Siemens MASTERDRIVES 6SE70 Bearing Current Protection Guide

Problem Overview: VFD-Induced Bearing Damage on a Siemens MASTERDRIVES 6SE70

A SIMOVERT MASTERDRIVES VC unit, catalog number 6SE7031-0TE60, has destroyed the bearings of a third-party induction motor after approximately two months of operation. The plant has now specified a Siemens motor as the replacement but must guarantee that the second motor does not repeat the failure. The technical question is therefore not whether the bearings failed, but whether the failure was electrical (caused by the drive) or mechanical (caused by the motor or its installation), and which protection measures and parameter changes in the MASTERDRIVES 6SE70 will give early warning of any subsequent bearing degradation.

Both hypotheses from the field discussion are plausible. Bearing grease starvation, misalignment, and overload will shorten bearing life on any motor, inverter-fed or not. However, when a healthy motor fails inside two months of PWM inverter operation and the raceways show the characteristic frosted or fluted pattern, the dominant root cause is almost always electrical discharge machining (EDM) of the bearing surfaces by high-frequency common-mode currents. This article documents the failure mechanism, the diagnostic procedure, and the layered set of mitigations—drive-side filters, motor-side insulation, MASTERDRIVES parameter changes, and thermal monitoring—that together bring the risk of a repeat failure to a tolerable level.

Failure Mechanism: Common-Mode Voltage, Shaft Voltage, and Bearing EDM

Every PWM voltage-source inverter, including the MASTERDRIVES 6SE70, produces a common-mode voltage at its output. The three phase-to-neutral voltages sum to a non-zero high-frequency component because the IGBT output stage switches each phase leg between the positive and negative DC bus rails. The resulting zero-sequence voltage has a fundamental at the switching frequency and a fast transient edge with a dV/dt that, on a 400 V class drive, is typically in the 3–8 kV/µs range.

This common-mode voltage couples through the parasitic capacitances inside the motor. The relevant capacitances are:

• Stator winding to rotor (Csr), typically a few nanofarads for a small-frame motor.
• Rotor body to inner bearing race (Cri).
• Outer bearing race to stator frame (Cof).
• Stator frame to ground (Csg).

These capacitances form a voltage divider. The rotor, including the shaft, is capacitively coupled to the stator winding on one side and capacitively coupled to the grounded frame through the bearings on the other side. The rotor therefore sits at a voltage (the shaft voltage, often called V_{rotor-to-ground} or the bearing voltage) determined by the capacitive divider and the instantaneous common-mode voltage.

Each bearing is, electrically, a capacitor (the lubricant film) in parallel with a switch (asperity contact). When the motor is running, the rolling elements are separated by a hydrodynamic oil film of a few hundred nanometers to a few micrometers. That film has a dielectric breakdown strength of roughly 15–30 V across the contact ellipse. As the shaft voltage rises, the film breaks down repeatedly and a high-frequency discharge current flows through the bearing. The energy of each discharge vaporizes a microscopic volume of metal from the raceway, leaving a micro-crater. Tens of millions of these discharges over a few thousand hours produce:

1. Micro-pitting on the inner and outer raceways.
2. Frosting, a matte grey appearance of the race surface.
3. Fluting, the characteristic washboard pattern perpendicular to the direction of rolling.
4. Increased vibration at the running frequency and its harmonics.
5. Premature grease degradation as lubricant is chemically attacked by the discharge.

The classic indicator of an EDM-related failure (as opposed to a mechanical one) is the fluted pattern and the presence of EDM micro-craters under microscope examination of the raceway. A grease-starved bearing typically shows a blue/brown discoloration from overheating and smearing of the rolling elements; an EDM bearing typically does not.

Three Categories of Bearing Current per IEC 60034-25

IEC 60034-25:2022 (Guide for the design and performance of a.c. motors with specific reference to the protection of bearings from electrical discharge) classifies the damaging currents into three families. Selecting the correct mitigation requires understanding which one is dominant in the installation.

The MASTERDRIVES 6SE70 is an IGBT-based PWM drive. Capacitive and EDM discharge currents are the dominant threats for small to medium frame sizes (up to roughly frame 280 / 75 kW). Circulating currents become significant on larger frames. The mitigation strategy in this article addresses all three categories.

Identifying the Affected Drive: 6SE7031-0TE60

The 6SE7031-0TE60 is a member of the SIMOVERT MASTERDRIVES VC compact family. The catalog number decodes as follows:

• 6SE70 — SIMOVERT MASTERDRIVES family.
• 31 — Compact (book-size) mechanical format, frame 31 (corresponds to a mid-range output rating).
• 0TE60 — Variant code. The TE block identifies the 3-phase 380–480 V class and the 60 suffix denotes the 60 A rated output current of this variant.

