Shaft Grounding and Bearing Currents in IE5 Synchronous Reluctance Motors: Bearing Protection on Drives and the Right Option

IE5 synchronous reluctance (SynRM) motors offer the highest efficiency class while always operating together with a frequency converter (drive/VFD). This mandatory drive operation improves the motor's efficiency and control quality; but it also brings a frequently overlooked risk: bearing current. The drive's fast-switching output creates a common mode voltage on the motor shaft, and unless suitable measures are taken this voltage discharges through the bearing, breaking down the oil film and damaging metal surfaces. Over time this shows up as EDM (electrical discharge machining) pitting, fluting (washboard pattern), grease degradation and premature bearing failure. In an expensive, critical motor like IE5, this damage means unplanned downtime and high maintenance cost. In this article we cover how bearing current arises, why EDM and fluting damage occur, how shaft grounding rings (AEGIS type), insulated bearings and other measures work, and which option is the right choice in which situation, in engineering terms and with a sound buying perspective.

Why Is a Drive Mandatory in an IE5 Synchronous Reluctance Motor?

A synchronous reluctance motor's rotor has neither windings nor permanent magnets; the rotor consists of a special lamination stack that channels the magnetic flux into specific directions. This structure provides high efficiency and low rotor loss, but it means the motor cannot start directly from the grid on its own. For the rotor to stay synchronous with the stator's rotating field, the flux must be controlled vectorially, which is only possible with a frequency converter (drive). The IE5 SynRM motor therefore always runs with a drive by design. While this requirement delivers the motor's biggest advantage, high efficiency, it also means that all drive-induced side effects (common mode voltage, du/dt voltage spikes and bearing current) must be accepted upfront. For this reason, in an IE5 motor bearing current should be treated not as an optional concern but as a natural consequence of the design.

How Does Bearing Current Arise? Common Mode Voltage

In a classic grid-fed motor the instantaneous sum of the three phase voltages is zero and no significant voltage appears on the shaft. A drive (VFD), however, produces voltage pulses switched very fast by PWM (pulse width modulation); the sum of these pulses is not zero at every instant. This non-zero sum creates a common mode voltage relative to the motor star point. Parasitic capacitances inside the motor (winding-frame, winding-rotor, rotor-frame, bearing) transfer this voltage to the shaft. Between shaft and frame the bearing's oil film behaves like a capacitor and a voltage (shaft voltage) builds up on the shaft.

When this voltage exceeds a small threshold, the thin oil film insulation in the bearing breaks down and a sudden current discharge occurs. The event is likened to EDM (Electrical Discharge Machining), because just like spark machining, each discharge tears a microscopic amount of material from the metal surface. A single discharge seems harmless, but repeated thousands of times per second the cumulative damage builds on the bearing ball and race surfaces. This is the essence of the bearing current problem: high-frequency, repeated, small but destructive electrical discharges.

Bearing Current Types and Damage Mechanisms

Drive-induced bearing currents are not a single mechanism. The main types are:

• EDM (discharge) current: The sudden discharge when accumulated shaft voltage breaks down the oil film; the main cause of pitting and fluting damage.
• Circulating current: Current induced by the high-frequency magnetic flux in the frame-shaft-bearing loop; dominant especially in large-frame motors.
• Rotor-to-ground current: If the shaft load (coupling/machine) is poorly grounded, current tries to flow to ground through the bearing.

The visible consequences of these currents emerge over time. EDM damage first creates a matte, frosted appearance (frosting) on the bearing surface; then small pits form. When continuous discharge and mechanical repetition combine, a fluting (washboard) pattern of regular, parallel grooves forms on the bearing race. This pattern reveals itself through increased vibration and a characteristic humming noise. At the same time the grease degrades chemically due to electrical discharge and heat, loses its lubricating property and accelerates bearing failure.

How Do Shaft Grounding Rings (AEGIS Type) Work?

One of the most common and effective solutions against bearing current is the shaft grounding ring (the AEGIS brand is widely known here). This ring, placed on the shaft, consists of conductive micro-fiber brushes. It creates a very low resistance, continuous conduction path between shaft and frame (ground). Thus the common mode voltage on the shaft flows safely to ground through the ring instead of discharging through the bearing. With no chance for a voltage to build up that would break down the oil film, EDM discharge and therefore pitting/fluting are prevented.

The main advantages of the shaft grounding ring are its very low mechanical wear, freedom from maintenance and applicability to existing motors. Because the micro-fiber brushes do not physically rub the shaft, they provide reliable conduction for life. In most applications this is the most practical and cost-effective solution; it is the priority choice especially for protecting the drive-end bearing.

Comparison of Bearing Current Mitigation Methods

The right solution is usually not a single method but a combination suited to the application. For example, applying a shaft grounding ring at the drive end and an insulated bearing at the opposite end is a common and effective approach.

Which Option in Which Situation? Selection Logic

The right option choice depends on motor power, frame size and the criticality of the application:

• Small-medium power, standard application: A shaft grounding ring alone is sufficient in most cases.
• Large-frame motor: Since circulating current is dominant, an insulated bearing at the opposite end together with the shaft grounding ring is recommended.
• Critical, continuous process: The safety margin is increased with extra measures such as hybrid/ceramic bearings and filters.
• Intervention on an existing motor: A retrofittable shaft grounding ring is the most practical solution.

