Assistance choosing a 3-phase transformer to supply a CNC machine

[Nescius]

Hi everybody,

I'm a mechanical engineer trying to wrap my head around choosing a transformer for a new CNC mill coming into our shop. We're a small operation, so I have ownership of all the technical aspects of the purchase, nearly all of which are squarely within my professional experience. The 3-phase power, however, is a new frontier.

Of course, a real electrician will perform the installation, but I need to have a fundamental understanding of the situation in order to get quotes for the work. Also, the machine represents a significant investment for our shop, so we're proceeding very methodically.

Documentation provided by the machine tool maker seems weak, at least to a layman. A call to the applications department of a transformer manufacturer was little help. Most surprising is the amount of bad and dangerous advice online regarding machine tool wiring.

I greatly appreciate any education that you can provide. Please correct any errors of terminology without mercy. So, here's our situation:

[li]Power company will supply our facility with 277/480 3-phase 4-wire wye.[/li]
[li]Machine tool specs name a range of acceptable input voltage that is nominally referred to as 220 3-phase.[/li]
[li]Machine peak power is 40kVA, maximum continuous power is 28 kVA. I'd guess that most people never use these machines at anywhere near these levels; we certainly won't.[/li]
[li]We're considering a transformer rated for 75kVA to support future machine purchases.[/li]

The following points are taken directly from the machine tool documentation:
[ol 1]
[li]The ground wire is required for operator safety and correct operation.[/li]
[li]A separate ground wire must be connected to the chassis of the machine.[/li]
[li]The ground wire must be supplied from the main building ground.[/li]
[li]Do not use a conduit as a ground wire.[/li]
[li]Do not use a cold-water pipe or ground rod to supply the machine ground buss[/li]
[li]Power can be Wye or Delta type.[/li]
[li]You must ground the power source. One leg or center leg for Delta. Neutral for Wye.[/li]
[/ol]

I wish that their language was a bit more precise or that they provided a diagram. In any case, as a layman with little knowledge of the applicable NEC requirements, I draw the following conclusions:
[ol 1]
[li]Transformer secondary could be either wye or delta.[/li]
[li]With a wye secondary, the center of the wye should be bonded to the equipment grounding conductor (EGC).[/li]
[li]With a delta secondary, either the center of one phase or the corner should be bonded to the EGC.[/li]
[li]The machine does not require a specific voltage between any phase and the ground lug of the machine.[/li]
[li]The EGC should be a wire and should run directly to the ground lug of the machine.[/li]
[/ol]

These conclusions lead to more questions:
[ol 1]
[li]It seems like there is no purpose for a "neutral" coming from the secondary of the transformer. Is this correct?[/li]
[li]Is there a reason to choose a particular primary arrangement, delta or wye?[/li]
[li]Is there a reason to choose a particular secondary arrangement, either delta or wye?[/li]
[li]If a delta secondary is chosen, since I don't need a particular phase-to-neutral voltage, is there a preferred method of bonding to the EGC, either center-of-phase or corner?[/li]
[/ol]

This post is getting awfully long, so let me stop here for now. I have attached a diagram of my understanding of the situation. I welcome harsh criticism of that as well.

I appreciate your help!

 

[Nescius]

Thanks, all.

The plan is to bond the center tap to the equipment grounding conductor within the transformer enclosure. However, I will proceed armed with the above warning regarding differing line-to-ground voltages. I will ask Haas directly on this issue.

itsmoked,

The machine is new from the factory. Unfortunately, I have not seen it in person yet; it's waiting at the rigger's warehouse right now. My awareness of an internal transformer comes from what little installation documentation I could scrape together. Presently, my most detailed information is a document from 2011 and Haas has clearly stated that they do NOT provide instructions like that any longer. The 2011 document is mediocre, but adequate. The "new" information is a weak webpage with some bullet points and specs, no instructions.

I need to put some pressure on Haas, or the factory outlet, to cough up the "real" documentation. They are obviously withholding it from us. Honestly, it's a bit disappointing.

In any case, you could be right; the internal transformer might not do any heavy lifting, so to speak.

