

Filtering Increases Motor Bearing Life Addition of KLC Significantly
Motor bearings form a circuit coupled through capacitance. A capacitor consists of two plates of different charge (voltage) levels. If an electric field can be generated between the plates, then there is capacitance. Both air and grease act as dielectrics through which such a field can be established. Once the field is established, the following fundamental equation holds true: I = C * dV/dt. This means that the current flowing through the capacitor (bearings, grease & air, bearing race) is proportional to the rate of change of voltage between plates and the dielectric quality of the grease and air (capacitor). If periodically the bearings exhibit metal to metal contact with the bearing race, then the capacitor is short circuited and more current can flow. Worse yet, when the bearings lose contact with the race, an arc may be drawn until the available energy is depleted for a given time period. The source voltage impedance (here, the drive is the source) is also a controlling factor in the amount of CM current flowing. Thus the equation truly looks like: Icm = (C * dV/dt) bearing factor + Z, where Z is the summation of all the CM circuit impedances other than the bearing impedance. For example, line impedance, ground impedance, shaft impedance, etc. Now that we have established a driving force, CM Voltage and a CM Current Loop, we can begin talking about how to change the results. Capacitive coupling is directly affected by switching speed. As noted in the equation: I = C * dV/dt. As switching speed rises, capacitive coupling impedance drops, thus increasing the amount of CM current for a given CM voltage. So, a desirable change would be to slow down dV/dt. Increasing common mode line impedance by the addition of a common mode inductor will limit CM Current. Optimum performance with this solution is achieved by custom inductors designed for each specific application. Common mode inductors are susceptible, however, to both saturation and high frequency permeability degradation. Another important factor is cost. Customdesigned common mode inductors can be expensive.



