bearing erosion ac motors

8/8/2019 Bearing Erosion AC Motors

1/17

Solving VFD-sourced Bearing CurrentProblems in AC Motors.


2/17

The 2 main sources of motor shaft voltage are:

1. Asymmetric Motor construction The stator and rotor magnetic circuits within the motor

itself are unbalanced (current path stays within the

motor)2. Unbalanced supply voltage (current path is

between the motor and VFD) Unbalance is between the motor and power source

Operation from a VFD creates an unbalanced supplycondition. The 3 phase output waveform is produced by

switching DC voltage On & OFF

Common Mode Voltage (CMV) results from the voltagewave forms from each of the three phases notsumming to zero


3/17

Common Mode Voltage (CMV):

Voltages should sum to zero butdont if the 3 phases are unbalanced.Neutral might reach 20-30 volts withsevere unbalance.

Problem is worse on 460 V VFDpower. Motor neutral reaches 375 V!

Unbalanced line voltage VFD voltage is inherently unbalanced


4/17


5/17

What effect does this have on the bearings?

Rotor charge builds up untilit exceeds the insulationlevel of the bearings oil film

The stored energyproduces an arc as therotor voltage collapses

It arcs from the inner racethrough the rollingelements to the outer race,

then to ground Current flow is

concentrated in a pinpoint-sized area where the ballmeets the races

Such high energyconcentration melts thebearing metal, creatingmicroscopic pits


6/17

Repeated discharges can

be seen on the raceways: Both races and rolling

elements are affected

Inner race & rollingelements will becomefrosted

Vibrations + arcing = Fluting Flutes form on outer race Noise, temperature &

vibration increasedramatically


7/17

Roller bearings are affected too!


8/17

Which motors are susceptible?

ALL OF THEM

All motors powered by VFDs, which by design produce CMVAll manufacturers/brands of AC Induction motorsAll motor frame size

All horsepower ratingsAll speeds

All bearing typesAll duty cyclesAll applications

* Variable Torque* Constant Torque

* Constant Horsepower All the time


9/17

To eliminate VFD-sourced bearing currents:

1. Reduce or cancel CMV before it reaches therotor:

Install CM loading filter (with RC network toground) at the VFD output Add Marathons patented Faraday shield in the

stator windings---OR---

2. Protect the motor bearings by one or acombination of devices:

Ground brush (bleeds off charge)

Insulated bearings (insulation blocks current) Shaft Grounding Ring (bleeds off charge)


10/17

Do Ground Brushes really work?

Yes! One brush will reduce shaft voltage by ~99%.......BUT

They wear out, requiring regular maintenance AND External brushes are not permitted in Div 1/Zone 1 or Div

2/Zone 2 Hazardous locations

What about insulated bearings? Yes! Insulated bearings are effectiveBUT They can be up to 10 times the cost..AND Extended lead-time Insulated bearings arent permitted in Hazardous locations

either


11/17

What is AEGIS Bearing Protection Ring

Technology? AEGIS SGR is a circumferential ring of conductive

micro fibers that reduce the shaft voltage to a safe

level AEGIS Shaft Grounding Ring provides a reliable

discharge path for shaft voltages AND

Because there is no wear pressure on the fibers AEGIS SGR is the most reliable and longestlasting solution to bearing currents

AEGIS Protects both the motor and attachedequipment


12/17

Comparison of Techniques.


13/17

Field add-on kits will be in stock soon!

Fits 90% of all Marathon Electricfooted motors Mounts on drive end bracket or

on encoders Takes up only 3/8 of shaft

No machining required Fits TEFC & encoders too No Div 1 or 2

Expect inventory during Q109


14/17

Kit components and instructions:


15/17

Bearing failures are costlytake care of themyou dont want to deal with this

For more information, request power point file Brgcurrent25.


16/17

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17/17

bearing erosion ac motors

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