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auto transformer selection isolated or non isolated ?

jamie1107

Plastic
Joined
Apr 8, 2008
Location
salisbury uk
hi
i have a 415v single phase welder (6.9kva) and im unsure if i should go for a isolated auto transformer over £400 or a non isolated auto transformer just £175 i have a very good earth system the spike which is chemically earthed is only a few feet from where the welder will be so my question is do i really need to spend all that extra money its a good quality murex welder and using a multi meter the earthing seems really good any comments
to be honest im not 100 percent sure of what the difference is and ho the isolation works or its function but the salesmantold me it was to do with earthing
thanks guys
 
On a welder, you should be fine with non isolated.

The difference between isolated, and non isolated transformers is the primary, and secondary are connected, or are the same winding on a non isolated transformer. So, one of the input leads is the same as an output lead, usually the neutral. Isolation transformers will comletely isolate you electrically from the earth/ground system of the building, it's a rather useless feature for a welder. So, as long as you've got it wired properly, it wouldn't be any different than wiring it to a properly grounded outlet of the proper voltage using a non isolated transformer. Anyways, I'm not sure autotransformer applies to an isolation transformer. The term autotransformer means a transformer where the primary and secondary are the same winding. It's just not possible to have an isolated autotransformer, so I don't think that term is appropriate. In order to be isolated, the primary and secondary CANNOT be connected together.


I'm assuming this is a step up transformer?? In which case an autotransformer (non isolated) makes even more sense. Think about it, the welder was DESIGNED to be grounded. Why would you need to isolate it from ground? :confused:
 
As JJ said, an auto-transformer is built from a single winding with a tap. The low end of the winding is common to the input and output circuits.

I can't see any reason why the transformer configuration would matter at the input to a welder. As long as the welder sees the desired supply voltage, and the transformer is rated greater than the current drawn by the welder, either type should work fine.

Be sure to check the voltage going into the welder under full load. You should anticipate a drop in voltage as you increase the current drawn through the transformer. Make sure the voltage exceeds the minimum input rating for the welder under load.

- Leigh
 
Out of curiosity Jamie where are you buying your transformer from? I often visit Southampton and may want to buy one some day.
Frank
 
hi
thats excelent its the cheaper option to the non isolated one
im not using anyone local airlink transformers do them and i think i will probably be using boost the phase convertor people as they are by far the cheapesc £179 for a 7.5 kw one seems very good
thanks all
 
Just because I am a semantics freak;

"Isolated autotransformer" would be an oxymoron: A transformer is an autotransformer, or it is an isolation transformer. You can buy an isolation transformer and reconnect it as a step-down autotransformer in which case you lose the isolation, but you cannot go the other way.
 
Assuming that we are talking about an oil cooled Oxford Bantam welder or similar sized machine, the primary winding of the welder provides isolation between the incoming mains and the output. If you were to run it on 415V single phase, both input lines would be 240V rms away from earth anyway. The ideal setup will be to use an autotransformer with the common terminal connected to live, other input connected to neutral and the welder connected between the output terminal and the common/live terminal. This will cause the average voltage from winding to case to be no worse than if connected to two legs of a genuiune three phase supply.

I'm lucky, I don't push my welder hard enough to need the 415V tapping, I just need a bloody great breaker on the 240V supply:D
 








 
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