vfd malfunction results in vibration at vfd output fundamental frequency?
vfd malfunction results in vibration at vfd output fundamental frequency?
(OP)
I’m posting this question on behalf of “Rotate” from maintenanceforums.com. He’s knowledgeable in vibration but not very familiar with vfd’s.
The entire thread is here:
http://maintenanceforums.com/eve/forums/a/tpc/f/37...
http://maintenanceforums.com/eve/forums/a/tpc/f/37...
The attachment to the first post of that thread is attached.
The basic question: Is there a malfunction of a vfd that can cause the 4-pole motor to vibrate at roughly the fundamental frequency on the output of the vfd. This frequency is slightly higher than twice the speed of the motor (for example 2.02 times). For example 50hz vibration for a 4-pole motor running at 24.75 rotations per second. If we didn’t have a vfd in the picture, we would refer to it as “line frequency” vibration (note this is not the common twice line frequency).
What type of vfd is apparently unknown.
The entire thread is here:
http://maintenanceforums.com/eve/forums/a/tpc/f/37...
http://maintenanceforums.com/eve/forums/a/tpc/f/37...
The attachment to the first post of that thread is attached.
The basic question: Is there a malfunction of a vfd that can cause the 4-pole motor to vibrate at roughly the fundamental frequency on the output of the vfd. This frequency is slightly higher than twice the speed of the motor (for example 2.02 times). For example 50hz vibration for a 4-pole motor running at 24.75 rotations per second. If we didn’t have a vfd in the picture, we would refer to it as “line frequency” vibration (note this is not the common twice line frequency).
What type of vfd is apparently unknown.
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(2B)+(2B)' ?





RE: vfd malfunction results in vibration at vfd output fundamental frequency?
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(2B)+(2B)' ?
RE: vfd malfunction results in vibration at vfd output fundamental frequency?
There is a 150Hz component on the output that arises due to the cyclic referencing of the DC BUS to each of the phases. This can become much stronger with a phase to earth or star point to earth fault in the motor. However, it's hard to imagine how this would manifest as a 50Hz vibration.
I'd be looking for something a bit more telling - the particular VSD perhaps or noise coupled into the accelerometer measurement.
RE: vfd malfunction results in vibration at vfd output fundamental frequency?
http://www.industry.usa.siemens.com/drives/us/en/e...
These faults could cause "line" frequency vibrations:
Interphase fault
Ground fault
Soft Foot/Eccentric Rotor
This M.E. is interested in motor vibrations!
Walt
RE: vfd malfunction results in vibration at vfd output fundamental frequency?
RE: vfd malfunction results in vibration at vfd output fundamental frequency?
If you have a situation where one of the output IGBTs is not being controlled correctly, that will cause a vibration at the frquency of the output of the VFD. This is usually quite violent, however it depends on how much current limiting there is on that phase.
Mark Empson
Advanced Motor Control Ltd
RE: vfd malfunction results in vibration at vfd output fundamental frequency?
Mark-can you offer any comments to help understand how that frequency arises?
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RE: vfd malfunction results in vibration at vfd output fundamental frequency?
Walt
RE: vfd malfunction results in vibration at vfd output fundamental frequency?
This DC component would result in a stationary torque field component, plus there would be an imbalance which would result in negative sequence fields.
The exact nature of the problem would depend on the cause, if an IGBT is turning partially ON over the full conduction cycle, there could be considerable power dissipation in that IGBT resulting in heat and changes in current, desaturation protection on the gate drive could shorten the conduction pulses without the same level of heat etc, so there are a number of scenarios that could cause an imbalance in the positive/negative conduction and this may be very small as caused by an additional switch ON delay in the gate drive etc.
I expect that the torque fields could quickly become very complex in the frequency domain.
Mark Empson
Advanced Motor Control Ltd
RE: vfd malfunction results in vibration at vfd output fundamental frequency?
RE: vfd malfunction results in vibration at vfd output fundamental frequency?
This was not uncommon in older Siemens VFDs. Haven't seen it lately, though.
If one phase of the motor is fed a lower current than the other two, you will not get a circular flux vector path, but an elliptic one. That definitely produces a vibration with the VFD fundamental frequency.
It is easy to find out if this is the problem if you use a clamp-on ammeter to check motor current balance. The VFD display cannot be trusted (and seldom shows current in individual phases), so a clamp shall be used.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: vfd malfunction results in vibration at vfd output fundamental frequency?
Thanks for all the input.
I'd think that if one phase ends up weaker than the other two or missing altogether, then the frequency of vibration would still be twice fundamental frequency, PROVIDED the weaker phase was half-wave symmetric. In this case we simply have a superposition of fundamental frequency fluxes which is itself fundamental frequency and should always result in twice frequency vibrations. An example would be unbalance supply to non-vfd motor...we know it vibrates at twice line frequency.
On the other hand, if the absolute value of a given phase differed between the positive half-cycle and the negative half cycle, then I could see fundamental frequency vibration. Perhaps this occurs if there is effectively an open circuit on the link from one pole of the dc bus to one phase.
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RE: vfd malfunction results in vibration at vfd output fundamental frequency?
I also see the article says this:
From this statement along with the symptoms which cause it, we pretty much conclude this author believes that most of the faults which cause twice line frequency vibration can also result in one times line frequency vibration.
I do have a hardcopy of another article which describes the same phenomenon in detail: "Line Frequency Magnetic Vibration of A-C- Machines" by R.C. Robinson, AIEE Transactions Part III Power Apparatus Systems Volume 81, 1962. Here is an excerpt:
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RE: vfd malfunction results in vibration at vfd output fundamental frequency?
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RE: vfd malfunction results in vibration at vfd output fundamental frequency?
Yep, all the potential motor faults still apply, except that they will occur at the VFD synthesized frequency and its harmonics, rather than at mains frequency and its harmonics. The rectification process in the VFD effectively isolates the drive from mains frequency. There are only two elements of mains frequency which persist - the DC bus ripple (6x mains frequency for a typically 6 pulse rectifier) and the common mode alternate referencing of the DC bus to each of the input phases (3x mains frequency). As electricpete illustrates - what then happens in the motor and in fault conditions is very complex. I'm willing to believe, though am not convinced, that some scenario could cause modulation of the 6x or 3x component and produce a mains frequency vibration.
I don't have any good references for you. This field is poorly represented in the literature at the moment. As I said this is a theoretical approach and pretty much comes down to lots of simulations and application of first principles to various VFD topologies - so apply the appropriate skepticism. I've been working recently on accurately modeling the spectral content of VFD driven motor currents, in particular in case of earth faults, and one strong and consistent phenomena is the lack of a mains frequency component in the output of the VFD.