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Shaft voltage
Shaft voltage occurs in electric motors and generators due to leakage, induction, or capacitive coupling with the windings of the motor. It can occur in motors powered by variable-frequency drives, as often used in heating, ventilation, air conditioning and refrigeration systems. DC machines may have leakage current from the armature windings that energizes the shaft. Currents due to shaft voltage causes deterioration of motor bearings, but can be prevented with a grounding brush on the shaft, grounding of the motor frame, insulation of the bearing supports, or shielding.
Shaft voltage can be induced by non-symmetrical magnetic fields of the motor (or generator) itself. External sources of shaft voltage include other coupled machines, and electrostatic charging due to rubber belts rubbing on drive pulleys.〔Hamid A. Toliyat, G. B. Kliman ''Handbook of electric motors'' CRC Press, 2004 ISBN 0824741056,pp.672-688〕
Every rotor has some degree of capacitive coupling to the motor's electrical windings,〔"Turning The Tables On Shaft Voltage And Bearing Currents", David W. Schlegel, Russel J. Kerkman, and Gary L. Skibinski, ''Electrical Construction & Maintenance (EC&M) Magazine'', June 1998, ()〕〔"How to Select a VFD", John Yoon (PE, LEED AP), ''Consulting-Specifying Engineer'', November 2010, ()〕 but the effective inline capacitor acts as a high-pass filter, so the coupling is often weak at 50–60 Hz line frequency. But many Variable Frequency Drives (VFD) induce significant voltage onto the shaft of the driven motor, because of the kilohertz switching of the insulated gate bipolar transistors (IGBTs), which produce the pulse width modulation used to control the motor.〔http://www.est-aegis.com/ASHRAE_Paper-Shaft%20Grounding-A_Solution_To_Motor_Bearing_Currents.pdf〕 The presence of high frequency ground currents can cause sparks, arcing and electrical shocks and can damage bearings.〔http://www.controleng.com/index.php?〕
==Counter-measures==
Techniques used to minimise this problem include: insulation, alternate discharge paths, Faraday shield,〔http://www.greenheck.com/library/articles/58〕 insulated bearings, ceramic bearings, grounding brush 〔 and shaft grounding ring.
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