Motor Bearing Damage and Variable Frequency Drives: - Diagnosing the Causes, - Implementing a Cure, and - Avoiding the Pitfalls Tim Albers, Director of Product Mgt, NIDEC Motor Corporation tim.albers@nidec-motor.com Tim Jasina, Sr. Application Engineer / Engr. Team Leader jasinat@nsk-corp.com Ken Fonstad, HVAC Sales / Application Support Principal Engineer ken.a.fonstad@us.abb.com
Anatomy of an AC in induction motor AC current in the stator This generates a current in the windings produces a rotor bars, which makes its own rotating magnetic field. magnetic field. The interaction of the two magnetic fields produces the motor’s torque. 2
Operation of an AC in induction motor AC Line-to-Line Voltage Voltage L1-L2 L1-L3 L2-L3 The frequency of the AC in the stator controls the speed of the motor. 3
How does a varia iable frequency driv ive (VFD) work? VFD 4
How does a varia iable frequency driv ive (VFD) work? VFD Rectifier DC Bus Inverter 5
How is is a VFD dif ifferent from the AC power lin line? Fast-rising pulses AC Line-to-Ground Voltage PWM Voltage Pulses Voltage L1 L2 L3 The voltage of the The voltage from the AC power line PWM VFD can change changes gradually from from minimum to minimum to maximum in maximum in less than about 8 ms 1 µs (0.001 ms) 6
How is is a VFD dif ifferent from the AC power lin line? Fast-rising pulses Air gap • Fast-rising voltage pulses can readily induce voltage across the air gap between the motor’s stator and its rotor • This can drive stray currents in the motor 7
Cause of current through a motor’s bearings Circulating currents • Asymmetries in the motor can result in a circulating current between the motor’s frame and the motor’s shaft • This seldom happens in motors < 50 HP; more common in 480 V and higher voltage AC motors 8
What causes bearing currents in in small ller motors? Common mode voltage AC Line-to-Ground Voltage Voltage L1 L2 L3 • When powered from a balanced center-grounded wye transformer, the net voltage with respect to ground is always 0 V ( no common mode voltage ) • Common mode voltage can induce voltage on the motor’s shaft 9
What causes bearing currents in in small ller motors? Common mode voltage + − • When powered from a VFD, only two voltages (+ & -) make the three-phase voltage to the motor • The 3-phase voltage applied to the motor isn't balanced with respect to ground 10
What causes bearing currents in in small ller motors? Common mode voltage + + − • If the stator has a net positive voltage, it will induce a positive voltage onto the shaft • If the voltage on the shaft is high enough, current will arc through the grease in the bearings 11
How can you detect the onset of bearin ing damage? • Noise from the bearings That’s a bit late • Motor vibration That’s a bit late and difficult • Measure the current That’s hard to do • Measure the shaft voltage There’s an idea 12
How can you detect the onset of bearin ing damage? Measuring shaft voltage • Use a brush to contact the motor’s shaft • Home-made: fine steel wool brush • Commercial: carbon fiber brush 13
How can you detect the onset of bearin ing damage? Measuring shaft voltage • Use a magnetic stand to hold the brush in place 14
How can you detect the onset of bearin ing damage? Measuring shaft voltage • Use a digital storage oscilloscope to capture the motor shaft voltage • Use the trigger function to capture the highest shaft voltages • Save the waveforms in computer files 15
How can you detect the onset of bearin ing damage? Interpreting the voltage waveforms 28.8 V peak 1 µs/div • The shaft voltage smoothly increases and then quickly drops due to a discharge 16
How can you detect the onset of bearin ing damage? Interpreting the voltage waveforms 12.8 V peak 500 ns/div • The shaft voltage oscillates, but no sharp drop is detected • No shaft current was present 17
How can you detect the onset of bearin ing damage? Remotely detecting bearing discharges • When a discharge occurs, it generates a pulse of electromagnetic noise, like radio static • A hand-held radio frequency detector can detect the “static” and count the discharges 18
How can you detect the onset of bearin ing damage? Non-contact detection of bearing discharges • Advantages • Little set-up is required • Counts the discharges over time • Concerns • Proper positioning of the meter is required for accurate readings • Practice and experience are required 19
