Premature failures of induction motor bearings have many causes. The growing use of Variable Frequency Drives with induction motors has generated no shortage of research by motor design engineers on this subject.
A primary culprit identified by this research is motor shaft voltages. Two sources of shaft voltages in induction motors are: an electrostatic charge introduced from outside the motor or an electromagnetic imbalance within the motor itself. Common sources of electromagnetic imbalances are: out-of round or off-center rotors, open rotor bars, magnetically non-symmetrical iron or windings that are not symmetrically placed in the stator.
To completely protect a motor from internal electromagnetic imbalance, one motor bearing must be insulated or grounding brushes must shunt the entire shaft assembly, including both bearings. To protect from an external imbalance (common mode voltages) both bearings must be insulated or a ground brush must be installed, or the source of imbalance must be diverted from the motor via filters. If the bearings of a VFD-powered motor are not protected, they will not obtain their expected B-10 life.
Jerry Muehlbauer, Senior Application Engineer at Regal Beloit Corporation, an IBT vendor, has prepared a technical paper that includes practical solutions on the subject of Solving Bearing Current Problems in AC Motors. Read the complete text below in the attached file.
Subjects covered within the article include:
- What causes bearing currents?
- Where does an electrostatic charge come from?
- What happens to the common mode voltage?
- How does a VFD produce a common mode voltage?
- What factors influence the CMV?
- What determines the amount of bearing damage?
- Can the CMV be reduced more?
- Can the transformer neutral be safely ungrounded?
- What if the neutral can’t be ungrounded?
- What protection is available for Explosion Proof motors?
To learn more about preventing premature wear or failure of electric motors, or for more information in general about motors and drives, see your IBT Sales Rep or ask us.