skip frequency

[ptonsparky]

When trying to skip frequency on a VFD to cut motor noise, how do you know which frequencies to skip?

Never really had the noise problem until yesterday on an irrigation motor. Not that the neighbors will complain, they are 1/2 mile away. 75hp rebuilt, 480v.

 

[ptonsparky]

Maybe, I don't want to skip a freq because the motor pretty much has a "hi pitch" to it from 0-60HZ.

 

[Jraef]

Skip frequency is typically used to jump over a speed where you detect there is a mechanical resonance that is potentially harmful, not just for noise. If your noise is a problem, you can play with the "Carrier Frequency", but be aware that as you increase it to reduce audible noise (whine), you are DECREASING the efficiency, so both the motor and the VFD will run hotter. In fact, some VFDs require that you de-rate them above a certain CF.

So the lower you can keep the CF the better the overall efficiency, but it will make that annoying whine.

 

[Besoeker]

Skip frequency is typically used to jump over a speed where you detect there is a mechanical resonance that is potentially harmful, not just for noise. If your noise is a problem, you can play with the "Carrier Frequency", but be aware that as you increase it to reduce audible noise (whine), you are DECREASING the efficiency, so both the motor and the VFD will run hotter. In fact, some VFDs require that you de-rate them above a certain CF.

So the lower you can keep the CF the better the overall efficiency, but it will make that annoying whine.

I agree with your point about skip frequencies and mechanical resonance.
That said, I have not yet had an application where I needed to enable this feature.
The carrier frequency has a bearing on audible noise perception and switching losses in the IGBTs.
As far as the motor is concerened, the leakage reactance usually takes out the higher frequency components so losses are not adversely affected.
Audible noise in the motor...
Our test bay is a fairly noise free environment. When we run a test on a VFD system, you can hear the mtor respond to the carrier frequency as the motor starts to rotate. Once under way, it isn't long before the shaft mounted fan drowns that out.
But my experience doesn't cover very small drives.

 

[Jraef]

I agree with your point about skip frequencies and mechanical resonance.
That said, I have not yet had an application where I needed to enable this feature.
The carrier frequency has a bearing on audible noise perception and switching losses in the IGBTs.
As far as the motor is concerened, the leakage reactance usually takes out the higher frequency components so losses are not adversely affected.
Audible noise in the motor...
Our test bay is a fairly noise free environment. When we run a test on a VFD system, you can hear the mtor respond to the carrier frequency as the motor starts to rotate. Once under way, it isn't long before the shaft mounted fan drowns that out.
But my experience doesn't cover very small drives.

In my 20+ years of involvement in VFD applications, I have had to use the Skip Frequency feature exactly twice, and once was just as a placebo to convince the user that we had actually done something (his problem was one of perception, not reality).

 

[weressl]

In my 20+ years of involvement in VFD applications, I have had to use the Skip Frequency feature exactly twice, and once was just as a placebo to convince the user that we had actually done something (his problem was one of perception, not reality).

Gear driven cooling tower fans seem to be the most notorious ones where resonant frequencies develop. (Often equipped with vibration switches too.) Have seen a few, played with the jump bandwidth, looked at the mech tech's vibraton profiling and worked the issues out like a charm. Consider it a greater invention than Wonderbread.......

 

[Doug S.]

Being in agreement with the previous posters, and seeing you are in central Nebraska... I am inclined to suggest you let'er whine.

My 2?
Doug S.

 

[ptonsparky]

Being in agreement with the previous posters, and seeing you are in central Nebraska... I am inclined to suggest you let'er whine.

My 2?
Doug S.

I agree and it is but when a customer asks I usually try to find an answer. Drives for irrigation wells are going to become more common in this area and some of the water districts are giving $$ for the installation of them.

Checking rotation was a lot less nerve racking as the LRA was limited at startup, just a failure of the motor to turn as it tried to ramp up.