120v Single Phase from RPC Generated 240 Three Phase

[MyLilMule]

I'm still a bit new to all of this, and I searched for this question, but couldn't find if anyone specifically asked what I would like to know.

I'm adding some three phase equipment to the shop. A couple of these machines will be in the middle of the shop floor. My intention is to build or buy a rotary phase converter and then use an extension chord from the RPC to power either one of these machines. One of the machines has a 120V power feed. Plus, it would be nice to have task lighting.

What does it take to step down the 240v three phase to 120v single phase for the power feed and lighting? Should I run yet another extension chord? Can I use a step down transformer, and if so, an example of something on which I could research more?

I saw James Clough add a step down transformer for his mill, but the $175 price tag for the transformer seems a little excessive when there must be other options. I know machine manufacturers do this all the time for control circuitry. I'm looking for a less expensive but an equally safe and adequate solution.

Disclaimer: I'm not a complete ignoramus as it relates to electricity, I know it can be dangerous and it can kill me. I am just looking to be pointed in the right direction for this specific application.

 

[Aukai]

You did not mention one of the cardinal rules of electricity, don't let the smoke out of the wire

 

[JimDawson]

Run another extension cord from a 120V outlet to run the powerfeed and lighting. While it is possible to grab the single phase from the RPC, it is not very practical for for what you want to do.

On my mill I have a 120V 4 plex outlet screwed to the back of the column for lighting and accessories, the outlet just plugs into the wall next to the machine.

 

[mksj]

An RPC just generates the third (wild) leg, L1 and L2 single phase is passed through, so all you would need is a neutral wire and either L1 or L2 to generate 120VAC. You would want a single pole breaker for the 120VAC, typically 15A or a dual pole if you wanted to split two 15A circuits which is what I do for my mill. You cannot use the RPC generated 3rd leg with neutral, nor should it be connected to any control circuits or transformers.

Many machines will use a step down transformer at the machine as an alternative and it also can provide some electrical noise isolation, it would be a 240VAC to 120VAC single phase transformer connected to L1 and L2 if run off of an RPC. True 3 phase the transformer would be connected to any of the two phases. Step down transformers in machines are typically just a few amps to drive electrical and lights, but not sufficient to run drives.

 

[Barncat]

If you build one you could use a plug with five conductors for l1,l2,l3, neutral and ground. Then your cord from the rpc has a neutral already built in to use for 120v. You could power the contractor coil and start stop circuit with 120v in the rpc also that way since you would need to run the neutral from the panel to the rpc. It would probably be cheaper though to run an extension cord from a wall outlet then to have a long heavy gauge 5 conductor cord and associated plugs. I doubt you can buy a pre made rpc that comes set up that way.

 

[MyLilMule]

Thanks for the input. I know how an RPC works. I was looking for an ALTERNATIVE to an extra extension chord, such as a step down transformer. I am willing to do the research, just needed a tiny little bit of guidance on what direction to look.

 

[Barncat]

Probably look for a 3 phase control transformer that you can add a center tap and create a neutral. Or three separate single phase transformer that are wired together as a three phase unit. I think that would work, although have never tried.

 

[JimDawson]

Thanks for the input. I know how an RPC works. I was looking for an ALTERNATIVE to an extra extension chord, such as a step down transformer. I am willing to do the research, just needed a tiny little bit of guidance on what direction to look.

In that case, just pull a neutral in with the RPC wiring, and you have instant 120V as long as you are connected neutral/L1, or neutral/L2, and not connected to the manufactured leg (L3). No transformer required, but a 5 wire cable and plug would be required.

 

[MyLilMule]

In that case, just pull a neutral in with the RPC wiring, and you have instant 120V as long as you are connected neutral/L1, or neutral/L2, and not connected to the manufactured leg (L3). No transformer required, but a 5 wire cable and plug would be required.

I had considered that, but where the RPC will go (as far as current plans) is a standard NEMA 6-50 outlet. No neutral.

But, I might just run a new circuit. If I do that, I'll run the neutral.

 

[Bi11Hudson]

There are several solutions to your question. Some are "drop in" simple, others not so much.

Scenario 1: If the motor that is the rotary converter is not isolated in any way, as advised above, two of the lines are just fed through. A neutral from those two can be carried through and at the load end will provide two 120 volt lines from line(n) to neutral. This should be done at a distrubution box like a breaker panel. By no means allow the neutral to be associated with the generated (wild) leg. If used with a breaker box, use a 'big box' hardware store box and feed the third phase through with no connection. Line 1 Black, Line 2 Red, Line 3 (wild leg) Blue

Scenario 2: Back in the day, building a "homebrew" RPC would use a 'delta' wound motor. It was more efficient, plus. . . Today, even some commercial RPCs use a 'wye' or 'star' wound motor. It is not as efficient but a heluva lot cheaper. For a star wound motor, sometimes a '9 wire' dual voltage motor is used. (240/480) In this case, the center tap of the motor (leads 4, 5, 6) can be used as a neurtral from where they are tied together. In this case, the output is like an 'old timey' 3 phase where the three lines are 240, and any line to neutral is 58%, ~135 volts. A little hot, but doable. A "Buck-Boost" transformer is often used here to get 120 volts.

Scenario 3: A commercial "control" transformer. Essentially an open transformer, the biggest concern would be the KVA rating. Kilo Volt Amps. . . At 120 volts, a 0.5 KVA would supply almost 5 amps. A metal cased "sand cast" transformer would have a similar rating, just cost a bit more. Most such transformers will have connections for 240-480 high and 120-240 low. As a rule, such a transformer will be lower cost than one dedicated to a specific range. I have several, from 0.25 to 1 KVA. None is in continuous service, just experimental stuff. . .

There are other solutions, such as 240 volt lamps. But most involve "non-standard" parts to accomplish. The most direct, requiring a length of white wire and a distribution box, is the concept of Scenario 1 above. The others, and some not listed, require a little more knowledge of electricity, beyond that of residential wiring.

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