# Wiring up a 30HP rotary phase converter



## General Zod (May 14, 2020)

So I picked this up recently.  She's a big momma..









































I got a deal on it, otherwise I wouldn't have gotten it.  

From what I gather, 240V 1-Φ power goes into A & B, motor goes to T1-T2-T3, the load, which in this case would be a welder connected via a 14-50R/P also goes to T1-T2-T3 (grounded separately of course.

The instructions from the Southern Converter website are lacking to be honest.   Hoping they answer my email.

Here is some reference info I found, which seems to share a common theme across the board...


















I think I have that right don't I?


----------



## Janderso (May 14, 2020)

Someone that knows something will be along soon. Looks right compared to my 10hp American Rotary set up.
You are set dood. 30 hp should do the job.


----------



## reddarc103 (May 14, 2020)

The company will surely reply to you as the diagram looks quite complicated. For more welding-related solutions please visit - RED-D-ARC website


----------



## Karl_T (May 14, 2020)

I would strongly suggest you get a 3 phase load center as a sub panel off your main panel. Wire L1 and L2 to top of this load center. leave "wild" leg L3 not connected.

Put your converter on a breaker off this panel, double check but looks to need a 100 amp breaker.

Now put all your equipment on its own breaker from this panel. really simplifies the wiring.  NOTE this way leaves two legs hot to your machines all the time, if you try to start them without the converter running bad things happen. My son leaves the circuit breaker off until he uses the particular machine, not idiot proof.

Karl

here's my son's shop unit








						Ultimate 3 phase converter
					

I will soon be helping my son build what I'll call the ultimate 3 phase converter.  he has just built the ultimate shop for his life - a 30' x 40' super insulated building with in floor heat.   This will be the fifth rotary converter I've built over the last 25 years.  There is a five year old...




					www.hobby-machinist.com


----------



## General Zod (May 14, 2020)

Karl_T said:


> I would strongly suggest you get a 3 phase load center as a sub panel off your main panel. Wire L1 and L2 to top of this load center. leave "wild" leg L3 not connected.
> 
> Put your converter on a breaker off this panel, double check but looks to need a 100 amp breaker.
> 
> ...



The system will be wired like the last pic I posted, essentially.  

There will be three breakers involved:
100A or 125A, 2-pole at the sub panel to provide 240V 1-ph power as OCDP to the system as a whole.
125A 2-pole right before the rpc panel to serve only as a means of disconnect
40A 3-pole at T1-T2-T3 (not between the panel and motor, just leading to the load) to serve as OCDP to the load which is a welder.  It consumes a max I1eff of 36A on 230V 3-ph.


----------



## General Zod (Feb 18, 2021)

I'm considering adding a soft-start even though the vendor says it's not necessary until one gets to a 60HP RPC size.  What would be the simplest way to accomplish soft-start?  I cannot run a smaller "pony motor" since there is no motor shaft protruding to attach to.


----------



## Barncat (Feb 18, 2021)

Interested to see how this progresses. I am in the material acquisition phase of building a 25 hp RPC. Could you tell me the value of the run caps they used? 
Why is the contactor labeled as 25hp for a 30hp motor? Is that standard practice? 
It may be difficult, but could you drill and tap the shaft stub to add a longer shaft for a pony start? Although if your home electric is robust enough, I don’t think you need to worry about pony or soft start, as long as this isn’t turned on and off many times throughout the day.


----------



## General Zod (Feb 18, 2021)

The caps have no markings on them that I can see.  As for the contactor, I'm not sure.  Maybe someone else can elaborate on that.

As for the drilling, there is no way for me to drill the shaft stub perfectly on-center and dead nuts straight in.


----------



## Karl_T (Feb 18, 2021)

I've not seen a soft start used on an RPC.

Many years ago I tried a 25 hp RPC and starting load dimmed all the lights too much. So, I added a 5 hp RPC to start first, then bring in the big motor a few seconds later. This worked so well, all my later ones are done this way.


----------



## General Zod (Feb 18, 2021)

Karl_T said:


> I've not seen a soft start used on an RPC.
> 
> Many years ago I tried a 25 hp RPC and starting load dimmed all the lights too much. So, I added a 5 hp RPC to start first, then bring in the big motor a few seconds later. This worked so well, all my later ones are done this way.


I did see that in your thread, thanks for the feedback.  Unfortunately I cannot do this with mine.  Since it is built specifically for RPC applications, there is no shaft to attach a small pony motor to get it somewhat going.   I don't even know which direction it turns, lol.


----------



## General Zod (Feb 18, 2021)

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 (Feb 18, 2021)

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 (Feb 18, 2021)

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 (Feb 18, 2021)

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 (Feb 18, 2021)

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 (Feb 18, 2021)

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 (Feb 18, 2021)

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


----------



## Karl_T (Feb 18, 2021)

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 (Feb 18, 2021)

Karl_T said:


> 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 (Feb 18, 2021)

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.


----------



## General Zod (Feb 19, 2021)

I see what you mean.   I'm willing to try it out.   Talked to someone with a 30HP and it seems the in-rush is quite a bit lower than what the motor spec says, although his motor is different.  That gives me some confidence.


----------



## General Zod (Feb 20, 2021)

So one thing that was brought to my attention is starting caps.  I have ten 270μF caps, and it was mentioned to me that I can add as many as I want to help with the starting of the motor, thereby lessening the in-rush.  I can easily get another 10 for $80 on Ebay.  What sayeth thou?


