Popping noise when decelerating motor.

[alex111]

Hello,
I have a vfd controlled 5 hp 3 phase motor that powers a vertically mounted boiler exhaust blower. When it is ramping down below 20 hertz (less than one third of its rated rpm) a clear popping or clicking sound occurs sporadically, until the vfd shuts down at its pre set minimum.

The sound can be heard by listening to the video at
https://www.youtube.com/watch?v=F4o8XcX8k2o&feature=youtu.be

The video is of a computer screen with the program Audacity playing the recording. It shows the spectrum of the sounds. There are clear horizontal bars at 4,8,12,16,20 kilohertz. In between the 4000 and 8000 hertz are occasional vertical red bars that correspond to the popping or clicking sounds.

The bearings are new, the shaft coupling is tight, the blower is sound, and it doesn't make this sound when run on a different vfd or when it 'coasts' to a stop.

My questions are
What is physically making the sound?
What is the cause of this malfunction?
How can it be prevented?

I have escalated this up through the local trade experts and they don't know. Which is a reasonable answer.

Thanks for considering this,

Alex
Ontario, Canada

 

[GoldDigger]

Is the VFD equipped with switchable braking resistors on the DC bus?
Monitor the bus voltage inside the VFD and see if it is jumping down at the time of the pops, possibly indicating an intermittent breakdown of some component when the voltage goes above a certain value. (Yet still a low enough value that the VFD does not fault and deliberately stop braking because of DC bus overvoltage.)

 

[milldrone]

This sounds like carrier frequency whine to me I'm guessing the carrier frequency of the drive is set at 4khz. If the noise is a problem try the highest carrier frequency the drive will allow.

 

[alex111]

Goldigger,
It does not have an external braking resistor. The DC bus voltage appear steady on the display.
I am inclined to add the resistor to a new vfd.

Plausible,
Thanks,
Alex

Is the VFD equipped with switchable braking resistors on the DC bus?
Monitor the bus voltage inside the VFD and see if it is jumping down at the time of the pops, possibly indicating an intermittent breakdown of some component when the voltage goes above a certain value. (Yet still a low enough value that the VFD does not fault and deliberately stop braking because of DC bus overvoltage.)

 

[alex111]

Milldrone,
Your right, the carrier frequency is 4khz, there is a whine there, but the popping is what bothers me. I'll switch frequency and record it to see/hear if there is a difference.
Thanks,
Alex

This sounds like carrier frequency whine to me I'm guessing the carrier frequency of the drive is set at 4khz. If the noise is a problem try the highest carrier frequency the drive will allow.

 

[Jraef]

Vertical pump you say? It's likely the thrust bearing. In centrifugal pumps, the hydrodynamic action of the pump itself produces thrust in the shaft, called "axial thrust" because it is at right angles to the direction of movement of the impeller. Axial thrust in a horizontally mounted pump is not as big of a deal because the pump shaft moves a bit and stays there. But when you turn the motor up to make it vertical, the axial trust moves vertically (up), essentially "lifting" the pump and in most cases, the motor itself. The problem comes with the fact that now you must contend with gravity, so at rest, the pump is at the bottom position and when you turn it on, the axial thrush lifts it to an apex position, slamming it against the bearing. If everything were tight enough to prevent movement, it would also add too much friction. So a special "thrust bearing" (usually a set of two bearings) is put into the motor shaft to allow that vertical lift to take place safely. The movement is only a small fraction of an inch, but it is still movement and distance.



Below some speed speed your pump is no longer creating the amount of vertical lift necessary to maintain it's apex position against the weight of the pump and motor. On your pump, I would say that 20Hz appears to be that speed where you are right on the ragged edge, so that "popping" sound is your motor rising, then falling, against the thrust bearing. You should program your drive to have that be a "critical frequency lockout" meaning you should not allow the drive to operate within a bandwidth of speed that includes 20Hz, say 15-25Hz to be safe. But just out of curiosity, why are you running a vertical pump at 20Hz at all? Most likely it stopped pumping at some level above that anyway, so if that's true, just turn it off.

 

[alex111]

Jraef,
It is similar to a pump, an air pump. However when switched on the impeller would pull down like gravity as it sucks from below.
Perhaps it would experience a lift on deceleration. None of this would explain why the popping sound it not there when I run it on a different vfd.

I was running it below 20hz when it was shutting off. Above that there is to much noise to tell if it is popping. I'm concerned that damage is being done ramping up and down under normal function between 25-55hz as it responds to control inputs to maintain a specified negative pressure in the boiler combustion chamber.
Alex

Vertical pump you say? It's likely the thrust bearing. In centrifugal pumps, the hydrodynamic action of the pump itself produces thrust in the shaft, called "axial thrust" because it is at right angles to the direction of movement of the impeller. Axial thrust in a horizontally mounted pump is not as big of a deal because the pump shaft moves a bit and stays there. But when you turn the motor up to make it vertical, the axial trust moves vertically (up), essentially "lifting" the pump and in most cases, the motor itself. The problem comes with the fact that now you must contend with gravity, so at rest, the pump is at the bottom position and when you turn it on, the axial thrush lifts it to an apex position, slamming it against the bearing. If everything were tight enough to prevent movement, it would also add too much friction. So a special "thrust bearing" (usually a set of two bearings) is put into the motor shaft to allow that vertical lift to take place safely. The movement is only a small fraction of an inch, but it is still movement and distance.

