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?

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,

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

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.

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

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.

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 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.

The Origin of the vibration does not need to be particularly strong – a slight amount can be amplified at any resonant frequency. Even higher orders will add to that lower resonant frequency.

The Centec bases were renowned for that problem with any single phase motor. Raglan lathes also suffered from vibrations at slow speed, if fitted with single phase motors. There are numerous (attempted) reported solutions to both these specific machines, none of which completely removed the problem – apart from fitting a three phase motor.

Check out the Tacoma bridge failure. Resonance caused by a specific wind speed. 34 knots, IIRC.

NDIY that makes a lot of sense – there were definate hot spots and sweet spots around my workshop where the sound was loudest or quietest. I used to stand at one of the null points during machining whenever possible. I presume that the dimensions of my workshop and placing of the mill happened to aggravate the problem and create the perfect storm. Perhaps a bit surprising that this should be so in a pretty cluttered room.

Anyhow It turned me into a committedbeliever in VFDs and 3 phase motors!

Johm