----------



## Barncat (Feb 20, 2021)

I am no electrical engineer, but I think capacitors are essentially a energy storage device. It seems adding more would require more current to initially charge them, thus increasing inrush. But that is just my speculation.


----------



## Barncat (Feb 20, 2021)

According to the example equation here, that would appear to be the case. If I am understanding it correctly.  https://www.electricaltechnology.or...increases-capacitive-reactance-decreases.html


----------



## General Zod (Feb 20, 2021)

Barncat said:


> I am no electrical engineer, but I think capacitors are essentially a energy storage device. It seems adding more would require more current to initially charge them, thus increasing inrush. But that is just my speculation.


Yes they are storage devices, yet RPC's include them as part of the _start-assisting procedure._ In my logic, they are not charging while the motor is already attempting to start, for it would make your second sentence true and be a contradiction to call them 'starting capacitors'. They must be fully charged prior to the motor actually attempting to start, and then either release their energy simultaneously or shortly before the motor windings are energized for it to get up to speed. That's how I see it. I don't know which of those two scenarios actually applies though.


----------



## General Zod (Feb 20, 2021)

Barncat said:


> According to the example equation here, that would appear to be the case. If I am understanding it correctly.  https://www.electricaltechnology.or...increases-capacitive-reactance-decreases.html


yes that is the basic theory behind them. To charge up with energy, they must consume electricity in the first place.  The real question that neither of us know is "when".  Exactly when do they charge, and exactly when do they discharge to function in their intended purpose as motor-starting-assistance.


----------



## Barncat (Feb 20, 2021)

General Zod said:


> yes that is the basic theory behind them. To charge up with energy, they must consume electricity in the first place.  The real question that neither of us know is "when".  Exactly when do they charge, and exactly when do they discharge to function in their intended purpose as motor-starting-assistance.


I certainly do not know the answer to that.


----------



## slodat (Feb 20, 2021)

In this type of application the capacitors create a phase shift which helps with starting the idler motor. The angular displacement of the three legs is what creates a rotating magnetic field in the motor stator windings. In a three phase ac induction motor that rotating magnetic field induces a voltage in the rotor, which produces a rotor field. These two fields then produce torque on the rotor. Very simplified explanation of that part.

In a generated 3 phase system the phases are 120 degrees apart. In a rpc application L1 and L2 are 180 degrees apart because they are two ends of a center tapped single phase transformer. A capacitor has a 90 degree phase shift (to keep things simple). This phase shift is what the starting caps are doing. This is enough to get the idler motor started and up to speed.

The simplified purpose of a capacitor is to oppose a change in voltage. This is done by a charge between two plates. When discharged, a capacitor opposes a change in voltage.. ie: for a brief moment it doesn't. The amount of time it takes to charge is a function of the circuit impedance and the capacitor's capacitance. The energy used to charge the capacitor is then "stored". When voltage is removed from the circuit the capacitor discharges, opposing that reduction in voltage. In this application the phase shift is what we are going for. The phase shift is caused by the capacitive reactance of the capacitor. This only happens in an ac circuit. An inductor is similar, but it opposes a change in current.


----------



## General Zod (Feb 20, 2021)

Thanks for the explanation.  How does the amount of capacitance play a part in reducing the in-rush? If you please.


----------



## slodat (Feb 20, 2021)

General Zod said:


> Thanks for the explanation.  How does the amount of capacitance play a part in reducing the in-rush? If you please.


I haven't done the calculations for this converter of yours. I'm not saying adding additional start capacitors will do what you want.

My advice is to connect it up and see what it does on an ammeter. You'll need a clamp-on ammeter, appropriately sized conductors and breaker. This is what I did and it all went really well. And, it all jived with what the manufacturer stated.


----------



## General Zod (Feb 20, 2021)

slodat said:


> I haven't done the calculations for this converter of yours. I'm not saying adding additional start capacitors will do what you want.
> 
> My advice is to connect it up and see what it does on an ammeter. You'll need a clamp-on ammeter, appropriately sized conductors and breaker. This is what I did and it all went really well. And, it all jived with what the manufacturer stated.


I was hoping to reduce the inrush and not blow up the transformer that feeds my house in case it is not appropriately sized.


----------



## slodat (Feb 20, 2021)

If that is truly your concern, you are definitely in need of a professional to help assess if you have adequate service. This is literally why the code exists. I'm happy to help and don't want to sound rude. But if you don't know, forum advice is definitely not going to suffice should you have a really bad day with this. 

With that said, let's start here: what does the manufacturer say and recommend as far as feeder circuit size? I posted photos of this info for my American Rotary RPC. If you install in in accordance with manufacturer's instructions, you are doing the right thing.


----------



## General Zod (Feb 20, 2021)

The vendor states 80A breaker with 4awg from breaker-to-panel, and also from panel-to-RPC.


----------



## General Zod (Feb 28, 2021)

I'd like to thank everyone that contributed to this long-winded thread.

[VIDEO]




[VIDEO]





With 2/0 copper wire feeding the juice within 2-feet of the RPC panel (all the way from the house main panel), the lights didn't even dim!  My 10HP compressor has more in-rush!


----------



## Barncat (Mar 1, 2021)

Good work. Nice to see it was balanced well enough to run the welder.


----------



## General Zod (Mar 1, 2021)

Yup, the voltages between the hot legs were within 1% of each other.  Therefore I highly recommend Southern Phase Converters.  I got this one on Ebay for $1200 shipped because someone bought the wrong one, so their loss was my gain..


----------

