Wiring up a 30HP rotary phase converter

[General Zod]

I might not even be able to start this thing at all. Just found out my meter box has 1/0 aluminum wire coming into it. That can't be good for ampacity. That's what 100A service? 125A tops? I don't wanna blow the transformer and lots of angry neighbors.

 

[slodat]

I just installed a 20hp American Rotary phase converter. American Rotary specifies supplying it with 2ga with a 93A FLA (full load amps) on the single phase supply and 54A FLA on the 3 phase output. The RPC is fed from a 125a breaker from my panel.

I put an ammeter on the single phase supply of the RPC. The starting current of the RPC is about 230a single phase for <2 seconds. When I start my 15hp lathe on the running rpc the sinlge phase current is about 260a for about 2 seconds. Running current is about 22a with the lathe spindle not running. I haven't hit more than 30a running the lathe.

A decent rule of thumb is starting current on an induction motor is about 7 x running current. To convert 3phase current to what the single phase side sees, multiply by √3. Crazy as it seems, the math works on in my case almost exactly. As it should. These starting surges are quite short in duration in the big scheme of things. I have a 200a service and I'm not running any other big loads when I start the RPC or the lathe. I temporarily wired this up prior to install and fed the RPC from a 70a circuit with 6ga THHN and didn't have any issues. My final installation was in accordance with American Rotary's instructions, which is what the code book requires.

If you trip your sub panel breaker starting the 30HP RPC, you could use a smaller RPC to get three phase going and then start your bigger RPC. I read your thread on another forum on this. I just have to ask... why are you using an RPC to power an inverter welder? Is it because the welder can't achieve its full output with only two legs coming in it's internal 3phase rectifier?

I did install a three phase load center on the output of the RPC via a disconnect switch.

 

[General Zod]

I see, thanks for the input. So since 30HP is 1.5x the "size" of a 20HP, then hypothetically mine should consume a lot more than 230A on start-up. Yikes.

As for the welder, I don't know about "can't", I just know that it doesn't. I don't know if it is a limitation, or purposely inherent in the design. It's already pulling 50A out of the outlet when running 350A welding current.

 

[slodat]

I'm venturing a guess here.. but it's possible/probable if the manufacturer is saying the welder needs three phase power to provide it's full rated output.. the welder's input stage components are sized for 3phase supply. When you provide only two legs of input to a three phase rectifier the rectifier power components have to be sized bigger.

 

[General Zod]

Yes that is completely true that it needs 3 phase for full output because It's on the electrical spec sticker on the machine. As to the reason why, who knows.

 

[slodat]

Yep. It's because the input side needs all three phases of input to create enough DC for the rated output. Just my opinion here but this isn't the application for a rotary phase converter. This is much more the application of a Phase Perfect. I know that's not what a guy wants to hear. An RPC is great for turning motors. Not great for powering a three phase rectifier.

 

[General Zod]

Well let's see how this all turns out.....

 

[Karl_T]

No, the motors are not connected. A small 3 phase motor just generates enough 3 phase to make the second one start easy

 

[General Zod]

No, the motors are not connected. A small 3 phase motor just generates enough 3 phase to make the second one start easy

Interesting. I need to do more research on how that would be actually implemented for my specific setup.

 

[slodat]

Generically speaking power electronics (rectifiers) don't love the so-called generated leg of an RPC. It works well for motors, which is where they are recommended. For the application you have, a Phase Perfect is what is typically recommended.