We get a lot of questions about the resistance (ohms) of our AEGIS Shaft Grounding Rings. Fair enough: Our rings work by providing a low-resistance path between the frame and shaft, so stray electric current will flow through the ring instead of through the bearings. It's natural to wonder how much resistance current feels when it flows through the ring. The answer is typically a few to several ohms*, but there's much more to the story.
When Ohm's Law Isn't Enough: Understanding AEGIS Rings and Effective Resistance
When you use a multimeter or ohmmeter, it uses Ohm's Law to calculate the resistance. But Ohm's Law only applies under certain circumstances. One requirement is that all of the current-carrying components must be physically connected.
When a shaft grounding ring is installed on a motor, its fibers contact the shaft. But the fibers vibrate when the motor is running, so as few as one to ten percent of them remain in contact. However, even when the fibers do not physically contact the shaft, they often remain close enough to allow current to flow. At fiber-shaft distances less than five microns, shaft voltage as low as 2V can drive current flow by field emission of electrons. If the field emission current gets intense enough, the air will electrically break down and conduct much better.
The value of the resistance from an ohmmeter does not capture this no-contact conduction. The "effective resistance" of AEGIS rings, including both contact and noncontact paths, is difficult to measure (though it can be done with the setup shown below). The effective resistance is not a constant property of a grounding ring; it depends on the size of the current flowing, its frequency content, and the shaft speed.
The resistance you would measure with an ohmmeter is significantly higher than the effective resistance when the grounding ring is in operation. The good news is that the measured resistance is an overestimate of – an upper limit to – the actual effective resistance.
But the fact is that you shouldn't worry about the resistance of an AEGIS ring. There are better ways to check whether a grounding ring is working. In our next post, we will explain why you should test shaft voltage instead of resistance.
AEGIS Rings 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.
Note: If you want to measure an AEGIS Ring's resistance, you can't just stick one ohmmeter lead in the fibers and one on the metal housing. Because they're so thin, individual fibers actually have high resistance, hundreds of ohms. But when the ring is mounted on a shaft, there are tens or hundreds of thousands of fibers touching the shaft, so the total resistance is much lower. To measure the resistance, you need to mount the ring on a motor shaft or a metal cylinder that touches fibers all the way around, and then measure between the cylinder and the ring housing.
