effect of the bearing currents to the motor

7/28/2019 Effect of the Bearing Currents to the Motor

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Effects of the Bearing Currents to theMotor Bearings

By:

Harris Joe Anches

Mentor:

Engr. Eugene Occea


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ABSTRACT

The vast majority of bearing failures in electric motors and

electric motor-driven equipment are due to mechanical andthermal causes. However, currents that flow through the

motor bearings can also create premature bearing failure. In

this paper, the utility and inverter driven motor bearing

currents and the potential current paths are identified and

methods to break these current paths through motor

modifications are presented. Methods to shunt these currents

away from the motor or driven equipment bearings through

improved high frequency grounding are also discussed.


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OBJECTIVES

To conduct a study of bearing currents that are present on the motor

bearings.

To identify the different causes that leads to the creation of bearing

currents.

To design a schematic diagrams showing the potential current paths.

To come up with a solution to shunt this current paths.

To come up with a solution to eliminate the causes of this currents.

To have an overall recommendation of the study.


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INTRODUCTION

Causes of bearing failure:

Mechanical and Thermal:

misalignment

vibration

incorrect lubrication

excessive radial or axial loading

lubricant contamination

inadequate maintenance.

Electrical Bearing currents


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What is a bearing?

derived from a verb to bear, or to support.

What is a bearing current?

Current that pass through the bearing due to the induced voltageon the shaft of the motor.

How does a bearing current caused bearing failure?


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What causes bearing currents?

o Electrostatic Charge

introduced from the outside of the motor or from an external

source.o Electromagnetic Unbalanced

Introduced within the motor itself or known as the alternating flux.

What is an AC motor?

An electric motor driven by an alternating current (AC)

Synchronous motor

Asynchronous motor or Induction Motor


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Induction Motor

an AC motor in which all electromagnetic energy is transferred by

inductive coupling from a primary winding to a secondary winding being

separated by an air gap.

Squirrel Cage - it is a cylinder mounted on a shaft.

Wound Rotor - the rotor windings are connected through slip rings

Utility Driven Motor

Variable Speed Driven Motor


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Fixed & Variable Speed induction motor

The vast majority of bearing failures in electric motors are

due to mechanical and thermal causes.

Bearing currents in the Electric motor are not new. In fact,

they have been around since electric motors were invented.

These currents are due to the induced voltage on the shaft.


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Causes& Effects

Lack of symmetry

Perfect symmetry is difficult to achieve due to automated

manufacturing methods and anisotropic properties of magnetic

materials, such as electrical steel.

Because of the low frequency nature of bearing currents in an AC

driven motors results in a current paths through what are generally

considered to be conductive materials (motor shafts, frames, bearing

races and bearing balls)


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lack of symmetry

Bearing at

RISK!

Because of the low frequency nature of bearing currents in an AC driven

motors results in a current paths through what are generally considered to be

conductive materials (motor shafts, frames, bearing races and bearing balls)

INSULATION

Ground

Putting insulating materials in the conducting current paths can eliminate

the low frequency bearing currents.


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Alternating flux or the electromagnetic unbalanced

Electric steel is not totally homogenous therefore flux paths in the

motor are not entirely symmetrical.

Out-of-round or off-center rotors

Open rotor bars

Magnetically non-symmetrical iron

Asymmetrically placed stator winding

Asymmetrical flux through the steel results in AC flux lines that

enclose the shaft.

Causes& Effects


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Bearing at

RISK!

electromagnetic unbalanced

This can drive a current down the shaft, to the bearings, through the frame

and back again through the bearings.

INSULATION


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Causes and Effects

Common Mode Voltage (CMV)

A non-zero value of the summation of voltages in a three phase motor.

Due to the unbalanced of the voltage drop in a three phase motor.

common mode coupling currents (dV/dt)


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Unbalance Source

common mode voltage

Phase A

Phase B

Phase C

=

CMV

0

Balance Source

Utility Driven motor / Sine wave DrivenInverter Driven motor


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stator winding-to-ground current stator winding-to-frame shaft coupling current stator-to-rotor coupling current rotor-to-shaft coupling current

What happens to the CMV?

When the CMV links the rotor, a charge will accumulate on the

rotor. Since the unbalance source is external to the motor, rotorcurrent will flow through the bearings to ground and back to the

source, taking the path of least resistance.

Stator

Due to capacitive coupled current from the stator winding to the

rotor across the air gap. This current finds a path that passesthrough a conductive coupling, and through at least one load

bearing, to the load ground, and to the driven ground.

Due to the capacitively coupled current between the stator winding andframe. This current flows through the stator winding insulation, which iscapacitively conductive at high frequencies. With a poor motor-to-inverter high frequency ground connection, this current will also flowthrough the motor frame, the motor bearing, motor shaft, conductivecoupling, load bearing, load ground, and finally to driven ground.

To reduce bearing damage, the preferred path for all these currents

is the stator winding-to-ground path. Here, no current flows throughthe motor or load bearings.

VSD

Bearing

Failure!


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How can this currents damages the motor?

Those potential current paths can cause damages on the races and rolling

elements of the bearing. Frosted appearance might be seen on the rolling

elements of the bearing

If mechanical vibration is present (which is true for all motors) the rolling

elements will bounce during rotation and create a fluting pattern.

This greatly increases friction and vibration and such bearing may catastrophically

fail in less than 6 weeks.

A 9 year life test at Regal Beloit confirmed the relationship between vibration and fluting.


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Drive endNon-Drive endInsulate

Ground

brush

VSD

How can we bypass these currents?

Insulate the drive end and the non-drive end bearings of the motor.

There might be a possibility that the current will flow through the loadbearing following the low impedance motor elements.

Not insulated

Insulate the drive end and the non-drive end bearings of the motor.

effect of the bearing currents to the motor

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