15HP RPC Build

I'm in the planning/procurement stage for my 15hp RPC build. So far the list of items that I have is:

Panel 16x20x8 Is this a good size?
Circuit Breaker 60A 2 pole
Contactor (K1)240V/24v 75 Amp 3 Pole with auxillary contacts
Potential relay (K2) Supco 9066
Terminal Block
Start Caps ~1050uF @70uF/HP
Run Caps ~225uF @15uF/HP
Control Xformer 50VA 240V/24V
Start Switch NO
Stop Switch NC
E-stop Switch
Volt/Amp Meters X3 (Do I need these)
Din Rail

How does this list look? Also, not completely sure how to wire the potential relay.

Does my schematic look correct?

RPC.pdf - Google Drive

I have no idea what that list of parts would cost, but just as an FYI they have control panels for 15hp RPC's on ebay starting around $250. You just supply the motor. I'm not so bright when it comes to electrical stuff, so I figured that's the route I'd take when I do 3 phase at home. For all I know the parts you list might only cost a hundred bucks but I just figured I'd throw it out there.

-Tim

How does K2 work in your schematic? Is it normally closed and does it open when the 3rd leg is generated?

Pete Deal said:

How does K2 work in your schematic? Is it normally closed and does it open when the 3rd leg is generated?

Click to expand...

I believe that it is normally closed... I should update the contacts to a normally closed set... Yes, open once the third leg is generated

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Usually the K2 potential relay would be NO. They have a small transformer in them to prevent them from dropping out once they are engaged. Most contactors will shutter when they receive(Undeceive) the designated proper voltage. A potential relay clicks in at its designated voltage, but it will not shutter until it gets to about half of it's designated voltage.

For example, they will engage at 200 V but won't shutter/disengage until they drop below 100 V.

This particular electronic device helps RPC's. when a machine has a hard start draw on the generated leg, It will not re-engage start capacitors. The arcing can be tremendous. A cheaper device would be a timer relay. But this involves set up and testing, and is installed on the two natural legs.

Making k2 normally closed does the rest of my drawing look correct? Do I split the run caps between the two legs or should both be 225uF?

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I'm not one to discourage honing one's skills on a good RPC project and the reward in no magic smoke emitted when you close the switch BUT, if your time is worth anything think about what Frenchy wrote. I have built a dozen different RPC's from scratch for myself and others using similar set-up's and all have worked well. However, when I finally started valuing my time I discovered buying the box from Phase Craft as an example with a warranty how could I go wrong. If you scrounge around you can probably buy everything you need for less than $150 but what's your time worth. Just food for thought!

R_Audano said:

I'm not one to discourage honing one's skills on a good RPC project and the reward in no magic smoke emitted when you close the switch BUT, if your time is worth anything think about what Frenchy wrote. I have built a dozen different RPC's from scratch for myself and others using similar set-up's and all have worked well. However, when I finally started valuing my time I discovered buying the box from Phase Craft as an example with a warranty how could I go wrong. If you scrounge around you can probably buy everything you need for less than $150 but what's your time worth. Just food for thought!

Click to expand...

Seconded. And I HAD all the parts needed, brand-new, and already paid-for.

Buying a Phase-Craft freed-up scarce time to apply to things I CANNOT as easily purchase, ready-to-use.

Now.. all I have to do is get off my ass and actually USE that "saved" time productively, but at least I no longer have a DIY RPC still on that endless RTWL.
("Round Tuit Wish List").

Your start switch and stop switch are labeled in reverse.

The emergency stop switch idea is a waste.

The 2 position overload is in the wrong place. I would use a three position overload.

There are no output terminals or wires for the target motor.

I wondered about that start circuit contactor chattering but did not know of a "potential" contactor. I built one once and solved this problem by using a start button with two sets of contacts. One to latch the start/stop circuit and the other I used to energize the start circuit contactor. This one didn't latch only pulled in as long as the start button was pressed. It worked pretty well i just had to hold the button in long enough for the motor to spin up.

Most of what you have looks pretty good. I am afraid that yon probably do not have enough microfarad in run capacitors.

Typically you should balance both sides of the natural legs as a 1/3 2/3 split to the generated leg. With a 15 hp motor don't screw around with anything less than 100 µF cans. My guess would be 400 µF of run capacitors. From the same leg the start capacitor's jump to the generated leg put 300 µF. From the other leg to the generated like put 100 µF.
Typically at idle to make the most of an RPC you want to measure your two natural hot legs to your generated leg and have a voltage being in the 250-259V.


Now there is typically three ways that work best to drop out the start capacitors.

-The cheapest way is using your finger to hold the start button for two seconds, and when you let go the start capacitor bank drops out through a contactor. This is free.
- The second cheapest way is to use a timer relay. About $5
- The third is using a potential relay. About $25.

There's quite a few helpful builds here in the archives. This is one of mine. Sorry, Photobucket drop some of my pictures. http://www.practicalmachinist.com/v...erters-and-vfd/20-hp-rpc-build-tranny-322027/

I had plans to build my own but just wasn't cost effective. I'm usually for learning by building but it wasn't worth it.

I ended up buying a 15hp panel from Crowman converters.

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Your 60 amp main breaker is may not be large enough. A 15 horse motor will draw about 75 amps full load from single phase, 45 amps from three phase. It may trip when you attempt to start the idler. If it does start, it will limit you to about a 10 hp or less load.

My 15 hp motor, and Croman box has had zero issues starting/running any load,machine i have hooked up to it..(60 amp breaker too)I picked mine up in person,since I ordered a 10 hp box, and the Motor turned out to be a 15hp instead. Super quiet setup.. It will run my 3 phase plasma cutter perfect, The free Tatung 15 hp turned out to be a great motor..

I even kill my Mill pump motor with a switch, and run the 5 hp spindle motor for extra juice if needed. Voltage are still good cutting 1.5" steel with the new three phase plasma. Yep 60 amp breaker..