Auto Isloating Transformer

There are a number of questions that you put below, that suggest that you are unsure of what you are doing. Electricity kills!

415 Volts is the UK between-phases voltage, so there is an implication that you are running on a 3-phase supply.

I am unaware (and I stand to be corrected, light curent background) but I have not heard of an autotransformer being used on a 3-phase supply

(Reply cross-posted to alt.engineering.electrical for a better response?)

Sola makes them or used to, They are three ganged variacs. . . I DO NOT FOLLOW MANY OF THESE NEWS GROUPS To answere me address mail to snipped-for-privacy@aol.com

That would be two (open delta) or three single phase transformers in a single assembly. In the industry, such an assembly is referred to as 'a transformer' rather than 'transformers' if it is nameplated as a single unit intended for three phase operation.

If someone told you to get an 'isolating' transformer, make sure you get someone else to do the actual design and installation.

An autotransformer is what you need.

---------------- Three phase autotransformers are quite common. Possibly not in this size range. Many 3 phase motor starters use an autotransformer but such units would not be suitable as the ratio would be fixed and they would be disigned for intermittent use. High voltage transmission systems often use autotransformers -e.g. 380/240KV and would likely be star-star with a tertiary delta winding to take care of triplen harmonics.

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

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How is the power supplied to your work place?

If you are in the UK for instance you may well have 11000 volt feed coming into a distribution transformer to supply 433v (unloaded) on the secondary.

Such transformers commonly have tapped HV windings to accommodate variations in the supply voltage; these taps are generally in five 2.5% steps i.e. Tap position 1 = 11550/433, tap position 2 = 11275/433, tap position 3 =

11000/433, etc - check the rating plate of the transformer for the exact details.

So for example if your transformer is currently on tap position 3 and you changed it to tap position 2 you would reduce the LV voltage by 2.5%, from

420 to 409.5 in the day and from 430 to 419.25 in the evening which would seem acceptable for your needs. It would also give you an acceptable phase voltage for any single-phase loads that it may be powering, (assuming that you are in the UK).

If you can change the tap position in this way please be aware that the tap switches are generally for use OFF CIRCUIT and NOT OFF LOAD which means that you MUST ISOLATE THE HV SUPPLY BEFORE CHANGING THE TAP POSITION.

IF THERE IS ANY POSSIBILITY OF THERE BEING A BACK FEED TO THE TRANSFORMER YOU MUST ISOLATE THE LV SIDE ALSO.

I've seen several failed transformers and heard of one engineer being killed in the UK this year through failure to properly isolate the supplies to transformers before changing the tapping position, it's not worth saving a few minutes switching time for.

Hope this is of some help.

Best regards John

If I can put my 5 penath in here!!!

There is no reason why an auto transformer cant be used on a 3 phase supply.......... are you cahps aware of the difference between an auto transformer and an isolating transformer???? They are very different beasts!

Also, 430V is prob. not too high for the input to the lathe.....if it is causing trouble then it will only be with the CNC section.... which prob runs off single phase anyway...so a much smaller (and cheaper :-) transformer could be used to run this section only if that really is the problem.

Adrian