Electrical Bearing Protection Q&A Part 10
Part 10 in a series of questions and answers from our Q&A webinars.
Q1. How do I implement a plantwide program to adopt AEGIS shaft grounding on all of our VFD-fed motors?
A1. We run into that every week. It's one thing for us to tell you, "This is the solution." But then how do you implement that solution? You've got dozens or hundreds of existing motors. Some of them already have bearing problems, and some don't. Obviously, the ones with problems need to be repaired and have AEGIS added or to be replaced with motors with AEGIS factory-installed.
But you don't want to stop there. Every motor on a drive is at risk. And it's way more cost-effective, even for 1-2 hp motors, to use shaft grounding and avoid the cost and hassle of replacement. Suppose you've got a $400 2 hp motor that fails once a year. OK, that's $400/year for a new one. If you swap in a 2 hp motor with AEGIS, that's a bigger expense in Year One... But then that motor doesn't need to be replaced, not for electrical bearing damage. AEGIS rings are usually good for 100,000 hours. That's quite a few years where you're spending $400 less each year. Now multiply those savings by every motor you have on a drive, and that adds up.
There are two philosophies on how to implement the program. Some companies just replaced or put rings on all of their motors at once. But usually it's phased in. One way to do it is to get a small inventory of motors with AEGIS, and as in-service motors fail or need work, repair or replace them with AEGIS-installed motors. Then you continue the acquire & swap out process until every motor on a drive has a ring.
And if you ever try to implement this and get hung up on something, give us a call. Our Regional Sales Managers would be happy to talk to you or visit your plant and help you phase this in.
Q2. I know that larger motors on drives are subject to high frequency circulating currents. Do they still have problems with shaft voltage discharge (EDM) current?
A2. The short answer is Yes. When a three phase motor is on a drive, the voltage in the three phases will never cancel out. There will always be some extra voltage in the motor windings. And that extra ("common mode") voltage is always going to charge up the rotor and create a shaft voltage. Now, we say "shaft voltage," but the real problem is that you have a voltage across the bearings. And if the shaft voltage is bigger than a few volts, it's going to arc through the bearings and damage them.
It's true that circulating currents are actually a bigger problem than shaft voltage in these large motors. You can interrupt (stop) circulating currents with one insulated bearing. Protecting one bearing from circulating current protects the other too. But insulating one bearing won't protect the other bearing from EDM bearing current from shaft voltage discharge. So to minimize the risk of electrical bearing damage in these motors, you need one insulated bearing and good reliable shaft grounding like an AEGIS ring. (And grounding of the motor frame to prevent rotor ground current, but that's another issue.)
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.