Isolation Transformer Source, Make, Model?

[foggysail]

Originally Posted by mjscottinnc

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.

A properly designed power transformer will have no trouble with locked rotor startup conditions. What usually happens is the input protection circuits (circuit breakers/fuses) open well before a transformer failure. Motor starters are used for large industrial motors. Those devices place internal resistors in series with the wiring and are then bypassed immediately after startup.

[foggysail]

Originally Posted by RaymondR

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

Originally Posted by 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!

The only thing I could find at Victron was autotransformers and that was the purpose of asking for their part number.

[foggysail]

Originally Posted by 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.

Do not make that assumption although I agree it would be nice if so. Most inverters, pseudo and pure sinewaves use what is know as H bridges to ‘make’ AC power.
Yeah, not a great sketch but the best I can do on my keyboard.

The tops of switches ‘a’ and ‘b’ are common, connected. The bottoms of switches ‘c’ and ‘d’ are also common. In an inverter the bottoms of ‘c’ and ‘d’ are are ground potential....if operated from 12vdc, it would be the same connection as the negative batter terminal. The tops of ‘a’ and ‘b’ are around 170vdc for a 120vac inverter output.

The way these things work is that switches ‘a’ and ‘d’ are closed together allowing current to flow from the 170vdc through the load ‘L’ for what is equal to 1/2 of an ac cycle while switches ‘b’ and ‘c’ are open. The next half cycle occurs when ‘a’ & ‘d’ open followed by ‘b’ and ‘c’ closing allowing current to pass through the load ‘L’ in the opposite direction.

Notice that each side of load ‘L’ varies between 170v and 0v!!!! THAT IS THE REASON MOST INVERTER OUTPUTS CAN NEVER BE CONNECTED TO A SYSTEM’S AC COMMON!!!!

SORRY about the quality of my attached sketch but taking a screen shot was the only way I could post it

[transmitterdan]

Originally Posted by mjscottinnc

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.

Transformers can easily supply high current for short durations unlike some inverters and generators. Transformers need not be rated for 4 times the average current. Don’t confuse different technologies. One is not like the other.

[dfelsent]

I found a surplus medical grade isolation transformer with appropriate windings for 115/230 50/60 shielded low leakage. Then I looked at the cost of the enclosure, breakers, and wiring devices. The victron unit looks good to me though I haven’t bought one yet as I rarely am near shore power.

https://www.victronenergy.com/isolat...-3600va-7000va

[sailcrazy]

foggysail....I looked back in some of my earlier notes, and found this computer address: https://shop.pkys.com/Victron-Energy...SAAEgICwPD_BwE. It is for a east-coast consultant called Peter Kennedy, and is for the Xfrm that I bought. It is a 3600watt Victron Energy Isolation Transformer with autodetect input voltage and 115/230v output. Hope this helps. As I wrote, since we installed this unit, we have had "0" issues with shore power, and works regardless of the marina shore power pole voltage.....our boat sees only 230vAC power, without issue!

[sailcrazy]

Sorry...I forgot the model no. It is: ITR 050362041. and no, while I used PK for advice and some purchases, I have no affiliation with him or his services. And from my perspective, he does seem to really know his stuff.

[foggysail]

Thanks sailcrazy for your above posts. I was poking around hoping that it was NOT an autotransformer that you purchased. I don’t know how this device makes auto adjustments but it must or it would not be sold as such.

There are what is called constant voltage transformers that were used in large amounts by the telephone company for years. They output a constant RMS voltage within a variable input voltage range. Transformers can be designed for both constant voltage outputs along with constant current outputs. A fine application for constant current transformers is found in microwave ovens.


Both of these are resonant transformers and use a capacitor. The output waveform on the constant voltage transformers are almost a square wave with a large voltage spike on the leading edge. Both are extremely sensitive to the line frequency.

[wsmurdoch]

Two methods of achieving power transformer output voltage stability compared...
https://www.edn.com/electronics-news...which-is-best-

[transmitterdan]

Originally Posted by wsmurdoch

Two methods of achieving power transformer output voltage stability compared...
https://www.edn.com/electronics-news...which-is-best-

Constant voltage transformers (a poor name IMO) are not suitable on most cruising yachts. They are extremely sensitive to frequency variations and they fail miserably when starting a motor such as an air conditioner compressor. For fixed frequency constant load usage a CVT is useable but very inefficient. There used to be a cottage industry of battery chargers based on some CVT designs. They were heavy and destroyed many batteries. Almost all boaters have consigned them to the dust bin.

Automatic tap changing voltage regulators are preferable to a CVT for a cruising boat. But nowadays most cruisers use their charger/inverter as a means to isolate sensitive equipment from power line fluctuations. Thus the KISS principle tells them to use a simple isolation transformer without any electronics to go wrong.

