How to deal with persistent bearing currents
On rare occasions, operators in the process industry may note that their low voltage motors experience a current flowing through the motor bearings, especially when they are used with a variable speed drive (VSD).
Left unchecked, these bearing currents will inevitably result in premature failure. Two new approaches can eliminate even the most persistent bearing currents. But 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 cause a current flow that results effectively in electric discharge machining (EDM) of the bearing, causing premature wear and ultimately, early failure.
This voltage can be generated in three different ways according to the size of the motor, how its frame and shaft are grounded, the electrical installation and the specific electronic characteristic of the AC drive.
The first of the three types is circulating current in larger motors – above IEC 280 frame size – when a high frequency voltage is induced between the ends of the motor shaft by the high frequency flux circulating around the stator. When this voltage reaches a level high enough to overcome the impedance of the bearing oil film, a circulating current starts to flow in the loop formed by the shaft, the bearings and the stator frame.
Second, in larger motors, shaft grounding current leaking into the stator frame needs to flow back to the inverter, which is the source of this current. Any route back contains impedance, and therefore the voltage of the motor frame is raised in comparison to the source ground level. If the motor shaft is earthed via the driven machinery, this increase of the motor frame voltage is seen over the bearings. If the voltage is high enough to overcome the impedance of the drive-end bearing oil film, a shaft ground current may flow via the drive-end bearing, the shaft and the driven machine back to the inverter.
Third, capacitive discharge current happens in small motors – below IEC 280 frame size – when the internal voltage division of the common mode voltage over the internal stray capacitances of the motor causes shaft voltages high enough to create high frequency bearing current pulses. This can happen if the shaft is not earthed via the driven machinery while the motor frame is earthed in the standard way for protection.
Top Three bearing current solutions
There is not one single technical solution that can be applied to cover all installations where bearing currents are considered to be an issue. Instead, it is a question of helping customers to make the optimal choice from a range of potential solutions according to the details of their specific application, with motor size being the key consideration.
There are three effective first-action choice solutions for bearing currents already well established on the market. These include: an insulated bearing at the non-driven end; common mode filter and earthing and cabling. Now, for the most demanding situations where one or a combination of these solutions cannot provide a complete remedy, two new enhanced approaches have been developed in the form of shaft grounding brushes and insulated bearings at both ends.
In detail, the three first-action solutions for bearing currents are:
1 – Incorporating an insulated bearing at the non-driven end
An effective solution for motors with an IEC frame size of 280 and upwards is to substitute the bearing at the non-driven end with an insulated bearing that prevents current flowing through this point. This can be applied as a retrofit or specified as an option on new motors. It must be noted that the current still has to go somewhere. So, while the motor will be protected, it is important to ensure that this approach will not create new issues in another part of the installation.
2 – Common mode filter used in conjunction with a VSD
For larger motors with a nominal power greater than 350 kW (IEC 400 or larger frame size), a common mode filter will reduce common mode currents and thus decrease the risk of bearing currents. The common mode filter might be installed internally as part of the motor and drive package, or could be fitted on the cable between the VSD and motor. Common mode filters do not significantly affect the phase of main voltages on motor terminals.
3 – Earthing and cabling of transformer, VSD, motor and load-train
For all installations, and especially motors with a nominal motor power over 30 kW, we recommend the use of grounding and motor connection cables with symmetric PE (Protective Earth) shielding across the entire system as this strongly attenuates motor shaft and frame voltages. This represents best practice even when bearing currents are not regarded as an issue. It should also be the first action taken prior to installing a common mode filter on larger motors
How to deal with persistent bearing current issues:
The two new enhanced approaches for installations where the first-action solutions do not provide a complete remedy are:
4 – Shaft grounding brush installed internally
An important new solution for motors in IEC frame sizes 132 to 250 is to install a motor shaft grounding brush that directs the current to the ground via the brush, rather than through the bearing. This protects the motor itself and the complete installation. The brush can be pre-installed on new motors by specifying a variant code. Or it can be retro-fitted on site.
5 – Insulated bearings at both the driven and non-driven ends
Another innovative approach, now being applied to motors in IEC frame sizes 71 to 250, is in the form of bearings installed at both ends of the motor with insulated outer races or rolling elements. This ensures that no current can flow to earth via the bearings. For smaller motors, as well as for special applications, hybrid bearings with non-conductive ceramic rolling elements can also be used. These hybrid bearings can offer other benefits such as longer re-lubrication intervals and a longer service life.
How can you measure bearing currents?
In most cases it is difficult for operators to measure any bearing current that might be present on a standard motor. But if bearing currents are suspected they can be detected using special equipment operated by experienced personnel. Should you think that you have an issue on your application Quantum, Controls Engineers can help
Summary – bearing currents can be avoided
While it is important to recognize that bearing currents can be an issue, they are responsible for a very small number of motor bearing failures. The most common causes for bearing failures are due simply to mechanical problems such as too high an axial or radial load, insufficient or wrong lubrication or foreign particles or moisture inside the bearing.
When a bearing current is present, there is no ‘one size fits all solution’. It is vital for the customer and motor and drive supplier to work together to identify the most appropriate solution for the specific application. Ensuring the correct use of grounding and connection cables according to best practice will always be an important first step. New solutions in the form of shaft grounding brushes and hybrid bearings are now showing significant promise as a cost-effective way of eliminating the potentially harmful effects of bearing currents.
When selecting a motor and drive combination contact our expert engineers and we will specify the correct matched equipment that will ensure that the potential issue of bearing currents is engineered out of the Driven System.
Source ABB Motors, WEG Motors, Schneider Drives