VFD Cable Technical Whitepaper
Mar 01, 2012
Selecting Cables for VFD Applications
For all their energy savings and process control benefits, variable frequency drive (VFD) systems have a downside too. When these drives fail, they can bring motor-driven industrial processes to a dead stop. To avoid this costly downtime, smart engineers carefully evaluate reliability when configuring a drive system.
All too often, however, the reliability analysis focuses solely on the VFD power electronics and neglects the most vulnerable component in the drive system–the power cable that connects the VFD to the motor.
The truth is, however, that power electronics have already become very reliable over the years. By design, they can handle the typical voltage spikes, inrush currents, harmonics and other power distortions that arise during VFD operation. Their controls can also prevent damaging electrical conditions or shut down the drive if power distortions rise to unsafe levels.
Cables do not have any such protection and can fail if subjected to electrical conditions that generate more heat or voltage levels than the cable’s insulating layers can tolerate. Cables in industrial settings may also experience mechanical loads and chemical exposures that lead to damage and premature failure.
Fortunately, it is possible to avoid VFD-related cable failures and the associated downtime by paying attention to a few cable construction details.
Materials. Not all insulation and jacketing materials are created equal when it comes to electrical performance. So it’s important to match cable materials to specific VFD application requirements. To take three examples:
High Potential For Electrical Damage.As the likelihood ofdamage from voltage spikes or other power distortions increases, consider VFD cables that make use of semiconductive layers between the conductors and the primary insulation.These semiconductive insulation systemshave for decades been employed in high voltage cables. More recently, they have been applied to VFD cables to protect against electrical damage.
Overload Conditions In Long Cable Runs. When the installations require long cable runs, the risk of capacitance loads triggering the VFD overcurrent protection system rises too. The use of cables with the right insulation system can minimize this risk. Cross-linked polyethylene (XLPE) insulation, for example, has a relatively low dielectric constant that reduces the capacitive effect in long cables.XLPE also has excellent thermo-mechanical properties that allow the XLPE insulation with withstand the heat generated by overcurrent conditions.
Precision Control. It may not be obvious, but insulation choices can influence the control response of VFD systems. For applications requiring precision control, an approprieate insulation system will minimize transfer impedance and improve the velocity of propagation to produce a more efficient control response.
Mechanical Properties. With VFDs typically installed in factory environments, cables should be engineered to withstand mechanical abuse and environmental exposures. Some of the key mechanical attributes in VFD installations include:
Flexibility. Enhanced flexibility pays off throughout the cable’s life cycle. During installation, flexibility makes handling and routing easier. In use, flexible cables are less susceptible to damage from bending.
Oil Resistance. For industrial applications where oil exposure is a concern, make sure that prospective VFD cables comply with UL Oil Resistance requirements.
Crush Tested. Consider whether cables have the crush resistance needed for the installation. Cables certified as TC-ER, for both tray and exposed run installation, have to pass rigorous mechanical tests for cable crush and impact resistance, including UL Standard 1569. Because these rugged cables need no conduit, they can drive down installation cost and time significantly.
Stranding. To minimize voltage drop and maximize efficiency, consider the cable’s circular mil area (CMA). Cables whose conductors have a large CMA have lower DC resistance than cables with a smaller CMA. Low DC resistance, in turn, translates to significantly lowers voltage drop across a given length of wire.