Lathe annoying 50Hz hum

[egaParticipant @ega]

Posted by Howard Lewis on 02/12/2019 10:18:39:

Have not read through all of the foregoing.

Would expect a PWM to produce a high frequency hum, which varied according the speed setting; so not that.

F W I W my Warco Economy Mill/Drill developed a 50 Hz hum.

It was coming from the F / R NVR switch. Having removed it, could find no fault. Refitted it, and tightened the securing screws down hard. No more hum – slack screw!

So it may be no more than something which is vibrating because the fixing is not tight enough.

Howard

I am often "guilty" of not having the time to read the whole of a lengthy thread before posting. CTRL-F sometimes helps but is there a better way to avoid duplication, etc?

My Taiwanese horizontal/vertical bandsaw recently developed a noise which turned out to be from the start capacitor rattling in its housing (it had been glued to the motor casing).

[Mark Simpson 1Participant @marksimpson1]

My Chinese Chester crusader deluxe was delivered with an annoying hum/buzz…. Several hours of head scratching, forum searching and general faffing about…

One of the relays in the elec box, part of the safety cutouts, was loose in it's seat… so it buzzed. properly re-seated and not buzzed since.

Good Luck!

[William HerschelParticipant @williamherschel23897]

Slightly off topic but I have to say that the first thing I did with my lathe was to remove all the so called ‘safety features’ which in my opinion are safety hazards. When you can’t actually see what’s going on for example is far more dangerous than being able to see what’s going on. So I know it’s not a relay to blame in my case.

[SillyOldDufferModerator @sillyoldduffer]

Did a bit of digging and I think/guess William has two problems.

The first problem, Im confident he's already diagnosed. It's that his motor mount / lathe, perhaps the tray or splashback, is acting as a soundboard and maybe resonating to make a slight hum loud enough to annoy. Cure for this mechanical trouble is to disrupt the effect by loading the metalwork with rubber pads, and/or stiffeners. The goal is to absorb vibration and spoil the soundboard by stopping or absorbing movement. A 'stiffener' might be a wooden strip fixed across a metal panel, or more fixing bolts, or glue. Check for anything loose and fix it down!

The second problem is more difficult: what's causing the hum? In theory it's a DC motor and shouldn't vibrate at a fixed frequency. But there are at least two sources of vibration. One is the Pulse Width Modulated power supply, the other is loose windings and/or laminations inside the motor. These will move as the windings in each pole are progressively energised and the armature rotates. On it's own the PWM is unlikely to hum at 50/60Hz because they operate at higher frequencies, perhaps 500Hz, which is a tone. So I wonder if the effect is due to a combination of PWM frequency, and the number of poles, and something in the motor being able to move. I'm guessing!

The cure for the combination fault is more difficult:

1. Replace the motor
2. Change the PWM operating frequency (may not be practical)

Funny noises can be horrible difficult to track down and fix. In practice they're often easier to reduce and tolerate than eliminate. Ear defenders?

Dave

[William HerschelParticipant @williamherschel23897]

Hi Dave,

Thanks for your analysis. Actually it’s not that serious a problem although without it the lathe would run very sweetly with little noise from the rest of the mechanism. I find sound is important when machining – it can often tell you important things especially when something is going wrong. I don’t think it’s anything loose in the motor since the hum does not change with the motor speed. In fact the hum is still there when the motor is effectively stationary. I believe that the rectangular pulses of the PWM power supply do in fact contain a spectrum of frequencies and the motor may be resonating at one of the lower tones. I think there are low bypass filters people have tried using inductors and capacitors so I might investigate that a bit more.

[SillyOldDufferModerator @sillyoldduffer]

Hi William,

Replacing my mini-lathe with a big machine was wonderful in every way apart from the noise. Reckon I could use a mini-lathe in a bedroom workshop without being a nuisance. And being able to hear metal or machine in distress is handy on a small machine. On a big one, the game changes – everything is louder. My WM-280 makes listening to the workshop radio hard, but isn't noisy enough for me to need ear defenders! It's too noisy to have in the house, but fine in my built-in garage.

You're right to see rectangular pulses as containing a spectrum of frequencies: they certainly behave as if they do in the real world. When pulses are presented to a DC motor, it mostly reacts to the DC component because the rising edges are too fast to have a major effect on windings. But pulse edges can and do have minor effects, including the possibility that windings will resonate at a particular frequency. My point about 'looseness' isn't that your motor is about to fall apart, rather that it's windings or laminates are able to move enough to create an audible pressure wave. The windings of most motors are firmly gummed together to reduce movement because flexing frays the insulation.

