Lathe annoying 50Hz hum

My Chinese lathe makes an annoying 50Hz hum which I always assumed was coming from the power supply as it remained constant irrespective of spindle speed. However when I connected an external power supply the hum was exactly the same. Then I took the motor off its mounting plate and held it in my hands and ran it up. It was silent. Even though the motor is essentially silent, somehow the small residual noise is amplified by the plate it is bolted to. Is there a tried and trusted way for eliminating this? The motor is rated 220V DC.

William, can you attach a thicker plate, perhaps in addition to the existing one? Some mastic between the two plates would help to dampen any vibration.

Bill

What do you mean by "external power supply"? DC or AC? Is it 50 Hz or 100 Hz? Mains hum from magnetic things like motors and transformers is often actually 100 Hz as the frame contracts slightly at the peak of each current half-cycle. Assuming your lathe has a commutator motor it is conceivable that it has a wound field that's supplied with rectified AC, while the drive in effect varies the voltage to the armature. But that wouldn't be consistent with the noise continuing if the external power supply was DC. Or was your external supply only supplying the armature?

Hi Bill,

Thanks for your reply. I’m wondering now if it’s not actually 50Hz mains hum which I suspected, but the frequency of the PWM power supply. It could explain why the hum is the same irrespective of the motor speed. I’ve just looked at various ideas to effectively smooth out the kick the motor gets at the PWM frequency. It may end up having to use some sort of mechanical damping like you suggest though.

On my Warco WM-16 mill I would often hear high pitched squealing coming from the motor housing when running at certain speeds, which I believe was a resonant frequency between the housing sheet metal and milling head.

To eliminate it I lined the housing with a self adhesive, sound dampening sheet used for computer cases.

Here is an example of the sheet, but I believe I purchased mine elsewhere.

Akasa Noise Reduction Mat.

Ed,

Exactly which lathe is it William? There are at least 4 different types of motor in use depending on the model.

My machine has a 3-phase motor and VFD, and the motor itself is fairly quiet. Can't say the same about it's cooling fans! Two of them run continually, one on the motor, one on the VFD.

Have you tried finding the exact source of the hum? ebay stethoscopes are cheap, but I've also used a short length of polished wood between ear and machine. If it's 50 or 100Hz I'd suspect something it the powewr supply is vibrating: sometimes possible to fix with a dab of epoxy or hot glue.

Dave

Hi William, can you confirm the model of lathe. PWM frequencies are normally much higher frequency, and some vary the frequency with motor speed.

Neil

Hi Neil,

The Lathe is branded as a Chester DB8VS in the UK although it probably is the basis for lots of similar machines. I bought an external PWM power supply which I thought was the solution but it gives exactly the same result. The motor is a one HP DC motor which I run around 150V. The hum is constant from zero RPM up to the max of a thousand or so with no variation in amplitude or frequency. As I said, when the motor is held in the hands, there is no noise.

Have you checked that it is not the coil in the NVR switch that is causing the hum ? Try disconnecting the output of the NVR switch and see if the hum is still present.

Les.

Hi Les,

Yes I bypassed all the original power supply and connected the motor directly to the external power supply.

Strange! Hard to account for a hum that doesn't vary in amplitude or frequency over a wide speed range, especially with a DC Motor.

The only thing I can think of is a mechanical resonance in the mounting, perhaps something loose? If it's mechanical, I would expect stuffing something resilient into the system (like a hand!) would effect it. Rubber washers, absorbent pads, altering the thickness of the plate and bulking up the stand-off pillars should help reduce accidental vibration. Single-phase motor mountings might be a source of parts - they're expected to buzz a bit.

Any chance of a photo showing how the motor is fixed to the lathe?

Dave

This is odd. PWM frequencies for DC motors are usually in the kilohertz range.

Maybe it's The Hum?

Neil

Hi Dave and Neil,

It’s a bit difficult as I’ve just put it back together. Essentially the motor was fixed to a steel plate about 5mm thick with four bolts at the pulley end. There was no support at the back end. This steel plate was fixed to a thicker steel plate on four stand-off posts about 40mm long. It is all very rigid. When the motor is taken off the lathe it makes the hum when in contact with a solid object like the bench top, but shuts up when held by just the hands. It is some kind of resonance but what is driving it I don’t know. It must be some kind of sound board effect. If PWM frequencies are in the kHz it’s unlikely that it’s that which I thought might be the case. I do live near the Bristol Hum but in that case they’ve got it synchronised perfectly with my On/Off switch.

Neil’s Hum that he suggested may not be so ridiculous as it sounds, excuse the pun, are you anywhere near any high voltage overhead cables, they have a field of influence that can be detected many metres away and have been attributed as the cause of numerous strange phenomena. I am immune to any strange humming or vibrations as I suffer with noise induced tinnitus, my tinnitus sounds as though I am stood next to a gas turbine engine screaming away so In away I would be grateful to hear an annoying 50 HZ hum.
Dave W

William, +1 for experiencing hum from a motor. I once had a 1.5hp 2850rpm motor on my Centec mill, the hum was so bad that I adopted the habit of wearing ear defenders, despite resilient mounts, belt drive and 50mm sand in the cabinet base.

ithen fitted 2 hp 3 phase motor fed by an inverter and it is whisper silent.

I have always suspected that the original motor had loose windings or delamination of the rotor.

either way it was absolutely the motor was humming and particularly loud. The motor did not run hot either which always surprised me.

I’d like to thank all the people who have contributed useful ideas to my question. I get the feeling that anybody who has retrofitted their lathe with a 3 phase VFD motor say they wish they’d done it sooner. So maybe I should give up trying to get rid of the hum cheaply and go down that route. It is a lot of expense compared to the cost of the lathe, but probably that’s why it was relatively cheap in the first place. But on the other hand my Chinese lathe can achieve remarkable precision on a good day, and sometimes it is good to pick up these as project machines and improve them. Stefan Gotteswinter has shown us how to do that with his remarkable YouTube series.

Just for the record, the hum suggestion was tongue in cheek!

It may be that the DC feeding the motor is ordinary AC rectified but barely smoothed, this may be enough to create a 100Hz hum which can be mistaken for 50Hz.

Neil

Hi Neil,

Yes that’s the way I took it.

In my limited understanding PWM and rectified AC (which has more or less vanished because of energy concerns) are completely different and there is no trace of AC on PWM lines. I was thinking there might be 50Hz or even 100Hz originally on the signal but I don’t think it’s possible with PWM. But an electrical engineer may correct me if this is wrong.

Hi William,

Can I suggest that the hum is 100Hz from the rectified AC used in the typical motor speed control boards that were very commonly employed before the VFD inverter systems started to take over. Chester and Warco have supplied this type of system for years. The 100Hz hum on my WM250 is very annoying. I would like to convert it to a VFD but don't think I can fit the motor in the space occupied by the original DC motor.

I hope this helps, Chris.

Hi William, I had a terrible buzz in a mount plate for the DRO Z axis on my Rong Fu mill. The buzz sounded like it would shake the top end to pieces. I remembered some work I did with tuned mass dampers at the day job, and started experimenting with various sizes of steel blocks clamped in various locations to try and change the resonance. Ended up with a 1/2" x 3/4" x 1" or so block of steel screwed to one particular spot on the mount plate. With it, no buzz at all. Without it, buzz and shakes like you would not believe.

Zero electrical or drive work done to fix it, just the tuned mass damper block. You might be able to reduce the buzz in your motor plate the same way. If the plate pivots on a rod or bolts, plastic bushings or rubber grommets may reduce buzz if some of it is originating at the pivots. Good luck.