Isolation Transformer Source, Make, Model?

[smac999]

it's not really a shorted 2ndary. the shield is the incoming shore ground and the 2ndary would be tied to the outgoing transformer ground. they are not the same ground. (the whole point of using a iso transformer). so it would be like shorted to a floating isolated point.

[wsmurdoch]

To see the various approved wiring diagrams for galvanic isolation transformers take a look at http://www.moyermarineforum.com/foru...4&d=1443364239 particularly Diagrams 6, 7, 8, and 9 beginning on page 37. (Thus copy of the standards is a few years newer than the one I quoted earlier but still not the current one.)

Bill

[foggysail]

Originally Posted by CharlieJ

Xfmrs with primary and secondary windings are required to have an electrostatic shield between the windings that is connected to the safety ground on the shore side. Switch mode xfmrs do not have windings so they are exempted from this requirement.

Can you provide a URL/source showing this requirement for transformers with an isolated secondary?

[CharlieJ]

ABYC E-11
11.7.1.1.2.1A metallic shield shall be located between the primary and secondary winding and be electrically insulated from all other portions of the transformer. It shall be designed to withstand, without breakdown, a high potentialtest of 4000 volts AC, 60 Hz, for one minute, applied between the shield and all other components such as windings, core, and outside enclosure.
NOTE: Breakdown is considered to have occurred when the current which flows as a result of the application of the test voltage rapidly increases in an uncontrolled manner.
11.7.1.1.2.2A separate insulated wire lead or terminal identified as the shield connection is to be solidly connected only to the shield, and brought out for external connection and shall be equal to or greater than the aggregate circular milarea of the largest transformer phase conductor(s).
11.7.1.1.2.3The shield and its connection are to be of sufficient ampacity to provide a sustained fault current path for either the primary or secondary windings to ensure operation of the main shore power disconnect circuit breaker when subjected to a fault current level in accordance with TABLE 4B.

[foggysail]

Thanks Charlie. I read more into your first post thinking that all transformers other than auto or pulse required a shield. When I was doing circuit design, we put shields in the specs and sometimes a double shield, one over the secondary and another over the primary. These were high voltage devices and our concern was to minimize the capacitance between the winding. But I do not recall any general purpose transformers using or for that matter needing shields. That is why I asked for more info.

Thanks for your post clarifying that it is an AYBC requirement.

[mjscottinnc]

Most power transformers have separate primary and secondary windows and are by default isolated.

All that is needed is a 2:1 step down transformer (220 to 110 or 240 to 120). Most can also be wired for just isolation 1:1
https://www.ebay.com/itm/173862945827

[GrowleyMonster]

Originally Posted by mjscottinnc

Most power transformers have separate primary and secondary windows and are by default isolated.

All that is needed is a 2:1 step down transformer (220 to 110 or 240 to 120). Most can also be wired for just isolation 1:1
https://www.ebay.com/itm/173862945827

For purely practical purposes you are right. However the typical step down xfmr does not meet the ABYC standards for an isolation transformer. So if you just want to be reasonably safe, it will do, on a budget, but it would not be compliant. Compliance is a burden but can make a difference when surveyed for a buyer. The good news is the iso xfmr is not something you would be replacing very often. A one time purchase. A smart purchase, if it can be squeezed into the budget.

[foggysail]

Squeezed into one’s budget is one thing, squeezing in a meager 3 kva single phase 60Hz power transformer weighing over 50# is another. And transformers rated for 50Hz are heavier. And don’t overlook the ease of proper wiring this toy.

Yes...... isolation transformers properly sized for the application provide electrical safety of ground isolation and that is all.

[RaymondR]

Originally Posted by mjscottinnc

Most power transformers have separate primary and secondary windows and are by default isolated.

All that is needed is a 2:1 step down transformer (220 to 110 or 240 to 120). Most can also be wired for just isolation 1:1
https://www.ebay.com/itm/173862945827

Be careful about that. A lot of the cheaper ones are devices called "autotransformers" which cannot be isolated.

