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I successfully built a working RPC

taiwanluthiers

Cast Iron
Joined
Dec 6, 2016
Location
Xinjhuan District, New Taipei City
So I just moved into a shop without 3 phase power. I asked the landlord about getting it and he said it would cost about 1200 dollars to do so. But since I only have about 3 machines (mill, bandsaw, and air compressor) that needs 3 phase, it's really not top priority. The landlord would pay the cost if I pay an extra 30 dollars a month in rent.

So I bought a RPC from someone, but unfortunately the idler motor it contains is basically rusted shut (the spindle will not turn no matter what). The sucker weights a ton, and I don't really know how to fix a stuck motor, so I went to the seller and told him. He gave me a static phase converter and told me to try it out.

So I did, and it turns out that it can get a motor spinning fairly easily, as long as there are no load. So I got the bright idea of hooking all my machines up to it, and using one machine's motor as the idler motor, I started that motor and everything else started working as though they are connected to 3 phase (albeit a bit slower start than grid level 3 phase power).

The problem is the air compressor... it is 5HP but it draws quite a big load at startup. If the circuit can't put out the necessary inrush the motor wouldn't spin up at all. So what I did was buy another 5HP 3 phase motor. Start that with the static phase converter. Once that spins up, I would start the band saw (which has a 5HP motor as well). The two motor combined gave enough juice to start the compressor even with the high startup load... might not be as fast as grid level but it got the job done...

I think I would want another 5HP motor for the simple fact that I really don't want to leave a bandsaw running unattended just so the air compressor can start up...
 
Take the motor apart on the rpc you bought and replace the bearing on each end. It's an electric motor with no shaft sticking out. 4 screws or rods and the end bell pops off. You can then slide the rotor out and replace the bearings on the bench.
 
A three phase motor is easy to take apart and put together. there are no extra moving parts, no centrifugal switch etc. Just three wires that connect to the fixed windings and a ground wire that is under a screw somewhere in the connection box. Removing both endbells and the rotor with bearings do not require you to touch any wiring.
A quality motor may even have the bearing numbers, four digits, on the data plate. Often the two bearings are slightly different sizes.
Bill D

PS: Excellent English, by the way
 
I was over 50 before I bought my first bearing splitter(seperator) to remove ball bearings. Should have bought one decades ago. For home shop type use you only need one or two sizes. Like one inch and two inch (30 and 60mm)will cover most you will ever see on a machine or motor. They are also useful when using a hydraulic press.
I thought they were a gimmick until I used one.
The trick is to buy one small enough it can get into tight spaces but still strong enough to have several tons of pull. This is a case where bigger is not always better
Bill D.

not recommendations, just pictures to see what I am talking about

Amazon.com: DAYUAN 14pcs 5 ton Capacity Bearing Separator Puller Set 2" and 3" Splitters Remove Bearings Kit : Automotive

How To Use A Bearing Separator - YouTube
 
Fooks sake! Landlord arranges the service & wiring, you pay a flat thirty bucks a MONTH? That's but ONE DAY cost of my 5 packs a day of smokes.

You lose no space, no sleep, have no hassle, and not even a demand charge top-up? Dollar a day fpr mains 3-Phase?

Most 'ere would consider that heaven!

Pay the freakin' thirty bucks and get yer azz back to doing something useful! Or at least more fun!

X2 - trying to operate a 3-phase air compressor on a RPC seldom-to-never works. The RPC just doesn't like the high-demand startup draw time after time and the compressor motor doesn't like having to struggle up to speed on low supply.

FWIW, I pay $21 a month for the meter fee for a storage building which goes a month without ever having the light turned on, much less powering up the air compressor.

jack vines
 
So I just moved into a shop without 3 phase power. I asked the landlord about getting it and he said it would cost about 1200 dollars to do so. But since I only have about 3 machines (mill, bandsaw, and air compressor) that needs 3 phase, it's really not top priority. The landlord would pay the cost if I pay an extra 30 dollars a month in rent.

