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VFD, High Pitched Sound?

pmach

Plastic
Joined
Nov 3, 2006
Location
PA.
I'm thinking about replacing the current Vari-drive on one of our mills to a VFD.

I have been told that they make a high pitched tone when they run.

True/false?

Any input would be great. What would you suggest to replace the 3hp motor and Vari drive on our import mill.


Thanks,
Mark
 
Thanks Neilca, Thats the kind of info I need.

I'm going to look into this a bit more.
 
VFDs operate with an internal "carrier" frequency on the order of 4,000 Hz. This is within the normal frequency range of human hearing.

Although this energy normally exists as electrical current, some of it will be converted to acoustic waves because the mechanical components act like little speakers. This is the whine that you hear.

The amplitude of the whine will depend on many factors, the most significant of which is proximity to the operator. If the drive can be mounted on the wall or in some position away from the operator, and controlled remotely, the whine level will be much less. Also, using enclosed drives like NEMA 4 or 4X styles, will reduce the whine.

You can adjust the carrier frequency of some drives above the hearing range of the operator, but this has possible negative consequences. This subject was discussed at length in the other thread.

One knowledgeable poster said that the whine also emanates from the motor. I find this hard to believe, since high-pitched noise requires small resonant features, not commonly found in motors. But I can't say he's wrong.

- Leigh
 
The high-pitched squeal/whine is pretty much exclusively from the motor, not the VFD itself (you may hear some slight buzzing from the VFD due to the transistors switching, but this would be pretty faint). The motor windings are basically vibrating at the VFD's carrier frequency, creating the sound you hear. Changing the carrier frequency changes the noise.
 
One knowledgeable poster said that the whine also emanates from the motor. I find this hard to believe, since high-pitched noise requires small resonant features, not commonly found in motors. But I can't say he's wrong.

- Leigh

Just like contactors can buzz, transformers and motors hum, the motor will convert the carrier frequency energy into sound waves. The VFD does not radiate the carrier as sound waves as a rule.

Look at it like this. Since the armature of the motor is free to move, then any electrical energy you place on the motor will be converted into a magnetic field which then acts upon the armature. Vibrate the armature at 4kc, and the armature will happily convert that into sound waves.

Can you guess what a loud speaker's voice coil and magnet assembly is called? Yup...a motor.

You could put the motor in a vacuum and that will silence it. Heat might be an issue since there is no air to carry the heat away.
 
The high-pitched squeal/whine is pretty much exclusively from the motor, not the VFD itself (you may hear some slight buzzing from the VFD due to the transistors switching, but this would be pretty faint). The motor windings are basically vibrating at the VFD's carrier frequency, creating the sound you hear. Changing the carrier frequency changes the noise.
Interesting thought, but I have two problems with it...
1) Acoustic resonance at 4 kHz requires a very small physical structure. I can't envision any component of a motor that's of an appropriate size.

2) The VFDs I have whine even when the motor's not running.

- Leigh
 
Just like contactors can buzz, transformers and motors hum, the motor will convert the carrier frequency energy into sound waves. The VFD does not radiate the carrier as sound waves as a rule.

Vibrate the armature at 4kc, and the armature will happily convert that into sound waves.

Can you guess what a loud speaker's voice coil and magnet assembly is called? Yup...a motor.
Buzzing and humming are basically line-frequency (60 Hz) based phenomena. This is hugely different than a 4 kHz whine.

If the VFD does not radiate sound waves, why is the whine the same from mine whether the motor is running or not?

If you vibrate a 5 HP motor armature at 4 kHz, NOTHING will happen. The mass of the armature is many orders of magnitude too great to respond to that frequency.

I've been repairing radios for over 50 years, and have never heard the term "motor" used in any way associated with a speaker or other reproducer.

- Leigh
 
Leigh, it's not a "thought"...it's a fact. I worked with VFDs for 13 years prior to changing industries a few years ago, and the high pitched squeal that people complain about comes from the motor, not the VFD. I'm not saying the VFD itself doesn't make audible noise, but it is negligible compared to the squeal from the motor at low carrier frequencies. This is a very well-known and understood phenomenon in the VFD world, so I don't know what else to tell you...maybe the sound you are talking about is something different than what we are referring to.

Here is an excerpt from an EC&M article on VFDs that I found when I googled "vfd motor whine":

Switching speeds of the IGBTs in a PWM drive can range from 2 KHz to 15 KHz. Today's newer PWM designs use power IGBTs, which operate at these higher frequencies. By having more pulses in every half cycle, the motor whine associated with VFD applications is reduced because the motor windings are now oscillating at a frequency beyond the spectrum of human hearing.

http://ecmweb.com/mag/electric_understanding_variable_speed_4/

I'm sure there are others....

John
 
Thank you all!

Looks like I'm in for yet another education.

This is the kind of information I was looking for.

Thanks again.
 
The whine might be the power supply for the control board.

