240 to 480 step up transformer questions

Hi, I'm actually not a Grumpy Old Machinist, but rather a CNC router guy. You guys seem to really know your way around 3 phase power, so hopefully you'll cut me some slack.

I just moved into an older building with 240V power and I'm trying to get my machines hooked up (CNC routers,both require 480 volts). The majority of electricians haven't returned my calls, one of them didn't want to work on an older high leg system, and the one who returns my calls and actually seems to want to do work and get money for it has proposed reverse feeding a stepdown transformer and running my machines with ungrounded delta.

I told him I feared for the safety of my VFDs, and he didn't seem to know a lot about VFDs.

Here are a few questions on my mind:

1. Is there any advantage to giving each machine it's own transformer? It seems like feeding both off of one is the most economical solution.

2. Is it worth paying the extra money for a transformer with 240 delta on the low side and 480 wye on the high side? Obviously this would have to be purchased new, so cost would be about double that of a used delta/delta.

3. If I do run 480V ungrounded delta to the machines, do I need some kind of additional ground fault protection? I read what motion guru wrote about the wholesale destruction of VFDs on an ungrounded delta system, specifically about the problems with MOVs.

Thanks for any help and advice.

Take a look at these threads, they cover the situation that your contemplating.

A Y output transformer is in your best interest, it can be developed in a few different ways, some simpler than others.

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/power-system-types-supplying-vfds-dc-drives-whats-grounding-ramifications-299531/

http://www.practicalmachinist.com/vb/cnc-machining/grounding-machines-326558/?highlight=machine+grounding

SAF Ω

Yes, that first link is the thread I was referring to. However, there are conflicting opinions expressed about the survivability of the MOVs. If I already owned the transformer I'd probably just disconnect the MOVs and leave it at that, although I'm still not exactly clear on whether there are any downsides.

Running on 480Y seems like the safest. I've seen some suggestions for adding a zigzag transformer to an ungrounded delta service to provide ground fault protection, but the design part looks complicated.

Hopefully somebody can address the question of using separate transformers or running both machines off one. One machine needs 15A and the other needs 30A, So 45A total, which looks like 45KVA would be suitable. The machines both have built in isolation transformers for the computers and servo drives, but the VFDs are exposed to the incoming lines.

What about the VFD models? Do they even have the option to disconnect the MOV's?

Ungrounded system would not need MOV disconnection, but has electrocution hazards to the operator in the event of a ground fault. The NEC requires a ground fault detection system on an ungrounded system, if you need to pass a inspection.

Corner grounded would likely require the disconnection of the MOV's, its drive model dependent.

SAF Ω

As I understand it, when an ungrounded delta system has a one line ground fault, it simply becomes an ad-hoc corner grounded system, and continues to function, with the conduit or ground wire becoming a current carrying conductor. As long as the ground path is intact, and can handle the load, there is no hazard. If that's not true, of course, there's a BIG hazard.

If the transformer is putting out 480 wye power it seems like a lot of things get simpler and easier, although perhaps somebody here has a cheap, easy way that I never thought of to add ground fault protection to an ungrounded delta system? Everything I've seen so far looks pretty complicated.

As far as intentionally setting it up as a corner grounded delta system, I do feel kind of uneasy about disconnecting the MOVs. If everything is fine when they are disconnected, why are they even there in the first place?

Albert Hofmann said:

As I understand it, when an ungrounded delta system has a one line ground fault, it simply becomes an ad-hoc corner grounded system, and continues to function, with the conduit or ground wire becoming a current carrying conductor. As long as the ground path is intact, and can handle the load, there is no hazard. If that's not true, of course, there's a BIG hazard.

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We covered an ungrounded and corner grounded delta last week for a 600V open delta transformer setup. Have a look at this post and the rest of the thread for some additional info on transformer grounding perspectives. Plug in your 480V when you see 600V in the thread.

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/open-delta-transformer-wiring-334614-post2969680/#post2969680

Albert Hofmann said:

If the transformer is putting out 480 wye power it seems like a lot of things get simpler and easier, although perhaps somebody here has a cheap, easy way that I never thought of to add ground fault protection to an ungrounded delta system? Everything I've seen so far looks pretty complicated.

As far as intentionally setting it up as a corner grounded delta system, I do feel kind of uneasy about disconnecting the MOVs. If everything is fine when they are disconnected, why are they even there in the first place?

