A machine tool equipment manufacturer's answer to an end user's request for electrical noise prevention results in thousands of dollars in lost production.
Ever wonder why even the best plans seem to falter, or good intentions result in bad situations? Here's a case history that shows how trying to be safe can sometimes make you sorry.
What's the story? A tool and die shop uses a desktop PC, equipped with a computer-aided-drafting (CAD) program, to download data to various machine tool controllers on the shop floor. Then, they store the computer-generated program coordinates for a metal forming machine.
The end user, concerned about the compatibility of the connection between its PC and machine tool controller, consults the machine tool manufacturer about power requirements. The tool manufacturer suggests the end user install an isolated power conditioner (a standard isolation transformer) at the 120VAC input to the desktop. According to the manufacturer, this will isolate any electrical noise on the desktop's branch circuit wiring so it won't affect data transfer. The company sends such a transformer to the end user for plug-in installation.
Where the problems begin. After downloading a design program to the machine tool controller, an operator initiates the machine to begin making prototype parts. However, the machine doesn't perform as expected. Some missing lines and commands in the design program are the source of the problem.
Time and time again, efforts to download the program result in missing programming language. What's worse, the missing program commands are never the same. This all adds to the frustration.
An electrical maintenance technician first looks at the communication cable connection between the PC and machine tool. It's properly configured. Then, he checks, cleans, and tightens all terminations as well as makes cursory checks of the machine tool hardware; still no problems. However, the machine tool only runs the desired program if he enters the program manually.
Lost production. The end user spends considerable time and resources to find the problem. Over the course of five weeks, in-house maintenance people replace the data cable (once); printed circuit boards in the machine tool controller (on four occasions); and the desktop PC (once). The end user also has the machine tool manufacturer visit the site (three times at considerable cost). Finally, the company has the CAD program distributor spend a week on site to troubleshoot the program.
Even after all this, still no answers. Finally, the end user considers the possibility of AC power and grounding anomalies as the source of the problem.
The investigation begins. Investigation of the AC power and grounding for the machine tool reveals voltages are within operating tolerance. In addition, there is no significant electrical noise on the circuit conductors. The equipment-grounding system for the machine tool is also adequate. The branch circuit feeding the power-conditioning device (isolation transformer) has voltages in nominal range as well as low equipment ground and neutral impedances.
Measurements of the power conditioner output voltages, however, reveal over 11V between neutral and ground. The equipment ground impedance is also unusually high.
A closer look at the isolation transformer reveals the machine tool manufacturer removed the required neutral-to-ground bond within the unit prior to shipping it. The manufacturer made this modification without the knowledge of the isolation transformer manufacturer or the end user. (Note: The machine tool manufacturer found no less than 50 customers purchased such altered isolation transformers.)
Looking back. Opening the neutral-to-ground bond at the isolation transformer creates high neutral-ground voltages. These voltages are a result of the distributed capacitance between neutral and ground created by the open connection. These high voltages cause random, intermittent changes in the chassis ground's reference voltage. In some cases, the voltage is high enough to cause a parity error within the bit structure of the PC program. As a result, the machine tool controller can't recognize the difference between logic 1 and logic 0 in some of the program code because its reference voltage is much lower than the desktop PC.
The end user has its maintenance technician install the neutral-to-ground bond and, miraculously, the data transmissions between the PC and machine tool take place without error.
In retrospect, this company was fortunate a ground fault did not occur within the PC because the open neutral-to-ground bond would not allow the fault to trip the overcurrent device in the unit.
The obvious result of such tampering is the voiding of the isolation transformer manufacturer's warranty. But, altering the design and construction of power conditioning equipment can cause more serious consequences like personnel shock, fire hazards, electrostatic discharge damage, and equipment malfunction.
Michaels is Field Technical Manager with BellSouth Communications Systems, Altamonte Springs, Fla. and Power Quality Consultant for EC&M.
