Understanding Isolation XMFR 4

[Senselessticker]

Does anyone know of a good document explaining how an isolation transformer *works*? That is to say: I'm wanting to know the fundemental distinct differences between a normal transformer and an isolation transformer. I'm having trouble finding such info by searching online.

Many Thanks,

Sense

 

[dpc]

The main difference is the name and the price that is charged. It's just a two-winding transformer. Sometimes they are provided with an extra electrostatic shield between the windings to limit the common-mode coupling between the windings. In addition, these transformers are often provided with high K-factor ratings.

Do a Google search on "shielded isolation transformers" and you'll get a bunch of hits.

Here's a starter:

http://www.blackmagic.com/ses/bruceg/EMC/isolatrans.html

 

[CJCPE]

I would say that any transformer that is not an autotransformer is an isolation transformer. However the term "isolation" isn't usually attached to a transformer unless it has a 1:1 ratio or has some special features. The term is more descriptive of the reason for using the transformer than it is a name for a particular type of design. In addition to K-factor and electrostatic shield, a transformer might be specified as a "drive isolation transformer." Drive isolation transformers were developed for use with DC drives. In addition to thier harmonic current design, they were design to match up with the KVA and short term overload ratings needed for drives.

 

[jraef]

I agree. Technically, any transformer that is not an autotransformer is an isolation transformer, but the context in which you are seeing it used is important for understanding those other issues. Describe the context of your question and we can hopefully enlighten you some more.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

It depends on your application.
An application for an isolation transformer may be for code compliance. One example would be Life Safety such as hospital circuits.
Compliance with CSA preliminary Standard Z32.1-M1986, Clause 5.4 Which mandates an electrostatic shield when the primary voltage of an isolating transformer exceedes 300 Volts.

Specifications or codes may call for ungrounded circuits over combustible fibres to limit fire hazard. This application may possibly be satisfied with an off-the-shelf dry type transformer. Possibly a 240/480 primary 120/240 secondary.
Drives may require isolation transformers with special characteristics.
We're all saying the same thiong in different words, senslessticker. And to repeat again let us know your application.
yours

 

[mc5w]

Some isolation transformers such as those for 12-volt underwater swimming pool lights have barriered junction boxes for safety purposes. Most new underwater lights that run directly on 120 volts nowdays are fiber optic so that the light bulb is in a dry location and the 12-volt underwater lights are light emitting diode.

If somebody wants you to install a 120 volt underwater light that has the bulb underwater run away from the job - what the CUSStomer has is a piece of dinosaur dung that they bought at a closeout sale. There are a lot of business people who cannot resist and alledged bargain not to mention that 1/2 of real estate snots are moronic enough to put up drywall before calling an electrician. I just simply refuse to do business with cheapskates and morons.

 

[Senselessticker]

Many thanks for all the responses folks.

The current application is this:

Quoting from the electrical requirements for a specific room of a new building: "3 isolated 120V circuits for sensitive instrumentation". The programming manual for this new building refers many times to "isolated 120 V circuits for instrumentation".

In the past I've seen seperate "instrument ground" grids installed throughout a facility, and also "clean power" panels that are fed from 480/208 Wye "isolation" transformers. I suppose the real issue is the context by which someone uses the term "isolation". I understand the grounding differences. But it appears the term "isolation" can have many different electrical interpretations. Is this correct?

 

[Senselessticker]

Please correct me if I'm wrong: It appears the purpose of a "drive isolation transformer" is to mitigate the noise/harmonic affects that VFDs put on a system (to comply with IEEE 519 perhaps) In my case...I'm concerning about the quality of power downstream of the transformer.

 

[waross]

Yes absolutely. Isolation depends on the application and the specs.
I would be rechecking the specs for any special requirements.
As I understand your post, the requirement could be filled with 3 "Off the shelf" 1500VA, dry type lighting transformers.
Depending again on the spec. you may be able to use an isolated panel, in whitch the circuits are isolated from the main power system, but not from each other. Then it would be one transformer for the whole panel.
yours

 

[LionelHutz]

Thats seems somewhat up to interpretation. But, I would read that as saying that you'll need to install a control transformer to provide the power for these circuits. I don't believe it's referring to the 3 circuits being isolated from each other and requiring a tranformer for each circuit but that's possible too.

However, it could also just mean the 3 circuits are to be dedicated to the instrumentation equipment only.

Maybe you can get whoever wrote the specification to clarify the requirement.