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transformer inrush #11388
07/11/02 03:34 PM
07/11/02 03:34 PM
J
Joe Carpenter  Offline OP
Member
Joined: Dec 2001
Posts: 50
USA
Can someone shed some light on this issue?
I have an Italian CNC machine that is setup for 240 volts 3 phase. We brought 480 to it and stepped it down to 240volts . no problem there. When the power to the machine is turned on the controller is energized.
What, if anything could happen if someone shuts down the machine with the transformer disconnect and not the machine disconnect and then restarts it the same way?
I am not totally familiar with transformer inrush current and the like. But could energizing it this way negatively affect the controller somehow?
The reason I'm asking is because we are having sporadic CNC controller malfunctions and I just realized one of the operators shuts down and restarts the machine this way.
We have had no problems with the machine since we installed it new 4 years ago till several months ago which coincides with the approximate time this new operater was employed.
We have installed three new CNC controllers and they all eventually started acting up the same way as the previous ones did. We have pretty much exhausted all possibilities and everything points to the controller. But is something affecting the CNC controller?
Any Ideas would be appreciated. I will furnish you with any additional info you would like.

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Re: transformer inrush #11389
07/11/02 08:24 PM
07/11/02 08:24 PM
J
jlhmaint  Offline
Member
Joined: May 2002
Posts: 202
Fredericktown, OH, U.S.A
we have alot of Mazak machine tools that are set up in the same manner we have never had the problem you are having but we allways power down at the controler then shut down the transformer. not sure if your problem lies in the transformer or a controler glitch. even if your guy powers the system down at the transformer when he powers back up he still should have to turn on at the controler. so it would be no different than when we have a power outage and our systems shut down and we have no problems what has the manufacture of the cnc said. what type of controlers are on the cnc and what type of cnc is it?

Re: transformer inrush #11390
07/11/02 10:46 PM
07/11/02 10:46 PM
J
Joe Carpenter  Offline OP
Member
Joined: Dec 2001
Posts: 50
USA
The controller is a master 124s. It is a numeric controller and the I/O's are actually set up with small internal mechanical relays.The machine does not use PLC technology which also makes for a lot of contactors outside the controller. These controllers are not very "smart". However To install new software we need to upload it from a laptop computer through a serial cable. The machine it's on is a panel cutting saw. I'll try to explain the way the machine operates in a nutshell.
Panel cutting programs are written for this machine on a PC using an optimizing program to get the best material yields. The programs are then uploaded to the controller via floppy disk. The machine operator places material (4x8 sheets of plywood)on the saw table and presses the cycle start button. The plywood is then gripped by grippers mounted to a moving fence. The fence moves back till it reaches the desired measurement (which is in the program). A beam then drops down on the material to hold it and a saw blade comes up from the bottom and travels along cutting through the material. at the end the saw drops, the beam raises and the fence moves the material into place for the next cut and the cycle repeats. The problem we are experiancing from time to time is that during the process of cutting up a piece of material the machine occasionally will let the fence move the material into position and then the beam comes down and immediatly goes back up and the fence moves the material to the next cutting position, totally skipping the main part of the cycle which would be the saw blade coming up and the saw moving through and cutting the material. This happens sporadiclly and follows no ryhme or reason. we narrowed the problem down to the controller. we replaced it and thing worked perfect for about 3 weeks and now we're back to the same old song again.
The manufacturer is unable to find the root of the problem. It appears unless we find something soon we will end up getting rid of the machine and getting something with a PC and PLC.

Re: transformer inrush #11391
07/11/02 11:23 PM
07/11/02 11:23 PM
B
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
What are the exact voltage/current/frequency ratings on the equipment label? What is the equipment total kVA rating versus the transformer-set rating?

It doesn’t sound like transformer inrush would cause controller problems. What type of stepdown-transformer arrangement is installed? Is it an autotransformer, or isolated/separately derived? Specifically, how are the secondary windings grounded?

The machine could be intended and rated to see something like 220Y/127V 50Hz, possibly not matching the furnished 240V 60Hz ungrounded, corner- or midpoint-grounded service. There may be no specific neutral-conductor requirement, but internal phase-to-ground surge protection is expecting to see ~130V and not 240V {or more on an intermittent basis.}

It’s possible that a slight change on operating sequence would bear out the variation/mismatch.

What specifically in the control sequence is failing? Or, is the entire control assembly being replaced with each incident? Any component-level failures observed or troubleshooting attempted?

