In preparation for this, my first-ever blog post, I had to switch my conventional way of thinking as a technical support specialist from asking questions in a logical and methodical manner to pinpoint and eliminate the root cause of an issue. Here, I am asked to present possible causes of a generalized hypothetical problem, a fundamentally different approach.
Some ground rules: I will assume the motor is operated within the nameplate rating; limit the discussion to 3 phase AC induction motors; and concentrate on motor generated noises, not on noises that are being “referred” or “transmitted” back to the motor from a mechanical issue located elsewhere. Having said this, properly designed, installed and maintained motors will perform trouble-free for many years. Motor noises often indicate malfunctions in the driven machinery. Therefore, motor troubleshooting involves the entire system, not just the motor. So please, thoroughly check out all possible external mechanical issues before looking into the motor itself.
Once we have determined the noise is not being “referred” or “transmitted” back to the motor from a mechanical issue, we can begin looking at motor issues. Motor bearing failures are time and time again the primary cause of failures, followed by fan blade interference which is fairly easy to detect. Other than these 2 issues, motors are very robust and failures will be rare. If a motor noise is not corrected the overall system performance will suffer, the noise will get progressively worse, and the motor will eventually fail.
After querying many colleagues who “live” in the motor Variable Frequency Drive (VFD) world, I have only been able to uncover a few such situations. All of them have been related to VFD applications.
In rare situations a motor will exhibit unusual noises but only in a certain carrier frequency range. Most modern VFD’s will allow you to configure a “skip” to eliminate this range.
Often a VFD is used in conjunction with an encoder to create a closed loop. This loop calculates an error value as the difference between the encoder feedback and the desired set-point. The response of this loop is controlled by a Proportional-Integral-Derivative (PID) loop located in the VFD. If the PID loop is tuned too tight, any little change in speed reference or load may cause the motor to oscillate creating a mechanical sounding noise in the motor.
I have uncovered one instance where the firing angle of the Insulated Gate Bipolar Transistors (IGBT) in the VFD were not synchronized correctly, again causing a mechanical sound in the motor. After an exhaustive troubleshooting session left the technician baffled, he finally contacted the VFD’s engineering support team to resolve the issue. The support team determined that the VFD needed replacing and the technician did not have the tools to diagnose such a problem on his own.
So, we have come full circle back to bearings. Just a closing note here: if you are using a VFD consider using a ‘Shaft Grounding Ring’ to mitigate Electrical Discharge Machining (EDM) damage.
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