We are beginning to investgate shaft voltage issues with motors in our plant. What is the best method to get an accurate measurement?

Thanks,

Just curious, but why are you investigating this area? This is not a periodic check and is usually the result of having lost mulitple bearings due to EDM. Are the motors in question on VFD's? If the answer is yes, then it is imperative that you check the power leads between the motor and the VFD for proper high frequency grounding (a single twisted copper ground wire will NOT work). Also, if the motors are over a 444 frame, then the outboard bearing must be insulated from the end bracket. If DC, then make sure the outboard bracket 'add-ons' (tach, RTD's etc.) are insulated as well.
That said, the best method for reading shaft voltage is to use a motor carbon brush mounted on a non-conducting 'stick' into a voltmeter, with the meter ground to the ground connection on the motor frame. It is also important to establish current levels as well. If a VFD is involved, this will require the use of a Rogowski Coil (air cored current sensor) due to the high frequency of the dv/dt switching. These currents can be as high as 5MHz to 10MHz!

IMO shaft current measurements are more informative as the measurement loop is a low impedance loop (short circuit cable between motor shaft ends / frame earth and the Rogowsky CT clipped-on this cable). Certainly with VFD a shaft voltage loop is sensitive for high frequency noise as such a loop is an high impedance loop.
Good luck!
Arie Mol, NL

What kind of frame is a 444 frame?