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What Are VFD Cables?

VFD cable stands for Variable Frequency Drive Cable. A VFD is a system used to control the frequency with which electrical power is supplied to an alternating current (AC) motor. This is done using a method called pulse width modulation (PWM). Controlling power is a means of controlling the rotational speed of the motor. For this reason, these systems are also called ASDs or “adjustable speed drives.” VFDs are used in many industrial applications. They can be found in the ventilation systems of large buildings, elevators, conveyors, and pumps. Usually, they are used to save money and increase efficiency by regulating the output of motors so as not to exceed system demand.

Construction

VFD systems are composed of three parts: the three-phase full-wave rectifier, the PWM inverter, and the motor. The semiconductor devices used in these systems move at fast speeds that create electromagnetic interference. If not contained, the radiofrequency energy produced can interrupt the operation of other nearby electronic equipment. This can be a major problem in any operation where a VFD is located near other electronic machinery.

Another problem is that stray electrical currents can cause premature motor bearing failure. In certain drive systems, high voltage reflections can also cause inverter-to-motor power cable to fail prematurely, leading to early in-service motor failure.

VFD Cable Options

Although there are several problems inherent in VFD systems, there are VFD cable choices you can make to minimize the risk of electromagnetic interference or system failures.

First, the electromagnetic interference problem can be limited by using impervious corrugated aluminum armor to shield the cable. This armor is referred to as CCW, which stands for “continuously corrugated and welded.” Though slightly less effective, copper tape, copper braid, or aluminum/polyester foil may also be used to shield the cable.

Next, the stray electrical currents which can cause early motor bearing failure may be restricted by using inverter-to-motor power cables constructed with three symmetrically placed grounding conductors and a low impedance shield. This construction minimizes overall current flow, therefore minimizing the stray electrical current in the motor bearings.

Finally, problems with high voltage reflections can be prevented by using 2000V cables for systems rated at 600V. A high voltage cable is recommended because peak voltages can be up to 2.5 times the nominal system voltage on some VFD power cables. Preparing for the extra volts can save a system from failure. However, for VFDs that operate at 480V or less, a 600V cable is usually sufficient.

So what is the best VFD cable to use? It is an impervious corrugated aluminum armored CCW cable with three conductors and three symmetrically positioned grounding conductors. This cable combines all of the problem-solving characteristics recommended above to create the best overall VFD cable.