"I quickly learned a lot about games to watch out for and enjoyed watching - very funny" Joel Grey - Purchasing - LPP Industries
Take a few minutes and enjoy the antics of the fictitous Burnham Electric. Entertaining as well as Educational Movie on tricks employed by a few that often are played on folks who let only the cheapest bid dictate who they give the job too.
Veiled in the shadows deep within the bowels of your motor, lurks an unknown evil.
This strange beast goes by the name “Shaft Currents”.
Shaft Currents are phenomena that are not fully understood and have come out into the light more in recent years – especially with the growth of Variable Frequency Drives controlling motors. Reed has seen a steady increase in the damage from shaft currents. What this weird monster does is eat up bearings. A variety of sources induces a voltage to flow from the motor shaft to ground. And when this current flows through the vulnerable bearings a destructive arcing occurs between the bearing races and balls. (The same effect can even occur in sleeve bearings.)
The photo below and those within the Service Bulletin on the right show you what a bearing ruined by Shaft Currents look like. Black fluting divots (similar to Electro Discharge Machining or etching machines used to print on metal) in a regular pattern can be seen on the raceways (where the balls ride) of the bearings. The balls will have either a frosted or pitted look. You or your repair facility should always be looking at the worn bearings at servicing to detect this event.
Sources of shaft currents:
VFD’s – high switching/carrier frequency; dirty waveform output, PWM types that create an electrostatic coupling
Motor design or manufacturing anomalies
Static electric friction charges from cooling vent passages, fans, pulleys & couplings
Ground loops from somewhere else in the vicinity
Arc-Welding which places the bearings in between the welder ground clamp and welding rod
How to stop these malevolent Shaft Currents?Besides the obvious onsite corrections, Reed and others have found 2 of the most effective silver bullets to kill this hideous creature and end his bearing eating cycle. Bullet #1 is to insulate the bearing housing (some prefer shaft journals). But be careful with RTD’s and other bearing temperature devices – if not changed to the proper type, you can again provide the path for Shaft Currents. Bullet #2 is the installation of a Shaft Grounding Brush that can be seen in the photo below. For ball bearing installations, in April 2005, SKF has introduced a line of aluminum oxide coated bearing races with the trademarked name INSOCOAT which they claim insulate against shaft currents. For more detail click on the file to the right for the 3rd in our featured series of Service Bulletins from the engineers at US Motors . Our thanks to Austin Bonnett – one of the sharpest motor engineers around.
A costly problem we run across occasionally involves a missing part. Some Reliance and GE 2-pole (3600RPM) TEFC (totally-enclosed fan-cooled) motors have very long stator designs. This length puts the drive-end bearing far away from the air stream of the cooling fan. Tocool the drive-end bearing, these designs incorporate a small, thin fan mounted on the drive-end with a flat sheet metal guard. This design works rather well… until….. Because the fan is small and fragile, they occasionally get broken or lost over the years. And when they are not replaced, it is difficult to visually detect that these fans were originally installed on the motor. These small odd drive-end fans are nothing like the big Main Motor Cooling fans that are always on a TEFC motor. What then happens is the drive-end bearing runs much hotter and has a shortened life. A motor that had provided successful service for years, suddenly just doesn’t seem to hold up like it used to. And the culprit is a $50 to $100 Missing Fan. Tips:
Check your 3600RPM motors for a drive-end fan before sending out for repair to make sure that any with a bearing fan are returned with this fan
If you have a 3600RPM motor that:
You just can’t keep grease in – it runs out
Has a drive-end bearing that is running much hotter than it used to
You just can’t keep bearings in
Then you may have one of these designs that are now missing the little drive-end bearing fan setup. Look on the drive-end endbell exterior for screw holes that would have been used to mount the drive-end fan guard. This would be your clue to the Mystery of the Missing Fan.
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I fixed them Squealin' Belts
Brute Force !
To combat broken shafts & waste in bad pulley setups, the Experts at US Motors have developed this Service Bulletin. US developed this training after countless investigations of real-world catostrophic failures - save yourself the heartache of waste... we can't tell you how many times the wrong answer to squealing/slipping belts has resulted in Brute Force broken shafts!
It may sound wierd, but a common Fish Scale (ya, you read it right--a scale to weigh fish) is a tool to check the RIGHT belt-tension. Contact Reed for this "fishy" story.
Our thanks to the engineer Jim Bryan at US Warranty for this Bulletin. (knows his stuff)
EASA is an international association dedicated to furthering quality advancements in the repair of electrical apparatus. While hamstrung by legalities that both protect rights yet shield manipulators of the legal system, EASA is one resource you will find valuable.
In our opinion, EASA is both worthy of support and frequent educational inspection.
The input of technical info into EASA is provided by many well-intentioned, genuine, and experienced folks from around the world. Give 'em a try... they do it for free.
