Isolation Transformer

Some isolation transformers have Faraday shielding, which keeps out stray magnetic fields, otherwise, they're mainly same as 1:1 transformers. Some constant-voltage isolation transformers have pretty much saturated cores but not necessarily good sine wave output voltages, acting rather as a flux limiting device.

A transformer by its nature has isolation - there's no ohmic connection between the primary and secondary. So, I'm not sure what you mean. If by "normal transformer", you mean one where the primary and secondary share a common ground, then the advantage of not doing this is "float" the secondary relative to the primary. This allows you to create a voltage that's not referenced to the primary ground. It's a good way to avoid ground loops. It also allows you interface systems that don't share a common ground.

The main reason I've used one is to avoid ground loops. After I had a power amp actually burst into flames due to a sneaky ground that I had missed, I started using them. They were particularly useful in the days of the cheap scopes and generators that we had to put up with. Nowadays, I rarely use one unless I'm worried about picking up noise and then I get one with a shield between the windings.

Tom

I am working in a VRLA mgf company. Our chargers rating is from 12V to 360V and currents from 10A to 10000A. To prevent workers from getting electric shock, all battery chargers are provided with isolation step-down transformers.

Isolation transformers are used to power a single elctrical device with a single operating voltage. They isolate the device from the line voltage as stipulated in regulations for protective isolation (eg VDE 0100). The secondary winding is connected neither to the core nor to ground. Primary and secondary are isolated from one another in such a way that even in the case of a wire break, no electrical contact between them is possible.

The highest permitted rated secondary load voltages are:

i. 250v for single-phase transformers

ii. 380v for three-phase transformers

The highest permitted rated secondary current is 16A. I do not think there is any one that can deliver as much as 10kA as claimed in kvsubramanyam's comment.

There are: I am using hundreds of battery chargers. Some are with 360V35A max. and some are 150V300A max. Submarine batteries are up to 220V and currents up tp 20000Amps. In that case many transformer are paralleled and rectification. Normal Arc welding transformers secondaries are 80 or 120 AC and current from 100A to 600A. Here also main purpose is Isolation from mains power supply which if comes in contact gives electric shock. 90% of all isolation transformers belong to this category. Rest to provide isolation of signals, etc. Some battery charger manufacturers are supplying charger that use step up isolation transformer up to 660V up to 300Amps. Visit "Bitrode - USA" or "Firing Circuits - USA" web site to gain more knowledge.

Sorry, I think there is a mix-up here. Isolation transformers are neither step-up nor step-down, they are 1:1 transformers, and I repeat the highest permitted rated secondary current is 16A. From all indications, the examples you have given are distribution transformers. They might have been used probably because the rated voltages of the equipment they feed do not match the operating voltage in your country. It could also be as a result of the high current level or the inrush of the equipment so as not to affect other loads in the entire electrical network.

I will visit the websites you mentioned and will get back to you on my findings.

Thanks for your comments. Please also visit the following.

www.powerdesigners.com and the link: www.powerdesigners.com/pdf/Transformer%20Isolation.pdf

good luck.

I visited the sites you recommended but didn't discover much. They seem to deal more on power electronics and where isolating transformer is mentioned they appear to be off-topic. In summary, they can not be considered authority on the subject of dicussion. All the same I was able to learn a few things.

I would like to throw more light on this subject, perhaps it would help. Isolating transformer belongs to the class of transformers called "Insulation Transformers".They are short-circuit proof or semi-short-circuit proof. Their windings are insulated from one another insuch a way that there is no possibility of electrical contact between the primary and secondary sides. Other types in this family include:

1. Transformers for Protective Low Voltage

2. Toy and Model transformers: These are the only transformers permitted for use in toys and models and are designed so that they can not be opened without special tools.

3. Bell Transformers: The output winding usually has several tappings (3v, 5v, 8v,12v). They are constructed for one input voltage only.

4.Torch Transformers

5. Thawing Transformers: These are used to thaw frozen pipes

6.Medical Equipment Transformers

You are right. I told more about power transformers that use "isolation" for mains power safety. More than 90% belong to this catogery (like battery chargers of cell phones/ lap tops, to VRLA, Battery eleminators, Electric Arc Welders, off-line SMPS, etc.). As some one commented they can be used with 1:1 ratio for signal isolation, to de-couple two electrical or electronic circuits electrically and so on. In India some loud speaker systems work on 110V. Here step-up isolation type audio-frequency range transformers are used (up to few watts only). I have got one Isolation Transformer designed (of 16 KVA single phase 1:1) to isolate one "Electronic Data Processing Office" that used many computers. The computers were getting currupted (in the year 1994) when used on mains along with UPS (standby power) support. That time it was a blind decision. I never used modern power analysers to study the problem. Only I have done some thing that solved the problem. The computers were working on MS-DOS ver 2.0

I agree with you. I personally feel that you got it more right than anyone else....you have also cleared up my thoughts on the subject too, thanks....