If these decisions are made upfront, the IE5 motor both preserves its high efficiency under drive operation and delivers long bearing life. Bearing current protection is a critical detail that preserves the value of the IE5 investment.

Early Signs and Diagnosis of Bearing Current Damage

Unlike a mechanical fault, bearing current damage is electrical in origin, so it is diagnosed by different signs. The most common early indicators in the field are: a humming or moaning noise that grows over time and becomes pronounced at certain speeds, a rise in high-frequency components in vibration measurements, bearing temperature climbing above normal, and grease color darkening. When the bearing is dismantled, a matte frosting on the race surface followed by regular parallel grooves (fluting) almost certainly points to electrical discharge damage. This pattern is clearly distinguishable from mechanical wear; mechanical damage is irregular and random, while electrical damage forms a regular, repetitive pattern.

The most direct diagnostic method is to measure the shaft voltage with an oscilloscope. In a healthy system the shaft voltage should be low and discharge-free; sudden drops at the peaks (discharge traces) show that bearing current is active. This measurement is also used to verify that a protection measure (such as a shaft grounding ring) is actually working. Adding this measurement to the preventive maintenance program significantly reduces unplanned downtime in critical motors like IE5.

The Role of Grounding and Cabling

Bearing current is not only about the motor's own options but about the whole system. The grounding quality between drive, motor and machine determines which path the common mode current takes. If a low-impedance, intentional return path is not provided for the high-frequency current, the current chooses the easiest path, often the bearing. It is therefore critical to use a shielded motor cable and to ground the cable shield at both ends (drive and motor) in a 360-degree, low-impedance manner. The shield offers the high-frequency current a safe return path instead of the bearing.

In addition, establishing a short, thick grounding connection (bonding) between the motor frame and the machine chassis reduces the potential difference and therefore the rotor-to-ground current. When grounding and cabling are done correctly, on-motor measures such as the shaft grounding ring and insulated bearing work far more effectively. Conversely, in a poorly grounded system even the best bearing protection may not be enough on its own. Bearing current protection is thus a holistic approach that begins with motor option selection and is completed with correct field practice.

The Cost of Running Without Protection

Because IE5 synchronous reluctance motors sit in the highest efficiency class, they are generally chosen for critical, continuously running applications. In such a motor, early bearing failure caused by bearing current creates not just the bearing replacement cost but a much larger indirect cost. An unplanned stop means production loss, line re-commissioning time and emergency maintenance labor. In continuous processes this loss is many times the price of the few protection options that were neglected. Bearing current protection should therefore be assessed not as an extra cost item but as a measure that secures the motor's reliability and the return on the efficiency investment. When the right options are chosen upfront, the IE5 motor's high efficiency advantage is combined with a long, trouble-free service life.

Commissioning and Checklist

When commissioning the IE5 motor with a drive, the following steps should be reviewed for bearing current protection:

• Confirm at the ordering stage that a shaft grounding ring (or a suitable insulated bearing) is present at the motor's drive end.
• On a large-frame motor, check that an insulated/ceramic bearing option is defined at the opposite end.
• Use a shielded motor cable and ground the shield at both ends in a 360-degree, low-impedance manner.
• Make a short, thick potential-bonding connection between the motor frame and the machine chassis.
• After commissioning, verify that the protection measure is effective by measuring the shaft voltage with an oscilloscope.
• Evaluate the drive's switching frequency and cable length per manufacturer recommendations; long cables can increase voltage spikes.

When these steps are applied upfront, the IE5 motor's high efficiency advantage is combined with a long, reliable service life. A small check skipped during commissioning can turn into a costly bearing failure later, so the checklist should be applied with discipline.

Frequently Asked Questions

Does bearing current occur in every drive-fed motor?

Every motor running on a drive has common mode voltage and therefore the potential for bearing current. But whether damage emerges depends on factors such as power, frame size, cable length, switching frequency and grounding quality. Since the risk is always present, protection should be planned upfront especially in continuously drive-operated motors like IE5.

Shaft grounding ring or insulated bearing?

The two work by different mechanisms and often complement each other. The shaft grounding ring drains the voltage safely to ground; the insulated bearing cuts the current path at the bearing. In large motors the safest approach is to apply a shaft grounding ring at the drive end and an insulated bearing at the opposite end.

Can fluting damage be repaired?

A bearing with fluting must in practice be replaced; the surface damage is permanent. The real solution is prevention: stopping the voltage discharge upfront with correct shaft grounding and/or insulated bearings preserves both the bearing and the grease life.

Choose the IE5 Motor with the Right Options and Fast Delivery from Stock

To run your IE5 synchronous reluctance motor safely on a drive, share your power, frame size, cable length and application criticality details with us. As HEM Motor, with manufacturer stock advantage and fast delivery, let us determine together the most suitable shaft grounding and insulated bearing option for your motor. You can review our articles on drive package selection, thermal behavior and cooling, grounding and EMC, VFD and bearing current protection and insulated bearings, then request a quote.