CNC retrofit power wiring

I know it's a taboo subject and one doesn't know unless they ask.

Beginning a DIY machine retrofit using off the shelf components. I have 220V 1P 4 wire supply HHNG (it's in a home, not industrial).

The components which power and control the machine use both 220 and 110 1P. In order to supply power to the 110 stuff, is it fine just to pull from one leg of the 220 or install a control transformer (I figure the transformer is the correct answer but wondering if the other option is fine for personal use)? The 110 is for powering things like the PC/monitor, 24V power supply, CNC board, stepper drives, case fans (pretty much everything except the VFD). 110 might draw 10A. The 7.5HP VFD will be derated so one leg will draw more current than the other so figure I pull the 110 devices from the non-derated leg.

Everything goes through a respective breaker and then the main breaker/disconnect on the enclosure.

If I am wrong, explain to me the why so I can understand instead of berating me without an explanation.

Thanks.

If you are talking about INSIDE YOUR EQUIPMENT, then you can get 110 VAC by a connection between one side of the 220 VAC feed AND NEUTRAL. That means that you MUST bring both a NEUTRAL and a GROUND connection from the breaker box to your equipment. You can not use ground instead of neutral.

The thing to look out for is will your 220 circuit supply enough current to power both the 220 VAC stuff AND the 110 VAC stuff in your item. So if you have a 20 Amp, 220 V circuit and your 220 stuff draws 11 Amps while the 110 volt stuff draws 10 Amps, you would exceed the 20 Amps on one side of the 220 V breaker and it will trip.

This is one reason why many designs that need both Voltages will use a control transformer. 10 Amps at 110 Volts will be only 5 Amps at 220 V. So then, using my same example, the total current on both legs of the 220 circuit will be 11 Amps plus 5 Amps or a total of only 16 Amps and the breaker is just fine. The control transformer with it's 2:1 step down balances the 110 V load between the two sides of the 220 V circuit.

I am not an electrician, but I have worked with a lot of high powered electronics and as far as I know, what I said above applies to any installation, home or industrial. I have never seen any installation where a 220 V line was split OUTSIDE of a piece of equipment. Even if there is a way to do it and it works, I think it would be bad practice. I would not do it that way.

I would also borrow one more thing from many of the professional, industrial pieces of equipment that I have seen and that would be to add a 220 V, two pole breaker at the point where power enters the equipment. That provides an additional safety factor and also a handy, power disconnect point. It is money well spent when you may have to work inside that item of equipment from time to time.

I am not so sure about what you refer to as a "non-derated leg" on your VFD. It seems to me that if the VFD uses 220 V, then it would draw the same current on both legs. I suggest that you check with the manufacturer on that one. In any case, there is no way that can happen if the VFD does not have a neutral connection. That imbalance in current has to flow somewhere.

In any case, that imbalance in current certainly is not for the full rated line input current of the VFD. So it would be only a fraction, probably a small fraction of that rated current.

Thanks. I'm doing what you mention in the second post. My panel is breakered out the wazoo for each supply inside and I have an external main breaker/disconnect on the outside. Doing my best to follow the little I know from NFPA 79.

It's a 7.5hp VFD and anything that large comes 3P input (did find a couple 1P that large but the price was astronomical for some reason) but can derate it where I have to jumper two inputs lines on the VFD. So now one leg, either L1 or L2 from my panel will have to supply more current to the jumpered inputs to the VFD. My thought process was then to draw power to the 110 devices on the line that was the "non-derated" line; the line drawing less current at the VFD. There's a whole lot of imbalace going on so I'm trying to balance through usage when everything is powered. I'm doing it slowly and checking through phases; not getting everything wired at once and then crossing my fingers when I flip the disconnect.

If I had a 1P VFD, I would certainly use a control transformer but I'm having to manage the imbalance from the 3P VFD.

Get a 220 volt power supply for the stepper drives. Then a very small DC supply for other items. Just me, but I would not want stepper drives and other items to share the same power supply. Amazon is your friend here.

If your VFD is pulling power from two hot wires, it will by necessity draw the same amount of power. The VFD is derated because you aren't using the full rectification circuitry, but it will still pull the same amps from the two available legs. Essentially a VFD that pulls 20 amps of 240 three phase will be rated to ~8.1 KVA. Now pulling 20 amps of 240 single phase gives 4.8 KVA. Generally you reduce to half, rather than the mathematically perfect 1/1.7, because the higher ripple is prone to cause undervoltage issues and is harder on capacitors.

I am in the 208-240 all of the way group. Either get power supplies that run on it directly, or get a transformer to supply everything else. Neutral connections, though rare, are some of the only connections that can deep fry your cabinet if disconnected, and are generally a nuisance to run. The balancing of the split phases is also a huge bonus for electrical systems that are operating near, at, or above capacity.

Well a "split" 240V, with a neutral, drawing two 120V from it, is no more than a simple common neutral setup that is often used in house wiring.

I do not like them, as the neutral is a problem, being shared, and it has to be wired up so that nothing requires breaking the neutral, all connections to outlets etc are pigtailed. Do-able, but a nuisance and often not understood by the next person to look at it. Stoves are also often like that, needing a neutral for some things.

I particularly do not like the idea of a machine that requires a neutral coming into it. Mostly for the same sort of reasons.

The transformer makes the machine standard and not a "special snowflake" that needs extra attention and consideration. It is "plug and play" in any 240V circuit.

While I have heard of one or two CNC that did want a neutral, in general standard wiring found in a shop is all they need. Why should yours be different?

I use this setup (LLNG) with everything in my shop. 240v Welder cart with 110v grinders, hydraulic pump 10hp ebay vfd on 7.5hp 3 phase motor using the neutral for110v controls ...similar to what you are doing....works great so far but I haven't pushed it yet..... L14-30 or l14-50 is a nema plug/receptacle that is standard for this use. The above point is accurate that this setup isn't very common in industrial use...... But no reason to spend the money on a transformer. Can always add it later if you move to a industrial park.

Strostkovy said:

If your VFD is pulling power from two hot wires, it will by necessity draw the same amount of power. The VFD is derated because you aren't using the full rectification circuitry, but it will still pull the same amps from the two available legs. Essentially a VFD that pulls 20 amps of 240 three phase will be rated to ~8.1 KVA. Now pulling 20 amps of 240 single phase gives 4.8 KVA. Generally you reduce to half, rather than the mathematically perfect 1/1.7, because the higher ripple is prone to cause undervoltage issues and is harder on capacitors.

I am in the 208-240 all of the way group. Either get power supplies that run on it directly, or get a transformer to supply everything else. Neutral connections, though rare, are some of the only connections that can deep fry your cabinet if disconnected, and are generally a nuisance to run. The balancing of the split phases is also a huge bonus for electrical systems that are operating near, at, or above capacity.

Click to expand...

Thanks. I guess it was explained to be differently. I thought the jumpered leg would draw more current. If it draws the same, then I have no problem installing a control transformer.

YOU can use a neutral to split a 240 phase for 110VAC controller power but put some sort of isolation in there like a UPS or something ( cleans, regulates and maintains power). keep the control running after servos and spindle die. At least the machine will be able to pick up where it left off and did I forget to mention ISOLATE the controller?

Just got the VFD going and it does draw the same amount of current from each leg. Cool, I thought wrong.

I'll be getting a transformer now. Thanks.