Variable Frequency Drives and Motor Cabling – Part II

Output cabling from a variable frequency drive (VFD) can act as a significant source of electromagnetic emission resulting from the cables’ characteristics in response to the high-frequency voltage pulses they send to the motor. In effect, the cables serve as antennae. For drives using IGBT’s to generate the voltage pulses – the most common method used in drives today – the emission spectrum coming from the output section of an AC drive may extend to 50 MHz; it is typically strongest in the 100 kHz – 10 MHz range. Below this range, electromagnetic interference (EMI) issues are rare, but in or above it the EMI emitted and its effects on cables and connected equipment can have a negative impact on drive system controls and any adjacent, sensitive equipment. Let’s examine more closely the role that cabling plays, and ways to mitigate EMI problems through cable specification and installation practices.

At high frequencies, current will flow primarily in the outer surfaces of the conductors due to skin effects, which raises the effective impedance of the cables and increases voltage and the potential for noise. Also, “stray” or “leakage” currents can arise via capacitive coupling and, if not effectively isolated, can radiate noise into the surrounding environment. Most of this noise is “common mode,” i.e, conducted via ground (“earth”) conductive paths; at these higher frequencies, series mode (i.e. phase-to-phase) noise is not usually significant. Common mode noise can affect the operation of signaling/data circuits, lead to motor circulating currents and bearing damage, and interfere with the operation of  adjacent connected equipment.

The length of the motor leads also plays a significant role, since it increases the area from which EMI can be emitted and presents higher impedance, all other things being equal. Added length also can lead to higher voltage at the motor terminals due to resonance effects.

So what can be done during cabling design and installation to reduce EMI and lower the risk of noise-related issues? Below are some basic recommendations:

• Provide a proper equi-potential grounding bond for the motor and VFD. This ground should present a low impedance to high-frequency current, so a flat braid is better than a round conductor of equivalent cross-sectional area. A good low-impedance ground will ensure that any stray/leakage currents are conducted back to the VFD. Unless equipotentiality can be assured, for example as with a common grounding plane or solidly connected  building steel, then more evaluation must be given to separating power grounds from signal grounds. Although there is quite a  bit of disagreement about this practice, the potential for ground loops to develop and influence sensitive data or high-speed signaling circuits cannot be discounted.
• Ensure proper sizing of the ground (earthing) conductor. Full-sized conductors are preferred to lower impedance.
• Routing: ensure proper distances are maintained between line and load-side power conductors and between any power and control conductors. Most manufacturers recommend at least 6 – 8 inches separation; a more confident recommendation is 10 – 12 inches and no parallel routing longer than 3 feet. Cable crossings should be at 90-degree angles wherever possible. Use of solidly bonded metallic conduit can reduce the distances of separation required, but at higher carrier frequencies and/or in high-speed signalling circuit applications metallic conduit alone may not be enough.
• Shielding: the method recognized most effective in mitigating EMI is the use of shielded power cables for the motor leads. These cables must be properly installed – shield bonded at both ends (unlike control cables); shields left intact (not braided) and bonded circumferentially (full 360-degree bonding), not pig-tailed; and no (or minimal) breaks in the cable, with any splices having the shields solidly and circumferentially bonded just as at the ends.

With proper planning and consideration, your VFD cabling installation can serve you effectively for many years. If you have concerns or questions, please let us know by visiting our Comments section, or contact us at simon.fan@vtdrive.com or visit us at www.vtdrive.com and www.vtdrive.tk. We’ll be glad to assist.