Suggestion to electricpete (Electrical) Nov 13, 2003 marked ///\\jb - what are you responding to?
///I am essentially responding to the original posting, namely:
""...but would like to know of any in line gizmo could be fitted to eradicate the eddy currents from rotor to stator.""
If motors are inverter driven, it does not mean that the motor input has the pure voltage sinusoidal waveform. If harmonic content is present, which actually is on the VFD output even when properly filtered dv/dt, carrier frequency filter, reactor, etc., the vibration posted in my above links can be caused by that harmonic content (not only harmonics, but also acoustical noise).\\
Was there anything there about bearing currents.
///Why are you questioning bearing currents?\\ I am trying to figure out what bearing currents have to do with 2LF, 4LF, 6LF vib.
///Bearing currents are milliamps. They can hardly cause any vibration until they damage the bearings. Then the vibrations are noticeable. In fact, all kinds of vibrations if the motor is firmly connected to the load. The motor-load set can experience the system vibration.\\ So far the only thing I know they have in common is they may both be caused by vfd's... but nothing I know suggests that they are related to each other.
///Not directly. There is a relationship of "cause" and "effect."\\ i.e. insulating a bearing may protect the bearing but is not going to change 2LF, 4LF, 6LF pattern vib in the least imho.
///Agreed.
Furthemore, I copy your posting from above and provide a comment within:
"""electricpete (Electrical) Nov 12, 2003
One comment:
Mechanical looseness can be excited by any forcing function.
///True. The forces caused by the harmonic content in the motor input should not be ruled out.\\ If the forcing funciton is unbalance
///What kind of unbalance? Electrical or Mechanical or both?\\ we will usually expect 1x, 2x, 3x etc. If the forcing function is magnetic force we will usually expect 2LF, 4LF, 6LF etc.
///The magnetic force is a very complex issue, e.g. magnetic eccentricity, distortion of electromagnetic fields by harmonic content, etc.\\
One question:
Is it being claimed that 2LF, 4LF, 6LF etc is caused by shaft currents?
///This is what appears to be sought in the previous comment. See my comments above.\\ I don't know for sure but that doesn't sound right to me.
///There can be various origins of shaft currents. Some of shaft currents (not necessarily bearing currents if there are slip rings and brushes to treat the shaft currents) can be relatively large, e.g. 200A or so on the ship propulsion shaft.
Also, subharmonics and interhamonics tend to produce mechanical vibrations. However, these are entering the motor stator from the motor power supply.\\