Cables used with variable frequency drives (VFDs) need to match the operating environment and other components in the system. Selecting the right VFD cable can help ensure the drive’s reliability and lifecycle.
The following are key specifications to consider when selecting a VFD cable.
Voltage and ampacity
Knowing both the motor and cable’s voltage and ampacity is perhaps the number one consideration in selection, said Neal Allen, Key Account/Industry Manager for HELUKABEL USA. Chris Daulton, Global Industrial Marketing Manager for Belden, added that the cable’s voltage must be able to support those of the VFD, so their ratings should be equal or higher than the VFD.
David Sedivy, Sr. Product Manager/Mining Segment Manager, TPC Wire & Cable, added that VFD motors are available in low and medium voltages, so voltage rating is a key characteristic to specify. “The voltage rating of the cable must meet or exceed the source voltage,” Sedivy said. “With respect to design, the cable must withstand the higher voltages generated within a VFD system due to reflected waves and voltage spikes from high speed switching circuits.”
Rick Orsini, product manager at SAB North America, added that in addition to motor voltage, horsepower should also be known. These specifications can be found on the motor plate. For example, a 100-hp drive might be powering four 25-hp motors, he said.
If you don’t have the AWG cable charts provided by cable suppliers, you need to use Table 430.250 (Full-Load Current (FLC), Three-Phase Alternating-Current Motors) in the NEC book to get the amps required. “For protection, it is required to multiply this value by 1.25 for FLC,” Orsini said.
Allen cautioned about undersizing the conductors to the ampacity or else the cables will fail. “The biggest no-no is using portable cordage (SO cord) for VFD applications. SO cord is usually not rated for the overvoltage produced, ranging from 600 to 1000 V, from VFD drives, and it is not tray rated,” Allen said. “Its lower quality insulation material and conductor insulation technique will cause these cables to fail.”
High-performance VFD cables are usually of the high-flex type, with higher strand counts, which is a better fit for high-frequency designs. Allen stressed how important it is to not use stationary cables for continuous flex or torsion applications.
Sedivy agreed, adding that physical characteristics of the cable are dependent upon the application environment. “Cables placed into dynamic flexing environments require a cable designed for flexing applications,” Sedivy said. “High conductor strand counts and braid shields provide a more flexible cable, and are a better choice for flexing applications.”
In addition, said Sedivy, consider other environmental impacts, such as abrasion, sunlight/UV, temperature and chemical.
Orsini added you must ask yourself a number of key questions. “Is the application robotic, installed in cable track or is it installed in overhead tray or pipe? If robotic, you will need a cable designed for continuous flexing. If in overhead tray, flexibility is probably preferred to make it easier to pull through the turns, bends and drops of a tray system saving on installation time,” Orsini said. “Check the stranding and shielding technique. Finer copper strands and a tinned copper braid shield is more flexible.”
Knowing load size is critical in helping to properly size the conductors, said TPC’s Sedivy. “Under-sized conductors pose the risk of increased conductor heating which may degrade the insulation material and lead to premature cable failure. Over-sized conductors add cost and increase cable OD which, in some applications, may create physical sizing issues,” Sedivy said. “Installers should keep in mind the NEC defines cable ampacity requirements for motor circuits per Article 430 of the NEC.”
HELUKABEL’S Allen added that an understanding of basic cable design is necessary in VFD cable selection. “The biggest choice to make here is having symmetrical grounds (three grounds and three power conductor) or a four conductor design (three power conductors and one ground),” Allen said. “Research shows a symmetrical ground structure provides a smaller cable outer diameter and reduced electromagnetic interference (EMI). A four conductor VFD cable is more flexible and better suited for continuous flex.”
Allen added that one must also consider other items used to build a continuous flex cable such as low friction tapes and fillers and cable machines that neutralize the cable twist during the cabling process.
Finally, jacketing and insulation are critical, said Orsini. After determining AWG size, next you must evaluate the environment and application. “Are there oils or chemicals present,” Orsini asked. “If yes, you will want a cable with a jacket that can withstand them.”
HELUKABEL’S Allen said that Chlorinated polyethylene (CPE), polyurethane (PUR), thermoplastic elastomer (TPE) and polyvinyl chloride (PVC) are the most common types. More exotic compounds are available but usually with limited demand. Stationary or continuous flex applications may dictate a better performance jacket material such as PUR.
Belden’s Daulton noted that special attention should be paid to cable insulation, as cables that use a heavy wall of thermoset insulation offer temperature stability and electrical benefits.
“A cable combats voltage spikes through its wall thickness and the insulation material used. Belden’s VFD cable uses thick XLP insulation which has much lower capacitance than THHN and generic control/tray cables that use PVC,” Daulton said. “The wall thickness of VFD cable withstands voltage significantly better and cannot melt from heat build-up.”
The XLP insulation and lower capacitance also extends transmission distance, Daulton said.
Allen pointed to the variety of insulation materials available for cable conductors. “The most common types are thermoset, such as cross-linked polyethylene (XLPE), PVC, semi-conductive (semi-conductive polymer/PVC) and PVC/nylon insulation, he said. “XLPE insulation can withstand higher heat and has a higher melting temperature than the others but it more expensive due to the crosslinking process required to insulate the conductors.”
In addition, semi-conductive insulation is believed to dissipate launch point, which occurs when the voltages exceed the capabilities of the cable. Launch points with fewer capabilities help extend the life of a cable.
Finally, said Allen, PVC/nylon insulation provides lower OD of the conductors and is extremely robust.
Orsini added that with PVC products, insulation thickness needs to be 20 mil, minimum.
Shielding, too, is an important design consideration. Our contributors will talk further about shielding in a post next week.