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20Hp Rotary Phase Converter Questions

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Cruzan80

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Hey All,
I just picked up a 20HP 3ph motor that I plan on turning into a Rotary Phase converter, and eventually running the output to a 3ph panel, so I can just wire everything together without having to move anything around.

I have been watching various RPC builds on Youtube, and seeing instructions on 10-20HP versions. The biggest issues I have are coming from their lack of documentation, and/or them using stuff they had "laying around".

In an ideal world, I do want to use a time delay to activate/remove a solid relay connected to a start capacitor. If I have to hold down a button to get the motor up to speed, that is not the end of the world.

When seeing these set-up, I get the 240V single phase comes into a 3ph contactor, and this goes out to the motor (Main On/off switch). However, I have seen several people reference 120V contactors, even with these high amp ratings. What would the benefit be (if any) for 120V contactors instead of a 240V contactor?

I have also seen various ways people are tying in the start capacitor with this, especially in a fully automated fashion.
Some of them seem to be running this off the load side of the contactor, which seems odd to me. Ideas on sizing the start capacitor?

I am guessing that I size the contactor to the size of the breaker in the panel upstream (in this case, 60A). Is that correct? Do I need any extra contactors downstream, if I have a breaker between it and any other motors?

Anyone have a rough idea of the capacitor sizes I will need to get this balanced? Trying to get the biggest MFD ones I can to save room, but also know I need some smaller ones to fine tune. Thinking 4x 100MFD (2x for each line to ghost), 3x 50s (mix of line to ghost and line/line), and 2x 25, 10, 5s (tweaking). Does this sound like a good assortment?

Also any suggestions on On/Off switches to tie into this Contactor?

Right now, I am planning on drawing out all the steps/wiring before I start to order stuff, to make sure I have everything I need. Any suggestions on where to buy any of this?

I know enough about electrical to know where my limits are, and when to reach out to others. I am sure there are other parts I have forgotten to put in this, as it is after a long day at work.

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930dreamer

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Last edited:
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larry_g

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larry_g said:
http://metalwebnews.com/howto/ph-conv/ph-conv.html

read the above link to understand a bit of basic information on rpc's.

Another thread, https://www.garagejournal.com/forum/showthread.php?t=367420&highlight=hanrahan

lg
no neat sig line
Click to expand...

The link to the metalwebnews above will get you the hanrahan writeup on RPC's. I followed this plan to build a RPC many years ago and the said converter is still running in my shop. If you search Hanrahan on the garage journal you will find ~ 6-7 threads about RPC's.

lg
no neat sig line
 
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Bert_

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Lots of factory built ones have no start capacitors at all. The run capacitors are enough to start it.

If you want simple you don't even need any contactors. Last one I wired was a 60hp, fed it with a 600A fused disconnect that also served as the controller.

Which brings up a question, I don't know if a 60A breaker is even enough to start the idler. I would think 100A would be minimum even if you're only running a light load. 10-15hp + idler...
 
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Cruzan80

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Thanks, Larry. That is one of the websites I am working from.

Bert, from what I have seen, it pulls about 30A with no load based on others videos. I dont plan on overloading it with too many machines (right now I have a 12/14 cabinet saw, 7.5hp, and a 1.5hp knee mill). Planning on adding a spindle sander and maybe a air compressor down the road.

The idea on this is to build it once, and get as much set up for long-term as I can, so I dont have to keep tinkering with it. Also why I am going to watch for a cheap 3ph panel, even with only two machines right now...

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alfredeneuman

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Bert_ said:
Which brings up a question, I don't know if a 60A breaker is even enough to start the idler. I would think 100A would be minimum even if you're only running a light load. 10-15hp + idler...
Click to expand...

A true (not SPL) 10hp 1Ø motor draws between 38 and 50 amps and should be on a 90-100 amp breaker
 
Last edited:
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Cruzan80

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Ok...But this isnt a single phase. Nameplate says it pulls almost 50A at full load on the 20HP, 3ph. No plans on loading it up that high. Figured upsizing a bit was a good idea, but not double...

