How to identify "mechanically" induced vibration and acoustics noise?
How to identify "mechanically" induced vibration and acoustics noise?
(OP)
Hi,
My background is in electrical engineering. I wish to analyse the vibro-acoustics of a 3-phase permanent magnet brushless machine. I'm aware that vibration or acoustics noise can be generated "electromagnetically" (due to MMF, airgap field, etc) or/and "mechanically" (e.g. misalignment, eccentricity, unbalanced load, bearing, brush/commutator, etc.). As I need to analyse the vibro-acoustics of the PM motor at various loads, the PM motor must be attached to a DC machine (armature winding connected to a variable resistive bank), which acts as a load during testing (see attached image). I'm aware that, in practice, it is very difficult to avoid misalignment and eccentricity. In other words, there will be some degrees of misalignment or eccentricity in real world setup. As I'm interested only in "electromagnetically" induced vibrations and acoustics noise, I wish to distinguish "electromagnetically" induced vibrations and acoustics noise from that of mechanically induced.
I've the following instruments: dynamic signal analyser, PCB accelerometer, and sound pressure meter. The accelerometer is attached (using wax) to the motor under test.
Questions:
[Case-1] The PM motor is not excited, i.e. the winding terminals of the PM motor are left open. In this case, the PM motor is driven by the DC machine (i.e. the DC machine acts as a motor now), e.g. at 1000rpm (rated speed of the PM motor). What is the information that we can extract from the vibration reading measured by the accelerometer? Can we use this experiment to identify "mechanically" induced vibrations?
[Case-2] If the winding terminals of the PM motor are connected to a 3-phase resistive load (in such a way that the current flows in each phase winding is the rated phase current of the motor), and like in Case-1, it's driven by the DC machine at 1000rpm, what is the information that we can extract from the vibration reading measured by the accelerometer?
As I'm a novice in vibro-acoustics test, please correct me if any of my comment is wrong. Please also kindly suggest if you have better options to identify "mechanically" induced vibration and acoustics noise of my experimental setup.
Thanks
My background is in electrical engineering. I wish to analyse the vibro-acoustics of a 3-phase permanent magnet brushless machine. I'm aware that vibration or acoustics noise can be generated "electromagnetically" (due to MMF, airgap field, etc) or/and "mechanically" (e.g. misalignment, eccentricity, unbalanced load, bearing, brush/commutator, etc.). As I need to analyse the vibro-acoustics of the PM motor at various loads, the PM motor must be attached to a DC machine (armature winding connected to a variable resistive bank), which acts as a load during testing (see attached image). I'm aware that, in practice, it is very difficult to avoid misalignment and eccentricity. In other words, there will be some degrees of misalignment or eccentricity in real world setup. As I'm interested only in "electromagnetically" induced vibrations and acoustics noise, I wish to distinguish "electromagnetically" induced vibrations and acoustics noise from that of mechanically induced.
I've the following instruments: dynamic signal analyser, PCB accelerometer, and sound pressure meter. The accelerometer is attached (using wax) to the motor under test.
Questions:
[Case-1] The PM motor is not excited, i.e. the winding terminals of the PM motor are left open. In this case, the PM motor is driven by the DC machine (i.e. the DC machine acts as a motor now), e.g. at 1000rpm (rated speed of the PM motor). What is the information that we can extract from the vibration reading measured by the accelerometer? Can we use this experiment to identify "mechanically" induced vibrations?
[Case-2] If the winding terminals of the PM motor are connected to a 3-phase resistive load (in such a way that the current flows in each phase winding is the rated phase current of the motor), and like in Case-1, it's driven by the DC machine at 1000rpm, what is the information that we can extract from the vibration reading measured by the accelerometer?
As I'm a novice in vibro-acoustics test, please correct me if any of my comment is wrong. Please also kindly suggest if you have better options to identify "mechanically" induced vibration and acoustics noise of my experimental setup.
Thanks






RE: How to identify "mechanically" induced vibration and acoustics noise?
Cheers.
RE: How to identify "mechanically" induced vibration and acoustics noise?
First step is to look at the pattern of the vibration (primarily the frequencies, also the directionality and possibly the time waveform) to try to discern the nature of the vibration. If vibration is at running speed I would think usually mechanical. Vibration at number of poles times running speed would likely be electrical. Bearing fault frequencies are mechanical. Vibration which is highly directional is usually amplified by mechanical resonance.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: How to identify "mechanically" induced vibration and acoustics noise?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: How to identify "mechanically" induced vibration and acoustics noise?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: How to identify "mechanically" induced vibration and acoustics noise?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: How to identify "mechanically" induced vibration and acoustics noise?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: How to identify "mechanically" induced vibration and acoustics noise?
To electripete:
You wrote: "...First step is to look at the pattern of the vibration (primarily the frequencies, also the directionality and possibly the time waveform)..."
May I know what does "directionality" mean in your statement?
Thank you very much.
[IMG]http://
RE: How to identify "mechanically" induced vibration and acoustics noise?
The DC machine is a shunt DC machine. It's field and armature winding are connected to two different regulated DC power supplies.
RE: How to identify "mechanically" induced vibration and acoustics noise?
How may poles does the PM machine have?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: How to identify "mechanically" induced vibration and acoustics noise?
And what are the units of velocity used?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: How to identify "mechanically" induced vibration and acoustics noise?
The accelerometer is placed on the PM motor body as shown in attached experimental setup. I use PCB accelerometer 303A03 and you may find its specs here:
ht
The PM motor has 12 slots and 10 poles (or 5 pole-pairs). By the way, which "velocity" do you mean? The Y-axis of results shown in my previous post were not scaled to the actual units.
Thank you very much
RE: How to identify "mechanically" induced vibration and acoustics noise?
Kindly refer to the spec sheet of 480C02 as well as 303A03 (ht
RE: How to identify "mechanically" induced vibration and acoustics noise?
RE: How to identify "mechanically" induced vibration and acoustics noise?
The pattern of harmonics of 50hz are seen in the blue (300cpm) spectrum: 150, 200 (not labeled), 250, 300 (labeled 296?), 350. These are very likely electrical.
I don't quite understand the 416 and 472. Could be lots of things. Do you happen to have the bearing part numbers?
You lost me when you started talking about hammer. I assume these are running vibration tests, not impact tests. Where does the hammer fit in?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: How to identify "mechanically" induced vibration and acoustics noise?
First of all I wish to apologize for mentioning "Impact Hammer" in my last post in this thread. I did carry out several Impact Hammer tests (please refer http://www.eng-tips.com/viewthread.cfm?qid=252742), but they are nothing to do with the test mentioned in this thread.
The test results shown in my previous post were obtained based on the test setup and procedure as described in Case-1 (please refer to my first post of this thread), except that now the DC machine runs at 300rpm and 400rpm. As the terminals of the PM motor (which is driven by the DC machine) are left open, I suppose the PM motor does not produce any current. Unfortunately, I do not have the bearing part numbers. As the DC machine is a brushed DC shunt wound machine, do you think the brush/commutator assembly will contribute to the vibration spectrum shown above?
By the way, is there any vibration source that does not rely on motor speed (those spikes marked by YELLOW circle in the results)?
Thank you very much
RE: How to identify "mechanically" induced vibration and acoustics noise?
RE: How to identify "mechanically" induced vibration and acoustics noise?