Reed Electric

Shaft Current Horrors !

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 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.

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Reed Electric - LA
5503 S. Boyle Avenue, Los Angeles, CA 90058
Ph: 323-587-2284 / Fax: 323-587-2142

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5375 Louie Lane, Reno, NV 89511
Phone: 775-824-7333 / Fax: 775-824-7344