Variable frequency drives emit electrical noise, which can interfere with other circuits. This interference can cause motor torque loss or cause motors to stall out or fail completely. It can also reduce system efficiency, pose a hazard to personnel in the area and create false trips of drive overcurrent. To protect those circuits, VFD cable shielding is critical.
This shielding should be constructed of either a foil and tinned copper braid or a 5-mil copper tape with minimum overlap of 25%, said Rick Orsini, product manager at SAB North America. “The higher the percentage of overlap, the better the protection will be,” Orsini said. “The goal is to keep the noise within the cable and not allow it to interfere with other cables.”
Armored cables also offer effective shielding, as they too provide a low impedance path for common-mode noise to return to the drive. According to Chris Daulton, Global Industrial Marketing Manager for Belden, limiting run length of VFD cable parallel to instrumentation cables to less than 10 ft can also reduce radiated noise.
As David Sedivy, Sr. Product Manager/Mining Segment Manager, TPC Wire & Cable, pointed out, “The shield system in a VFD cable serves two main purposes, to contain radiated electrical noise and provide a low impedance path to ground. Radiated noise can potentially interfere with external circuits and create nuisance lock-outs or error conditions in controllers. Shielding should effectively reduce radiated noise levels, and therefore the risks associated with radiated noise. To a lesser extent, the shield also inhibits external noise from interfering with signals internal to the cable.”
Sedivy added that another issue with VFD systems is common mode noise. This can cause damage, especially to motor bearings. “In simple terms, common mode currents will find the path of least resistance to ground. Often, this path is through the motor bearings,” he said. “To control common mode noise we need to provide a low impedance path to ground. A low impedance shield system can provide this path of least resistance and help reduce the effects of common-mode noise.”
Electromagnetic interference (EMI) usually affects the environment more than the cable. Frequency changes, pulsing, reflective wave phenomenon, and coupling effects are more likely to affect the cable than EMI, said Neal Allen, Key Account/Industry Manager for HELUKABEL USA.
SAB’s Orsini agreed, adding that EMI/noise does not affect VFD cables if grounded properly. “Using a grounding gland at the enclosure (metal) and motor will remove the noise from the shield and take it to the earth ground for the enclosure and motor,” Orsini said. “There are also two PE terminals on the motor and drive. Affixing the shield to one of the PE terminals will continue the protection. In essence, you create a Farrady Cage to keep the noise contained and safely removed to earth ground.”