Low Voltage And Shore Power Transformers

[Brian Degulis]

Hello All

We spent some of the weekend at a marina with our 61'MY. I pluged into 2 50Amp recepticles they were marked 208V that's pretty common around here since alot of the marinas are wired to 3 phase 208V. So you get 2 legs of the three phase and between those legs you get a nominal 208V and from either leg to ground you get a nominal 120V. My actual readings on the meters where 190V (under load) between legs and 95V from either leg to neutral.

I'm thinking that the low 95V reading is because I'm going thru shore power transformers which I think simply split the total available voltage. In other words there is no conection between the docks neutral and the neutral on the boat. Is my thinking correct?

If the above is true what can be done about it? The low 95V was not suficient to start my refridgerator/freezer and was also causing problems with some other stuff. I know I'm going to run into this alot around here on past boats it wasn't much of a problem because I didn't have the shore power transformers. So using 208V for the 240V loads was not so bad and because I was conected to the docks neutral I still got 120V from either leg to neutral.

At my home dock I have single phase 240V 120V and everthing works perfectly with the transformers.

I don't realy know much about these shore power transformers I'm assuming it's an isolation transformer making the boats power independant from the grid or dock it's pluged into? When I got the boat the neutrals at the shore power recepticles on the boat had been cut and taped off. I discovered this while replacing one recepticle it didn't seem right to me so I reconected them I'm not seeing how that would have anything to do with this problem.

I know some of you guys are very well versed in this stuff so maybe you can let me know just how these things work and what I can do to solve this problem.

Thanks in advance

Brian

 

[randy]

Brian, by connecting the neutral wire to the receptacle and to shore power when you pluged it in you have defeated the isolation. Undo what you did.

Randy

 

[SKYCHENEY]

The isolation transformer should boost the output voltage back up to 120/240v. If this is not happening, then something is wired wrong. Does it do this with both transformers or just one?

 

[yachtsmanbill]

Is this uncommon? I would guess that it changes the taps on the transformer. I was also told Hatts used to use these for service in foreign ports of call. As in 50hz in Europe. I havent touched it as everything appears to be spot on as far as volts and amps are concerned. ws

 

[SKYCHENEY]

I like that Bill. I've never seen one before, but that's a nice thing to have. I have noticed a notation on my wiring diagrams of which taps to change for 50hz operation (not that I'll ever need it).

 

[luckydave215]

I wish I had an easily adjustable isolation transformer like that. I too have a low voltage problem in my marina (208, which usually sags down to 203 under load) instead of 220 and the only solution was to intall a "buck/boost" transformer between the dock panel breaker and the shore power cord.
They're common sights on my docks.

 

[Brian Degulis]

"The isolation transformer should boost the output voltage back up to 120/240v. If this is not happening, then something is wired wrong. Does it do this with both transformers or just one?"

Both are exactly the same to the best of my knowledge and isolation transformer does not boost voltage but only induces the input current from one set of coils to another. I don't think anything is wired wrong it all apears to be original.

As for the neutrals being re conected and defeating the purpose of the transformer I'm thinking if that where true then I wouldn't have had this problem. If I was conected to the docks neutral I would have had the nominal 120V from the hot leg to neutral on the boat.

Brian

 

[Trojan]

Some ISO trans can be changed to boost the voltage. Most are 1 to 1. The more expensive trans can be changed.

BILL

 

[Brian Degulis]

The Hatteras blue book for the boat says that the transformers can be changed to use the European 220V 2 wire system but it involves changing several taps on the transformer.

Brian

 

[randy]

Brian,

There are automatic voltage regulating transformers available and were installed in some boats, expensive to buy. Some can be set up to change the taps to either 208 or 240 incoming to 240 out with a switch or rewiring. With mine there are two shore circuits but only one is required,so shore #1 has a 240/240 and #2 a 208/240. Just have to know what the shore votage is.
I had an auto reg trans when I got the boat but it was noisy and draws 7amps for it own use so I change it. Still have it if anyone is interested.

The neutral you connected to the shore power receptacle is obviously not connected to the centre tap of the transformer. It should be disconnected for the safety of anyone working on the ac power without this uncommon knowledge.