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Corner grounding may not even be an approved option for your drive, only your drive manual can tell you.

Everything is not fine when they are disconnected, your removing the protection they provide when disconnected. Meaning vulnerable to transient voltage spikes in your drive. Lack of a common reference for the drive to dissipate common mode currents and noise.

It's not to say that it won't run that way, but subjects the drive to possible harsh treatment and extra noise on the system.

SAF Ω

Does the transformer need to be bonded to building steel or have a ground rod installed?

The transformer output coil needs to be bonded to the supply circuit grounding conductor, and the building steel.

Installing a dedicated isolated ground rod is a very bad idea, and doesn't accomplish the desired effect of a low impedance fault current return path, or a path to dissipate common mode noise from the drive.

SAF Ω

I asked about bonding the transformer over at the Mike Holt forums, and one of the moderators (Don in Illinois) said that since it's a separately derived system it needs a grounding electrode:

24 to 48 step up transformer questions - Page 2

Don's got over 23,000 posts there, so I feel like I should seriously consider what he says.

Although I should note that for many years I operated a Strippit CNC punch press that was connected to a step down transformer which had no ground rod and while it had plenty of other problems the Fanuc control and servo motors worked perfectly.

Machine Grounding for VFD Equipped Machines

Don_resqcapt19, in the Other Thread gave you the correct reply. However I get the feeling you don't fully understand, what qualifies as a grounding electrode.

It appears that you view a grounding electrode as only a ground rod. A rod is one of several electrodes that CAN qualify, but is one of the POOREST CHOICES out of the ones that are approvable. If you used a separate rod at the transformer location, it still would be required to bond it back to the service grounding electrode system or the building steel, metal water service pipe. A isolated rod by itself, is NOT approved and is also a very bad choice for your equipment protection.

In one of the earlier post's I gave a link to a video, that gets into the details of why and why not, on grounding electrode systems. It's a good reference to comprehend, as to the why's and what for's, for 30 minutes of your time. It covers CNC'S specifically, at the 10 Min mark, but really should make time for the whole thing.

Here it is again..Grounding Myths

In the Other Thread Jaref and and Mike Kilroy gave you a good explanation on why not a delta transformer output for input to your machine supply. I think you got that point, because you now see the need for the Wye output.

However if it's not connected properly, it will do you little good or even has the potential to do harm, as explained in the video. Don't cheap out, to save on the labor and cost of 25' of wire to properly bond the transformer output to the building steel, it could cost you a lot more later. Not to mention machine problems.

"Albert Hofmann" Although I should note that for many years I operated a Strippit CNC punch press that was connected to a step down transformer which had no ground rod and while it had plenty of other problems the Fanuc control and servo motors worked perfectly.

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That transformer likely had no rod connection, which is good, but likely did have a connection to the building steel AND one of the required service electrodes. This is why it worked fine, a ground rod has nothing to do with it, other than potential harm.

It's also imperative that you check the main service entrance of the building, to ensure that the building steel and water system are boded properly to the electrical service. Without that connection properly made, the bonding connection of your transformer to the building steel will be compromised.

One last point. You kept looking for a cheap way to implement a ground fault detection and protection system. To my knowledge, nothing in this department is cheap. Further, ground fault detection does nothing to to protect your system from being ungrounded. You would still be subjected the same problems of an ungrounded system, even with ground fault detection. When it did detect and operate, all it would do is shut you down. It wont protect your drives or provide the proper path to the transformer coil, to dissipate common mode current's and noise, as well as conduct large ground fault currents, in order to trip the circuit protection.


SAF Ω

It's been a while since I thought about that Strippit punch press, and I now recall that in their installation instructions they specified that a ground rod be driven next to the machine. This is just my one anecdotal account of a CNC with a ground rod that worked OK. That transformer was bonded to building steel and passed inspection.

It struck me as odd that you never once mentioned the fact that installing this type of transformer creates a separately derived system. If you try to explain this again it might help to use the term the code uses, and also emphasize what a large umbrella term "grounding electrode" is.

Sorry you found it annoying that I poked around a bit to see if I could save hundreds of dollars using an alternative solution before I spent the money on the transformer I bought. Much as I wish it wasn't, that's still a meaningful amount of money to me, so I put the question out there in front of the most knowledgeable people I could find. Clearly the answer is no.

