Isolatioon or Auto Transformer?

I am planning on buying a transformer to run several of my machine tools. My RPC supplies 240 volts and the machines are 575. So I'm looking for a transformer to step up the voltage, one that is 240 primary, 600 secondary with the taps so I can buck or boost. I have tried to find a used transformer for some months when I find one the right voltage it is a Delta - Y step down unit. So I have decided to buy new and get on with life.

Here is my question: Are there any issues with using a transformer with a RPC that would favour a isolation transformer over a auto? I know that the isolation transformers are a more robust design but does the fact that the RPC is manufacturing the 3rd leg make a difference? I thought that on an isolation unit the coils were completely separate but the auto was different somehow.

Just wondering, Thanks.

Doug

D Rennie said:

I thought that on an isolation unit the coils were completely separate but the auto was different somehow.

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Hi Doug,

An "ordinary" transformer has two windings: primary and secondary. The primary connects to the supply, and the secondary connects to the load. This provides isolation between the two circuits. In some situations this is an important safety concern, but not in your applciation.

An "auto-transformer" has a single winding with three leads. One lead goes to each end of the winding, and the third is connected to a tap in the middle. If you envision the winding drawn vertically, the lead at the bottom is connected to both the supply and the load.

The highest voltage appears across the wires at the two ends of the winding, i.e. top and bottom. The lower voltage appears between the middle and the bottom. So if you hook the middle to the supply and the top to the load, you have a step-up function. If you reverse the connections, with the middle to the load, you have a step-down function.

Since there's only one winding, this type of transformer provides no isolation between supply and load.

Auto-transformers may be smaller and less expensive than regular transformers of the same rating because there's only one winding.

- Leigh

Are there any issues with using a transformer with a RPC that would favour a isolation transformer over a auto?

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Yes the use of an isolation transformer allows the production of star (Y) type power with an arbitrary star point which is very handy if you have a machine which expects 4 wire power with the star point at a potential near earth. European machines are the most likely to need this. It is a good idea to keep the star point near the earth potential for any machine which is run off star (Y) type power just to minimise the potential across the machine insulation.