Skip Frequency?

[Strahan]

I have been asked a question from one of our electricians that I'm having a hard time answering. As he was looking through a VFD manual he came across the "skip frequency" parameter and asked me to explain how it worked. As far as I know we do not use this feature on any of our drives and we have many. I have not personally been involved where we use this. The manual is very vague on the explanation. I get the impretion it is to "filter" so to speak the drive from operating at certain frequencies (maybe for harmonic reasons) but I don't know the benefits or reasons for using this. The max we run our drives at is 100HZ not sure if this parameter is even relevant at these frequency levels. Can someone help explain?

 

[iwire]

You just can't beat Google. :smile:

Skip frequency. All rotating machinery ? from motorcycles to industrial fans and pumps ? have mechanical resonance points. These are the frequency points at which vibration can rapidly damage that specific equipment. If you're aware of these points and avoid them ? by either accelerating beyond or decelerating below them so the motor doesn't run at those points ? you can prevent the rapid damage. VFDs with skip features allow you to do exactly that. In fact, most drives offer multiple skip frequency parameters to mitigate different resonance points.

The skip frequencies do not affect normal acceleration and deceleration. The drive output will ramp through the band, uninterrupted. When the operator issues a command to operate continuously inside the established band; however, the drive will alter the output to remain outside the band until a new command is issued.

If you know the mechanical resonant frequencies of your equipment, you can program the drives to ?skip? through operation at those frequencies. That is, your equipment will run at those frequencies only momentarily, rather than continuously ? just long enough to arrive at a safe frequency of operation. How can you determine what these resonant frequencies are? You may find this information in the equipment manual. A more common method is simply observing the equipment for noticeable changes in heat (for example, at bearings), noise, or motion when the operating frequency changes.

 

[ohm]

You just can't beat Google. :smile:

All ships have a resonant frequency which is determined durinig sea trials. This is a certain shaft RPM where vibrations occur and can be very detrimental to the ship. Once noted standing orders are to "skip" through them.

I believe this is true for aircraft and all expensive rotating equipment.

 

[Strahan]

Ok if I'm getting this right this is to avoid running motor at certain resonant frequencies that may cause excessive vibration ect.. is this only active on accel and decel? i still don't see how this is really beneficial. if you determine the resonant frequency just simply don't run at that level. I only see this being in use during accel and decel and really how much will this help?

 

[iwire]

i still don't see how this is really beneficial. if you determine the resonant frequency just simply don't run at that level. I only see this being in use during accel and decel and really how much will this help?

You did not even read the info I posted did you?

One more try, condensed.

The skip frequencies do not affect normal acceleration and deceleration. The drive output will ramp through the band, uninterrupted. When the operator issues a command to operate continuously inside the established band; however, the drive will alter the output to remain outside the band until a new command is issued.

 

[LarryFine]

You did not even read the info I posted did you?

I did, I did! :grin: Way cool!

Likewise, I read years ago that foot-soldiers are told to break step, i.e., not march in step, to aviod weakening bridges and such. I don't know if it's still done today.

 

[brian john]

And Bridges and high rises and guitar strings and......

 

[Strahan]

You did not even read the info I posted did you?

One more try, condensed.

Sorry:wink: Trying to do this while at work don't tell the boss I just briefly skimmed through it and opened mouth before brain was in gear. I got it now thanks guys

 

[weressl]

You did not even read the info I posted did you?

One more try, condensed.

You are specifying not a single frequency, but an upper and lower limit around the critical frequency. Depends on the resonant effect this window could be large, 2-3 Hz above and below or narrow, 0.25 in both directions. This largely depends on the driven inertia.

When in the P&ID control mode this could be a problem. If the 'jump' window is wide enough - the upper and lower frequencies around the critical frquency - the controller can go into a hunting mode and wind up and down around the window trying to get to the speed that the controller is calling for.

 

[ronaldrc]

Bob

I for one will say I learned something here tonight, Thanks