Machine designers who choose VFD cables for their motor-driven systems want to be sure their drive is reliable in order to take full advantage of the precision and energy savings VFDs can offer. Because VFDs produce unbalanced three-phase vector sums that give rise to various unwanted paths to earth, VFD cables must be shielded and properly grounded.
VFD Cable Grounding Basics
One of the most common questions SAB North America gets asked is what is the best way to ground VFD cables? Here are some of our recommendations:
The best way to ground a shielded VFD cable is on both sides of the cable connection — at the drive and at the motor. That’s because increases in voltage and common mode current (CMC) can travel towards the motor and through the stator, causing the ball bearing on the stator to fail prematurely. To address this, we suggest installing a shielded VFD cable with a tinned copper braid that can be pigtailed and terminated to the second PE terminal on the drive and on the motor.
Additionally there are VFD cable designs that include a single drain wire or multiple drain wires. Grounding a drain wire by itself offers less copper mass than pigtailing the shield, consequently resulting in poorer grounding reliability. We recommend pigtailing the shield together with the drain wire or wires. This offers a much larger copper mass to sufficiently drain the noise and create a better ground path.
Create a Faraday Cage
Of course, engineers also want to avoid electromagnetic interference (EMI) issues. In VFD systems, EMI can affect surrounding circuits and cause data degradation or even a total loss of data. You can further guard against EMI by incorporating a grounding gland at the enclosure and motor housing. Grounding glands, such as the EMC-2 and EMC-4 from SAB North America, channel electricity through the enclosure and motor to ground to create a 360-degree contact at the enclosure — essentially replicating a Faraday cage. And when combined with the braiding-and-draining method, EMC grounding glands provide complete protection against unwanted CMC noise. In fact, grounding glands can eliminate the need for drain wire.
SAB North America EMC grounding glands with strain relief are impact resistant, watertight and easy to install. The EMC-2 resists corrosion, while the EMC-4 with bi-directional installation offers free radial and axial cable movement without damaging the braid.
Proper Shielding and Grounding Ensure All-around Protection
Machine designers depend on VFD drives to operate with precision, but their reliability can be compromised by unwanted ground paths. VFD cables need proper shielding and grounding both where they connect to the drive enclosure and where they connect to the motor. SAB North America shielded VFD cables and EMC grounding glands can help create the all-around protection VFD systems need against noise and the common mode current issues that can cause bearing failure.
For more information about VFD cables and glands, download our brochure: