[rdsn]

I have a 16" x 8" mini lathe. It's one of those generic, gray, chinese mini-lathes, no brand name associated.

Amonst other deffects, which I can work out, there's an irritating whining noise (high pitch noise).

I have removed the motor and checked that the noise comes from the motor.

I have also disconnected the motor from the lathe and connected another motor I use for other stuff and that I know to be quiet.

Regardless of the voltage provided both scream if connected to the lathe's electronics.

I read that the problem is the frequency used by the PWM module.

What can be done to get rid of this noise? I see a couple of adjustment screws in the electronics board but I don't know what they do.
(I haven't changed anything yet)

I can provide a video and a couple of pictures later. Thanks

[Al_The_Man]

What is the nature of the motor? DC ? AC? BLDC?
Al.

[rdsn]

It's a 750W motor with brushes which has a 220V sticker on it. I believe it is a DC motor, It only has 2 wires running from it.

Here's a link for a video (in reality the noise is much higher than in the video): https://bit.ly/2ZmsjjB

Here are a couple of pictures, I know they're not great but they are what I have for the moment.:
Adjustable components (don't know what they do): https://bit.ly/329sITs
PWM module (?): https://bit.ly/326tb99

[Al_The_Man]

Did it suddenly start this or was always there, It appears it may me a simple PWM drive, if so often the main reason is too low a PWM frequency.
Al.

[rdsn]

It was always there from new.

[hfjbuis]

PWM module (?): https://bit.ly/326tb99

This picture is of the back from the digital RPM readout. This is connected to the 5 Volt powersupply (2 pin connector) and a proximity switch (3 pin connector). I have the same on my small lathe.

The 2 potmeters are probably for setting the minium and maxium rpm. I have a similar (not identical) board on my mini lathe.

[rdsn]

Yes, you're right. Yesderday when I got home from work I tried to connect those wires to another PWM module and that checked out, it is only for the readout (of course, the "in" and "out" are disconnected).
Also, disconnecting that module, all works well but without RPM reading.

The potentiometer wires go straight to the circuit board so the PWM module is integrated there as well. The question remains, is there something that can be done to replace/filter/change the frequency driving the motor?

[__Britt]

is there something that can be done to replace/filter/change

Maybe... it depends on the type of circuit that's driving the motor. See if you can identify the power output device on the board that is closest (electrically speaking) to the motor... or failing that, post a bunch more detailed photos so we can try to figure it out...

[hfjbuis]

@RDSN
That is hard to say. My board controller version doesn't make that noize. To modify the board you need schematics and even then, it not guaranteed to work.

You could however replace the controller. If you google, you will find a lot of them.

One of my lathes has a Siemens DC motor controller. I can't take a picture of it because the back is standing against a wall and I can't move (+ 260 kg) that thing without taking it appart.

[rdsn]

I will both try to identify the power output device and take detailed pictures to post here.

Preferably I would like to avoid modifying the board and just add something in series with the motor (if possible) or just bypass the original PWM module and add a different one. But first show you guys some pictures of the board in order to identify it. Thanks

[Al_The_Man]

You can try a choke in series with the motor, but with PWM you should not normally have to do that, this is one advantage to PWM.
It must be running a very (abnormally) low PWM frequency in order that you can hear it, the ones I have used are completely silent, including the Treadmill versions.
Al.

[rdsn]

I have added some close up pictures here: https://bit.ly/2zyEtXj

Also, I'm attaching a schematic of the lathes wiring (sorry about the drawing, I'm not an electronics guy).

[rdsn]

Any suggestions, please?

[hfjbuis]

That wiring diagram doesn't help. A schematic of the board is needed, but that's not made public by the seller

I am sorry, but I only see mounting a standard controller as possibility!

