Reliable VFD cables Boost Productivity, Minimize Downtime

Variable frequency drives (VFDs) are a mainstay of the industrial world due to their remarkable ability to improve the efficiency of motor-driven equipment. As part of a complete VFD package, high quality cable is one of the most important components in terms of achieving maximum productivity and minimizing downtime. 

When designing a robust VFD cable, the materials used in its production are critical to ensuring that the cable’s electrical properties will guarantee peak performance. 

For system engineers and others involved in specifying VFDs, cable quality should be one of the most decisive factors.

What's Inside:

• VFD theory simplified
• The problem with cable
• The importance of selecting the right insulation material
• How to maintain cable integrity with proper shielding
• Regulatory compliance issues

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The Essentials of Electronics and Electrical Equipment: Shielding

Effective EMI control prevents spurious signals from entering or leaving an enclosure. Shields and filters are the predominant techniques for controlling EMI.

Shields can involve many combinations of foils, conductive inks, paper, and adhesives. For example, there are now shields that consist of silver ink printed on 5-mil-thick polyester. After printing, a curing process removes nonconductive solvents. The result is a homogeneous shield that does not crack or delaminate when bent. The shield can be mechanically fastened and grounded with solder tabs. The cost is about the same as that for conventional laminated designs.

Carbon and stainless-steel fibers, combined with thermoplastics, provide an effective shield in many applications. Carbon fibers are usually classified as either PAN (polyacrylonitrile) or pitch. PAN fiber composites are selected for their high strength. Also, because PAN fiber has a higher aspect ratio (length to diameter ratio) than pitch, less is needed to provide a given conductivity.

Pitch-based fibers are not as strong as low-modulus PAN fibers. However, pitch fibers process easily into high-modulus products, making them attractive for stiffness-critical and thermally sensitive applications. The third carbon additive commonly used is carbon black. Carbon-black plastics are inexpensive, and are primarily for applications requiring high surface conductivity that allows dissipation of static charge.

Five factors affect plastic conductivity. The first is fill aspect ratio, which is proportional to conductivity. Second is loading level, also proportional to conductivity. The lowest fill loading needed to produce conductivity (generally defined as 10⁵Ω/sq) is called the critical concentration.
The third factor is resin type. The amount of fill needed for the critical concentration depends on the resin. For example, because nylon has a crystalline structure, its surface becomes conductive at lower fill concentrations than materials such as amorphous polycarbonate.


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Cable Glands For Full EMI Protection

Winning the war against electromagnetic interference (EMI) on today’s factory floors requires attention to detail. One of those details involves the ability of cable glands to contribute to a reliable grounding system.

Left unprotected, cable glands transmit electrical noise that can wreak havoc on motor-driven industrial processes. Many types of cable glands feature shielding to keep EMI at bay, yet the effectiveness of that shielding  can vary widely from product to product.

The ease of installation can vary as well. Some shielding options have difficult termination methods or grounding connections that can drive up labor cost and time.  

With SKINTOP^® MS-M Brush, our engineers have addressed both the shielding and installation issues. As its name suggests, this cable gland features a brush-type grounding connection that:

•  Enhances EMI control. Unlike other connection methods, such as the grounding clamps commonly used on drive systems, the brush provides continuous 360° contact around the cable’s screen braid. This continuous contact protects against EMI by lowering the resistance of current ground path and providing a low impedance connection between the cable shield and the housing.

•  Eases Installation. Making an EMI-free connection between cable and the SKINTOP cable gland is simple. Installers simply insert the cable, push the screen braid into the brush and tighten the cable gland assembly. Other grounding methods require far more effort to make the termination. Earth sleeves, for example, can serve as an effective grounding method, but their termination process requires precision measurements and exacting preparation of the stripped area.

In addition to its innovative brush-type connection, SKINTOP^® MS-M Brush offers IP 68 protection and resists temperatures up to 100ºC. Until recently, it was available only in metric sizes, ranging from M–25X1.5 to M–110X2.0. This month, we’ve introduced NPT sizes from ¾” to 2”.

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