Re: transformer inrush #11392
07/12/02 05:40 AM
07/12/02 05:40 AM
T
Trainwire  Offline
Member
Joined: Mar 2002
Posts: 360
Strasburg,PA,USA
------
The machine does not use PLC technology which also makes for a lot of contactors outside the controller.
------
Sorry if I am suggesting something that you have already checked, but have you looked at all of the contacts on the limit switches and the contactors? If the operator is shutting it off and on at the transformer, would it be possible to arc across one of these? The symptoms sound like the controller is "confused" and thinks that the saw is done, when in fact it hasn't even started. The magnetic field collapsing in the transformer when it is turned off is going to make all kinds of crazy transients.
For what it's worth,
Trainwire

Re: transformer inrush #11393
07/12/02 06:54 AM
07/12/02 06:54 AM
J
Joe Carpenter  Offline OP
Member
Joined: Dec 2001
Posts: 50
USA
Transformer is an isolated 30kva 60hz 480 delta H.V. - 240 delta L.V. ungrounded. Machine requirements are 230 volt 23kva 60hz. Controls and controller are supplied by smaller transformers inside the machine. All voltage readings are within 5% of required. When the controller is replaced it is sent back to the manufacturer to be refurbished.
Trainwire,
you are correct, it has a lot of contactors and limit switches which makes for a lot of potential problems. This area was checked out quite extensively and a few questionable components were replaced. this corrected some of our other more minor glitches but the main problem remains.
Also another thing which just started happening more frequently is that for no apparent reason the operating software corrupts and becomes totally unusable till it is reinstalled. this happened to 2 different controllers in 2 days.
To my original question about turning the machine on and off with the transformer disconnect, This is no longer being done this way since the last new controller was installed. But the problems persist.

Re: transformer inrush #11394
07/12/02 08:24 AM
07/12/02 08:24 AM
J
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
I was at a site that had similar problems, in that they were "losing" their PLC memory. The control panel also contained 6 servo controllers. The system was fed by an ungrounded step down transformer (480 - 240Y136). The common ground point in each cabinet was not connected directly to neither the machine steel nor the power grid ground reference.

This situation is typical of a system with poor, to non-existent, signal reference points. Electronic devices are sensitive to voltage gradients caused by multi-point grounding, especialy if the connections are bad. Ground reference point problems typically do not appear until several years into the life of the facility (due to the ground connections oxidizing and/or becoming loose).

The solutions suggested were:
Confirm the connection of the transformer, because the typical solution to noise caused by servos is to put them onto a 240Y136V grounded wye transformer.
Create a common direct "ground" reference point for the different control cabinets on the machine, this point needs to be connected to the power grid ground also.


It has been 8 weeks and I have not heard of any more problems.

[This message has been edited by JBD (edited 07-12-2002).]

Re: transformer inrush #11395
07/12/02 11:50 AM
07/12/02 11:50 AM
B
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
For a typical dry-type transformer reference, see http://www.electro-mechanical.com/7700-94.pdf

JBD's experience may apply here. That is, lack of a bonding jumper on the "240V" secondaray circuit. JBD is referring to a drive-isolation type transformers that, by convention, would be 460V delta primary 3-wire and 230Y/133V 4-wire wye secondary.

Joe C, your 480V-240V dry-type is most likely 480V 3-wire delta primary, and 240V 3- or 4-wire delta secondary. If there is an X4 connection, that is called a 'lighting tap' and this gives a true 240/120V 3-phase 4-wire delta service (similar to what a utility serves a little rural grocery store or gas station.) It is fairly close to those on page 21 at 7700-94.pdf Look over voltage ratings, and also wiring diagram 17 or 21 on pages 46 & 48.

JBD's version is more like on page 30, top chart and third column, labeled 460V∆ to 230VY similar to diagram 19E but with a special 230Y/133V wye-connected secondary as opposed to the more common 208Y/120V voltage rating.

Joe C., the solution MAY BE to ground your transformer secondary at the X4 terminal, but only if the other X terminals are presently ungrounded. Make absolutely sure that all primary and secondary grounding, conductor sizing and overcurrent protection complies with NEC Article 450. If you do not completely understand the under 600-volt requirements of Art.450, get someone who does to check your work.


[This message has been edited by Bjarney (edited 07-12-2002).]

Re: transformer inrush #11396
07/12/02 12:52 PM
07/12/02 12:52 PM
J
Joe Carpenter  Offline OP
Member
Joined: Dec 2001
Posts: 50
USA
The transformer I have is a sola-hevi-duty model T5H30S. It does not have an X4 terminal. The only terminal it has that can be grounded is X6 for 120 volts which we do not need and do not have connected. I will still double check for proper equipment grounding throughout the machine


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