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Cruzan80

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I get that all the current for every 3ph machine will go thru the single phase connection to the idler. Just not sure I am going to be putting more than 7.5-10hp 3ph on at a time, which is where I dont get the math equalling 100A (I am figuring on around 40A draw). Just because the 20hp says ~50A on the tag, doesnt mean it will pull that without a load on it, if all I am asking it to do is generate a third leg, with capacitors to help balance it.

Again, I am not trying to argue, just dont see what I am missing. I have found sqrt(3) as a common factor (1.73) to change 3ph amps to single phase. So for a 100A service, it means I can run 50hp of 3ph (roughly) plus a converter.

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Bert_

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10hp 3 phase is 25A roughly. 25x1.732=43A on single phase just for that one motor. That only leaves 17A for current in the converter. And you don't want to load the breaker to 100% unless you're fine with the breaker tripping often when you start a motor.



Cruzan80 said:
Again, I am not trying to argue, just dont see what I am missing. I have found sqrt(3) as a common factor (1.73) to change 3ph amps to single phase. So for a 100A service, it means I can run 50hp of 3ph (roughly) plus a converter.

Sent from my Phone 2 using The Garage Journal mobile app
Click to expand...

50hp would draw about 200A on single phase. I've powered 50hp motors from single phase. Did one last last fall with a vfd, fed with a 300A breaker...
 
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Cruzan80

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Gotcha. So it is not the 7.5hp I have now (19A FLA 3ph) that would necessarily be an issue with 60A (19*1.73=33, giving me 27A for the idler) but trying to make sure I size everything for the largest possible load I would expect, to be future-proofed? Would I need the wire gauge to be sized from the contactor to the idler motor the same as from the "home" breaker to the contactor?

Part of the reason I was using the 60A feature is that was the highest rating I could find contactors for that were reasonably priced.

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Bert_

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If you only plan on running a 7.5 hp motor then a smaller converter would probably be easier. 10hp converter would run great on a 60A breaker.

A 20hp idler is going to have trouble tripping a 60A breaker with startup current.

If you want to keep the 20hp idler you can find used 90A contactors on ebay. Or just run it with the breaker.
 
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Cruzan80

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The largest I can realistically see myself running is a 10hp air compressor motor.

The 20hp idler has already been bought, for $50. Out here, I would be spending probably 4x the price to downsize.

Any links to 90A contactors (just so I can find naming conventions)?

I have no problem upsizing the breaker/wiring going to this. It is going to be on a subpanel in the detached shop, which already has a 100A feeder. So 90+A is my limit (need a couple amps for lights...)

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slodat

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As a reference point, I have an American Rotary 20HP RPC. The instantaneous single phase starting current (going in to the RPC) is 200 amps on the idler for the second or so it takes to get rolling. When I start my 15HP lathe, the starting current is 260 amps for 2 seconds or so. The lathe starts unloaded (not spinning the chuck). These are the single phase currents going in to the RPC. This testing was done with some temporary 6a THHN wiring supplied by a 70a breaker at the panel. Panel is a 200a service. The reason this isn't tripping breakers is the amount of time at that current is quite short. Breakers use an inverse time characteristic.

My final installation, now that I know this is all working and what currents I'm working with, will have the RPC fed by 2ga THHN and a 125a circuit breaker, going to a 3 phase panel via 6ga THHN. American Rotary's full load single phase current is 93a and 3 phase is 54a. In my case, I'm using American Rotary's suggested wire sizes.
 
R

restorick

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Mar 6, 2008
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SE Wisconsin
Cruzan80 said:
Hey All,
I just picked up a 20HP 3ph motor that I plan on turning into a Rotary Phase converter, and eventually running the output to a 3ph panel, so I can just wire everything together without having to move anything around.

I have been watching various RPC builds on Youtube, and seeing instructions on 10-20HP versions. The biggest issues I have are coming from their lack of documentation, and/or them using stuff they had "laying around".

In an ideal world, I do want to use a time delay to activate/remove a solid relay connected to a start capacitor. If I have to hold down a button to get the motor up to speed, that is not the end of the world.

When seeing these set-up, I get the 240V single phase comes into a 3ph contactor, and this goes out to the motor (Main On/off switch). However, I have seen several people reference 120V contactors, even with these high amp ratings. What would the benefit be (if any) for 120V contactors instead of a 240V contactor?