For ratings, parameter list, and firmware compatibility refer to the Siemens Industry Online Support portal and the operating instructions for the SIMOVERT MASTERDRIVES VC. Relevant parameter numbers used in this article are documented in the MASTERDRIVES parameter list, which is the primary engineering reference for the 6SE70 series.

The relevant characteristics of this drive family for the bearing-current problem are:

• IGBT output stage with default switching frequency of approximately 3 kHz (parameter adjustable up to 8 kHz on most variants).
• No internal common-mode filtering beyond the standard line-side EMC filter.
• External option modules for dv/dt reactors, sinusoidal filters, and common-mode chokes are available and should be retrofitted when the motor is replaced.

Mechanical Verification: Was the Old Bearing EDM-Damaged?

Before changing the drive configuration, the failed bearing should be inspected. The checklist below distinguishes an EDM failure from a mechanical one and therefore determines which mitigation is the highest priority.

1. Visual examination of the outer and inner raceways. Look for the fluted washboard pattern perpendicular to the rolling direction. Fluting is pathognomonic of EDM bearing current damage. If the raceways are smooth and the damage is concentrated on the rolling elements (smearing, peeling), the failure is mechanical.
2. Microscope examination at 10–40×. EDM damage shows micro-craters of 5–50 µm diameter with resolidified metal at the rim. Mechanical wear shows scratches and adhesive transfer.
3. Grease analysis. Blackened, hardened grease indicates overheating (mechanical overload, misalignment, or insulation breakdown of the film from a long-term EDM process). Light-coloured grease with metallic micro-spheres indicates EDM.
4. Bearing insulation resistance test. Use a 500 V megohmmeter between the outer race and the housing. For a non-insulated bearing the reading is <1 MΩ. For a properly insulated bearing (if the new motor has one) the reading should be >1 MΩ at 500 V.
5. Motor load history. Compare the logged output current from the MASTERDRIVES with the motor nameplate FLA. If the motor was running inside its rated current band, a thermal/mechanical failure is unlikely.

Drive-Side Mitigation: Output Filters and Switching Frequency

The first layer of protection is to reduce the common-mode voltage and the dV/dt at the motor terminals. On a MASTERDRIVES 6SE70 this is achieved by adding output filters and by selecting an appropriate switching frequency.

For a 6SE70 driving a motor in the 22–30 kW range, the recommended minimum filter package is a common-mode choke on the output plus a dv/dt reactor. If the cable run from the drive to the motor exceeds 50 m, upgrade the dv/dt reactor to a full sinusoidal filter. The cable itself is part of the protection: use a symmetrical three-core cable with a continuous concentric earth shield (e.g., CY-JZ type) and avoid running motor cables parallel to signal cables.

The switching frequency is set in parameter P340 on the MASTERDRIVES 6SE70. The default is typically 3 kHz. Increasing the switching frequency reduces motor acoustic noise and current ripple but increases the rate of EDM events. For a motor on a 6SE70 that is not specifically designed for high-frequency inverter operation, reducing P340 to the lowest acceptable value for the application (often 2 kHz) reduces both dV/dt and the EDM event rate. Verify motor current ripple and thermal performance after the change.

Motor-Side Mitigation: Insulated Bearings and Shaft Grounding

The second layer of protection is to block or dissipate the bearing current at the motor.

Insulated bearing on the non-drive end (NDE)

Standard solution on inverter-rated motors. The outer race of the NDE bearing is coated with a thin ceramic or aluminium-oxide layer that provides >1 MΩ resistance at 500 V DC. This breaks the capacitive and EDM current paths through that bearing. The drive-end (DE) bearing is left uninsulated so that any common-mode current that does reach the shaft is conducted to the frame through the DE bearing and the frame ground.

Hybrid ceramic bearing

The rolling elements are silicon-nitride ceramic. Ceramic is electrically insulating, so the EDM current path through the bearing is eliminated even without an insulated race coating. Hybrid bearings are the most reliable solution for inverter-fed applications but are more expensive.

Shaft grounding ring (SGR)

A brush-type grounding ring is mounted on the motor shaft and connects the shaft to the motor frame through a low-impedance path. The brush material is typically silver-graphite or a fibre-based conductive composite. A properly specified SGR holds the shaft voltage below 5–10 V peak, well under the lubricant breakdown voltage, and therefore prevents the discharge from forming. The SGR is paired with an insulated NDE bearing so that the discharge energy is routed to ground through the brush instead of through the bearing.

For the new Siemens motor on the 6SE70, the recommended package is either a hybrid ceramic bearing at the NDE, or a standard insulated NDE bearing plus a shaft grounding ring. If the motor is a standard Siemens 1LE/1LG series specified with the inverter-rated option, it will already have an insulated NDE bearing; confirm this on the nameplate (often shown as "Ins. bearing NDE" in the options list).

