# crazy loud minilathe VFD (??) whine - HELP!



## Mr. Science (Mar 3, 2019)

Okay - so i bought one of the infamous 8x16 chinese mini-lathes from ebay after getting dicked around endlessly on alibaba. I read a lot of posts here and elsewhere about the various 'what-to-expect's from these things - only to find myself with a totally unique problem. The issue seems to be a loud squeal (electronic - NOT MECHANICAL) from what i am guessing to be the driver board (or possibly motor) but mine is much worse than other examples i have yet heard.

Here is a sample:





I wrote to the seller about this explaining how the sound gets LOUDER as the RPMs go UP (watch end of video) and if they could get me any info about the solution to this. Seller responded thusly:



> "my friend,i send the video to the factory,they said the machine works well in your video,about the voice,they said it is normal,you just use it without any worry about that,they said it is the pulse compensation,it is like a technical term,after you use several times,the voice will be decrease,so you do not worry about that "




i am guessing something is out of spec here - i have seen the same model lathe operating on various youtube videos WITHOUT the offending sound (sounds like an alarm of some kind like a smoke detector) - i'm pretty comfortable with electronics work (power supplies and control electronics notwithstanding) but i'm wondering if anyone might have a clue what this is ... I'm not sure about it being a 'carrier' signal due to the behaviour of the sound ... (seems different in nature to other descriptions of VFD noise) but it would be great if someone had some more insight into this. I estimate the frequency to be somewhere in the realm of 2400 hz if that rings any bells with anyone. Thanks

PS - this is my first post here and the start of an exhilarating and frightening jump into the world of 'machining' ...!


----------



## JimDawson (Mar 4, 2019)

It really sounds like the PWM carrier.  2.4KH would be in the range I would expect.  I am guessing there's no way to change it like you could in many VFDs.

Putting a choke between the between the controller and the motor might help.  The actual noise could be coming from the motor itself.


----------



## Ulma Doctor (Mar 4, 2019)

you are hearing the carrier frequency delivered by the pulse width modulation board.

if you can adjust the carrier frequency to around 8,000 to 10,000 Khz, the whine will be noticeably lower in pitch and it will run a heck of a lot quieter.


----------



## Mr. Science (Mar 4, 2019)

yes i'd read that elsewhere - not sure how adjustable it is - though i find it hard to believe that EVERY example of the same model has this problem - that's just poor electrical engineering - i'm guessing I'll have to dig through the driver circuit and find the out of spec capacitor or whatever it is ... at worst I'm expecting to have to upgrade the motor and VFD entirely ... but i was just wondering if anyone else had come across this problem - i'd seen videos of others with VFD noise - but NOTHING even close to being this loud


----------



## kb58 (Mar 4, 2019)

I just completed setting up a VFD on a mill and like others said, you're hearing the carrier; you can tell because its frequency never changes. What can make it bad is that some part(s) of the lathe/motor can mechanically resonate at certain frequencies, and yes, it can be loud. Mine was similar until I increased the carrier frequency Assuming your VFD is not user-configurable, your options include trying to get VFD instructions (maybe from the VFD manufacturer), or, replacing it with a more user-friendly VFD (many here use the Hitachi WJ200). A hail-Mary would be to replace the motor, hoping that the next one doesn't resonate the same, or there's just learning to live with it and to wear ear plugs. I know that isn't what you wanted to hear, but non-adjustability is typical on the small units.

Search around the web for something like "mini lathe VFD noise" and see what others have come up with.


----------



## markba633csi (Mar 4, 2019)

You might give a shout to Pete at: www.olduhfguy.com and see what he says about it.
If it is a two wire dc motor as opposed to brushless you could replace the whole controller with an scr type unit such as KB electronics but I suspect it's a brushless type.
mark


----------



## JimDawson (Mar 4, 2019)

Maybe a reactor would help between the VFD and the motor






						AC Line Reactors | Drives | Products | AutomationDirect
					

AC Line Reactors from AutomationDirect, the best value in industrial automation - low prices, fast shipping, and free award-winning service.



					www.automationdirect.com


----------



## Mr. Science (Mar 4, 2019)

kb58 said:


