ElectroStaticTech
Field Test Report
Shaft Voltage Testing
Wal-Mart Distribution Center
VFD Driven Conveyor Motors
Purpose:
1. Assess the application of AEGIS™ Shaft Grounding Ring for protection of VFD Driven Motor bearings.
2. Identify presence of destructive shaft voltages that can discharge in electric motor bearings and cause electrical pitting damage, bearing lubrication degradation and catastrophic “fluting” damage in the motor’s bearings.
3. Assess best practices to protect VFD/Inverter driven motor systems from Bearing Failure and Induced Shaft Voltages and Electrical Discharge in Bearings.
4. Provide recommendations for applicable VFD/PWM inverter driven AC motor systems used in conveyor systems.
Test Date: March 1, 2011
Location: Wal-Mart Distribution Center
Menomonie, WI
Conveyor Motor – 15 HP on VFD
Systems:
(1) Location 0115F, DC motor conveyor, PWM drive
(2) Location M102F, 3HP Reliance, 182TC, conveyor motor
(3) Location 205F, 15HP Baldor Super E, 254T
(4) Location 16R01, 5HP Reliance, conveyor motor
(5) Location VB156200, 1 HP conveyor motor
History:
System upgrades to VFD driven conveyor systems are on-going to improve productivity for Wal-Mart Distribution Centers. Motors in the “critical path” are vital and must be of the highest reliability. A survey of various motors was conducted by Electro Static Technology to determine if newly installed VFD driven motors are vulnerable to potential unplanned down time from VFD induced bearing currents.
Test Measurement Objective: Measure shaft voltage using AEGIS™ SVP - Shaft Voltage Measuring Probe to determine induced voltage on the motor shaft. These VFD induced voltages are likely to cause motor bearing currents and bearing “fluting” failure.
Test setup: AEGIS™ SVP Tip applied to the motor shaft while operating. The probe tip was connected via a Fluke 10:1 probe to the Fluke 199C 200MHz Scopemeter.
Test Setup: Shaft Voltage
Tests conducted:
1) Shaft voltage reading with no shaft grounding
2) Shaft voltage reading with AEGIS™ Shaft Grounding
Typical Shaft Voltage Readings Results of electrical bearing discharges
Slide from AEGIS™ Conference Presentation
Motor Shaft
Oscilloscope:
Fluke 199C
200 MHz
2.5 GS/s
Holder/Magnetic Base
Fluke 10:1 Probe
AEGIS™ SVP Probe Tip
System 1 Test: 0115F, DC motor conveyor, PWM drive.
No shaft grounding on motor
Reading 1: Shaft Voltage Reading DE
Fluke 199C Setting: 2 v/div; 500 µsec/div
Observations: 8.96 v pk – pk with positive and negative voltage spikes present. Voltage spikes indicate possible bearing discharges.
AEGIS™ Shaft Grounding Applied to Motor
Reading 2: Shaft Voltage Reading DE
Fluke 199C Setting: 2 v/div; 500 µsec/div
Observations: With AEGIS™ Shaft Grounding applied to the motor shaft, the voltage was reduced to 2.72 v pk-pk with low level discharges at the conductive micro fibers and presence of ground noise.
Motor Shaft end
System 2 Test: M102F, 3HP Reliance, 182TC, conveyor motor.
No shaft grounding on motor
Reading 1: Shaft Voltage Reading DE
Fluke 199C Setting: 2 v/div; 2 msec/div
Observations: Measured 9.6 v pk – pk with positive and negative voltage spikes typical of bearing discharges.
AEGIS™ Shaft Grounding Applied to Motor
Reading 2: Shaft Voltage Reading DE
Fluke 199C Setting: 2 v/div; 2 msec/div
Observations: With AEGIS™ shaft grounding applied there was a 1.12 v pk-pk consisting primarily of ground noise with one low level peak. The shaft voltages is discharging at the AEGIS™ conductive micro fibers. AEGIS™ shaft grounding provides “path of least resistance.”
System 3 Test: 205F, 15HP Baldor Super E, 254T conveyor motor application
No shaft grounding on motor
Reading 1: Shaft Voltage Reading DE
Fluke 199C Setting: 10 v/div; 2 msec/div
Observations: 74.8 v pk – pk with positive and negative voltage spikes present of ~20-30 v pk. Lack of six step common mode voltage wave form indicates bearing current discharges in the motor bearings.
AEGIS™ Shaft Grounding Applied to Motor
Reading 2: Shaft Voltage Reading DE
Fluke 199C Setting: 10 v/div; 2 msec/div.
Observation: AEGIS™ shaft grounding reduced the voltage to 5.2 v pk-pk ground noise present on the motor shaft.
Procedure: Tested with and without AEGIS™ conductive micro fiber shaft grounding device to motor shaft while measuring shaft voltage with the shaft voltage probe.
No bearing discharge spikes present on the motor shaft. AEGIS™ shaft grounding provides “path of least resistance.”
System 4 Test: 16R01, 5HP Reliance, conveyor motor
No shaft grounding on motor
Reading 1: Shaft Voltage Reading DE
Fluke 199C Setting: 2 v/div; 200 µsec/div
Observations: 9.4 v pk – pk with common mode wave form. Some transients are observed with potential for bearing discharges.
Measured presence of shaft voltage only. No opportunity to take the reading with AEGIS™ Shaft Grounding due to limited access to the motor shaft.
System 5 Test: VB156200, 1 HP conveyor motor
No shaft grounding on motor
Reading 1: Shaft Voltage Reading DE
Fluke 199C Setting: 2 v/div; 200 µsec/div
Observations: 13.3 v pk – pk with common mode voltage spikes and potential for bearing discharges.
