August 20, 2011

Electrical Fluting: How To Identify And Correct This Problem

by: Nate Morse

What Is Electrical Fluting?

Electrical fluting (also known as electrical pitting) is a bearing failure phenomenon where a current is passed from the rotor through the inner race to the ball or roller bearing, through the outer race and finally to ground by the frame of the motor.

If you’re like me the first thing you think after reading the above statement is what the heck! How is a current getting to the motor rotor in the first place? Not being an engineer, the closest thing to an explanation i can come up with is this: a current is being induced to the motor rotor due to inconsistencies in the power, which is being supplied to the motor.

This weird current racing through the balls of the bearing causes pits in the inner and outer race of the bearings. As the arc passes through the grease, it heats it up to above 2000°f, which reduces the ability of the grease to do its job, leading to further damage of the bearings.

Now that we know what electrical pitting is, how do we identify it?

In the HVAC world, we see electrical fluting as associated with motors which are being operated by variable frequency drives. However, a VFD is not always the problem. Dirty power can cause electrical pitting. It is also possible for a vfd to cause electrical fluting in the bearings of the pump, fan, compressor or whatever the motor is driving.

The only way to be 100% sure that electrical fluting is occurring (prior to bearing replacement) is to test the motor with an oscilloscope or a meter which measures radio frequencies caused by the racing of electricity. Having said that, it’s a pretty good bet that you don’t have either of these on your truck so what do you do?

First take your headphones off and listen the bearings. The sound of a fluted bearing is like hitting the rumble strips on the side of the highway going 60 mph. Next, look at the grease. At 2000°f the grease will burn - turning it black (unless someone has put black grease in the bearing). Electrical pitting usually affects the outboard or opposite of drive end bearing.

Why? Because it’s almost always a smaller bearing which makes it easier for electricity to arc. So if you’re thinking the inboard or drive end bearing of the motor is fluted, be careful not to misdiagnose the problem. Just because a motor is on a drive doesn’t mean electrical pitting is the problem. The last thing you should do after the bearings have been replaced is to cut the old bearing open and inspect it. Compare the bearing to the koyo bearing failure chart.

How Can We Prevent Electrical Fluting?

One of the best ways to prevent electrical pitting is to install ceramic bearings on the motor. Ceramic bearings are constructed with steal inner and outer races with the balls or rollers made of ceramic. The ceramic balls will not conduct electricity thus breaking the path to ground. Another way to reduce the possibility of electrical pitting is to install a grounding brush kit. The grounding brush is installed on the end bell of the driving end of the motor around the shaft. The brushes are made of a flexible conductive material which provides an area of lower conductivity to ground than the bearings.

There is a potential problem with installing ceramic bearings on the shaft of a motor. I have learned that if ceramic bearings are installed on both ends of the motor shaft high currents could build up in the shaft of the motor. Electricity being electricity will eventually find a way to ground and if it can’t go to ground through the bearings where’s it going to go? Through the windings of the motor that’s where; this causes another problem - motor failure!

To ensure we don’t have this problem, we could install one ceramic bearing and one standard bearing. Not the best practice, as electrical pitting can now affect the inboard bearing. We want to eliminate the possibility of electrical pitting, not just prolong the process. We could install 2 ceramic bearings and grounding brush kit - this will positively eliminate the possibility of electrical fluting but this method is a little pricey. Most havoc equipment will be properly protected by a combination of the ceramic bearing mounted on the outboard side and a grounding brush on the inboard side. This method protects the outboard bearing through insulation and the inboard bearing by providing an easy way to ground while also preventing high currents from building up in the rotor.

About Nate Morse

Nate is one of our Hero Technicians out there in the field everyday doing what he loves. We are proud of Nate. He has been an outstanding AirTight employee!

About AirTight

AirTight is a full service mechanical and mission critical environments contractor with a heavy emphasis on service, maintenance and repair. AirTight specializes in mission critical cooling and electrical infrastructures, as well as the comfort cooling surrounding them. AirTight provides data-center and UPS assessment services in conjunction with select partners.

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