 

[mc5w]

Ideally you should use a Drive Isolation Transformer, 460 volts delta primary, 133Y230 volts secondary with the primary taps set at the HIGHEST voltage possible usually will be 5% Full Current Above Normal ( FCBN ) to bring down the secondary voltage to around 133Y230 volts. The Drive Isolation Transformers have the ability to handle harmonic currents from industrial rectifiers.

Definitely solidly ground the secondary and then use 3 120 volt primary 12 volt secondary buck-boost transformers in the wye-wye autotransformer configuration to knock the voltage further down to 121Y210 volts. I am serious about this. A LOT of European solid state drive manufacturers such as Kollmorgen are under the FALSE impression that our industrial voltages are 127Y220 to 133Y230 volts and 254Y440 to 265Y460 volts like South American 60 cycle countries and Cuba have. If you expose a Kollmorgen 460 volt drive to 289Y500 or 300Y519 volts ( I have seen both on Cleveland Public Power at 5 AM in the morning during the summer ) a Kollmorgen drive will fry like bacon.


After you put in that you can try changing the primary taps of the drive isolation transformer to 2.5% FCBN and see if that gives you around 220-225 volts and if so leave it there. Also, you should size the feeder and branch circuits to 140% of full load current to account for the matter that these drive very likely will be Periodic Duty not Continuous Duty. Also, National Electrical Code wire and circuit sizing is driven by the Cheaper-Than-Dirt-Cheap Club and going for a larger wire size and the largest fuses and circuit breakers will be prudent.

Also, THWN/MTW wire only has a 60 degree Celsius rating when exposed to oil or gasoline not the 75 or 90 C rating that it has as building wire. A potential gotcha.

Also apply whatever surge protection you can at the main distribution panel for the 277Y480 and on both sides of the drive isolation transformer. Do not skimp on this.

 

[Nescius]

mc5w,

Thank you for taking the time to reply. I must admit that some of what you wrote is over my head and I did unload the new transformer at the shop today, but I'm glad to learn more.

I gather that a "drive isolation transformer" is designed to better withstand SCR loads that create unique demands on a transformer. As a layman with some hobbyist electronics experience, I can digest this in only a very basic way...current spikes associated with SCR switching and larger physical forces on the windings, perhaps?

Then, is it a good thing that my plain vanilla transformer is greatly oversized, better able to endure the punishment of this type of duty? (75kVA transformer supplying a machine that can draw at most 28kVA continuously, and will usually draw only a fraction of that)

Maybe it goes deeper than that and the DIT construction benefits the load as well?

The machine specs give the acceptable voltage range as 195-260 Volts. The plan was to select primary taps that give an output voltage right in the middle of that range. I have no idea about the details of the electronics in our machine, but it sounds like you'd recommend we choose taps to minimize the output voltage.

Surge protection is even more foreign to me than what we've been discussing so far. In this context, does "surge protection" mean devices to guard against lighting strikes?

Thanks again.

 

[itsmoked]

Go with the nominal voltage your machine requires. Your machine also will have NO Kollmorgen anything in it so you have nothing to fear there though that's good to know info.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mc5w]

You also need to install voltmeters so that you can monitor for when the power company jacks up their voltage during the summer so as to overcome undersized wiring in residential neighborhoods. For what your voltage range is 208 volts open circuit during the Winter would be prudent. You should also install an overvoltage relay that will sound an alarm or even cut power to the machine when the voltage exceeds 255 volts. An example was that about 1/2 or 1 hour after First Energy restored power on August 15, 2003 the open circuit voltage in Seven Hills was 125 volts. During the warm half of the year Cleveland Public Power jacks up the voltage on their 6,900Y12,000 volt system to more along the lines of 7,230Y12,500 volts so that people who are on 2,400 volt 3-wire 3-phase distribution will get more voltage for their air conditioners. At onbe place iun Cleveland supplied by CPP during the summer our service were reading 125/250 volts singlemnpohase and 289Y500 volts and in 1 service 300Y519 volts! Here in Columbus, Ohio American Electric Power's voltage inside my apartment reads 123-124 volts even though I live about 150 feet from the supply transformer in a 100 unit apartment building.