AEGIS™ Shaft Grounding Applied to Motor
Reading 2: Shaft Voltage Reading DE
Fluke 199C Setting: 2 v/div; 200 µsec/div
Observations: With AEGIS™ shaft grounding applied there was a 1.36 v pk-pk consisting primarily of ground noise. The shaft voltages are discharging at the AEGIS™ conductive micro fibers. AEGIS™ shaft grounding provides “path of least resistance.”
Test results:
System 1 Test, Shaft Voltage Measurement: Before and after AEGIS™ Shaft Grounding Applied
Equipment: Conveyor motor; Location 0115F; DC motor conveyor, PWM drive. No shaft grounding/bearing protection installed.
Reading 1: 8.96 volts peak to peak (no shaft grounding)
Reading 2: 2.72 peak to peak (AEGIS™ shaft grounding applied to shaft)
System 2 Test, Shaft Voltage Measurement: Before and after AEGIS™ Shaft Grounding Applied
Equipment: Conveyor motor; Location M102F; 3HP Reliance, 182TC, conveyor motor. No shaft grounding/bearing protection installed.
Reading 1: 9.6 volts peak to peak (no shaft grounding)
Reading 2: 1.12 volts peak to peak (AEGIS™ shaft grounding applied to motor shaft)
System 3 Test, Shaft Voltage Measurement: Before and after AEGIS™ Shaft Grounding Applied
Equipment: Conveyor Motor; Location 205F; 15HP Baldor Super E, 254T. No shaft grounding bearing protection installed.
Reading 1: 74.8 volts peak to peak (no shaft grounding)
Reading 3: 5.2 volts peak to peak (AEGIS™ shaft grounding applied to shaft)
System 4 Test, Shaft Voltage Measurement: No Shaft Grounding Applied
Equipment: Conveyor Motor, Location 16R01, 5HP Reliance, No shaft grounding bearing protection installed
Reading 1: 9.04 volts peak to peak (no shaft grounding)
System 5 Test, Shaft Voltage Measurement: No Shaft Grounding Applied
Equipment: Conveyor Motor; Location VB156200, 1 HP conveyor motor. No shaft grounding/bearing protection installed.
Reading 1: 13.3 volts peak to peak (no shaft grounding)
Reading 2: 1.36 volts peak to peak (AEGIS™ shaft grounding applied to shaft)
Shaft Voltage Measurements (Motors 1 to 5 HP)
Shaft Voltage Measurement s (Motors 1 to 5 HP)
Conclusions:
(1) All VFD driven motors tested, systems 1 through 5 had voltage present on the shaft with sufficient voltages to produce electrical discharge machining (EDM) damage in the bearings.
(2) Systems 1 through 3 showed voltage spikes typical of bearing discharge patterns.
(3) System 4 and 5 showed the common mode voltages typical of potential bearing discharge.
(4) In tests of systems 1, 2 and 3 and 5 the AEGIS™ Shaft Grounding Ring technology was applied to the motor shaft during operation. In all cases the AEGIS™ shaft grounding technology discharged the voltages to ground.
(5) Application of AEGIS™ Bearing Protection Ring decreased the shaft voltage by 75% to 93% provided the path of least resistance for destructive shaft voltages.
(6) Operation of electric motors by variable frequency drives (VFD) resulted in destructive shaft voltages which result in system degradation, significantly reduced reliability, and potential catastrophic bearing failure from:
a. Electrical discharges in the motor bearings
b. Deterioration of bearing lubrication during operation through heating and contamination from the EDM pitting
c. Increased energy consumption from increased friction as bearings deteriorate
d. Increased life cycle costs from operation, maintenance and repair
(7) Based on the voltage measurements and wave form analysis, it is clear that EDM bearing damage is currently occurring in systems 1, 2, and 3 and is highly likely to discharge in systems 4 and 5.
(8) No voltage discharge or bearing protection was installed on the motors.
References:
1. NEMA MG1 Part 31, section 31.4.4.3
a. States “Shaft voltages can result in the flow of destructive currents through motor bearings, manifesting themselves through pitting of the bearings, scoring of the shaft, and eventual bearing failure”
b. Shaft grounding may be used to divert the current around the bearings to protect motors and attached equipment.
2. March 2005 Journal of Electrostatics “Statistical model of electrostatic discharge hazard in bearings of induction motor fed by inverter” by Adam Kempski et. al. “Electrical Discharge Machining (EDM) bearing currents have been found as the main cause of premature bearing damages in Pulse Width Modulation (PWM) inverter fed drives.”
Recommendation:
1. AEGIS™ Shaft Grounding for VFD driven AC and DC Conveyor Motors: To protect bearings from destructive shaft voltages in all VFD driven pulse width modulation (PWM) drive conveyor systems, the AC or DC motors should incorporate AEGIS™ shaft grounding as a standard in all applications per AEGIS™ Application Notes.
a. Install AEGIS™ SGR on DE or NDE of motor.
b. Apply to clean shaft and coat shaft with AEGIS™ CS015 Colloidal Silver Shaft Coating to ensure highly conductive surface and minimize potential for corrosion or rust.
Note: All motors over 100 HP: (1) Install AEGIS™ SGR on drive end as described above; (2) Apply CS015 to motor shaft where fibers touch; (3) Install insulated bearing on opposite end to AEGIS™ SGR.
2. AEGIS™ SGR Installation:
a. AEGIS™ shaft grounding rings may be installed on new motor or when replacing bearings.
b. Specify AEGIS™ SGR in all new motors from the motor manufacturer.
c. AEGIS™ rings may be installed on coupled equipment using AEGIS™ supplied conductive epoxy “split ring” kits.
d. If installed directly to the motor end bracket ensure that the shaft shoulder is measured and the correct AEGIS™ ring is selected for this measurement.