The local power co. will be happy to give me 400A of 3 phase for a small hook up fee. 18,000!!! I am 150' from the wire!
 
No idea who makes the motor or even what power it is. Name plate isn't there.

In fact I tried hooking it up just to see if it would work (maybe an internal brake or something??), and it hummed for half a second then tripped the breaker. Plus I can see exposed copper in places within its wiring. So I wouldn't even rule out that the winding is screwed. The motor weights like 100kg or more. It took mechanical assistance for me and 1 other guy to get it into the shop.

So part of the challenge is to get it apart and then figure out exactly what went wrong. That's assuming the winding isn't eaten through either, as it looks like this thing has been outside for probably 8 years or so.
 
in some places you have 3 phase power in the form of 2 phases of Y service but you don't know it.

you can indeed hook up a 3 phase motor from line to line and line to netural, and you can delvelop about 2/3rds nameplate hp from the motor without over heating. but you have to find the hidden star point of the motor and connect it to your netural or ground. it will self start.


in the case of the OP.. just pay the 1200 bucks and get your 3 phase. the landlord willing to amortuize the cost by only charging you an extra 30 a month is pocket change.
 
I bet this is what is going on, as when I measure the voltage between the two legs I am getting 230v, similar to what I get when I measured the leg to leg voltage back in my old shop (that has 3 phase service).

When I measure single phase I get 249-250v when I measure line to line.

My induction cooktop is also showing 230 volt too, as it displays line voltage when you shut it off.

I guess I can try...

Edit: For the voltage balance, it seems a little unbalanced, by about 30 volts. But when I start subsequent motors in the network the voltage became better and better balanced. With only the idler, it's 230 -260 -260, but with one other idler motor running it becomes 220 - 230 - 230. When all the motor in the network is running minus the air compressor, it's around 218 - 230 - 230.

Tried connecting the motor just to the 2, and it just buzzes and not start (and draws a LOT of amps too). The static phase converter still has balance caps in it, and those run caps alone will start it.
 
So I'm just going to give an update:

2 5HP motor will start my 5HP compressor under load, but it's not always reliable, as one day it will start a bit slow but another day it will trip breakers. So I decided that I'm going to put a solenoid onto the unloader in order to no load start the compressor, as the start is just much easier and more reliable than trying to start them under load. As if I start the air compressor under no load it's no different than using the RPC to start another 5HP motor not connected to anything, and once it's up to speed it will pump.

I asked the landlord about adding the 3 phase and he was really not wanting to do this. He said one of the biggest reason is that he's not sure how long I'd live there and felt if he spent the money to add 3 phase and I move out he'll be stuck with the bill, and that it wouldn't add any value to the property as he really doesn't want the property used as a factory. Then he said something about the electrical box MUST be outside, cannot be inside at all because a meter reader must be able to see it anytime (it's the guy the power company sends out to read the meter every 2 months). However the alley it's on is very narrow and he's afraid a truck can run into it causing power outage.

I figure having a solenoid on the thing is good anyways as it would impart much less stress on the electrical system and the motor. Some of the higher HP compressors sold here uses solenoid to no load start the motor before it actually pumps, presumably to lessen the stress on the motor too. I can't imagine how much amps a 10HP or higher compressor will draw if it's trying to start under load, I suspect no electrical systems can handle it. Actually I had seen a 7.5HP compressor spin but not pump before, and the reason it wasn't pumping was because the solenoid was not closing.
 
Well, I'm pretty sure without an unloaded start the thing will not start at all. I used to have a 2HP belt driven compressor, and if the unloader didn't work, then the compressor would not start at all. That was a 2HP using a single phase motor by the way. I think unloader used in my 5HP compressor is simply not adequate to start unless the supply current is much larger than its running load amp because after a few turns the pressure would have built up to about 60 psi and using any sort of converter it wouldn't start at all. I imagine higher HP compressor with larger cylinders would require unloaded start or else it would never start, and those unloaders integrated with pressure switches is simply not adequate for the job.
 








 
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