How to Handle Constant Bearing Currents

On rare occasions, operators in the process industry may notice that their low-voltage motors experience current flowing through the motor bearings, especially when they are used with variable speed drives (VSDs).

If left unchecked, these bearing currents will inevitably lead to premature failure. ABB has a well-established portfolio of first-action solutions. There are now two new ways to eliminate the most persistent bearing currents. However, making the right choice requires careful consideration of the specific application conditions.

What causes bearing currents?

When voltage is present on the motor shaft, it can overcome the insulating effect of the bearing lubrication film. This can cause current flow, which effectively leads to electrical discharge machining (EDM) of the bearing, which can lead to premature wear and ultimately early failure.

This voltage can be generated in three different ways, depending on the size of the motor, how its frame and shaft are grounded, the electrical installation and the specific electronic characteristics of the AC inverter.

The first of these three types is the circulating current in larger motors, when the high frequency flux circulating around the stator induces a high frequency voltage between the ends of the motor shaft. When this voltage reaches a level sufficient to overcome the bearing oil film impedance, circulating current begins to flow in the circuit formed by the shaft, bearing and stator frame.

Second, in larger motors, the shaft ground current that leaks into the stator frame needs to flow back to the inverter, which is the source of that current. Any return path contains impedance, so the voltage in the motor frame rises compared to the source ground level. If the motor shaft is grounded through the slave machinery, an increase in motor frame voltage can be seen at the bearing. If the voltage is high enough to overcome the impedance of the drive-side bearing oil film, the shaft ground current may flow back to the inverter through the drive-side bearing, the shaft, and the driven machine.

Third, capacitive discharge currents occur in small motors when the internal partial voltage of the common-mode voltage across the motor's internal stray capacitors causes the shaft voltage to be high enough to generate high-frequency bearing current pulses. This can occur if the shaft is not mechanically grounded through the drive and the motor frame is grounded in a standard manner to provide protection.

Extended portfolio of bearing current solutions

There is no single technical solution that can be applied to all installations where bearing current is a concern. Rather, it is a matter of helping customers make the best choice from a range of potential solutions based on the details of their particular application, where motor size is a key consideration.

Hebei Nicer has established three effective preferred bearing current solutions in the marketplace. These include: insulated bearings on the non-drive side; common mode filters and grounding and wiring. Now, for the most demanding cases where one or a combination of these solutions does not provide a complete remedy, two new enhancements have been developed, namely shaft grounding brushes and two-end insulated bearings.

In detail, the solutions for bearing currents are:

1 - Insulated bearings at the non-driven end

For motors with IEC frame sizes 280 and above, an effective solution is to replace the bearing at the non-driven end with an insulated bearing to prevent current flow through that point. This can be done as an option for retrofit applications or specified for new motors. It is important to note that the current must still flow somewhere. Therefore, while the motor will be protected, it is important to ensure that this method does not create new problems in another part of the installation.

2 - Common mode filter in combination with VSD

For large motors with nominal power greater than 350 kW (IEC 400 or larger frame size), a common mode filter will reduce the common mode current and thus reduce the risk of bearing currents. The common mode filter can be installed internally as part of the motor and drive package or on the cable between the VSD and the motor. The common mode filter will not significantly affect the phase of the mains voltage at the motor terminals.

3 - Grounding and wiring of transformers, VSDs, motors and load systems

For all installations, and especially for motors rated over 30 kW, we recommend using ground and motor connection cables with symmetrical PE (protective ground) shielding throughout the system, as this will greatly attenuate the motor shaft and frame voltages. This is best practice even if bearing currents are not considered a problem. This should also be the first measure taken before installing common mode filters on large motors.

4 - Internally mounted shaft grounding brushes

An important new solution for motors in IEC frame sizes 132 to 250 is the installation of a motor shaft grounding brush that directs current to ground through the brush rather than through the bearing. This protects the motor itself and the entire installation. By specifying a variant code, the brush can be pre-installed on the new motor. Or it can be retrofitted in the field.

5 - Insulated bearings on the drive side and non-drive side

Another innovative method, now available for IEC frame sizes 71 to 250, takes the form of bearings mounted at both ends of the motor with insulated outer rings or rolling elements. This ensures that no current can flow through the bearings to earth. For smaller motors and special applications, hybrid bearings with non-conductive ceramic rolling elements can also be used. These hybrid bearings can offer other benefits such as longer relubrication intervals and longer service life.

How do I measure the bearing current?

In most cases, it is difficult for the operator to measure any bearing currents that may be present on a standard motor. However, if bearing currents are suspected, they can be detected using special equipment operated by experienced personnel. Hebei Nicer has gained extensive experience in performing these measurements on motors and drives in a variety of different applications worldwide.

In summary - bearing currents can be avoided

While it is important to recognize that bearing currents can be a problem, they can cause very few motor bearing failures. The most common causes of bearing failure are simply mechanical problems, such as excessive axial or radial loads, inadequate or incorrect lubrication, or foreign material or moisture inside the bearing.

When bearing currents are present, there is no "one size fits all" solution. It is critical for customers and motor and drive suppliers to work together to determine the most appropriate solution for a particular application. Ensuring that grounding and connecting cables are used correctly according to best practices is always an important first step. New solutions in the form of shaft grounding brushes and hybrid bearings are now showing great promise as a cost effective way to eliminate the potentially harmful effects of bearing currents.