Shaft voltage occurs in

electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate for ...

s and generators due to leakage, induction, or capacitive coupling with the windings of the motor. It can occur in motors powered by

variable-frequency drive A variable-frequency drive (VFD) is a type of motor drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and, depending on topology, to control associated voltage or current va ...

s, as often used in heating, ventilation, air conditioning and

refrigeration The term refrigeration refers to the process of removing heat from an enclosed space or substance for the purpose of lowering the temperature.International Dictionary of Refrigeration, http://dictionary.iifiir.org/search.phpASHRAE Terminology, ht ...

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 , pp.672-688 Every rotor has some degree of capacitive coupling to the motor's electrical windings, but the effective inline capacitor acts as a

high-pass filter A high-pass filter (HPF) is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency ...

, 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 transistor An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch, which, as it was developed, came to combine high efficiency and fast switching. It consists of four alternating lay ...

s (IGBTs), which produce the pulse-width modulation used to control the

motor An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power g ...

. The presence of high frequency ground currents can cause sparks, arcing and electrical shocks and can damage bearings.

Counter-measures

Techniques used to minimise this problem include: insulation, alternate discharge paths,

Faraday shield A Faraday cage or Faraday shield is an enclosure used to block electromagnetic fields. A Faraday shield may be formed by a continuous covering of electrical conductor, conductive material, or in the case of a Faraday cage, by a mesh of such mat ...

,Are Bearing Currents Causing Your Motor Failures?
Greenheck insulated bearings, ceramic bearings, grounding brush and shaft grounding ring.

Faraday shield

An electrostatic shielded induction motor (ESIM) is one approach to the shaft-voltage problem, as the insulation reduces voltage levels below the dielectric breakdown. This effectively stops bearing degradation and offers one solution to accelerated bearing wear caused by fluting, induced by pulsewidth modulated (PWM) inverters.

Grounding brush

Grounding the shaft by installing a grounding brush device on either the non-drive end or drive end of a VFD electric motor provides an alternate low-impedance path from the motor shaft to the motor case. This method channels the current away from the bearings. It significantly reduces shaft voltage and therefore bearing current by not allowing voltage to build up on the rotor.

Shaft grounding ring

A shaft grounding ring is installed around the motor shaft and creates a low impedance pathway for current to flow back to the motor frame and to ground. Various styles of rings exist such as those containing microfilaments making direct contact with the shaft or rings that clamp onto the shaft with a carbon brush riding on the ring (not directly on the shaft).

Insulated bearings

Insulated bearings eliminate the path to ground through the bearing for current to flow. However, installing insulated bearings does not eliminate the shaft voltage, which will still find the lowest impedance path to ground. This can potentially cause a problem if the path happens to be through the driven load or through some other component.

Shielded cable

High frequency grounding can be significantly improved by installing shielded cable with an extremely low impedance path between the VFD and the motor. One popular cable type is continuous corrugated

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

sheath cable.

References

External links

*{{cite web , title=Technical guide No. 5 – Bearing currents in modern AC drive systems, url=http://www05.abb.com/global/scot/scot201.nsf/veritydisplay/8c253c2417ed0238c125788f003cca8e/$file/abb_technical_guide_no5_revc.pdf , accessdate=May 23, 2011 , archiveurl=https://web.archive.org/web/20110720013656/http://www05.abb.com/global/scot/scot201.nsf/veritydisplay/8c253c2417ed0238c125788f003cca8e/$file/abb_technical_guide_no5_revc.pdf , archivedate=July 20, 2011
A Unique System for Reducing High Frequency Stray Noise and Transient Common Mode Ground Currents to Zero, While Enhancing Other Ground Issues Meeting Notices and Rule Changes
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