Variable frequency drives have an Achilles Heel. Even though the VFD itself is protected against damaging electrical conditions, the drive's power cables can fail when subjected to heat or voltage levels greater than their insulating layers can tolerate. Power cables can also be irreparably damaged by heavy mechanical loads or corrosive environmental conditions. Fortunately, it is easy to avoid VFD cable failures--and the ensuing downtime--by paying attention to cable design. This technical paper from Lapp outlines the key design features engineers should consider when picking reliable cables for VFD applications.

Industrial connectivity includes any component that is in the path of delivering power or control signals to do useful work. Typical connectivity components include connectors and terminal blocks, motor starters and relays.

A flexible printed circuit is as much a mechanical device as it is an electrical device. Conductors must be laid out such that the circuit functions properly and reliably. Unlike a rigid printed-circuit board (PCB), flexible circuits bend, flex, and otherwise contort to fit the final assembly. These bending and flexing operations can severely strain improperly routed internal conductors.

The industry standard IPC-T-50 of the IPC Association Connecting Electronics Industries defines a flexible circuit as, “A patterned arrangement of printed wiring utilizing flexible base material with or without flexible cover layers.” A typical flexible circuit is formed by stacking four different types of primary layers: the base layer, a metal foil or conductor layer, an adhesive layer that bonds the other layers together, and outer insulating (cover) layers. Multilayer boards stack these four basic layers as needed to complete the circuit design.

The base and cover layers are typically a flexible polymer film that creates the foundation of the flexible circuit and provides most of the physical and electrical properties of the circuit. A number of materials may be used as base films, but most flexible circuits today use polyimide films because of their excellent electrical, mechanical, chemical, and thermal properties.

Normal base-material thickness typically falls between 12 and 125 μm (0.5 to 5 mils), but thinner and thicker bases are possible. It should be obvious that as the base material gets thinner, the circuit becomes more flexible.

The metal foil layer provides electrical connectivity for the circuit. While different metals may be used, the most common metal found in flexible circuits is copper. Its high malleability, along with good conductivity, makes it an ideal material for flexible applications.

Rolled and annealed (RA) foils are the most common choice, though thinner foils may use electrodeposited (ED) copper.

The bonding-adhesive film, as its name implies, affixes the metal foil layer to the base material, bonds base layers to each other, and also adheres covers to the circuit. As with base films, adhesive films are available in different thicknesses, which are usually determined by the application. For example, different adhesive thicknesses are used in the creation of cover layers to meet the fill-needs demanded by different thickness copper foils. The most-common adhesive films used today are made from a modified acrylic or epoxy base.

When circuits bend or flex, material towards the outside of the bend must stretch to cover the expanded radius, placing that material in tension. Materials inside the bend, however, see the force of compression as the inside-bend circumference shrinks.

At some point in the middle of the material stack is an area that sees little to no tension or compression. This area is called the neutral-bend axis. In a flex circuit, it’s loosely defined as an imaginary planar region with no thickness that undergoes neither tension nor compression during bending or flexing. As different layers in the flexible circuit move further away from the neutral-bend axis, the forces of tension and compression become more severe and damaging.