Friday, July 26, 2013

The Right Cable Connection

The Proper Choice Begins With a Few Fundamentals: Application Specific, the Ability to Function Reliably in Harsh Environments, and It Must Satisfy Regulatory Requirements and Customers

06/03/2013
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Just as clothes can make the man, cables can make — or actually break — the machine. The wrong wiring can cause electronic drives to prematurely fail, lead to regulators shutting down machines, or result in the loss of important data. On the other hand, the right cable can allow an automation solution to work as designed.
The proper cable choice begins with a few fundamentals. The first is application specific: "Is it going to be a power, control or signal?" asks John Gavilanes, director of engineering at cable manufacturer Lapp USA. "That would dictate what cable you're going to be using."
The second basic requirement is the ability to function reliably in the often harsh environment of a plant floor.
The third broad necessity is to satisfy regulatory requirements and customers, such as using a specific color-coding for cabling based on its voltage.
Fundamental Focus
With regard to the basic function, a key difference between power and signal wiring applications has to do with the voltage and current specifications. Control or signal wiring typically sees only a few volts, and need only support milliamp currents. Power wiring, in contrast, might have to handle hundreds of volts and amps of current. Thus, the gauge of the wiring could be 14 or less, meaning the diameter of the conductor is 1.8 mm or larger. Signal and control wiring, on the other hand, might be as small as 24 gauge or 0.6 mm.
Power and control-signal wiring were once quite distinct. That difference is becoming blurred due to the increasing use of variable-frequency drives (VFDs), which change speeds in response to a varying input voltages and current waveforms.
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The result is a solution that can be more responsive and more energy efficient than a traditional, single-speed drive, but it does demand more from the cabling. Waveforms could be traveling down the wire at megahertz rates, with voltage swings from zero up to hundreds of volts, and currents ramping through the complete range. What's needed is determined by the motor. A 5 hp motor, for instance, might demand 7.6 A and 460 (3 ph) V with switching at 20 MHz.
Sending such a waveform down a wire can have various effects. Some can show up in the drive itself, if the cable is not up to the task. These waveforms generate high-frequency ground currents in the return path. If the cable isn't constructed to deal with this, the current will find other ways to ground. For instance, this could be through the drive bearings, leading to arcing that pits the bearing races.
"The bearings will eventually be so worn that premature bearing failure will result," says Isaac Muller, applications engineer at cable maker Nexans. "That's usually a good first sign of a VFD cabling issue."
Environmental Concerns
Outside the drive, varying voltages and current at a high frequency can create significant electrical noise. "A lot of these Ethernet cables are installed very close to drive systems, and in most factories drive systems are often the single largest source of noise," says Peter Cox, industrial project manager at cable maker Belden.
The solution involves installing shielded cable, with conductive sheaths protecting the wiring. The need for such shielding might become apparent after the installation and use of VFDs.
Resistance to electrical noise is an example of the basic need for wiring to function correctly in a given environment. Other common factory-floor issues involve oil, temperature extremes or crushing force. Oil resistance is a particular concern, and is a good reason not to use standard Ethernet cable. The insulation of such cables is typically not oil resistant. In fact, it could be oil-soluble.
Certain environmental issues arise from the automation solution itself. "For instance, some applications require that cable flex and bend, perhaps with a twisting motion thrown in," Cox says. "In such situations, the changing geometry and distance between conducting strands can lead to signal attenuation. This happens because the impedance changes, resulting in reflections. Testing has shown return loss margins from such effects can be as high as -5 dB. Belden's solution is to use its Bonded-Pair technology, which ensures the distance between individual strands does not vary even as the cable is flexed, coiled, compressed or twisted."
Satisfy the Regulators
A different kind of solution is needed for regulatory requirements, the last broad basic specification that cabling must meet. Often this involves product ratings with regard to voltage, suitability for use in a hazardous or dangerous setting, and flame resistance. The latter, for instance, could involve exposing cable to flames for anywhere from 30 seconds to 40 minutes, with the requirement being that the wiring either not catch fire or only suffer damage over a limited distance. Such testing can't be done solely by the wiring manufacturer.
"Have they been evaluated by a nationally recognized testing laboratory?" asks Eric Bulington, chief engineer for wire and cable for Anixter.
Organizations such as Underwriter's Laboratories in North America and Europe's International Electrotechnical Commission can have thousands of ratings that apply to wiring and cable. There might be dozens of listed types of wiring specified. There also could be another class of wiring designed to be used on a machine or between its components that enables the end product to meet requirements.
The waveforms generate high-frequency ground currents in the return path. If the cable isn't constructed to deal with this, then the current will find other ways to ground. This could be through the drive bearings, leading to arcing that pits the bearing races.
At times, the regulatory and environmental specs of wiring overlap, according to Katina Kravik, CEO of cable maker Northwire. For instance, wiring that goes into and out of a wet location has to meet designated approvals, a case where a specific rating is needed due to an environmental conditions. Another example is RoHS compliance. Here, lead use is restricted.
Ratings can be complex and satisfying them costly, with testing done by a host of different nationally recognized laboratories. The output is often a report and certification that an inspector will go over in order to ensure that a system can be operated.
However, sometimes not meeting regulatory requirements isn't hidden. For example, a standard requirement for all wiring is that it be a certain color to signify voltage level or other characteristics. Failure to meet such basic specifications is easily detected, says Andy Pringle, North American OEM commercial manager at Rockwell Automation. "The local inspector could say you can't start it up because it doesn't meet code," he says. 
http://www.controldesign.com/articles/2013/hogan-the-right-cable-connection.html

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