I have also seen various ways people are tying in the start capacitor with this, especially in a fully automated fashion.
Some of them seem to be running this off the load side of the contactor, which seems odd to me. Ideas on sizing the start capacitor?

I am guessing that I size the contactor to the size of the breaker in the panel upstream (in this case, 60A). Is that correct? Do I need any extra contactors downstream, if I have a breaker between it and any other motors?

Anyone have a rough idea of the capacitor sizes I will need to get this balanced? Trying to get the biggest MFD ones I can to save room, but also know I need some smaller ones to fine tune. Thinking 4x 100MFD (2x for each line to ghost), 3x 50s (mix of line to ghost and line/line), and 2x 25, 10, 5s (tweaking). Does this sound like a good assortment?

Also any suggestions on On/Off switches to tie into this Contactor?

Right now, I am planning on drawing out all the steps/wiring before I start to order stuff, to make sure I have everything I need. Any suggestions on where to buy any of this?

I know enough about electrical to know where my limits are, and when to reach out to others. I am sure there are other parts I have forgotten to put in this, as it is after a long day at work.

Sent from my Phone 2 using The Garage Journal mobile app
Click to expand...

I went your route a number of years back for a smaller (5hp) RPC. Worked well, but it was very time consuming trying to get things tuned. I have a 10hp and 20hp motor that I'm considering as a base for another RPC, but I'll go the panel route instead.

Have you considered purchasing a pre-made/configured panel instead? I've dealt with these guys: https://www.northamericaphaseconver...erters/rotary-phase-converter-control-panels/

and they not only offer a great product but have outstanding tech support.

Good luck!
Rick
 
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restorick

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SE Wisconsin
Cruzan80 said:
The largest I can realistically see myself running is a 10hp air compressor motor.

The 20hp idler has already been bought, for $50. Out here, I would be spending probably 4x the price to downsize.

Any links to 90A contactors (just so I can find naming conventions)?

I have no problem upsizing the breaker/wiring going to this. It is going to be on a subpanel in the detached shop, which already has a 100A feeder. So 90+A is my limit (need a couple amps for lights...)

Sent from my Phone 2 using The Garage Journal mobile app
Click to expand...

Compressor motors are another beast altogether. Inrush current is significant. You will want to consider upsizing your panel to compensate for that current.
 
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Cruzan80

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Yeah, the issue is the price I am paying for pre-built. Just dont have that in the budget.

The compressor would rarely start under any load, so shouldnt be that bad. I dont have one this size yet, but may if the right deal comes along. Otherwise, just a 5hp 3ph compressor.

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Bert_

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I've wired several commercially built phase converters. I haven't used a contactor for most.

You could run yours straight off a 100a breaker or disconnect.

The wiring of most commercial phase converters is simple. Literally a motor with some run capacitors from the generated leg to one utility leg. If I was building a converter I would do the same.
 

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Norcal

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Cruzan80 said:
The largest I can realistically see myself running is a 10hp air compressor motor.

The 20hp idler has already been bought, for $50. Out here, I would be spending probably 4x the price to downsize.

Any links to 90A contactors (just so I can find naming conventions)?

I have no problem upsizing the breaker/wiring going to this. It is going to be on a subpanel in the detached shop, which already has a 100A feeder. So 90+A is my limit (need a couple amps for lights...)

Sent from my Phone 2 using The Garage Journal mobile app
Click to expand...


A 90A contactor is a NEMA size 3.
 
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Bert_

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alfredeneuman said:
Doesn't the idler motor require overload protection that a contactor lacks?
Click to expand...

I used to think so, non of the ones I have installed calls for it. I installed a set of 42kva converters in parallel last summer. For that set up the install instructions referenced definite purpose contactors but no overload protection.

From my understanding the the capacitor current must all flow through the idler when it's unloaded. This is the worst case. As you add load the capacitor current flows through the load and the idler actually sees less.

That would mean there is no danger to the idler from overloading. If you overload it the voltage on the generated leg drops and you trip overloads on your other motors.
 
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