VFD/ Motor Noise

Probably. Many VFDs use an 'electronic gearbox' to sustain low revolutions from a high-frequency source. What is happening is the incoming waveform is being 'chopped up' into little bits at high frequency, and then reassembled differently to produce the rotation required. Have a listen to the class 317 electric trains operating out of Kings Cross station as they accelerate and all will become clear.

Class 332 does it as well.

Regards,

Nerdy.

1. MOTOR IS TO BE PREMIUM EFFICIENCY TYPE, BESIDES BEING INVERTER DUTY.

2. NOISE IS NORMAL WITH CERTAIN MANUFACTURED MOTORS AND IT IS NOT WITH

OTHER MOTORS EVEN THOUGH THEY ALL MEET THE SPECIFICATIONS.

3. TRY A TOSHIBA MOTOR.

4. IN FUTURE TRY TO SPECIFY MOTOR AND VFD BY THE SAME MANUFACTURER.

Exceptional!!

ALL motors will do it on ALL PWM drives, brand or matching is completely irrelevant. It is an inherent condition in all PWM VFDs, which means 99.9% of all VFDs now.

If you are able to withstand the noise, or the noise is just a mall part of your overall machine noise, put up with it. it is better for the motor and the VFD.

Surely you can, Your motor runs cool--that's what matters!

Marvellous Tutorial!

Your Message:

Well stated.

Personally I find the noise bothersome, so if there is the capacity to run at the higher carrier frequency try it.

My ears are "old" and lacking the high frequency range(and have their own whistle), so it usually doesn't take much to shft it out of my hearing, .... but the young dogs complain!

I know this is a old post but very good explanation.

I had design and instilled a process line that I contract the controls out. I never experience the high pitch before, and when I questioned they did not give me a satisfying answer.

Thanks

p911

Could be that circuit uses Supersonic Frequency!

mm

hello mm

Could be that circuit uses Supersonic Frequency!

Don't know.

This was an extremely high pitch, it was quite irritating and when the pitch varied, at certain resonance actually hurts your ears.

The problems that I had was it was a cheese grinding operation and I had to reduce the Db levels, I believe it was in currently excess of 75 Db.

The customer did Db reading of it replacement equipment undr operations, but like an place if you have a constant sound it (Pitch) it does not have to be in unacceptable Db levels either, it can still do damage to your hearing.

Customer accepted it because it was at an acceptable level according to OSHA. Its just that I never experienced this high pitch hum at that level before.

p911

It is the magnetic noise due to saturation .The inverter produces wave forms which are far from sinusoidal type and harmonics are induced which cause saturation and the shrill noise u hear is produced. more of the saturation takes place in the teeth area. it is a combination of various factors and try using a inverter of good quality.

Thanks to all

I am guessing here but maybe part of the reason for the noise is when the outer motor windings are not absolutely rock solid with the stator and parts of them can just move a tiny bit in sync with this frequency.....

A high quality motor with resin (or whatever they use!) completely embedded coils may not make the noise anywhere near as much.....cooling may then become a problem!

Hysterisis/Magnetostiction is always there.

Even at 50/60 Hz you hear the HUM. It does not bother you.

At 3000 Hz the core stampings SING!

Only expected.

So sing along.

Well put! Thanks.

Thanks for the info JRaef. I am just getting into VFDs and have another question. We just installed a large (24 foot diameter) warehouse fan and have 2 more on order. All 3 will be close (75 feet apart) to each other. Will there be a frequency beating sound and if so is there a fix or just experiment with the CF of the drives? Thanks -- JHF

With high frequency sound you will find areas of constructive and destructive noise interference.

I believe the dominant noise you will hear is the fan itself, but that is without knowing the db of the fan. My experience is there is a beat from the blade passing fixed mounting structures and with multiple fans running at slightly different loads they will have slightly different beat frequencies and set their own harmonics.

I have not had experience with the fan sizes you mention, only up to about 2 ft electric, and aircraft propellers at about 72" @ 2200 rpm.

I agree, more likely the fan sound will be the dominant noise problem. Besides, I'm not exactly sure what "frequency beating" means. Beating sounds in fans are usually mechanical from the blades, which would have no connection to the carrier frequency issue.

If you have tried tuning a musical instrument, such as a piano where there are 2 or more strings tuned to (almost) the same frequency, as the pitch of the 2 strings approach the same frequency there is a noticeable increase and decrease in volume as constructive and destructive waveforms are produced. When the frequencies match they "sing" together

In twin engine light aircraft, as the engines are set (almost) to the same rpm this beat is also very noticeable, and can be very annoying, especially on long duration flights. It is sometimes easier to deliberately offset the rpm than to nail the 2 together, or adjust them now and then to at least listen to a different beat.

