Adding a transformer

[Bigbadbubba01]

I bought a an older Acra cnc mill that requires 360-480 Volts to run. I have a 20hp rotary phase converter and I just picked up a 30kva 3 phase transformer that is Hv 480 lv 208. My question and I am really surprised I could not find an answer for it is. Do I wire it feeding the idler motor that is 220/440 or after as it is leaving the rotary phase converter. Also I have seen the pro and cons of wiring the transformer delta/ whey is there any preferred method in this situation? I would really appreciate any help I am going crazy not having a mill working.

 

[BT Fabrication]

make sure you have the right style of autotransformer for step up voltage and not step down, they are wired wye style and not delta which can give you problems with amperage draw.

 

[Gordon Heaton]

Well, if you step up the voltage before the converter you only need a single-phase transformer. In that case you are powering the idler and the load with the transformer. Idler power is not free and can be substantial when under load. It seems to me that boosting after the RPC with your 3ph transformer makes more sense, but I'm NOT an EE or an electrician and have only basic knowledge of such things.

 

[JST]

Well, if you step up the voltage before the converter you only need a single-phase transformer. In that case you are powering the idler and the load with the transformer. Idler power is not free and can be substantial when under load. It seems to me that boosting after the RPC with your 3ph transformer makes more sense, but I'm NOT an EE or an electrician and have only basic knowledge of such things.

There is a point to what you say. You certainly would have to supply both the load, and the idler losses if you go for a high voltage idler, which does require a bigger transformer VA.

In both cases there is a transformer in series with all the lines, so no difference there. It may be a question more of what transformers you have available. Single phase are fairly common, 3 phase may not be as common, especially in any particular voltage ratio.

But, having the transformer after the PC might have another advantage. If the transformer is tapped, you might be able to use it to directly boost the generated leg, as opposed to doing it with capacitors.

 

[SAF]

How about the nameplates of your RPC, mill and transformer?
A picture of the mill control panel’s content would also be helpful.