> I just completed setting up a VFD on a mill and like others said, you're hearing the carrier; you can tell because its frequency never changes. What can make it bad is that some part(s) of the lathe/motor can mechanically resonate at certain frequencies, and yes, it can be loud. Mine was similar until I increased the carrier frequency Assuming your VFD is not user-configurable, your options include trying to get VFD instructions (maybe from the VFD manufacturer), or, replacing it with a more user-friendly VFD (many here use the Hitachi WJ200). A hail-Mary would be to replace the motor, hoping that the next one doesn't resonate the same, or there's just learning to live with it and to wear ear plugs. I know that isn't what you wanted to hear, but non-adjustability is typical on the small units.
> 
> Search around the web for something like "mini lathe VFD noise" and see what others have come up with.




thanks for the response ... yes i figured it MIGHT have been the carrier - though others have said it should get quieter with increasing RPM - i'm finding just the opposite. It wouldn't be very surprising to find out that it may not be adjustable - i kind of expect that - but i SHOULD be able to change the frequency by subbing in different value resistors or capacitors somewhere it seems to me (?). I was just hoping someone here might know these circuits well enough to get me started in the right directlon - or at least explain how and why this is happening and how it might be avoided ...


----------



## Mr. Science (Mar 4, 2019)

markba633csi said:


> You might give a shout to Pete at: www.olduhfguy.com and see what he says about it.
> If it is a two wire dc motor as opposed to brushless you could replace the whole controller with an scr type unit such as KB electronics but I suspect it's a brushless type.
> mark



it's a brushed motor apparently ...


----------



## kb58 (Mar 5, 2019)

Mr. Science said:


> thanks for the response ... yes i figured it MIGHT have been the carrier - though others have said it should get quieter with increasing RPM - i'm finding just the opposite. It wouldn't be very surprising to find out that it may not be adjustable - i kind of expect that - but i SHOULD be able to change the frequency by subbing in different value resistors or capacitors somewhere it seems to me (?). I was just hoping someone here might know these circuits well enough to get me started in the right directlon - or at least explain how and why this is happening and how it might be avoided ...


Assuming that it's digital, that's unfortunately unlikely. The whole thing gets dicey when messing with a 230v circuit without a schematic, like walking out blindfolded on to thin ice. Unless you can get information from the VFD mfg, it gets dangerous guessing. Either put in a VFD that can be adjusted, get information from the mfg, or just live with it.

For what it's worth, a friend has a $25k CNC mill and the squeal on his drives him nuts.


----------



## Mr. Science (Mar 5, 2019)

kb58 said:


> Assuming that it's digital, that's unfortunately unlikely. The whole thing gets dicey when messing with a 230v circuit without a schematic, like walking out blindfolded on to thin ice. Unless you can get information from the VFD mfg, it gets dangerous guessing. Either put in a VFD that can be adjusted, get information from the mfg, or just live with it.
> 
> For what it's worth, a friend has a $25k CNC mill and the squeal on his drives him nuts.



WELL you have a to figure out the circuit first obviously ... silly to tamper with what you don't understand ... i popped open the control board on front and the business end at the back and found a couple of trimmer pots ... potential good news


----------



## Mr. Science (Mar 5, 2019)

oh well ... i guess neither of the trimmers seemed to have any effect ... one controlled the speed and the other who knows what so i didn't mess with it too much


----------



## markba633csi (Mar 5, 2019)

The trimpots probably are for speed trim and torque compensation, hard to adjust on the fly without some test equipment
The good news is if it's a brush type motor then you could swap controllers- go back to old fashioned tech like KB or similar- you can find them on Ebay for as low as 50$ sometimes, look for models like KBIC-125
You'd be trading high frequency whine for 60 cycle ac hum
M


----------



## Mr. Science (Mar 5, 2019)

markba633csi said:


> The trimpots probably are for speed trim and torque compensation, hard to adjust on the fly without some test equipment
> The good news is if it's a brush type motor then you could swap controllers- go back to old fashioned tech like KB or similar- you can find them on Ebay for as low as 50$ sometimes, look for models like KBIC-125
> You'd be trading high frequency whine for 60 cycle ac hum
> M




thanks - well i'm up to my ears in 'test equipment' (oscilloscopes meters power supplies etc) but may opt for an alternative - will look into it thanks - still trying to wrap my head around what specifically is 'oscillating' to make the carrier pitch though ...


----------



## JimDawson (Mar 5, 2019)

Mr. Science said:


> thanks - well i'm up to my ears in 'test equipment' (oscilloscopes meters power supplies etc) but may opt for an alternative - will look into it thanks - still trying to wrap my head around what specifically is 'oscillating' to make the carrier pitch though ...



Just put a scope on the motor leads and you'll see the PWM carrier signal.  The actual noise is probably coming directly from the motor, resonance.  You might try a 1mF cap across the motor leads and see if that quiets it down a bit, or maybe a ferrite ring or two will help.  Maybe whip up a low pass filter, might help.