Regards,
Randy

 

[Brian Degulis]

Hi Randy

The neutral you connected to the shore power receptacle is obviously not connected to the centre tap of the transformer. It should be disconnected for the safety of anyone working on the ac power without this uncommon knowledge.

I understand your logic but aparantly it was hooked up when the boat was built as the wires where obviously cut and the ends taped off. I pulled the cover off one transformer and your right the neutral wire does not enter the transformer enclosure. I don't know where it goes if anywhere?

I have learned about the buck boost transformer so that's an option. I also called Wards Marine Electric on the advise of Hatteras and they seem to feel that the existing transformers might be capable of being tapped to increase voltage.

I'm going to give Jefferson transformer a call with the model # and see if they can tell if or how it can be done. If it's possable then it would just involve a selector switch to connect the taps to the correct terminals for high or low dock power.

Brian

 

[randy]

Brian,
The wire should go to the panel where you select the shore power location.
Just because the neutral was taped off does not mean it was not factory work. It probably was as that is the way mine is.

Randy

 

[Brian Degulis]

OK I have solved my problem I spoke to the transformer manufacturer and learned that the transfomers installed in my boat (Jefferson Electric) where capable of being re tapped to boost voltage around 11%. I also called Hatteras and they were nice enough to send me a wiring diagram showing just how it's done. It requires re wiring the transfomer and adding a selector switch to be able to select 2 diferent pairs of tapps to power the primary coil. So I got 2 65A 2 pole selector switchs wired it up and I now have 2 settings. In the low setting the transformer output is the same as the input on the high setting the output is 11% above the input. So if dock power is 208V on the high setting the boat will have around 231V.

Thanks to some on this list who were nice enough to take the time to share there knoledge tru PMs. I also got an education on just how these transformers work as far as grounding and isolation.

A 208v dock might be usable with a 240v system without a transformer then you would have 208V on the high side so your about 13% low. But since you carry the neutral from the dock the low side will still be 120V.

With 208 on a transformer you don't carry the neutral on the boat so you will have 208V on the high side and only 104V on the low side. The tranformer is ratioed so your low side will be only 1/2 of your high side which compounds the problem.

Thanks
Brian

 

[TedZ]

You might want to check out this site. These are not that expensive.

http://www.international-electrical-supplies.com/voltage-regulator.html

4-wire 208 does not have 104 volts from any leg to ground. These are three phase supplies (you may not see all phases on any one particular power pole) and the calculation for the leg to neutral voltage is 208 divided by the square root of 3 (phases).

So 208 / 1.73 = 122 VAC which is what they are. This is precisely why you cannot "make" 110 VAC from a three phase 240 VAC supply. The "110" ends up being 138 VAC, outside of the tolerances of 110 loads whether lights or motors.

208 is very popular with power companies because of the flexibility outlined above.

Ted

 

[Brian Degulis]

4-wire 208 does not have 104 volts from any leg to ground. These are three phase supplies (you may not see all phases on any one particular power pole) and the calculation for the leg to neutral voltage is 208 divided by the square root of 3 (phases).


You misunderstood what I said. I said if you are using 2 hot legs from a 208V 3 phase source which is what a lot of marinas are providing. And you run the 208V into a transformer without using the docks neutral. Then the transformer will split the 208V into 104V to provide the low side. This is exactly what will happen on a boat with typical isolation transformers.

Brian

 

[TedZ]

I am not sure what you are saying. I assume i am just not following this thread.

Depending on how the transformer is wound and what the requirements are, the transformer may or may not divide the legs. Only balanced isos split them and in that event as i have said ad nauseum, the neutrals should continue from the dock.

Ted

 

[Pete]

This situation is really two different subjects of electrical distribution to deal with. Let me separate the two and see if it adds any clarity.

First is the shore neutral connection to the boat transformer secondary. From the dockside shore power connector on the dock, the neutral runs back to the location of the shore 50 amp breaker. It then joins the common neutral for the dock and runs back to the source of the power where it is joined to the green wire safety ground and then the two are connected to earth ground using a grounding rod. This is true if your dock has a central distribution transformer or not. Thus that neutral in the 50 amp dockside outlet ties you to every other boat on the dock, just what an isolation transformer is designed to eliminate. The neutral on your boat is created at the isolation transformer secondary center tap and is tied to the bonding system back at the DC engine room panels. Thus should you carry the dock side neutral on your boat and tie it to the boat AC neutral, you have tied your boat bonding system to every other boat that shares the same power source. Exactly what you do not want to do, and why you paid extra for an isolation transformer.

