L&S Electric, Inc.

Understanding Shaft Current Problems in AC Motors

Shaft Current History

• Shaft currents are not a new problem.

• Technical papers written on the subject date back to the 1930s

• Shaft currents come from multiple sources– Electromagnetic Asymmetries– Variable Frequency Drives (PWM VFD)– Capacitive coupling

Shaft Current Damage

• When shaft currents flow the result is damage to the bearing races and rolling element.

• The damage can be minimal (frosting) or more severe creating a pattern of fluting on the inner and outer races.

• Either way, the bearing life is shortened.

Severity of Bearing Current Damage

• The photograph below shows signs of the pitting that can occur.

Severity of Bearing Current Damage

• Severe cases of shaft bearing currents produce a fluting pattern on the inner and outer races.

Severity of Bearing Current Damage

• Fluting on inner race caused by shaft bearing currents.

Shaft Currents

• We refer to these currents as shaft currents as it is the current that causes the damage

• There is no practical method to measure these currents, so we measure the magnitude of the voltage instead.

• How much voltage is acceptable?

Acceptable Shaft Voltages

• Most manufacturers use a rule of thumb of 100 mV for ball bearing applications and 200mV for sleeve bearing applications.

• NEMA MG-1 part 31.4.4.3 suggests a limit of 300 mV measured end to end on the shaft.

Measuring Shaft Voltages

• Test Equipment:• Oscilloscope: Fluke Scope meter 199C• 600 MHz 2.5GS/s• Probe: Fluke VP 210• 200 MHz 10 to 1

Sources: Electro-Static Technology, L&S Electric

Electromagnetic Asymmetries

• Circular residual fluxes resulting from electromagnetic asymmetries in the construction of AC motors can generate voltages end to end in the shaft during operation.

• Examples of electromagnetic asymmetries are gaps in the iron (segmented laminations), uneven air gap, and circulating currents in the parallel circuits of a three phase winding.

• This condition primarily exists in larger frame sizes (above 500 frames).

Current Path

• The current path is from the motor frame through a bearing to the motor shaft, down the shaft, and through the other bearing back to the motor frame.

Source:: Electro-Static Technology

VFD Produced Bearing Currents

• Variable frequency drives (PWM VFD).• VFD rectifies incoming AC voltage.• Resulting DC voltage is applied to the DC Bus.• The DC is “chopped” into positive and negative

pulses to simulate an AC sine wave.• The DC pulse width is varied simulating

a variable AC sine wave which changes in frequency.

• The change in frequency allows us to change the motor speed.

VFD Produced Bearing Currents

• Common Mode Voltages– A three phase motor operating on true sine

wave power is balanced and has a common mode voltage that is always zero.

– With VFD produced voltages the balance no longer exists.

– DC is either positive or negative so at any point in time the three phases are ++- or +--

Common Mode Voltages

• Unbalanced line voltage

• Voltages should sum to zero but don’t if phases are unbalanced.

• Neutral might reach 20–30 volts with severe unbalance

Sources: Electro-Static Technology, Marathon Electric

Common Mode Voltages

• VFD voltage is inherently unbalanced.

• Problem is worse on 460 volt VFD power.

• Motor neutral reaches 375 volts.

Sources: Electro-Static Technology, Marathon Electric

Capacitive Coupling

• VFDs are generators of high frequency common mode voltages.

• Common mode voltages are coupled to the rotor through parasitic capacitances between the stator winding and the rotor.

• The currents generate circular time varying magnetic fluxes which induce shaft end to end voltages.

Capacitive Coupling

• A static charge builds up over the entire rotor surface.

• The rotor charge discharges to ground through the bearings and returns to the VFD.

Sources: Electro-Static Technology, Marathon Electric

What effect does this have on the bearings?

• Rotor charge builds up until it exceeds the insulation level of the bearing’s oil film

• The stored energy produces an arc as the rotor voltage collapses

• Current flow is concentrated in a pinpoint sized area where the ball meets the races

• Such high energy concentration melts the bearing metal creating microscopic pits in the surface of the races

Sources: Electro-Static Technology, Marathon Electric

Factors Affecting Shaft Currents

• Long cable runs from drive to motor.

• Poor grounding connections.

• Higher switching frequencies on the PWM drive.

• Later designs of VFDs have higher switching frequencies.

Solutions for Eliminating Shaft Currents

• Insulating of bearing housings.

• Installation of grounding brushes.

• Use of insulated bearings.

Insulating of bearing Housings

• Both housings need to be insulated to eliminate the effects of capacitive coupling.

• Housings wear with usage and must be replaced.

• Cost to insulate housing is high.

• Insulation can be compromised by contamination, etc.

Grounding Brushes

• Is an effective method to mitigate all types of currents.

• Some brush designs are good for the life of the motor.

• Two brushes may be necessary on larger frame motors.

• Two brushes required to eliminate effects of capacitive coupling.

Insulated Bearing Designs

• Ceramic rolling elements.• Bearing life is as good as standard bearings or

better.• SKF Insocoat bearing.• Races are coated with an insulating material.• Rolling element same as standard bearing.• Cost for these types of bearings can be as much

as double that of standard bearings.

Comparison of Solutions

Comparison of SolutionsAEGIS SGR TM Insulating

SleeveCeramic/ Hybrid Bearing

Copper or Bronze Metal Brush

Carbon Black Brush

Conductive Grease

Protects Motor and Attached Equipment

Yes No No No No No

Long-term Effectiveness

Yes No No No No No

Easy to install Yes No No No No No

Contamination Proof

Yes No No No No No

Low Lifetime Cost - High Return on Investment

Yes No No No No No

Effective at any RPM

Yes No No No No No

Maintenance Free Operation

Yes No No No No No

Sources: Electro-Static Technology, Marathon Electric

L&S Electric: we continually work hard to keep you running!