[sailcrazy]

We did just that, thinking it would be an EZ, quick fix for our power incompatibility problem when we first got to the Caribb. Bought the Xfmr on EBay, paid the (expensive) shipping to Puerto Rico, and installed it. The first one lasted about a week, the second about a month(???). Both were oversized for our load, so we did not overload them. We finally got wise, stopped throwing $$ away, and bought a Victron auto-sensing Isolation Xfmr. It's worked perfectly now for 4 years. Recommendation.......forget the cheap EBay route; do it right and save the hassle!

[CharlieJ]

Post #69: Further to your comment, the link led to a xfmr rated at 350 VA. Without getting too technical, that is a 350 Watt unit. A 30A/120VAC shore power cord provides 3600 Watts. So the eBay unit was dramatically undersized and would fail quickly in service.

[foggysail]

Originally Posted by sailcrazy

We did just that, thinking it would be an EZ, quick fix for our power incompatibility problem when we first got to the Caribb. Bought the Xfmr on EBay, paid the (expensive) shipping to Puerto Rico, and installed it. The first one lasted about a week, the second about a month(???). Both were oversized for our load, so we did not overload them. We finally got wise, stopped throwing $$ away, and bought a Victron auto-sensing Isolation Xfmr. It's worked perfectly now for 4 years. Recommendation.......forget the cheap EBay route; do it right and save the hassle!

Your transformer failure experience is unusual if they were operated within their rated capacity. There are tens of millions of power transformers used in multiple applications everywhere. Many are found on street poles that have been operating for well over 50 years and still going strong. Material is a small part of a transformer’s manufacturing cost.

EDIT: I just looked at Victron’s transformer line and must ask if you would provide the part number of your so called auto sensing transformer.

[GrowleyMonster]

If it is auto sensing, it is more than just a transformer. A transformer is just core and coils. A lot of switching power supplies are erroneously described as transformers. I will say though that any switching mode PS or inverter will have isolation between input and output as a matter of course.

[sailcrazy]

To foggysail and GrowlyMonster....I'm not on my boat right now, and do not have the Victron model nos readily available, But a quick check of the Victron webpage will quell your doubts. The unit IS an autosensing isolation Xfmr! It senses incoming power, and adjusts accordingly. In our case it is connected between our shorepower inlet and the AC distribution panel. If the incoming power (3-wire) is 230 v, the power my boat systems require, it passes the power thru (although it does act as an isolation device); if the incoming power is 115v, it provides isolation, and then runs it through the Xfmr so the output is 230v. And yes, foggysail, I did size my Xfmr purchases properly. As I recall, my initial buy was a 6000 watt unit. I plugged the shore power into it(115v, 30 amp=3450w), then plugged the output into the boat shorepower inlet. It lasted about a week before it just quit! I took it apart, looking for a broken connection or a burnt wire-nothing, it just quit! The second unit was a 12000w unit. As I said, it lasted less than a month. And even IF there was a 200% start-up surge load, and I had a full 30amp load on it, it should have been an easy load to carry. Maybe I'm just unlucky, maybe it was the salt water environment, or maybe is was just crappy materials and construction??? I'm glad you have had good luck, with the EBay cheapies, but I surely did not. And after buying 2 of them, and paying shipping to Puerto Rico(they are heavy!), I can NOT recommend them to anyone on a saltwater boat. Hence, my post!

[mjscottinnc]

Originally Posted by foggysail

Your transformer failure experience is unusual if they were operated within their rated capacity. There are tens of millions of power transformers used in multiple applications everywhere. Many are found on street poles that have been operating for well over 50 years and still going strong. Material is a small part of a transformer’s manufacturing cost.

EDIT: I just looked at Victron’s transformer line and must ask if you would provide the part number of your so called auto sensing transformer.

One of the mistakes people make in sizing a transformer is to use the average or listed current for a device. If you are running a motor or compressor, really anything inductive, the transformer has to be large enough to supply the startup current, some times called locked rotor current, which is typically 6 times the running current. If the device you are running says anything like 'uses 5 amps, use a 20 amps normal or slow blow fuse' that is a motor or compressor. While we are at it, and before anyone says 20 amps is only 4 times. Fuses operate bases on time at or above the rated current. A 20 amp normal fuse which is loaded at 25 amps may take 10 minutes or longer to blow, that same fuse will blow instantly at 100 amps. It's based on current and time. There are also fast acting, normal, and slow blow fuses and each acts differently.