Electrical Bearing Protection Q&A – Part 22
Q1. What are the pros and cons of shaft grounding rings compared with shaft grounding brushes?
A1. Our understanding is that carbon block brushes wear a lot faster than our AEGIS® rings. There are some other factors, too, but these brushes have a couple of strengths.
Carbon brushes can conduct dc current better than high-frequency ac current. These brushes have been used for over 100 years in dc motor commutators. With grounding rings, it's the opposite. So you wouldn't want to use an AEGIS ring in a commutator. But VFD-sourced shaft current is high-frequency, so AEGIS rings are the better fit there.
There are also cases where you have dc/low-frequency and high-frequency current. So sometimes you want a carbon brush and an AEGIS ring. Wind turbine generators are a great example. Neither a carbon brush nor a grounding ring alone is enough. But together, they can protect the generator bearings.
Q2. Can a shaft grounding ring protect a motor from nearby welding?
A2. We wouldn't recommend it. Welding current can be hundreds of amps, continuously. Depending on the size of the rings, SGRs can handle up to 70 amps, and PRO rings up to 250, but those are peak ac current ratings, not continuous dc.
Q3. If you use a shielded VFD cable, should the shield be attached to the grounding ring?
A3. No. When you use shielding, it should be connected with glands at the motor junction box and at the VFD. It's important to get this right because doing it wrong will lose most of the shielding's benefits. There's more detail in this blog post by Southwire. At any rate, you should always follow the drive manufacturer's instructions.
Q4. If we specify shaft grounding rings be used on VFD-controlled motors, do we also need to specify high-frequency grounding straps explicitly?
A4. Short answer: Yes, if you want HFGS to be installed. We recommend using HFGS to bond the motor and drive, and motor and load. But the grounding ring is more important. If the motor is well-grounded, you can usually get by without an HFGS. But there are cases where you definitely want to use at least one. In those cases, you need to specify them explicitly.
AEGIS Rings also come with a 2-year extended warranty against bearing fluting damage. No other form of protection against VFD-caused bearing damage offers a warranty like this.
To learn more about AEGIS shaft grounding and best practices for electrical bearing protection, sign up for a training. We offer monthly live training webinars, and - pandemic restrictions permitting - we can also visit your facility to review your exact application.