Bearing Protection

What causes bearing damage?

Premature bearing failure is a major cause of motor downtime and costly repairs. Variable frequency drives (VFD) used on AC and DC motors produce induced electrical currents on the motor shaft. Common mode voltage occurs when the voltages on the three output lines of a drive do not sum "instantaneously" to zero. Normal balanced "sinusoidal" 3 phase voltage is always instantaneously summed to zero (volts) with respect to ground. PWM line voltages are on or off, (the transition times are small with respect to pulse width and are not used for zero summing anyway), thus cannot sum to zero with respect to ground. This current seeks the path of least resistance to ground which is typically through the motor bearings. When the current passes through the bearings electrical arcs take place leading to fluting, pitting, and bearing damage. The fluted or damaged bearing surface causes noise, vibration, and premature bearing failure. Bearing failure due to these induced electrical currents is one of the most common causes of motor downtime. Carbon brush technology in a variety of bearing protection kits for motor shafts, effectively divert induced electrical currents away from the bearings, protecting them from damage and failure.

Effective bearing protection will divert the current from the motor shaft and provide the lowest resistance path to ground in an economical product. The result is the lowest shaft voltage and maximum discharge to ground through the kit rather than through the bearings or other components. Unlike other options, carbon brush technology uses a proprietary silver graphite material that provides superior voltage minimization.

Advantages include:
  • Longevity: Will typically outlast the unit it is installed on.
  • Most effective technology proven to minimize shaft voltage and damaging currents.
  • Most effective technology proven to minimize shaft voltage and damaging currents.
  • Provides lowest resistance path to ground.
  • 5 times more effective at reducing shaft voltage than the fiber ring which significantly reduces the risk of current flowing through the bearing.
  • Low to high current carrying capacity.
  • Stock kits fit most motors and multiple shaft sizes.
  • Easy to install and maintain.
  • Numerous options for wide variety of conditions.
  • Internal mount options avoid exposure to outside and contaminated environments.

Shaft grounding option

Shaft grounding is a common method across industries however it does not reduce or protect equipment from the actual problem. The common mode current is directed around the motor bearing but still exist and can cause damage in other areas of the system such as: erratic behavior of VFDs and PLCs, premature motor insulation failure and cable damage.

Optimum solution is achieved by installing custom inductors on the output of the drive designed for each specific application. Common mode inductors are susceptible, however, to both saturation and high frequency permeability degradation. Another important factor is cost. Custom-designed common mode inductors can be expensive. However filters on the market such as the “dV Sentry” by MTE has proven to provide common mode and rise time reduction, as well as peak voltage protection. This gives greater motor protection over time.