[mjscottinnc]

Wow, so many experts. Just makes me think that my 45 years as an electrical engineer I must have been sleeping. Ya right. Back to reality, transformers are built on an iron core, the size of the core for the most part dictates the max power it can supply. Winding space, max wire gauge, number of turns and like. Overloading a transformer, even temporarily, means that as the current rises the voltage goes down, and it can also deform the waveform. That pesky P = E*I equation. Constant voltage transformers are actually two types of transformers in one. They used to be called ferro-resonant transformers. Within limits they attempt to maintain a fixed output voltage when the input voltage varies. Not going into the physics at this point. I'll step back and let the experts take command. Goodbye and Cheers

[transmitterdan]

Originally Posted by mjscottinnc

Wow, so many experts. Just makes me think that my 45 years as an electrical engineer I must have been sleeping. Ya right. Back to reality, transformers are built on an iron core, the size of the core for the most part dictates the max power it can supply. Winding space, max wire gauge, number of turns and like. Overloading a transformer, even temporarily, means that as the current rises the voltage goes down, and it can also deform the waveform. That pesky P = E*I equation. Constant voltage transformers are actually two types of transformers in one. They used to be called ferro-resonant transformers. Within limits they attempt to maintain a fixed output voltage when the input voltage varies. Not going into the physics at this point. I'll step back and let the experts take command. Goodbye and Cheers

As you rightly point out, the self impedance of the transformer will cause the output voltage to reduce as the load current increases. But this effect should not cause waveform distortion. And if it happens only briefly (<1 second) it will not cause significant overheating.

Magnetic flux in the core is mostly unaffected by load current. The two variables that affect transformer magnetic flux are primary voltage and frequency. Flux increases with increasing primary voltage and/or decreasing frequency.

[redneckrob]

Originally Posted by mjscottinnc

Wow, so many experts. Just makes me think that my 45 years as an electrical engineer I must have been sleeping. Ya right. Back to reality, transformers are built on an iron core, the size of the core for the most part dictates the max power it can supply. Winding space, max wire gauge, number of turns and like. Overloading a transformer, even temporarily, means that as the current rises the voltage goes down, and it can also deform the waveform. That pesky P = E*I equation. Constant voltage transformers are actually two types of transformers in one. They used to be called ferro-resonant transformers. Within limits they attempt to maintain a fixed output voltage when the input voltage varies. Not going into the physics at this point. I'll step back and let the experts take command. Goodbye and Cheers

You can mitigate the core saturation to some extent by getting a dual 50/60 hz model for use on 60 hz if you're worried about distortion/harmonic issues. I do have to wonder how significant the locked rotor load issue is, given you the extra load from the motor is exponentially decreasing at the same time the voltage drop is on the transformer. We're talking max 15 amp motors on a min 30 amp circuit here for typical boat AC stuff. (Just 25 years as an EE here)

[botanybay]

Originally Posted by transmitterdan

Constant voltage transformers (a poor name IMO) are not suitable on most cruising yachts. They are extremely sensitive to frequency variations and they fail miserably when starting a motor such as an air conditioner compressor. For fixed frequency constant load usage a CVT is useable but very inefficient. There used to be a cottage industry of battery chargers based on some CVT designs. They were heavy and destroyed many batteries. Almost all boaters have consigned them to the dust bin.

Automatic tap changing voltage regulators are preferable to a CVT for a cruising boat. But nowadays most cruisers use their charger/inverter as a means to isolate sensitive equipment from power line fluctuations. Thus the KISS principle tells them to use a simple isolation transformer without any electronics to go wrong.

The victron unit has two primary windings and two secondary windings on a torroidal core.

When hooked to 220v range power the primary windings are in series, when on 110v range power the primary windings are in parallel.

The secondary windings are the same, to produce 220v power both secondary windings are in series, to make 110v power the secondary windings are in parallel.

Thus for 220v input, 220v output:
Primary's in series, secondaries in series, current limit on both primary and secondary 16A

For 110v input and 110v output:
Primarys in parallel, secondary in parallel. Current limit of 32A on both primary an DC secondary

For 220v input and 110v output
Primaries in series, secondaries in parallel. Current limit on primary 16A current limit on secondary 32A

For 110v input and 220v outpur:
Primaries in parallel and secondaries in series. Current limit on primary is 32A and current limit on secondary is 16A.

The output for this victron auto sense unit is set with short jumpers inside the unit and is either parallel secondaries or series secondaries.

The input senses if it should be in the high range (and thus puts the primaries in series) or in the low range (and thus puts the primaries in parallel)

It does not adjust beyond that. So on my dock I have 208v input (high range) and get either 208v (if output is high range) or 104v (if output is low range)

Since all of the European equipment is happy running 208v 60hz that is how I have it configured. Primary auto selects parallel or series, output is always series configuration.

Inverter programmed to draw no more than 14A so about 2900 watts available to work with (14x208)

I could run 110v neutral to hot and draw 32a getting 220v output and have 3600 watts to work with. However the current would be twice as high.

Have been using in a liveaboatd configuration for 5+ years, working well.

[RaymondR]

Dunno about the U.S. but in Australia an isolation transformer is often a requirement on building sites. Maybe search for industrial rather than marine.