Shaft-Voltage occurs in motors powered by variable-frequency drives. Variable Frequency Drives (VFD) induce shaft voltages onto the shaft of the driven motor because of the extremely high speed switching of the insulated gate bipolar transistor

Insulated gate bipolar transistor

The insulated gate bipolar transistor or IGBT is a three-terminal power semiconductor device primarily used as an electronic switch and in newer devices is noted for combining high efficiency and fast switching...

s (IGBT) which produce the pulse width modulation used to control AC motor

AC motor

An AC motor is an electric motor driven by an alternating current.It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the...

s in heating, ventilation, air conditioning

HVAC

HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...

 and refrigeration

Refrigeration

Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

 systems. 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 dishcharge paths, Faraday shield, insulated bearings, ceramic bearings, grounding brush and shaft grounded 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

Bearing

Bearing may refer to:* Bearing , a term for direction* Bearing , a component that separates moving parts and takes a load* Bearing capacity*Bearings, album by Ronnie Montrose in 2000...

 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 device provides an alternate low-impedance path from the motor shaft to the motor case. This 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 (SGR) is similar to a grounding brush, except that this brush makes use of conductive micro fibers, creating a low impedance path from the motor.

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

Electrical impedance

Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

 path between the VFD and the motor. One popular cable type is continuous corrugated aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

sheath cable.

External links

• 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 from Electrical Manufacturing and Coil Winding