----------



## markba633csi (Mar 5, 2019)

Yes most likely the motor singing or a choke if there is one, or both. I'm sure the mfg. made only a feeble attempt at knocking down the harmonics and overshoot-  Power components and ferrites cost money which adds to the sales price
mark


----------



## Mr. Science (Mar 5, 2019)

yes some sort of blocking filter (cap or coil) was my first idea - but doesnt' the motor need to receive that signal for whatever reason? (minilathe voodoo? doesn't it need the PWM signal?)


----------



## Mr. Science (Mar 5, 2019)

cleaning up after the manufacturer is sort of half the fun of mini lathes though it would seem ...
i'm pretty stoked ... can't seem to measure ANY detectable runout on the spindle .... maybe i got lucky ...


----------



## markba633csi (Mar 5, 2019)

Well, they are monitoring the current for sure and probably the armature voltage too, so if you smooth the waveform it should still work the same way, I would think- you don't want to lose too much power in the output filter though-  I'm sure there is room for improvement. If you can get ahold of some ferrite rods or E cores and some 16 to 18 gauge wire you could experiment, maybe cannibalize some PC power supplies. Try to pinpoint the noise first


----------



## kb58 (Mar 5, 2019)

It's probably the wire coils and/or the core in the motor physically vibrating. I got a dose of that when I rewound a 60Hz transformer and jammed the core plates back in, not shellacking them like they were before. It made one hell of a buzzer, it did...


----------



## JimDawson (Mar 5, 2019)

Mr. Science said:


> yes some sort of blocking filter (cap or coil) was my first idea - but doesnt' the motor need to receive that signal for whatever reason? (minilathe voodoo? doesn't it need the PWM signal?)



No, the motor does not need the squarewave to run.  So knocking the corners off of the spikes won't change the operation.  Given that it apparently a brushed DC motor, it will run on pure DC.  The PWM is really just controlling the torque, while keeping the voltage at maximum, but changing the wave duty cycle.  Gives better low end torque performance.


----------



## tq60 (Mar 5, 2019)

Imagine flipping a light switch really fast.

The light is now dim.

Regardless of how fast it is same.

Leaving it on longer the light is brighter.

This is the pwm process in a nutshell.

The pulses are exciting something as they are full voltage but very little duration.

The motor does not spin up as the shirt period of full voltage does not create enough force to do anything.

But anything with a coil will move or try to move.

Given you have electronic toys and your handle you already understand pwm...we are helping others too.

Locate a microphone, tiny from headset or old school ear bud.

Connect this to your o-scope or an audio amp and attach the item to a pencil.

Now get the lathe to make the noise and with the microphone Starr touching things to find it.

Could be motor windings as well as inductors on the control board.

If motor many things to do from epoxy to filtering and coils on control board may need to be epoxied in place.

Touching them with the eraser end of pencil to see if sound changes is simple test.

Lots of possibilities....enjoy the hunt!

Sent from my SAMSUNG-SM-G930A using Tapatalk


----------



## Mr. Science (Mar 5, 2019)

tq60 said:


> Imagine flipping a light switch really fast.
> 
> The light is now dim.
> 
> ...




yes good idea with the pencil eraser thanks

hoping not to have to pull the motor apart to find the ringing bit ... I just spent two weeks trying to get my table saw motor back together after replacing the bearings ...


----------



## Mr. Science (Mar 6, 2019)

snooped around inside - definitely coming from the pulley end of the motor ... if i'm going to go through the pain of removing the motor i wonder if i should just wait and put up with the noise for now until i opt to replace the motor and drive circuitry with something better ... only issue with replacing is that it looks like quite a tight fit!

i may end up just trying the filter-the-signal option .. for now


----------



## macardoso (Mar 6, 2019)

Hate to say it, but that is just an example of a low quality VFD generating excessive switching noise. If it is programmable, you can change the carrier frequency to a higher value to bring it out of the audible range for you (but maybe not your dog's). Inadequate grounding/bonding of the VFD may also play into it.

If you are putting a choke between the VFD and the motor, be careful. There are components specifically designed for this purpose and standard line filters are typically not designed for the large voltage spikes.

Edit: Just read it is a brushed DC motor. Whoops!


----------



## stupoty (Mar 6, 2019)

Ulma Doctor said:


> you are hearing the carrier frequency delivered by the pulse width modulation board.
> 
> if you can adjust the carrier frequency to around 8,000 to 10,000 Khz, the whine will be noticeably lower in pitch and it will run a heck of a lot quieter.




