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Robotics Tech Papers

Selecting the Right Motor Supply Cable

Alpha Wire

Selecting any cable involves getting the right performance without overspecifying—and overpaying—for what you don’t need. For powering motors, the choice ranges from PVC/nylon-insulated wires in a conduit to precision VFD cables. The most obvious requirement is a cable that meets the power-handling requirements of the motor over the distances between drive and motor. For short distances and modest powers, a PVC cable may work fine. As distances and power levels increase, so do the electrical stresses on the cable’s insulation and jacket; a cable designed specifically as a motor supply cable is then advisable.

The Special Case of VFD
Variable-frequency drive (VFD) technology only complicates things further by inducing more stresses on the conductor insulation. While pulse width modulation (PWM) provides an excellent way to control a motor’s speed and torque, it creates several phenomena that can affect the power, create noise, reduce the life of the cable, and disrupt operation of the drive system. Cables designed specifically for VFD applications withstand these phenomena. And, equally important, they do nothing to make the situation worse. A generic motor supply cable is not designed with the specific requirements of VFD system in mind actually cause problems.

Cross-linked polyethylene (XLPE) has the properties needed for VFD applications, making it a better choice than standard polyethylene or PVC. XLPE provides superior corona resistance, low capacitance for longer runs, and excellent low-temperature properties. Its excellent dielectric properties help prevent breakdown from the stresses of standing waves resulting from impedance mismatches, harmonic distortion, in-rush currents at motor startup, and voltage spikes.

A round, symmetrical cable gives the best electrical performance and resistance to deleterious effects. One reason is that round symmetry creates more uniform electrical characteristics in the cable. The electrical characteristic of one conductor doesn’t differ significantly from another. For example, having the ground wires places symmetrically around the cable provides a more balanced grounding system.

Other Issues
There are also the usual issues to be addressed, such as shielding in noisy environments, the need to meet regulatory requirements, physical routing and cable termination. In general, these apply equally to whatever type of motor supply cable you select.

Ask an Expert
You should always begin by following any cabling guidelines offered by the drive and motor manufacturer. If these guidelines are insufficient, then consult the cabling experts at Alpha Wire. We have the experience with motor/drive applications and the in-depth understanding of cable performance to make sure you get the cable you need.

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