Vibration On AC motor with VFD
Vibration On AC motor with VFD
(OP)
Hello,
I am not an Electrical Engineer but I need to measure the vibration on a motor that is controlled by a VFD for variable speed.
I have read that a VFD converts the AC current to DC and then converts again into AC but something that is called modulated (PWM), which gives a different frequency.
I have some questions regarding this:
First question is if the motor design (electrical) needs to be different than an AC motor running on a fixed frequency?
Next will be if minimizing all the mechanical sources for vibration, like rotor unbalancing, shaft runout, etc, the motor still vibrates on some frequencies?
Is there any relation or conversion between PWM and frequency (Hz)?
What is best to measure vibration on this case, Vel, Displ or Acc?
Any advice will help me a lot on this matter.
Thank you.
I am not an Electrical Engineer but I need to measure the vibration on a motor that is controlled by a VFD for variable speed.
I have read that a VFD converts the AC current to DC and then converts again into AC but something that is called modulated (PWM), which gives a different frequency.
I have some questions regarding this:
First question is if the motor design (electrical) needs to be different than an AC motor running on a fixed frequency?
Next will be if minimizing all the mechanical sources for vibration, like rotor unbalancing, shaft runout, etc, the motor still vibrates on some frequencies?
Is there any relation or conversion between PWM and frequency (Hz)?
What is best to measure vibration on this case, Vel, Displ or Acc?
Any advice will help me a lot on this matter.
Thank you.





RE: Vibration On AC motor with VFD
RE: Vibration On AC motor with VFD
RE: Vibration On AC motor with VFD
Because the motor can now operate at any theoretical speed (within limits), it can begin to have resonant interactions with mechanical harmonics from any number of sources that may have always existed in a load system, but were never a problem before because the motor accelerated through them quickly. For that reason, most VFDs have what is called "critical frequency lockout" (usually more than one) where they can be told to NOT run within a speed range in which you have found mechanical resonances or harmonics.
Don't know what you are asking about with the term "relation" in that next question. Please elaborate.
Same for the last question; "best"? Best for what? Those 3 metrics are looking at different issues.
RE: Vibration On AC motor with VFD
respectfully
RE: Vibration On AC motor with VFD
There is of course some relationship between the amount of modulation and the output frequency since the basic purpose of the modulation is to provide the optimum stator voltage which is roughly proportional to the stator frequency.
RE: Vibration On AC motor with VFD
RE: Vibration On AC motor with VFD
What is the reason for wanting to measure vibration?
If you are doing it just for historical info then you need a plan. For example at what hertz/speed will you record? Does the system change alot or only run at 2 or 3 present speeds. Where on the motor are you going to measure? Can a vibration meter be used? This is the easiest
If you are trying to "lock out" frequency that are causing vibration then go through then, measure and lock
Hop this helps, sorry about the homework remark.
RE: Vibration On AC motor with VFD
Bearings are monitored so they can be replaced when vibration levels start approaching critical numbers. The measurement is mostly done using velocity pick ups and level is mostly in millimeters/second or the corresponding imperial speed unit.
Quite often, especially when the measuring equipment is using the so called envelop technique (which also is called high frequency measurement) there are problems with variable speed drives. I have been to places where the apparent vibration level is up to tens of mm/s, but no vibration can be felt or heard. That is usually due to EMI from the motor cable that leaks into the vibration pick-up and cabling.
Sometimes there are also magnetoelestic vibrations from the PWM voltage that makes the stator iron vibrate at a high frequency (this you can hear quite well). This frequency is then picked up and the algorithms in the measuring equipment tries to transpose the HF energy down to "bearing frequencies" and the result is often very confusing since there is no correspondence between the resulting spectrum and what the bearing was supposed to do.
Solution is often to just ignore the spectrum - or go back to classic "measure in baseband" technique. Sometimes a motor filter helps. But du/dt is not enough. It has to be a sine filter - or close to one.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...