Resonance is very common in stepper motors: in this photo I've fitted a home-made rattle damper to an experimental set-up to see if I can reduce the effect.

The damper is the aluminium drum with six holes and ball-bearings on top of the motor. It's not part of a Colt six-shooter! When the motor resonates the idea is that the ball-bearings fly about and transfer energy to counter vibrationt. In theory damping is further improved by filling the holes with oil and fitting a tight lid. Having to squish oil slows the ball-bearings down extending the time vibration is resisted. Not tried that yet due to breaking a tap in one of those little holes…

Might be interesting to wire up a small DC motor and see if I can deliberately upset it by fine-tuning PWM frequency to evil effect! Probably not, PWM control of DC motors doesn't get a bad-press.

Poor connections on a VFD/3-phase motor are bad news too. On them disconnecting a winding can cause the motor to behave like a car ignition system and generate nasty high-voltage spikes. Several thousand volts of pulse induced mayhem can puncture the motors insulation and fry the VFD's electronics. Although VFD's can be designed to manage the problem, best not to rely on it unless the VFD's blurb specifically mentions it as a capability.

Dave

[Anonymous]

Posted by William Herschel on 03/12/2019 12:26:00:

I believe that the rectangular pulses of the PWM power supply do in fact contain a spectrum of frequencies and the motor may be resonating at one of the lower tones. I think there are low bypass filters people have tried using inductors and capacitors so I might investigate that a bit more.

Using a low pass filter will ultimately produce a DC voltage, the value of which is proportional to the duty cycle of the PWM. Using a filter rather defeats the point of using PWM in the first place. Assuming a passive filter it will also require a physically large inductor, as it must carry the full motor current without saturating the core.

A fixed PWM signal will contain a fundamental frequency, which is the pulse repetition rate, and higher order harmonics. The amplitude of each harmonic will vary according to the duty cycle. There are no frequencies below the repetition rate of the pulse train. So for instance if the PWM frequency is 1kHz the spectrum will contain a frequency at 1kHz plus varying amplitudes of 2kHz, 3kHz, 4kHz and so on. There will be no component at, say, 100Hz.

Of course the above goes out of the window if the duty cycle of the PWM varies on a cycle by cycle basis, as in the output of the VFD. In that case the varying duty cycle is designed to produce a fundamental sine wave at a frequency below the repettion rate of the PWM. But for a DC motor the PWM will be fixed for all intents and purposes, unless one is playing silly duffers with the control knob.

I think I've discovered a new law:

Oscillation + ME = must be resonance

Just because something is oscillating doesn't necessarily mean it has to be at resonance.

Andrew

[William HerschelParticipant @williamherschel23897]

Hi Andrew,

Well thanks for that explanation. It seems the idea a low frequency component of the PWM signal being responsible was a load of old cobblers as they don’t exist. I assume you mean smoothing out the chopped up PWM signal with a filter to get pure DC is pointless after you’ve gone to the trouble of chopping it up in the first place. So I probably won’t do that. I withdraw the word resonance and replace it with vibration. I’m tempted to get a few batteries in series and see if the hum persists which might tell me something.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Johnston on 03/12/2019 14:54:11:

Posted by William Herschel on 03/12/2019 12:26:00:

…

…

I think I've discovered a new law:

Oscillation + ME = must be resonance

Just because something is oscillating doesn't necessarily mean it has to be at resonance.

Andrew

But, if something oscillates, what decides the frequency other than a resonance?

What I think might cause William's motor to hum, is something akin to a bell being struck by a hammer. The hammer blow is a pulse of energy that doesn't have a frequency in itself. However, the energy is converted to sound by the bell vibrating at its resonant frequency. Bong!

By analogy, the PWM pulse may be kicking William's motor in a way that pumps it into emitting a note. It's strange though, I have real difficulty imagining how the note can hold the same amplitude and frequency across the full range of motor speed.

Very educational this forum. Or thought provoking. Thanks to ME, I've managed to waste most of today in my armchair when I should be tackling my To Do List. At the moment no-one is getting anything for Christmas, bah humbug.

Dave

[William HerschelParticipant @williamherschel23897]

Hi Dave,

My hum is akin to the irritating sound a transformer in the street makes. I think I’d prefer a tintinnabulation around middle C as 50 Hz has nothing to recommend it. The voltage to the motor can be varied from about zero to 150V with the external power supply (an HHD6-G) but there must be some constant signal somewhere there to keep the sound constant over the range. It’s the same result with the power supply that came with the lathe by the way. It certainly is a humbug.

