Understanding Circulating Currents
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Electric motor bearings fail for a variety of reasons but the two most common reasons for electrical failure are circulating and common mode currents. Circulating currents are found in all motors. The severity of the problem and therefore the risk to the motor bearings is proportional to the motor frame size (the larger the motor frame the greater the risk). Let’s overview circulating currents, its cause, how it affects the bearings and how to mitigate them.
Circulating currents are a result of an asymmetry in the motor and the voltage it creates between the stator and rotor iron, but not related to the motor drive or method used to drive the motor. Specifically, the asymmetry in a motor comes from imbalanced windings, the differences in the iron core, and air gaps. Each motor winding has slight variations as compared to the other windings. These variations are in the wire itself, the winding method, wire insulation, etc. – no two windings are exactly alike. The same is true with the iron core, typically made from a stack of metal plates, one plate stacked alongside the other. Each plate is slightly different from the other and like the windings, no two are exactly the same. These variations along with slight discrepancies in the motor’s air gap as the rotor is turning to create an asymmetrical magnetic field in the motor.
The circulating current passes through and damages the bearings as they circulate through the motor (Figure 1 – shown in yellow). Manufacturers take careful consideration during design and manufacturing to reduce the asymmetry, but that level of quality comes at a cost many are not willing to absorb.
All motors experience this imbalance but only on large frame motors is the problem extreme enough to cause bearing failure. Naturally, large frame motors inherit more imbalances which leads to more circulating current. The rule of thumb is any motor frame over 100hp should be built with insulated bearings. It is common to use an insulated bearing to break the ‘circuit’ (shown below in blue). By adding only one insulated bearing the circuit, the circulating current, will be eliminated and the problem is corrected. Insulating either bearing will work but is typically only added to the opposite drive end (ODE) of the motor because it is less expensive and easier to replace.
Installing shaft grounding for common-mode discharges on the same end as the insulated bearing will reestablish the ‘circuit’ and will bypass the insulated bearing through the grounding device, the current will be forced to flow through the uninsulated bearing. For this reason, adding a grounding device to the same end of the motor as the insulated bearing is NOT recommended.
Please Note: Circulating currents are very different from common-mode current created by the pulse width modulated (PWM) signal used to drive the motor from a variable frequency drive (VFD). Insulating one bearing will eliminate circulating current but do nothing for common mode. Replacing both the drive end (DE) and opposite drive end (ODE) bearings with (expensive) insulated bearings will prevent damage to the motor bearings from either source but this typically only diverts the problem to downstream equipment like gearbox gears, load bearings, instrumentation, etc.
Circulating currents are directly related to the frame size and the quality of construction of the electric motor. Industry-standard recommends precaution be taken for motors over 100HP to prevent electrical erosion of the motor bearings due to electrical discharges. When common mode is also a concern, for motors driven from a variable frequency drive (VFD), it is also recommended to add shaft grounding. When adding a shaft grounding device like our Bearing Protection Kit (BPK) for common-mode current in addition to the insulated bearing to prevent circulating current take caution to ensure the shaft grounding kit is installed on the opposite side from the insulated bearing. For large frame motors driven from a VFD, it is recommended to insulate the ODE bearing to prevent circulating current and a Bearing Protection Kit (BPK) to the DE to prevent common-mode current. This will protect the motor from damaging electrical discharges from both sources.
Application Engineer II
Helwig Carbon Products, Inc. is the premier, family-owned, American manufacturer, of carbon brushes, brush holders, spring assemblies, bearing protection kits, metal graphite brushes, and mechanical carbons. Since 1928, we have forged innovations that have since become industry standards and requirements. We pride our self on manufacturing all of our products to the highest standard and performance because we know your application depends on it!