This invention relates generally to electric motors and, more particularly, to bearing assemblies for electric motors with adjustable speed drives.
A/C electric motors typically include a motor housing, a stator including one or more stator windings, and a rotor assembly. The rotor assembly includes a rotor core and a rotor shaft extending through the rotor core. The motor housing includes at least one endshield and houses at least a portion of the rotor assembly. Electric motors also typically include at least one bearing sized to receive and support the rotor shaft, and at least one inner bearing cap separated from the bearing. Typically, the bearing is positioned between an endshield and an inner bearing cap and facilitates rotation of the rotor shaft when the stator windings are energized. An adjustable speed drive circuit is coupled to an inverter and the motor to selectively vary the motor speed as desired in a particular application.
Improvements in inverter technology have led to increased use of adjustable speed drives with A/C induction motors. High speed switching of power supplied to these motors often results in charge build up between the rotor and the stator until a sparking voltage develops across the bearing. Once a sparking voltage develops, a spark discharges across the bearing. After the initial spark, a capacitive coupling between the rotor and stator provide a damaging “follow on” current through the bearing. This current damages the bearing thus affecting motor reliability and performance. Insulated bearings, shaft brushes, and modified drive circuits have been employed to reduce and/or eliminate damaging current flow through the bearings, but tend to increase the cost of the motor, reduce motor performance, or introduce costly maintenance issues.
Accordingly, it would be desirable to provide a low cost bearing assembly that reduces or eliminates current flow through the bearings to improve motor reliability and performance while avoiding costly maintenance issues.