Interesting side note , the PWM frequency can sometimes be worse at higher Hz depending upon how good your ears are, I was watching a video on the u tubes where someone sets the frequency higher and then declares it to be silent whilst I was watching the video going "owch that noise is offending my ears"  

I have my inverters set to 1.5khz (possibly 1khz) which I find to be the most comfortable for me.  






I stop hearing it somware after 17khz  might vary depending upon speakers/headphones etc.

Stu


----------



## Mr. Science (Mar 6, 2019)

macardoso said:


> Hate to say it, but that is just an example of a low quality VFD generating excessive switching noise. If it is programmable, you can change the carrier frequency to a higher value to bring it out of the audible range for you (but maybe not your dog's). Inadequate grounding/bonding of the VFD may also play into it.
> 
> If you are putting a choke between the VFD and the motor, be careful. There are components specifically designed for this purpose and standard line filters are typically not designed for the large voltage spikes.
> 
> Edit: Just read it is a brushed DC motor. Whoops!




how would that affect your advice (brushed vs brushless)?


----------



## macardoso (Mar 7, 2019)

Mr. Science said:


> how would that affect your advice (brushed vs brushless)?



Totally different control strategies. A brushed DC motor controller uses a unipolar PWM (fixed frequency) signal to set the speed (unless reversed) while a 3 phase induction motor VFD uses a variable frequency bipolar PWM signal which aims to generate sinusoidal current. 

If this were a 3 phase VFD for an induction motor, the sound you hear would be a the switching carrier frequency which remains constant (typically 4kHz) regardless of the output frequency of the drive (think of it like an overtone).

A brushed DC motor also has a PWM carrier frequency, but I don't know nearly as much about the operation of these motor controllers, so I will leave the troubleshooting to someone else


----------



## stupoty (Mar 7, 2019)

macardoso said:


> A brushed DC motor also has a PWM carrier frequency



The PWM for a brushed motor is just duty cycle so the sound may vary as the percentage of full power to the motor is changed,  at full speed it's the normally Full DC voltage and shouldn't "whine" (their may be exceptions to that but generally).  It can be a fixed frequency or variable frequency.

Stu


----------



## Mr. Science (Mar 14, 2019)

for those interested in this lathe ... i found a guy on youtube with the EXACT same build who is going over it in detail ... I feel lucky not to have the QC issues he seems to have (but then again I haven't totally taken this thing apart yet!) - though he has the same motor whine too (and says it toned down after some hours - which is nice to hear)







there are fifteen parts to this series.


----------



## Mr. Science (Dec 22, 2020)

OK - i think i solved it!!!!

spread the word.


----------



## ericc (Dec 22, 2020)

Did the pencil eraser find the problem?


----------



## homebrewed (Dec 22, 2020)

Yep.  Ceramic capacitors made with certain dielectrics are notorious for being piezoelectric.  No only can they act as miniature (unwanted) speakers, they also can become microphones.  Capacitors made with so-called "high K" dielectrics (like X7R and X5R)  are worst and they also have really large voltage coefficients -- their capacitance can go down to 50% or less of their rated capacitance when they have a voltage across them!

However, that source of noise isn't consistent with your observation about it coming from the pulley end of the motor.  But higher frequency noise can be difficult to localize by ear, unless you use something like what was shown in the video.


----------



## macardoso (Dec 22, 2020)

Ok, I’ll admit I did not read this entire thread but I did want to chime in. I work on Variable Frequency motor drives (VFDs) all the time at work.  Usually pretty big ones but the principles are usually the same.

The high pitched whine or chirping is caused by the way the VFD controls the motor. The VFD rapidly switches the outputs on and off swinging the voltage from rail to rail of the DC bus (~170VDC on a 120V drive, 325V on a 240V drive). This occurs many times per second at the carrier frequency. This is almost always 4kHz. The pulse width is modulated to generate the desired waveform to the motor. This high change in voltage (dV/dt) generates electrical and mechanical stress in the system. On the control board, the actual switching elements (IGBTs) might make noise, but commonly it is the windings in the motor mechanically vibrating at the carrier frequency. Better quality motors whine less.

Most drives allow you to select higher carrier frequencies (8 and 12kHz are common) to reduce audible noise at the expense of increased motor and drive heating. This is done especially in elevators and HVAC since the noise would be bothersome in an office setting.

Your lathe sounds particularly bad, could be a cheap drive or something else. Hope you get it fixed.


----------

