitable one for sale, let me know.


I found it after a search, it is an isolation transformer, 0--235/240/245/250/255/260 input, output 110V at 35A.

It's roughly 8" X 8" footprint and 13" high, terminals on top, threaded studs for output and terminal block for input.

I'll see if the missing cheek can be replaced, but you can have that for £50, you'd need to collect though, it's 35kgs!




























It is vacuum impregnated and has a 30minute overload factor of 150% of full rating. There is an earth screen between primary and secondary windings, it is a properly rated industrial transformer.

Peter


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## raynipper (Aug 4, 2008)

Looks just like the inside of my 5.5KVA one.
Mine is a WYSEPOWER with four outlets in a metal case.

I also have a HEAYBERD - LONDON Auto Transformer Type ZE 3000 in metal case.
A LEEWAY type PT 2/2 2KVA in metal case.
A SMART 125 watt one for the chiristmas lights.

But would sell all and any right now.

Ray.


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## simonmd (Dec 20, 2013)

Looks like a beast, only problem is im in Spain so collection isn't an option.


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## listerdiesel (Aug 3, 2012)

With the numbers of m/h forum people going out every week, I'd have thought that wouldn't pose too many problems, or talk to one of the local transport companies who do the fresh fruit run to the UK.

We get one tonne and heavier engines moved around the country and Europe, so a 35kg transformer is relatively small beer.

Peter


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## simonmd (Dec 20, 2013)

I'll bear it in mind. I'm still working on the cosmetics and waterproofing, the electrics will be a job for the new year so i'll be in touch as and when I need one.


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