Leigh, the whine you hear when the motor is off might be the switching supply for the control board. I dunno if it uses a switching supply or not, but that coule explain it. It might also be the control circuit for the fan, some of them use a PWM setup. I know it's NOT the IGBTs when the motor is off because they're not being driven with a carrier when the motor's off. The ammount of movement for a high pitch sound is MINIMAL. and a motor is quite capable of producing those sounds. Not only that, but the shell of the motor, and the windings can move enough to make a little noise. not just the rotor. The motor IS the source of most of the noise generated by the carrier frequency, it doesn't take much movement to generate sound waves in that range. It may not be the armature making the noise, but it's definitely something in the motor, doesn't matter what size it is. I guarantee a 5hp motor CAN generate whine grom a 4khz carrier.

As far as a speaker's voice coil being a motor, it IS technically a linear motor. Allthough it's usually not refered to that way. I've heard of it called that before, it's not the common terminology though.
 
...
The motor IS the source of most of the noise generated by the carrier frequency, it doesn't take much movement to generate sound waves in that range. It may not be the armature making the noise, but it's definitely something in the motor, doesn't matter what size it is. I guarantee a 5hp motor CAN generate whine grom a 4khz carrier.
...

I expect that the noise is produced as a result of a magnetostrictive effect in the magnetic materials that make up the motor and transformer cores.
See: http://en.wikipedia.org/wiki/Magnetostriction
At one time, laminated magnetic cores were used as ultrasonic transducers for high power cleaning tanks. The whining noise you hear from a laser printer or a camera flash is due to this effect. Windings are potted to prevent abrasion of insulation from vibration.
 
I've been repairing radios for over 50 years, and have never heard the term "motor" used in any way associated with a speaker or other reproducer.

- Leigh

Leigh,

I have been designing speaker systems off and on for several years and it is a common term in that circle. Do a google search for "speaker motor". Anyway the definition of a speaker motor is a speaker without the cone. If you think about it, a speaker motor is a very accurate description since you are converting electrical energy into mechanical energy.

As far as reproducing 4kc, 4kc is fairly easy to reproduce even in something as large as a electric motor. Granted it is not efficient, but it does do it.
 
As far as a speaker's voice coil being a motor, it IS technically a linear motor. Allthough it's usually not refered to that way. I've heard of it called that before, it's not the common terminology though.

I guess it depends on who you are talking to. Speaker designers (not speaker system designers) use the term frequently. Some speaker rebuilders use the term as well.
 
Junkyard funny you should mention the term linear motor in reference to speakers.

I cut the magnet and voice coil assembly out of a large speaker and placed the voice coil on a linear guide and used it as a current to force transducer for changing the set pressure on a low pressure regulator.

Strictly speaking a voice coil assembly is a current to force transducer, the primary current standard used to be derived by measuring the force generated by opposed voice coils with the reference current flowing.

As for VFD noise, most of it comes from the motor on my machines, there is some noise from the drive but it is minor. Finned motors seem to be worse, I suspect the unfinned explosion proof motors will be quiet. Resonance doesn't seem to be much of a factor, compared to motor current. Some motors fins will ring at 4 to 10Khz when plucked but I haven't tried to excite this with a VFD.
 
Motor whine

I have 3 vfd's, one is a Hitachi SJ200 240v 3hp unit on a compressor, the whine is definatly from the motor. Another is a Hitachi SJ300 400v 3ph 7.5hp and the whine comes from the motor on this, mostly at low speed as this is a variable speed lathe.

The first one I got is the same as the Teco unit sold in the US, it is 240v 2hp and I use this to power a Bridgeport and also a 2hp lathe. This one does not produce a whine from either the Bridgeport or the lathe!!!!

Can anyone explain this??
 
"I've been repairing radios for over 50 years, and have never heard the term 'motor' used in any way associated with a speaker or other reproducer."

It is a linear motor.

It is possible, under certain circumstances, to have the speaker produce a "motoring" sound, which is really a periodic, and sustained oscillation which is a consequence of the exciting force (within the voice coil) and the restoring force (within the suspension), and certain external components, some of which may form the exciting means, and some of which may influence the "damping" of the system.

If the exciting force just equals the restoring force, a continuous oscillation can be produced, usually at the resonant frequency of the electromechanical system.

There is a very good textbook on the theory of oscillators by a Soviet writer, which book is available as a reprint from Dover. The mathematics involved in defining and explaining oscillators is way beyond the post-graduate physics/engineering level.
 
My AC tech NEMA 4x VFD makes a high pitch whine whenever it is powered up. The pitch does not come from the motor, it comes from the VFD. It does not change when I change the carrier frequency.

When the motor is running, there is a second high pitched noise from the motor that changes frequency when I change the carrier frequency.

-Joe
 
The whine from the drive is the power source for the control board.

Some VFDs use a standard transformer/rectifier/cap arrangement to reduce the input voltage for the control circuitry. Some use a small switching supply. The ones that use a switching supply make a whine from the switching frequency. It's similar to the whine from the carrier frequency, but it's a separate circuit. The noise is usually comming from the core of the inductor vibrating in the switching supply.

Kiwi, your drive may have had the carrier frequency adjusted above where the motor makes any noise you can hear. The higher it is, the quieter the whine will be. Eventually it exceeds your hearing range, or the motor's ability to vibrate that fast. :cheers:
 








 
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