Increasingly common, variable frequency drives (VFDs) are used to control motor speed — elevating the frequency of power pulses/sec fed to motors for faster turning, and decreasing that frequency to make motors turn more slowly. As for the motors controlled this way, they work best when power pulses are well regulated and current is injected in a flow optimal for sustained speed and acceleration consistency. However, VFD conversion of ac to dc to variable ac is not perfect, and introduces nonlinear spikes, wave reflections, inrush currents, and other signal mutation. The current from this nonlinear power does not support motor requirements, because change in voltage does not generate the same change in current: The distorted current either undermines or exaggerates the voltage change, resulting in high-voltage stress and heat.

What's worse, connecting cable can actually magnify these signal imperfections. To address this problem, some VFD connections include unshielded tray cables (consisting of single-conductor lead wire installed in conduit) or welded armored cable. But armored cable and lead-wire conduit are heavy, difficult to install, and necessitate huge bending radii — and they do not completely solve noise and corona discharge problems, or effectively address VFD noise issues. On the other hand, well-selected motor and control features coupled with specially insulated cabling minimizes VFD distortion without the fuss.

Switch spikes and cable length

For a rectified 460-V system, a drive's inverter — a fast-switching transistor — must rise from zero to 650 V and then go back to zero 20,000 times each second. This results in a very fast rise time. Several things can happen as the inverter switches and sends its pulses through the cable to the motor. The 650-V normal voltage can overshoot — spike — by 2,000 V or more. Even though these voltage spikes are very quick, lasting for only a few millionths of a second, they can cause considerable damage as power distortions created by VFD rectifiers are sent back through source power systems.

Long cable exasperates the issue: Greater lengths make for greater inductance and higher voltage-spike overshoot, so long power-supply cables have more numerous and more intense voltage spikes than shorter cable. Why is that? Long cables connecting motors to drives give a reflected standing wave more opportunity to go into phase with itself, thereby doubling voltage and current. (Similar to a wheel that appears frozen when actually in motion, reflected frequency waves look electrically to be standing still.) These standing waves can cause damage if they bounce off the large impedance difference at the cable-motor attachment and reflect back to the drive. So standing waves can easily turn 650 V into 1,300 V. At the spot in the cable where this happens, insulation is severely stressed, can eventually overheat, and might even allow puncturing, causing the cable to fail.

To minimize spiking: The typical conductor inside a VFD cable is 12 to 2 AWG and is rated for 1,000 V and 3,000 V_peak so cable can withstand reflections and standing waves. VFD cable is also UL and CSA standardized for these conditions.

Too, cable specifically for use with VFDs differs from ordinary motor power-supply cable in that it disperses spikes that VFD drives generate. A semiconductive layer relieves electrical stress experienced by VFD cable during high-voltage spikes.

VFD cable construction includes semiconductive composite insulation applied over the conductor. This inner jacket provides an additional layer of protection between the insulation and braid shield — and lowers the capacitive interaction between the braid and power conductors. The semiconductive layer disperses rapid voltage rises to protect the primary insulation from damage, letting the cable operate without a disruption in service. PVC-insulated VFD cables typically include an extruded thermoplastic semiconducting layer applied directly over conducting copper strands. (Also, the inner semiconductive jacket of some cables is sized to fit an MSSC-type shielding connector for full 360° shield grounding.)

Semiconductor-insulated cable protects motors as well. Windings are the delicate part of the motor; their lacquer insulation is extremely thin, and any flaws under shock can cause arcing and motor failure. But motor life is improved about 6% when connected to VFD cables, which reduce both the number and magnitude of reflected waves with their improved impedance.