Rotary Phase Convertr Australia

G'day,

I originally asked a question in the following thread but I was advised to open a new thread with my question. The original is here - Rotary phase converter won't start Monarch 12C

The basis of the question was that the OP couldn't run his 3 phase machine on his rotary converter. He tried a few things, including removing the motor and having it bench checked and then he replaced it and it seemed to work OK. The thread didn't come to a conclusion as to what the original problem was.

During the trouble shooting though, it was asked if he had a transformer in his RPC. Apparently, because Australia has single phase @ 240v with only a "live" and a "neutral", we need a transformer to step it up to 415v prior to the RPC. He said he doesn't have a transformer because they are too expensive. He also mentions he runs a larger 7.5 hp machine with no problem.

So I am confused as to how he is running his 7.5 hp machine on a DIY converter without a transformer.

Here is a video from an Australian RPC manufacturer. He measures the Voltage drop between two of the phases and gets 415v, then he measures between a phase and neutral and gets 240v. So if he was to measure an RPC made in the states, he would get 240v between the phases and 120v between a phase and neutral...is that correct? YouTube

So if I have my head around it... the 3 phase motors in the States run on 3 phases of 240v and in Australia we run 3 phase motors on 3 phases of 415v...yes?

Sorry for the long post and multiple questions, I have spent the last three days on the PC trying to figure this out and I seem to be going down the hole after the rabbit.

OK. The situation is that 415V and 240V are essentially the same thing... in a way.

Your 3 phase is 3 legs of 240V to neutral. The 415V is between hot legs. For single phase, the powerco just runs one leg and neutral to your panel.

With 415V 3 phase of that type, the "wye" voltage is 240V (line to neutral) and the "delta" voltage is 415V (line to line)/ Motors may be wound for either the "wye" voltage, so the actual motor coils are for 240V, or for the 415V "delta" voltage, which means they require 415V. A 415V wye motor therefore can operate on 240V delta, if reconnected in "delta".

So, you have a choice. You can either reproduce the 3 phases by adding two phases via a rotary converter, or you can use a VFD. . In many cases this could reproduce the 240V to neutral /415V line-to-line system, but it is somewhat of a strain 0n the idler motor.. Some motors may be able to be easily reconnected for 240V line to line, or "delta" connection, which gives you another option.

A VFD needs to be fed with 415V in order to get the needed voltage, so the transformer is used to boost the 240V to 415VAC to feed it. That gets you the 3 phase 415VAC from the VFD to run usual motor. The downside is that the voltage is no longer relative to neutral, and the voltage to earth may be more than 240V. Most all motors will be OK with that, but it is something to remember.

The usual US rotary converter only needs to "create" one "generated leg", as our 240V is supplied as two hot wires, and out motors are commonly set up for 240V from hot wire to hot wire (instead of the 415V you use).

If you have access to 240V motors, or can reconnect the existing motor to accept 240V delta, then you can make a rotary converter of the same sort as is used in the US, "creating" just one "generated leg" and ending up with what we in the US would call "corner grounded delta" 240V. That would also work fine.