Id Identif ifying the cause of bearin ing damage Not all bearing damage is caused by bearing current • There are also mechanical causes for bearing damage • Before implementing a solution, it is important to understand the cause of the problem • Study the damage to the bearings • Ideally, the grease in the bearing should not be cleaned away before the bearing is submitted for evaluation 20
Id Identif ifying the cause of bearin ing damage Images of damaged bearings • Classic bearing fluting, likely caused by shaft voltage discharges 21
Id Identify fying the cause of f bearin ing damage Bearing electrical arc damage process • Damage occurs when current passes through bearing • Oil film between balls and raceways acts as an insulator • Film is only microns thick at most • Shaft voltage can exceed ‘dielectric strength’ (when insulating property fails) of film 0.003mm • Arcing occurs – melting a minute area of surfaces • Melted metal enters grease or is rolled over onto raceways • Initially forms a dull grey ‘frosted’ or matte appearance • Fluting can develop with continued arcing 22
Id Identif ifying the cause of bearin ing damage Initial electrical arc damage • A frosted ball path appears • Potentially electrical arc damage • Potentially lubrication contamination • Extreme magnification required to verify • Scanning electron microscope • Confocal microscope 0.007mm 23
Id Identify fying the cause of f bearin ing damage Advanced electrical arc damage – Fluting • Overlapping arcs • Fluting’s repetitive appearance related to dynamic system of arcing and rolling motion 24
Identif Id ifying the cause of bearin ing damage Images of damaged bearings • Electrical arc damage similar to electric discharge machining (EDM) process • Bearing surface material removed 0.0012mm 25
Identif Id ifying the cause of bearin ing damage Images of damaged bearings • Likely caused by shaft voltage discharges, made worse by a large radial load • With light radial loading, the insulating grease layer is uniform and thick • A large radial load reduces the thickness of lubricant in the load zone 26
Id Identif ifying the cause of bearin ing damage Possible solutions for this problem • Belt drive application: • Reduce the belt tension • Use larger diameter sheaves on the motor and the load • When the load is shaft-mounted: • Mount the load as close to the motor as possible • Reduce the weight of the load • Use external bearing(s) to support the load • Increase the minimum speed 27
Id Identif ifying the cause of bearin ing damage Why increase the minimum speed? • Test on three motors with radial loads Effect of Motor Speed on Breakown Voltage Bearing Insulation Breakdown Voltage 50 V 40 V 30 V Motor 1 Motor 2 20 V Motor 3 10 V 0 V 33% 50% 67% 83% 100% Motor Speed 28
Id Identif ifying the cause of bearin ing damage Not all bearing damage is caused by bearing current • The motor made a “clicking” noise when it was rotated slowly • A fluting pattern is too dense to make such a sound • The shaft voltage 2.84 V and hand-held detector showed no discharges 5 µs/div 29
Id Identif ifying the cause of bearin ing damage Study of the bearings • The races of the bearing showed regularly spaced marks • The spacing of the marks was the same as the spacing of the balls in the cage of the bearing 30
Id Identif ifying the cause of bearin ing damage Diagnosis: False Brinelling • The motor was driving a shaft- mounted fan wheel • It appears that the problem was caused by vibration during shipping 31
Identif Id ifying the cause of bearin ing damage Other mechanical damage: Brinelling • Brinelling is generally caused by a sharp impact on the bearing • This causes an indent in the bearing’s race • It often indicates damage during the assembly of the equipment 32
Id Identif ifying the cause of bearin ing damage Summary • Not all bearing damage is the result of electrical discharges through the bearings • Mechanical damage • Lubrication problems • Too little • Too much • Contamination • Overloading • It is important to know the problem before developing a solution 33
Avoid idin ing bearin ing dis ischarge currents A variety of solutions are available 1. Proper wiring 2. VFD adjustments 3. System maintenance 4. High frequency toroids 5. Blocking the discharge 6. Re-routing the discharge 34
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