Can someone please explain to me the difference between a transformer and an isolation transformer.I read up on them but all I get out of it is that they are mainly for very sensitive equipment and they have a ratio of 1:1.
Joe there are multiple transformers out there including isolation transformers.
One of the most common ones you will find is the step-down transformer used in commercial and industrial applications. It is simular to most utility transformers. Commercial /industrial are gennerally served by 480/277Y VAC. A step down is used to transform this voltage down to 208/120Y for single phase loads. They still provide isolation, but also step down.
Some other types but not all inclusive, are constant voltage, ferroresonant, zig-zag, auto-transformer, current transformer, potential transformer, K-factor, signal, impedance matching, balum, filters, mixers, and others. Each has a specific service.
Sending current through the primary of a transformer creates around the coils a magnetic field. The secondary coils detect that field, and respond by sending current out the secondary conductors. The voltage in the secondary will be Ns/Np times the voltage in the primary. The current in the secondary will be Np/Ns times the current in the primary. Here, ?Ns? is the number of turns in the secondary, and ?Np? is the number of turns in the primary. The power in the secondary will be the same as the power in the primary, except for internal power losses. That is the essence of a "transformer."
If Ns and Np are equal, there will be no change in voltage or current, but the primary and secondary circuits will be isolated from each other. That is the essence of an "isolation transformer."
Just to add to what Dereck has already stated (and this is a huge, varied subject your question touches upon) but simply stated...
Transformers are a major class of coils with two or more windings electrically isolated from each other. The ratio of primary to secondary turns of these windings determines the output voltage. Transformers that have the ability to transform voltage to higher levels (step-up) will have more turns on the secondary coil. Conversely, step-down transformers will have more turns on the primary coil. If the number of turns on the primary and secondary coils are equal, i.e., 1:1, the voltage and current is transferred unaltered from the primary to secondary -- these are often called Isolation Transformers.
Isolation transformers in turn have many applications; industrial, audio, telecom, utility, medical, military, etc., etc. I happen to have one on my boat, isolating the shore power connection to the boat's on-board electrical system to help prevent galvanic corrosion. You mentioned sensitive equipment which is another major use as isolation transformers can be very effective at cleaning up dirty street power, eliminating/reducing ground loop, ground line noise, and interference.
I'll stop at this point and let your questions dictate to what extent you want more info. or technical details. I know a little, but Dereck is extremely knowledgable, the real expert here, so you are in good hands.
Edit: Just after posting my reply I see charlie steped in which is fine. So some of my verbage unintentionally duplicates what he has said. Now Joe, you have two real experts to answer any additional questions you may have.
Don: Great question - this one might top 50 posts.
My 2 cents -- answer to your question depends upon its context.
One answer might be the NEC does not permit isolation transformers to be installed in such a way to isolate the customers neutral from the utilities neutral.
Another answer, again depending upon the context of your question, might be the NEC does permit a Separately Derived System, which a isolation transformer may create, providing 250.30 provisions are met. Simple bonding of the chasis or enclosure of the SDS (e.g., the medical type bennie mentions) is acceptable.
Yes, the Article 517 application is one case of a truly isolated system. The second one is also possible, but only if the 480 source is an ungrounded source. That is not very common in this area any more. My only point is that is very difficult to install a code compliant, isolated system where there is a neutral needed for the isolated system.
Mike mentioned the isolation transformer that he uses for his boat. If that transformer was requried to be installed per the NEC [boats are not in the scope, 90.2(B)(1)], it would not be able to be installed as an isolated system.
Bob: Electrical engineering and applied technology dictate the definition of a separately derived system.
The explanation in the Handbook, and Soares Grounding Book, is putting a different spin on the definition.
The Soares illustrations are insinuating the neutral conductor is switched by the transfer switch. Anyone can see this is not true. This is an irresponsible and potentially dangerous schematic.
The Soares Book insinuates the green wire does not make an electrical connection and is not a circuit conductor, only the neutral (white) wire does this. Yet both terminate at the same place.
The Soares Book states that the only transformer configuration that is not a separately derived system is an auto-transformer. This is pure fantasy. No document available to substantiate this statement.
A grounded wye to grounded wye must have the neutral common to both. It didn't make the cut.
The schematics in the Soares Book are mostly of MGN system transformers, definitely not power sources for separately derived systems.