We used to use 24 inch fans to cool SCR assemblies, and a lot of the noise was the blade passing the SCR assembly that blocked airflow from the blade. When you got several of those going in a line up, you not only had the racket of each fan, but there was also a definite beat frequency set up. It was very fatiguing listening to it over several hours or days.

Nice explanation, I understand now (which I take it was your intended goal).

With that in mind then, carrier frequency will have nothing to do with 'frequency beating" which I take to mean is a mechanically derived sound problem. CF has only to do with the whine often heard in VFD driven motors.

But if you do end up with a mechanical resonance between the 3 fans, you may be able to use the VFDs to get rid of it, or at least avoid it. the VFDs will likely have a programmable feature that allows you to set up "critical frequency lockouts". These are bands of frequencies in which you do NOT want the VFD to stay in. If the system commands a spee that falls into one of the programmed avoidance bands, the VFD automatically skips over or stays under the bad to avoid resonant mechanical harmonics..

You have definitely got the idea!

I have recently worked in a warehouse with fans like you describe. There was no discernible "beating" or sound cancellation as I would walk past the row of fans. The carrier frequency was very audible (and more than my 60-year old ears liked). The fan rotational speed was quite low (below 250 rpm), and I didn't notice any other sounds.

Back to the generally good post from JRaef, I have worked with VFD's which did not have a fixed carrier frequency. They did this by making the width and spacing between pulses continuously variable (instead of using multiples of the carrier frequency). The motors attached to these drives did not sing, but were very quiet. Therefore, I would say that the claim of 99.9% of VFD's do this is off by a modest amount.

Insalling a load reactor between the VFD and the motor should reduce the level of carrier frequency noise, because it is smoothing out the waveform.

--JMM

<

Installing a load reactor between the VFD and the motor should reduce the level of carrier frequency noise, because it is smoothing out the waveform.

--JMM

load reactor> better still will be a Low-Pass filter.

Any business savvy CR4 Type to promptly enter the market with logo?:

<Control your Motor Speed with anybody's VFD- but you need not SING any more doing that.

Cut in XYZ's All-Quiet T-O-D-A-Y!>

I don't know the spec's for a low-pass filter with the power capacity typically required for VFD applications. Every VFD manufacturer whose products I have installed supplies tables to size a load reactor. I suspect the filter would be more expensive and harder to size properly. Although the filter is elegant, the KISS principle is hard to ignore.

--JMM

The filters are usually a 3 phase reactor that represents about a 3% impedance. There are also small RC circuits and MOV's that they can attach to the motor to try to absorb high frequency voltage spikes that are damaging the windings and help suppress standing wave phenomenon.

No doubt you came across what was called "whisper wave" technology that varies the CF throughout the sine wave simulation in order to optimize low audible noise with better efficiency. I'm very familiar with that, it was developed by PDL in New Zealand (now absorbed by Schneider). Great feature, many other manufacturers have copied it now.

The thing is, it doesn't change my comment about 99.9% of VFDs having this issue, because what I meant is that 99,9% of VFDs now use PWM, and 100% of PWM drives have this issue with making motor noise. The "whisper wave" and any other technology use to mitigate it still just does what I also said, moves the noise up to a sound frequency we humans cannot hear. But it is still there.

Ask your dog, he will know...

,

<The thing is, it doesn't change my comment about 99.9% of VFDs having this issue, because what I meant is that 99,9% of VFDs now use PWM, and 100% of PWM drives have this issue with making motor noise. The "whisper wave" and any other technology use to mitigate it still just does what I also said, moves the noise up to a sound frequency we humans cannot hear. But it is still there.

Ask your dog, he will know...>

JR You are telling the truth,all the truth and nothing but the truth.

But you don't mind somebody making some money -Promising- to mitigate a perceived Serious Problem

Thanks for the comments and the suggestions. Our new fans just hit the door and we will start the install after hours later this week. I'll let you know what happens. -- JHF

We would much appreciate the feedback.

Speaking in parables: It so often it appears like a child poses a question and a bunch of old ladies start discussing it while the child has quietly walked away shaking their head.

(Or a lazy student needs help with a problem, but the answers come the day after the dead line so we get no feedback!)

<discussing it while the child has quietly walked away shaking their (his?)head>

Never too late to learn

Haste is Waste

Well, we finally finished our fan project. None of you probably ever had a project start with "Lets get it done" from management, then 1/3 rd in "NO, NO - No overtime, don't interupt the warehose during business hours, just get it done."

Anyway, no problem with unusual noises from the 3 fans being close to each other. Some hum from the motors, but not bad especially compared to the huge noise the old high speed fans made. You could not hear yourself think over them! -- JHF

Suggest you look back at Comment#10

mm