You should also use antioxidant compound on copper wire that is formulated for use on copper wire. Ilsco Deox, Penn Union CuAl Gel, Burndy Penetrox E, and Thomas & Betts Kopr-Shield are examples. Deox and CuAl gel can be used to lubricate box lug setscrews but the latter 2 cannot because Penetrox E and Kopr-Shield contain colloidal copper to make them suitable for the threads of brass or bronze conduit and copper clad threaded sectional ground rods. One of the things the you WILL NOT find on the Consumer Product Safety Commission website was that when Dr. Jesse Aronstein was working on the Aluminum Wiring Failures Problem he tested copper-to-copper connection methods as a matter of scientific control to determine if it was the method or the material or both. These are the things that he determined:

1. Copper oxide is just as much a fire hazard as aluminum oxide which was what the US Army Signal Corps was teaching their electrcians during World War 2. I have seen more in the way of copper wiring burn up than aluminum. I have used silicon carbide abrasive paper + elbow grease + Ilsco Deox electrical grease on the copper wiring for a replacement starter motor in a car. 3 years later I had to drop the starter motor and change the power relay. Ilsco Deox passed the 3 year salt spray test. Deox also caused the battery terminals to have ZERO sulfuric acid creep.

2. You do indeed need to fan out all of the wire strands, cut off the sharp tips, clean each one with silicon carbide paper ) 220 or 240 grit for larger wire sizes and aluminum, coat them with electrical grease, and recompress with a hose clamp. When I was a 3 year old grandfather taught me how to bunch clean the wire strands of lamp cords and extension cords with diagonal cutters which works for wires #10 AWG or smaller with 19 or more wire strands.

3. Other methods of connecting stranded copper wire that most people use only have vhave the outside layer of wire strands conducting even if the circuit is direct current. The Ericson company in Willoughby, Ohio determined that when most people make a crimped or box lug connection to copper wire they only have the outside wire strands conducting. This issue is why long before Dr. Aronstein was born Erico invented exothermic welding of copper wire. Burndy has told me at a trade show that some of their customers have built heat sinks for welding Burndyweld lugs to insulated building wire.

4. A steel wire brush is 100# INEFFECTIVE at removing aluminum oxide and that Burndy Penetrox is also 100% ineffective at penetrating aluminum oxide. What you need to remove aluminum oxide are either #220 or #240 grit silicon carbide abrasive paper or a tungsten inert gas welder. I demonstrated when using 4-wire aluminum service entrance cable to hook up a plastics machine with 60,000 watts of electric heaters in it that you can sand each wire strand dry and then put Deox on it right away. I have told Dr. Aronstein by email and the telephone that you do not have to grease the wire strands and then clean them. There are also a few other issues such as cutting off the sharp tips of the strands before cleaning them and deliberately overstripping the insulation so as to have a strand bending area in order to fan them out. Use a 1/4 inch screwdriver shaft to maintain a minimum bending radius of the strands. I still have not figured out who to degrease my hands frequently without using perchloroethylene brake cleaning spray. Next time I go to a trade show I will go to the industrial hygiene booths first and see what solutions they have. If you a solution for degreasing my hands please let me know. Also, for compact stranded wire cleaning the concave surface of the wire strands and the other 3 sides is essential to interstrand conduction and having all wire strands conducting regardless of whether it is copper or aluminum. Yes, there is compact stranded aluminum.

I know this sounds like a lot of extra work but for how much coipper wire and aluminum wire costs these days would the customer want half the wire strands conducting of all of them conducting?

You can contact Dr. Jesse Artonstein at protune@aol.com .

 

[waross]

There are probably a few million copper wire connections made each year without any cleaning whatsoever that stand the test of time.
However if you have the misfortune to work with stranded wire that has been exposed to the weather for a long time, then cleaning is a very good idea.
At times I have encountered copper wire with corroded strands. I have often been able to trim a few inches or more off the end of the cable and find clean copper. Sometimes this is not possible and I have to clean the strands.
The worst case of copper corrosion that I have seen was a bad connection on a DC press motor.
The bad connection heat cycled and caused enough copper oxide to form that the connection acted as a copper-oxide rectifier. The motor would run forward but not in reverse.
Aluminum oxide is difficult to see and unless removed within the preceding few minutes is always there on unprotected aluminum.
Copper oxide is visible and distinctive. Clean copper is no problem
Anyone who connects discoloured copper without cleaning it first gets what they deserve.

Bill
--------------------
"Why not the best?"
Jimmy Carter