[William HerschelParticipant @williamherschel23897]

Having just said that I’m wondering now if there is an AC component of 50 Hz on the line which would account for the constancy of the hum. Ground loop springs to mind. I guess I’d need an oscilloscope to find out.

[Anonymous]

Posted by SillyOldDuffer on 03/12/2019 16:44:49:

But, if something oscillates, what decides the frequency other than a resonance?

You can make an oscillator from a resistor, capacitor and a CMOS inverter. The frequency is set by the resistor and capacitor and, to some extent, the trigger levels of the inverter. Not a resonant circuit in sight.

Andrew

[Anonymous]

Posted by William Herschel on 03/12/2019 18:11:34:

Having just said that I’m wondering now if there is an AC component of 50 Hz on the line which would account for the constancy of the hum. Ground loop springs to mind.

I suspect that may well be the case. Ideally you'd need an oscilloscope to check the waveform. Unlikely to be a ground loop, more likely to be inadequate smoothing capacitance after rectification. One test is to firmly hold a wooden dowel on various parts when the hum occurs. If the hum changes you've found what is oscillating, but not the why. But once you know the what the why is easier to track down.

Andrew

[William HerschelParticipant @williamherschel23897]

Thanks for all the contributions. I think I’ve ground to a bit of a halt on this. Time to consider the options.

[Howard LewisParticipant @howardlewis46836]

Anything will oscillate if the exciting force is great enough. The object will oscillate at the frequency of the exciting force, but not necessarily be in resonance.

Everything has a natural frequency, it will only go into resonance if the exciting frequency coincides with the natural frequency.

As the frequency of the exciting force approaches the resonant frequency, the amplitude of vibration will increase dramatically, some times with fatal results at resonance.

This is known as the Dynamic Magnifier effect. If the equipment survives, the amplitude will decrease equally rapidly as the exciting force moves away from the resonant frequency.

Notice how, pushing a child on a swing can send them higher and higher, for little effort if the push is applied at the resonant frequency of the "child + swing" system. The exciting pulse arrives at the same time as the natural movement of the swing. Applying the force 180 degrees out of phase will bring things to a halt very quickly.

Racing motorcycles, and naturally aspirated engines sometimes make use of this. Indeed the two stroke engine often used an exhaust pipe tuned to resonate at the frequency of the port opening to scavenge the cylinder.

Similar tuning can be used to provide ram, for induction, at certain speeds, but not over a wide speed band.

Motorcycle racers used to tune the exhaust to "get on the meg"aphone. In the same way that car engines would "get on the cam" Being open handed, nature will ensure that if you halve or double the speed, the effect will be not negative back pressure, but positive, stuffing the exhaust back into the cylinder!

It was said that a steel (as opposed to air ) sprung lorry would overturn if it passed through the average UK roundabout at 22 mph. The rate at which the body rolled would be the same as natural frequency of the suspension, so that the roll increased so rapidly as to turn the lorry over.

Air suspension, which stiffens as load is applied, has a variable natural frequency, and so is less prone to such problems.

Howard

[John HaineParticipant @johnhaine32865]

William, I note that your external PSU was an HHD-6G, looking that up I see that it has outputs for both field and armature windings. Does your motor have a wound field that you connected up to the F1 and F2 outputs? If so that would probably be the source of the problem – it is highly likely that the field supply is derived from the mains with a simple rectifier and minimal (if any) smoothing, so it would have a large 100 Hz voltage and therefore current component, which could cause such a hum from the magnetic circuit. If so, connecting a large electrolytic, the right way round, across the field (one with an adequate voltage rating) should make an improvement.

Just as an aside, people keep talking about a 50 Hz hum – you may get 50 Hz hum on an audio system, in valve days because of pickup from the heater wiring for example, or from earth loops, but virtually any time electrical equipment hums it is 100 Hz, as the alternating magnetic field "tightens up" loose laminations and suchlike every half cycle at the current peak.

[William HerschelParticipant @williamherschel23897]

Hi John,

No it uses just the two wires. A good idea though. I take the point about 50Hz.

[Gerhard NovakParticipant @gerhardnovak66893]

I should have read this thread before purchasing my lathe. I can confirm the noise it makes is disturbing. Exactly as discribed in this thread… I have an SX2 mill standing next to the lathe, absolutely silent runner!

[Author]

Posts