View attachment 10684

Below some speed speed your pump is no longer creating the amount of vertical lift necessary to maintain it's apex position against the weight of the pump and motor. On your pump, I would say that 20Hz appears to be that speed where you are right on the ragged edge, so that "popping" sound is your motor rising, then falling, against the thrust bearing. You should program your drive to have that be a "critical frequency lockout" meaning you should not allow the drive to operate within a bandwidth of speed that includes 20Hz, say 15-25Hz to be safe. But just out of curiosity, why are you running a vertical pump at 20Hz at all? Most likely it stopped pumping at some level above that anyway, so if that's true, just turn it off.

 

[Jraef]

Jraef,
It is similar to a pump, an air pump. However when switched on the impeller would pull down like gravity as it sucks from below.
Perhaps it would experience a lift on deceleration. None of this would explain why the popping sound it not there when I run it on a different vfd.

I was running it below 20hz when it was shutting off. Above that there is to much noise to tell if it is popping. I'm concerned that damage is being done ramping up and down under normal function between 25-55hz as it responds to control inputs to maintain a specified negative pressure in the boiler combustion chamber.
Alex

Wow, how the heck did I miss the word "blower" and saw pump? It sucks getting old...

Switching to a different VFD and having the sound go away is a new piece of info.
So a question for you. Is the old VFD by chance set up for Sensorless Vector Control, but the auto-tune was never done, then the new VFD is set up for V/Hz control? Asking for SVC without performing an auto-tune or manually entering all the motor circuit data can result in some unexpected behaviors.

 

[Stus Dad]

Hunting

Hunting

Sounds to me like the drive is hunting with no or little load on it. Does the drive do the ramping or is there a reference it is trying to match from another source? It could be a fundamental frequency issue at that rpm, which has been addressed by an earlier thread by bypassing that frequency range. It just sounds to me like it is banging back and forth like it is trying to follow a reference, but the load is low enough that it can't manage the change in inertia. Are you using any PID control? One drive may regulate better at slower speeds. Cogging at lower speeds is common to some drives, but it doesn't sound like cogging. Carrier frequencies normally range in newer drives from about 2000 to 16000. This has been mentioned, but it is certainly worth a try to change it to something that doesn't correspond with the frequencies you are showing issues with. Try something that isn't divisible by 2000- you mentioned 2000, 4000, 8000, 12000 as points of issue. There could be some fundamental frequencies in the mechanics that correspond to your carrier frequency or multiples of it. Lower carrier frequencies are better on motors and wiring, but you could experiment here. Just something to try.

 

[alex111]

Jraef,
No oth vfds were operating V/Hz. The vfd that doesn't cause popping had a ramp frequency control to avoid excessive voltage from inertia, although the DC bus voltage does rise a fair bit. The one that pops has a DC voltage control option that keeps it very steady. If I un-enable it then it faults on high DC bus voltage.
Alex

Wow, how the heck did I miss the word "blower" and saw pump? It sucks getting old...

Switching to a different VFD and having the sound go away is a new piece of info.
So a question for you. Is the old VFD by chance set up for Sensorless Vector Control, but the auto-tune was never done, then the new VFD is set up for V/Hz control? Asking for SVC without performing an auto-tune or manually entering all the motor circuit data can result in some unexpected behaviors.

 

[alex111]

Stus Dad,

The ramping up and down during normal operation is cause by the MOP up and MOP down inputs. The plc sends that input based on greater -than and less- than function related to milliamp signal from a Dwyer vaccuum transducer.

I have tried changing the carrier frequency. It changes the nature of the background hum or whine but the popping is still there.
I am curious as to what could cause that sound, short of a short or arc.
Alex

Sounds to me like the drive is hunting with no or little load on it. Does the drive do the ramping or is there a reference it is trying to match from another source? It could be a fundamental frequency issue at that rpm, which has been addressed by an earlier thread by bypassing that frequency range. It just sounds to me like it is banging back and forth like it is trying to follow a reference, but the load is low enough that it can't manage the change in inertia. Are you using any PID control? One drive may regulate better at slower speeds. Cogging at lower speeds is common to some drives, but it doesn't sound like cogging. Carrier frequencies normally range in newer drives from about 2000 to 16000. This has been mentioned, but it is certainly worth a try to change it to something that doesn't correspond with the frequencies you are showing issues with. Try something that isn't divisible by 2000- you mentioned 2000, 4000, 8000, 12000 as points of issue. There could be some fundamental frequencies in the mechanics that correspond to your carrier frequency or multiples of it. Lower carrier frequencies are better on motors and wiring, but you could experiment here. Just something to try.