[grg12]

Check if chips on controller board have readable numbers (on my board all chips had identifiers ground off). If yes - try to Google them, they probably used standard pwm controller - there is chance it will have simple rc timing circuit and changing one component will move pwm frequency to inaudible range (inaudible to you - your dog will still don't like it).
My controller board started "acting" a few months ago - motor lost torque. I reverse engineered the schematic (using "informed guess" on chips without numbers). My board is quite simple - just a phase cut controller (glorified light dimmer) not pwm like yours.
It "acted" because all electrolytic caps dried out - age and low quality I guess.


Wys?ane z mojego SM-N950F przy u?yciu Tapatalka

[__Britt]

Sorry... been having a few PCB problems of my own here...

Check if chips on controller board have readable numbers

Of particular importance in determining what overall type controller you have is the components attached to the heatsinks... if they are SCRs or TRIACs; then that indicates a simple phase-cut type like grg12 has... which shouldn't be capable of making high pitched noises; as they run at 50/60hz....

If however, as I suspect from the photos, it's got a switching power supply for converting the mains AC into DC; and then a power MOSFET or IGBT that outputs an actual PWM signal to the motor; it may be possible to put a suitably rated capacitor across the motor or a choke inline with it.

If you are determined to get rid of the squeal; and are willing to replace the motor controller if your modification steps don't work; then you might as well try throwing a capacitor or choke at it "blind"; if the board gets killed in the process (unlikely, but always a possibility when working without a schematic & datasheets) then you are only out the cost of the capacitor and/or choke - as you would have had to replace the PCB anyway...

Just make sure that anything you buy for it has adequate voltage & current ratings - unless, of course, you wish to have a "Photonic Induction" experience (if you don't know who I'm talking about, look for him on YouTube).

[grg12]

Do not connect random condensators parallel to motor. It may destroy switching elements on controller board.
PWM controller used in this type of application switches output between full supply voltage and 0V (ground) (for example 0.5ms full voltage, 0.5ms 0V - 50% cycle at 1kHz frequency). If you connect capacitor to output it will try to fully load it at the moment "high" phase starts resulting in short high current spike that may destroy switching elements.
You can use carefully calculated combination of choke and condensator (LC filter) to smooth output of pwm controller (and effectively convert it to DC power supply) and it will quiet down your motor.

Wys?ane z mojego SM-N950F przy u?yciu Tapatalka

[__Britt]

Do not connect random condensators parallel to motor. It may destroy switching elements on controller board.

That depends on the output stage of the drive; if it is a MOSFET or IGBT directly coupled to the motor outputs, with no protection... and you put a large enough cap with low enough ESR, with nothing else added... yeah, that'll kill the output switch. If the drive has short circuit protection; it may just shut down. If there is any sort of "line filter" on the output for reducing EMI, that may function to limit the inrush current going into a bare cap bridged across the motor enough such that the switching components aren't over-stressed... or not. It may also mess up any sort of feedback loop it has. Without a schematic I'm just... guessing.

My suggestion earlier about adding something "blind" is intended as a last resort - that's what I'd do when my next step was to replace the drive anyway.

I kind of wish I could put the thing on my bench and probe it with my oscilloscope... it would answer a few questions...

[grg12]

I was standing with oscilloscope probes in hand over controllers board - connected to mains and some lightbulbs (dummy load). I wanted to see what's going on... then I remembered that this "light dimmer" doesn't have any real "ground" (it's powered through bridge rectifier so "ground" is half the time connected to phase wire). Luckily I didn't cause any fireworks displays this day
I managed to repair this thing using multimeter about three days before Amazon package with a isolation trafo arrived.
Mains voltage (especially 230V) isn't a thing you can be careless about...

Wys?ane z mojego SM-N950F przy u?yciu Tapatalka

[__Britt]

Yup!
https://youtu.be/xaELqAo4kkQ

I have both an isolation transformer and variac on my bench for just that reason. Photonic Induction and ElectroBOOM may be fun to watch on YouTube, but I’d rather not have that experience myself...

I also happen to have one of those cheap Chinese portable battery powered single channel scopes... which would come in handy for something like this (just make sure to double check it’s voltage range - some of them don’t go very high; and don’t have a x10 probe).