Second is the 208 versus 240 volts. Both voltages are achieved by using two phases of 120 volts. In the case of 240 volts the two phases of AC voltage are 180 degrees out of phase, which gives you the greatest difference between phases as each phase reaches a peak of 167 volts. This results in 240 volts RMS (Root Mean Square) which simply is a measure of the equivalent DC voltage. In the case of 208 RMS volts two 167 peak volts phases are also used, but in this case the peaks of the AC voltage are 120 degrees apart so the maximum difference between phases is less, thus less RMS voltage. The Hatteras wiring system takes the two hot phase wires and connects these to you isolation transformer primary. The shore power neutral is not connected and can be seen tied off on most boats. The transformer primary converts the incoming voltage to magnetic energy in the transformer core and the transformer secondary winding converts the magnetic energy back to electricity. At the point the primary winding of the isolation transformer converts the incoming 208 volts to magnetic energy all phase relationships are lost. The secondary then creates single/split phase electricity using the two end points of a single coil and uses a center tap on the coil to establish a boat neutral.

BTW, I have wired a selector switch to my isolation transformer using the formerly unused European low voltage taps and it works just fine. I get about a 9% boost on these taps allowing 208 volts to be boosted to the high 220's. You may also find a newer marina with between 215 and 220 volts. These use a different distribution transformer in a Y configuration and these voltages can be boosted to around 240 volts.

Just know what you are doing, let me repeat, just know what you are doing before you dig into the isolation transformer. Serious injury to people and gear can result from the smallest of errors. BE SAFE

Pete

 

[Brian Degulis]

BTW, I have wired a selector switch to my isolation transformer using the formerly unused European low voltage taps and it works just fine. I get about a 9% boost on these taps allowing 208 volts to be boosted to the high 220's. You may also find a newer marina with between 215 and 220 volts. These use a different distribution transformer in a Y configuration and these voltages can be boosted to around 240 volts.


That's exactly what I did both transformers now have a rotary switch for 215V or less on the dock line or 216V or more on the dock line. So it brings the 208V up to around 230V. Big diffrence works great.

Brian

 

[TedZ]

Sounds like you folks have this figured out well.

For phase relationships in transformers which are far from "lost" and this can be important see http://www.allaboutcircuits.com/vol_2/chpt_9/4.html

As far as grounding/neutral is concerned you can read this if you like.
http://www.powermanagementdesignline.com/howto/51201145

It says in part when comparing an iso to a dedicated line for power.
"An isolation transformer does not regulate the incoming line and therefore does not provide the main function provided by a dedicated line. An isolation transformer is not permitted by safety regulations to interrupt the grounding wire. Therefore the transformer cannot provide any reduction in Inter-System Ground Noise, which a dedicated line can provide."

Looking at the wiring diagrams by manufacturers if iso xformers is the best way to determine how things need to be connected for each particular xformer design. If the manufacturer doesn't know, who would?

Ground loops, hum etc. are not typically why we as boaters use iso xformers. Generally, it seems that folks are worried about spikes where isos help, and power conditioning for voltage where isos do nothing more that any xformer with taps could do, some dynamically.

As far as trying to isolate neutrals/grounds from every other boat's n/g's obviously the majority of folks without isos are not isolated. That is what shore side GFI's are for.

If you like to float your isos with respect to the rest of the world, i.e. grounds and neutrals not connected to anything except your boat, wire it that way. If your transformer secondary or anything on the boat faults to ground, as there is no fusing on the secondary the ground/neutral will be hot. Maybe every GFI on the boat will trip but the g/n is still hot!!

That simply doesn't work for me. If it is an acceptable risk (and it probably is on well maintained electrical installations) for you, do it!

Ted

 

[krush]

I think there may be a misunderstanding of the fundamentals of electricity and electricity distribution in this thread. Props to pete for trying to explain it


I never knew marinas were lazy enough to run 208 to the docks, I figured they would split their 440/watever phases and use independent center tap transformers to get two 120v lines like in your house.


Of course, if everything could be 3phase our electric motors would be much cheaper, simpler, and more efficient.