Nobody in this thread or the one over at Mike Holt said anything about the question of feeding two machines off one transformer versus each machine having it's own. I hope I'm OK with one.

Separately Derived Systems & Grounding Electrodes

Albert Hofmann said:

It struck me as odd that you never once mentioned the fact that installing this type of transformer creates a separately derived system. If you try to explain this again it might help to use the term the code uses, and also emphasize what a large umbrella term "grounding electrode" is.

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I think your right, I'm a little odd. I assumed that if the discussion was centered around talk of a 3Φ ungrounded delta winding, we were talking about a separately derived system, as all isolation transformer outputs are separately derived. Code terms are fine, but I like to focus on the intent, instead of the legal verbiage, ungrounded delta or Wye, to me is a little more specific than just separately derived. Call it what you like, both terms apply to your 3Φ delta or Wye output coil.

Here's the NEC definition of separately derived,

Separately Derived System. A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct connection from circuit conductors of one system to circuit conductors of another system, other than connections through the earth, metal enclosures, metallic raceways, or equipment grounding conductors.

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I'm not sure if that makes it clearer, but pretty much includes all transformer outputs unless it an autotransformer.

Grounding electrode can mean different things to different people. The NEC definition:

Grounding Electrode. A conducting object through which a direct connection to earth is established.

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In regard to a grounding electrode for a separately derived system. 250.68 and 250.68C are the NEC sections that apply to separately derived systems.

250.68 Grounding Electrode Conductor and Bonding Jumper Connection to Grounding Electrodes. The connection of a grounding electrode conductor at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at a separately derived system and associated bonding jumper(s) shall be made as specified 250.68(A) through (C).

(C) Grounding Electrode Connections. Grounding electrode conductors and bonding jumpers shall be permitted to be connected at the following locations and used to extend the connection to an electrode(s):
(1) Interior metal water piping located not more than 1.52 m (5 ft) from the point of entrance to the building shall be permitted to be used as a conductor to interconnect electrodes that are part of the grounding electrode system.
(2) The metal structural frame of a building shall be permitted to be used as a conductor to interconnect electrodes that are part of the grounding electrode system, or as a grounding electrode conductor.
(3) A concrete-encased electrode of either the conductor type, reinforcing rod or bar installed in accordance with 250.52(A)(3) extended from its location within the concrete to an accessible location above the concrete shall be permitted.

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So in the case of a ungrounded separately derived delta or Wye output coil you have only 3 choices. A water service pipe within 5' of it entrance to the building, The structural steel frame, or A concrete encased electrode.

A ground rod is not one of the 3 given choices. The 2 you may have available is the water pipe or the building frame. I would hazard to guess that the building frame is closer than the water service entrance. A driven rod is not forbidden to add as a supplemental electrode, but doesn't qualify as one of the required electrodes.

There are many other types of grounding electrodes given in 250.52 but the only the 3 listed above qualify as the required electrode, for a separately derived system (Delta or Wye Output coil).

Albert Hofmann said:

Sorry you found it annoying that I poked around a bit to see if I could save hundreds of dollars using an alternative solution before I spent the money on the transformer I bought. Much as I wish it wasn't, that's still a meaningful amount of money to me, so I put the question out there in front of the most knowledgeable people I could find. Clearly the answer is no.

Nobody in this thread or the one over at Mike Holt said anything about the question of feeding two machines off one transformer versus each machine having it's own. I hope I'm OK with one.

Click to expand...

I'm not at all annoyed that you poked around to save money, it's your responsibility to do so.

As to using one transformer on multiple machines, no problem if it's sized properly to handle the load, you can connect as many as you like.

For motor loads the transformer should be sized at a minimum of the motor FLC X 1.5 at a minimum, most members here feel that X 2, is a safer multiplier to use. It really depends on the nature of the actual loads. Ideally you would want current readings from the equipment operating in it's actual use, to know for sure what the real world load actually is. then you might be able to size the transformer closer to the actual load profile.

All I know about your loads is that you said combined that they require 45A @ 480V.

45A @ 480V = 37.5KVA, 37.5KVA x 1.5 (min) = 56 KVA minimum, 37.5KVA x 2 (recommended)= 75KVA

Your loads may get by with less, but we really don't know what they consist of, only a bulk number was given. So only an approximation to size can be given in return.

SAF Ω