# Checking out an rpc



## Tamper84 (Nov 6, 2012)

I found a used rpc for sale on my local Craigslist for 125 bucks. I don't know how big or anything yet. What should I check out to make sure it still works? Anything in particular I should look out for?

Thanks,
Chris


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## RWL (Nov 7, 2012)

Ideally you should see it connected to another 3 phase motor which it will start and run, and ideally that motor would be the largest HP that the rpc says it can start.

Bring your voltmeter.  In the absence of a motor to connect it to, start the rpc and see what the voltages are that are being produced by the three output legs of the rpc.  Two of them should be pretty close to the line current going into the rpc and one will likely be different.  IIRC that third leg will be different by about 10%.

If you can't even plug it in and run it, you'll have to get a feel for the seller and his story about why he's selling it.  Some of the things you could do is open the enclosures and look for any burned wires and sniff for anything that smells burnt.  Most of them are fairly simple devices and even if it is non-functional, it's unlikely that all of the components would have to be replaced.  

Assuming the rpc is for around 2-3 HP motors, at the price that's being asked, you almost couldn't buy the components for that price.  I realize that some have put together rpc's that they consider free or close to it because they already had acquired the components cheap in other deals, but if you're sourcing the components yourself, it's likely to cost around $100 and probably won't look as nice as a commercial unit.


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## Tamper84 (Nov 7, 2012)

Thank you for the information. I dont really have any knowledge of electronics or the like. I havent even read on how to build one, if I could build one for that cheap, it would be awesome. But finding a 3 phase motor to use around here, idk about...hmmm Im going to have to do some checking. But I do believe this one is a home brew concoction. Tell ya guys what I will post a link to it..  http://wheeling.craigslist.org/tls/3329674622.html  Tell me what you think, if you can.

Thank You,
Chris


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## PeteH (Nov 7, 2012)

A good general rule when buying electronics (or electricals, for that matter) is "if you can't see it actually running, treat it like scrap".  There are NO guarantees on CL stuff.

Also -- Seems to me you could find a new VFD in that size for not much more than the ask on that RPC.

-Pete


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## Old Iron (Nov 7, 2012)

That is a home brew but those do fine most of the time, You use the smaller motor to get the other up to speed then throw the switch.

I prefer self starting ones my self, I've built three 2 for myself and one for a friend. The first one I built all I had was my time in it and the second one I had 200.00 in and also the third they were all 10 HP.

Used 3 phase motors go for about 10.00 a hp, He doesn't say how many HP that one is or you could go by that. If it is big enough for your needs you could use it like that till you could build the electrical part of it.

Paul


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## RWL (Nov 7, 2012)

Tamper84 said:


> But I do believe this one is a home brew concoction. Tell ya guys what I will post a link to it..  http://wheeling.craigslist.org/tls/3329674622.html  Tell me what you think, if you can.
> 
> Thank You,
> Chris



Definitely home built.  There are no balancing capacitors so the voltages and currents won't be as close as they could be.  A lot of guys run them that way without problems as long as the machine's motor isn't being continuously heavily loaded.  If 3 phase motors are scarce in your area, the price may be worth it just to be able to get up and running, but given the time to shop, around here my guess is that the components would be:  3HP 3 ph motor - $30,  small single phase motor - $25  Fused switch box - $5.  Pulleys and belt - $5 to $20 depending on your resourcefulness.  Scrap lumber - no charge.  My gut is that that rpc is worth around $60 - $75 to the seller, and he may have built it for a lot less than that.

  When I built my rpc, my recollection is that I paid around $100 for the components.  Some of the more expensive components were 4 prong Hubbel plugs and sockets (abt $20 new each), and 12 Ga SJ cord (around $1.50/ft IIRC) so I could connect different machines to it.  Prior to actually building it , I had a prototype that was just the 3 Ph motor and a switch on a fusebox both screwed to some 2x4's.  I pulled a rope to spin the motor and threw the switch.  It worked, so I proceeded to add components.  The naked motor and switchbox had cost me around $15 total.  Version 2 was to add a start capacitor (about $10 new) and a switch.  Power up the switch box with the start cap in the circuit and then flip off the start cap switch when the motor was up to speed - around 1 second.  I had been a litle nervous when I was doing these experiments, but it worked just like the guys in the forums told me it would.


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## RWL (Nov 7, 2012)

Tamper84 said:


> Thank http://wheeling.craigslist.org/tls/3329674622.html  Tell me what you think, if you can.
> 
> Thank You,
> Chris



PS.  My approach to that sale would be to go with as much cash in hand as I was willing to spend (often I'll take more because if he's like the rest of us, he'll have other stuff lying around he might want to sell) and look at it.  Ask the seller his best price on it - most people build a little fluff into their prices and see what he really expects.  It will probably be in the $100 range.  Then tell him you had more like $60 in mind (I based this on what you could buy the components for).  That will be above his cost to build it and you can negotiate from there.  If he won't budge, it's up to you to decide whether to build your own or buy a variable frequency drive.  $125 is not an unreasonable price, but I think you could do a lot better.

Important points in negotiating - go see the machine first.  Don't try to negotiate over the phone before ever seeing it.  

Cash is the coin of the realm.  Don't expect to pay by check, even for expensive machines. 

If it's already priced a lot lower than it should be, just pay the money and smile.  Some will disagree since occasionally you can get an even better deal - a tool gloat, but you run the risk of insulting the seller, or if knowlegeable people learn of the price you may earn a reputation as being a little crooked or hard hearted.

Know your market and prices.  For example, I'll be selling a nice Clausing lathe with a taper attachment and other essential tooling for a reasonable price when I get around to cleaning it up a little and figuring out what to let go with it, but I won't budge below my asking price.  I'm in an area (north central PA) where they don't come up for sale often, and three different Craigs lists are within an hours drive.  Rather than inflate the price a little for negotiations and potentially be a little higher than some buyer's price range, I'll ask what I want to get for it.  My response to buyers who want to negotiate is that if it doesn't sell on one Craigslist, I'll move to another a month or so later.  If after the three lists, it doesn't sell, I'll know I've overpriced it, and reconsider.  My gut is that anyone who's been shopping will recognize they're getting a good deal and snatch it up fairly quickly.


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## Tamper84 (Nov 7, 2012)

Thanks guys for all of the info. Hmm..I may have to think of building my own then. Is there any specific info out there that I should be reading up on? I didnt realize 3 phase motors could be cheap as what you all say. I would learn something too if I build it myself. Thats a good thing! 

I would like to go with an rpc so I have one phase converter and run everything off of it. Although I dont even have any machines yet* And I mean yet lol. I just saw this pop up on Craigslist so I figured I would ask about it.

Thanks,
Chris


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## DaveD (Nov 7, 2012)

Find out what the HP is on that 3 phase motor. That will determine what you need to feed it.

For example, I have a 10 HP one hooked to a 220v 50 amp feed and it draws a instantaneous peak current of about 90 amps at startup. Then it settles down to probably 6 amps with no load on it. It's wired with #6 copper. Output is #12 feeding a 5hp 3 phase lathe.

Assuming that Craigslist one works, it's at least a 5hp motor,  and you are comfortable cleaning that wiring mess up a little, I'd offer $75 for it and probably pay  $100 for it.

At least it will get you running. Problem I see with building a 'better' one is it takes time to find the parts (motor, relays, resistors, capacitors, disconnect, etc) and build it, plus the delta cost, if what you want to do is start making chips now.

I probably have $600 into mine, everything new.


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