3 Shaft Grounding Alternatives to Help Protect VFDs

As the demand for automation and energy efficiency increases, many facility managers, building owners, and engineers are switching to variable frequency drives (VFDs). These solutions are among today’s most sustainable technologies, enabling motors to function at reduced speeds without sacrificing torque. VFDs ultimately work to reduce power consumption in electric motors, saving at least 30% in energy costs. At the same time, VFDs often need more protection than other systems.

Common-mode currents induced by VFDs may cause electrical discharge machining (EDM) that can damage electric motor bearings, shortening their service life and compromising the system’s reliability. Many rotational machines like pumps, generators, and motors use shaft grounding rings to mitigate this issue. These rings allow for easier grounding of electrical currents that would otherwise build up on the machine’s rotating shafts, preventing subsequent fluting and pitting on the bearings’ raceways.

The grounding ring channels electrical currents along a ring and brush, redirecting them away from the bearings. In theory, shaft grounding rings effectively reduce voltages, but only until they are contaminated by grease or dust. In addition, the rings don’t reduce the damaging currents—they only divert them to the ground grid, where these currents create electrical EMI issues with any electronic device, such as relays and sensors connected to the same electrical grid.

Although grounding rings may help protect motors from damage, there are some other alternatives available, but only one that is the most effective: Nanotech^® common mode choke cores.

Alternatives to Grounding Rings

The alternatives to grounding rings include shielded cables, insulated bearings, and common mode chokes. Unfortunately, shielded cables and insulated bearings have their own pitfalls as with the grounding ring, leaving the common mode core choke as the best alternative to solving common mode current issues.

Shielded Cables

One alternative to grounding rings is to run shielded cables from the VFD to the motor. The shielded cable acts as a Faraday cage, effectively isolating the VFD and motor from the external environment by providing a direct ground path back to the VFD. Unfortunately, the shielded cable is usually not connected properly, which negates the shielding on the cable. In addition, shielded cables add an extremely high cost to the VFD/motor system. In instances where standard cables are buried beneath the floor, replacement is usually not an option.

Insulated Bearings

Another alternative to grounding rings is to install ceramic insulated bearings, which form a high-impedance electrical barrier for damaging electrical currents, protecting bearings and motors.

As with grounding rings and shielded cables, there are many problems associated with insulated bearings. First is the cost. These bearings cost significantly more than traditional bearings, and they still require maintenance and must be replaced periodically. Secondly, the insulated bearing does nothing to reduce the common mode currents. It only traps the motor’s common mode currents, allowing them to circulate and cause excessive heating in the windings. Finally, the common mode currents can eventually overcome the insulation within the bearing and destroy the bearing.

Common Mode Chokes

The most effective alternative to shaft grounding rings is Nanotech^® common mode chokes. By installing Nanotech^® common mode chokes around the VFD’s output conductors to the motor, the damaging high-frequency common mode current is absorbed in the source of the noise (VFD’s IGBT/SiC/GaN switching) well before the damaging currents reach the motor. As with the other solutions, these common currents are absorbed instead of redirected or blocked.

Advantages of Nanotech^® Common Mode Chokes

There are several advantages to using Nanotech^® common mode chokes over traditional alternatives. Some of the most notable advantages include:

• Improved Performance: Nanotech^® common mode chokes provide superior filtering performance, reducing EMI and improving signal quality. In addition, Nanotech^® common mode chokes are the only solution that reduces conducted and radiated EMI interference that affects relays, sensors, and other electronic devices connected to the electrical grid. This becomes extremely important when unshielded cables are used.
• Installation: Installation can be done in less than an hour, eliminating long downtimes associated with the other alternatives.
• Higher Reliability: Nanotech^® common mode chokes are a permanent, no-maintenance lifetime solution, unlike all the other alternatives. Because they do not rely on physical contact with the shaft, common mode chokes are less susceptible to wear and tear and can provide longer service life.
• Cost-Effective: Common mode chokes are typically less expensive than traditional grounding rings, making them a cost-effective solution for improving system performance.

Why treat an electrical problem with a mechanical device when the electrical solution of Nanotech^® Common Mode Chokes is readily available?

Innovative Nanotech^® Cores From MH&W

You can prevent common mode currents from causing damage or interference to your VFDs, allowing for efficient and cost-effective operation. Choose Nanotech^® to reduce electrical noise and ensure your high-power inverter systems continue working as they should. Contact MH&W for more information.