Insofar that almost any transformer (other than auto of course) should isolate, they are not called "Isolation" transformers as their primary job is usually to step an AC voltage up or down....

Isolation transformers perform a slightly different job in that they "isolate" the user from the mains supply, generally to make sure that an inadvertent contact to that isolated voltage will not cause a shock....

It could also be for other reasons of isolations of potential one from another or signal isolation etc...many uses....

This is quite a difficult usage to describe fully & correctly.....I have it in my mind, but putting it down on paper (the screen) is not quite so simple...

Isolation transformers are mandatory in the UK for electric shaver outlets in bathrooms, and all the shaver outlets available are of the isolation transformer type. The principle is that if the person doing the shaving comes into contact inadvertently with one of the two conductors feeding the shaver, possibly through the moisture that is always high in a bathroom, then the person does not receive a fatal electric shock. The outlet conductors are isolated from electrical earth.

A shaver outlet is the only type of outlet permissible in a bathroom in the UK.

In other countries, like Switzerland for example, this criterion does not apply.

Isolation transformers act as a barrier to any connection with the AC line or mains voltage. As you know we have this foolish procedure of grounding one side of the AC line. That means anything on the line can be live to ground. With an isolation transformer this stops.

You can get some capacitive coupling, but it is not dangerous

The main advantage for me would be to survive ;-).

Does anyone know others ?
Regards Uwe

No

I think that generally speaking when someone says Isolation transformer they mean a 1:1 transformer that does not change the voltage in any way, it just is an extra level of safety for the end user as the voltage he is using is not in any way "related" to the mains voltage supplying the transformer. This means that even if he touches the 230 volts, there is no path back to earth and he will a) probably feel nothing at all or b) only a very slight tingle at worst!! No current will flow and he will not be electrocuted.

Step up and step down transformers should do the same as long as the insulation between primary and secondary windings is adequate.

Some transformers are Auto transformers and are potentially lethal at any voltage as there is no insulation barrier between input and output!!!

In my RN days, we used a shore supply for lighting of 110 volts, that was ungrounded, sometimes the cables got pulled off accidentally, they were only held by a big thumbscrew and washer, I (and the others in the electrical supply branches) "Greenies" would reconnect "live" with lots of intentional sparks to amaze the onlookers.....as long as you made sure that a spark did not burn you, nothing happened. It was a great party trick!!!!

Before anyone tries this, you should measure with a meter the voltage difference between the live conductor and the ground potential, no voltage = no danger!!!

But as with all things electrical, be aware that accidents can happen!!!!

Did I ever tell you how we would search for ground faults on 220 volts DC Aircraft Carriers (and similar), using two fingers on the same hand in a fuse box across fuse positions?? if you want to know just ask me as it would be off topic!!!

It is also used for minimizing the short circuit current in the secondry side.

I understand isolation transformer also aims at to block harmonic current by selecting type of primary /secondary connection . e.g delta primary etc.

The word "blocking harmonic currents" is correct only to some extent. Harmonic currents un-neccessarily heatup primary/ secondary windings.

It is exactly what the name implies: Isolation. The primary is DC isolated from the secondary. and the main purpose is to isolate the power source from the load or vise versa. and/or eliminate ground loops. Windings ratio could be any. "Normal transformer"?, what is that?. You probably mean "Auto-Transformer" where actually there's no secondary, where the primary and the "secondary" windings are shared (tapped).and are DC connected.

(My wife says that a normal transformer is one that doesn't need a shrink.)

Hope it helps.

Wangito.

"The primary is DC isolated from the secondary"

Wangito, electrically there is no difference between DC and AC (Sinus or Pulse or any shape) isolation - there may be only differences in magnitude and frequency dependancy. If you need very secure isolation then there are so called "partial discharge free" transformers which are tested up to several kV, depending on the application.

Partial discharge can be "heard" with a sensible AM receiver. You increase the primary to secondary voltage until the noise appears and lower the voltage until it dissapears again and then you have the partial discharge voltage of your tested device.
Regards Uwe

Wrong sir,

THERE IS a difference between AC and DC. That is, pure constant DC, no pulse, no ripple. Hence, NO transformation. to obtain induced voltage in a transformer, we need to have a changing magnetic field. No change, no induction. As far as DC is concerned, the transformer windings represent a pure Ohmic (DC) resistance. Not so for AC, where apart from the Ohmic resistance there is also a complex "X". As to the rest of your posting, I don't think the original poster meant to ask about transformation noise or alike. So your detailed answer is a bit off-topic. I believe the poster is interested in (although never bothered to react) step-up step-down transformers, isolated, and auto.

Wangito.

Wangito, of course its generally accepted that there are differences between AC and DC - but my theme was primary to secondary ISOLATION, not the function of transformators and not the resistance of windings depending on AC and its frequency.
Regards Uwe