MASTERDRIVES Parameter Configuration for Motor Protection

The 6SE70 has several parameters that influence how the drive behaves during a bearing-related fault (rising temperature, rising current, decreasing insulation) and that should be reviewed when a new motor is commissioned.

Confirm the exact parameter numbers and ranges against the MASTERDRIVES parameter list for the firmware version installed in the unit. Different firmware versions of the 6SE70 use slightly different parameter menus; the principle is the same but the parameter numbers and limits may vary.

Motor Thermal Protection with KTY and PTC Sensors

The recommended temperature management is a three-level scheme:

1. Alarm 1 — pre-warning. Triggered at 110–120 °C (Class F insulation, with 155 °C rated limit). Indication only, no drive action. Used to schedule inspection.
2. Alarm 2 — bearing pre-warning. A separate KTY or PT100 in the drive-end bearing housing (optional) can be monitored on a second analog input. Triggered at 90–100 °C; indicates lubricant breakdown or impending mechanical failure.
3. Trip — over-temperature. Triggered at 130–140 °C winding, or 110 °C bearing housing. Drive latches a fault, stops the motor in a controlled ramp, and requires manual reset.

The KTY sensor must be wired with shielded, twisted-pair cable routed separately from the motor power cable. The shield is grounded at the drive end only. Verify the wiring resistance compensation in the MASTERDRIVES analog-input configuration, since long cable runs (10 m or more) can introduce 5–10 °C of error if the lead resistance is not compensated.

Standards Reference: IEC 60034-25 and NEMA MG1 Part 31

Two standards govern the selection of mitigation:

• IEC 60034-25:2022 — Guide for the design and performance of a.c. motors with specific reference to the protection of bearings from electrical discharge. Defines the three categories of bearing current, the test method for the shaft voltage (V_rotor), and the recommended mitigation by frame size.
• NEMA MG1 Part 31 — Definite-Purpose Inverter-Fed Polyphase Motors. Defines the performance and construction of motors designed for inverter duty, including insulation class, bearing insulation requirements for frames > 500, and surge withstand capability.

For a 6SE70 driving a Siemens motor, the key decision points from these standards are:

1. Frame size < 250: Insulated NDE bearing is the standard mitigation; a shaft grounding ring is optional but recommended for long cable runs.
2. Frame size 250–500: Common-mode choke + insulated NDE bearing + shaft grounding ring.
3. Frame size > 500: Insulated NDE bearing + shaft grounding ring + common-mode choke; some installations also require insulated DE bearing.

Commissioning Procedure and Baseline Measurements

1. Verify drive and motor nameplate data. Voltage, current, speed, power factor, and insulation class must match the MASTERDRIVES configuration. Re-run the auto-tune (P372 = 1) with the motor uncoupled from the load to populate the equivalent-circuit parameters.
2. Install the output filter and shaft grounding ring before first run. Common-mode choke between drive output and motor terminal box; SGR on the motor DE face. Confirm SGR brush wear indicator is in the green band.
3. Verify bearing insulation. Megger test on the insulated NDE bearing (or both, if fitted) at 500 V DC. Reading > 1 MΩ; typical 5–50 MΩ for a new insulated bearing.
4. Measure baseline shaft voltage. With an oscilloscope and a 100:1 probe, measure peak rotor-to-frame voltage on the first run. Target < 5 V peak for an SGR-protected motor, < 15 V peak for an insulated-bearing-only motor. The shaft voltage probe is connected to a brush riding on the rotating shaft and a separate clip on the motor frame.
5. Confirm thermal monitoring. Heat the KTY sensor (or substitute a decade resistance box) and verify that the warning and trip thresholds trip in the MASTERDRIVES at the programmed temperatures.
6. Capture a baseline vibration spectrum. Record accelerometer readings on the DE and NDE bearing housings at no-load and at full-load operating speed. Save the FFT spectrum as the baseline; subsequent monthly spectra are compared to this baseline to detect fluting-related bearing frequencies (typically at the ball-pass-inner-race, ball-pass-outer-race, and ball-spin frequencies).
7. Log the cable run. Record cable type, length, and routing. Cable length > 50 m generally requires a sinusoidal filter on the 6SE70 output.

Preventive Maintenance and Condition Monitoring

The mitigation package above addresses the electrical root cause. The mechanical life of the bearings is still governed by grease life, alignment, and load. The recommended maintenance schedule for an inverter-fed motor on a 6SE70 is:

Document every measurement in the motor's logbook. Trending is the only way to detect a slow EDM process before it becomes a catastrophic failure.

Troubleshooting Matrix: Bearing Damage on MASTERDRIVES-Driven Motors

Frequently Asked Questions