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Cyclical Mill Vibration & Power Discussion

P

PeterT

Ultra Member
Premium Member
I've been sorting through a kind of a background harmonic resonance issue on my (single phase 220V mill). A few forum buddies have been in the loop. I made some good 'mechanical progress', its certainly much better than it was.
One of the things that came up in discussion is apparently some residential lines can be noisier than others. I 'think' the power companies are supposed to be within some kind of tolerance band, I may have read +/- 5% somewhere. I'm using the word power loosely because I'm not sure if that means voltage specifically or the waveform or levels of AC current.

Hypothetically, with a 3HP belt driven motor just idling unloaded, would it be normal for its RPM to vary (the same example 5%) purely based on what's coming in the wall? Is this something that can be quantified with simple test instruments to really needs some kind of scope? Other than my 2HP SP lathe (which is bucking a gear box at idle) I don't have anything else to compare against. How about the motor itself, does it have anything internal that might react or dampen input power in this regard? Now there very well may be some mechanical reasons behind this, but many of the obvious things have been evaluated in detail. I'm just curious about this aspect because it arose in discussion.

I didn't think this was a thing in a typical city setting, but I also recall some people had to get old lines 'fixed' to solve flickering issues. And I seem to remember some of our field offices in more remote settings had to have (I think they are called power conditioners) installed because of even common office machines, computers, faxes, appliance type devices going glitchy on them - end of the line effects or however they worded it.

 
I don't think that there is any way to check except via oscilloscope. AC induction motors (which you almost certainly have) can certainly be affected by changes in the input. Especially a non-sine waveform or a supply from the grid that is too small / close to its limits.

Without knowing more it is hard to say, but there aren't obvious reasons why the motor itself would fluctuate if it is mechanically sound
 
@PeterT I believe your +-5 refers to the supply voltage. The 120V in your house can be between 115V and 125V. The voltage shouldn’t be oscillating between those limits though. It should be fairly steady and move slowly as the system load increases or decreases.
“Flicker” from the utility perspective is when a large load turns on and drags the voltage down. Motors are the worst. None of these should causes resonance though, as they’re either a one-off occurrence or slow acting.
In the city where you’ll have a number of neighbors off the same transformer there could be a load causing harmonics that could lead to issues. Utilities don’t allow those types of loads but likely wouldn’t notice one if it was on the low side of a residential transformer.

You could try a strobe light to see if the speed of your motor is oscillating. There are phone apps that you can use to measure the motors rpm. They use the camera flash as a strobe. They definitely not lab grade but might give you a visual.
 
I sort of assumed that he already knows that the speed of his spindle is going up and down, and is trying to figure out why. Trying to decide if it is some further mechanical problem with the motor mount or something about belts (the setup is not clear to me) or if it is an electrical problem

I don't think that there is a way to find that out without putting an oscilloscope on the line. If the line is 'clean' or even mostly clean, electrical supply can be ruled out
 
I'm pretty certain it's mechanical & I could probably write a short thesis on what was speculated & evaluated leading up to this. Last week the motor came off & run up independently (perfectly smooth). Head was evaluated piece by piece down to the bottom. Everything looks clean & in great shape. One idler shaft/bearing sub-assembly felt a bit loose so that was replaced thinking has to be it. Resulting in 90% of the concerning noise is gone. But you can hear the faintest rendition of prior, cyclic noise in the background. So now is it a chicken & egg thing?

Its a (low mileage) TV style Taiwan 935 mill. So yes a harmonic noise could be originating from any of 32 different parts. But all the likely culprits, sheaves, belts, keys, bushings... all check out like new. But because the motor runs at constant RPM & the harmonic 'seems to be' relatively constant (just different intensity at various RPM) one theory mentioned to me is maybe the motor itself is somehow varying = electrical issue.

But you know when you hear a noise in your car & you've got it narrowed down to somewhere between the left headlight & rear bumper... & starts a helmet fire in OCD personality types? Its that kind of thing. I'll save this discussion for when I've had a chance to run it in with new belt & bearings. Lots to say & show that may benefit others. But thought I'd inquire with the electrical experts on this particular power variation subject because sparks are not my strong suite.
 
Johnwa said:
You could try a strobe light to see if the speed of your motor is oscillating. There are phone apps that you can use to measure the motors rpm. They use the camera flash as a strobe. They definitely not lab grade but might give you a visual.
Click to expand...
Hmmm... interesting, @Johnwa
I actually thought about a strobe to try & see if I could capture a resonating belt flapping or sheave spring wobble... to kind of freeze the action so to speak when I heard the resonance. My only exposure to strobes was like car timing where it was triggered off ignition. How would I synch it to motor RPM? (or maybe send me to an app I can do some reading).
 
sparky here, there are a few factors to consider: incoming power, can be anywhere from 108v (I've personally seen this) to 127v, 207v to 277v. advertised power in Ontario is supposed to be 125/250v, then +/- 5%, although adherence to that is vague, most of us call it 120v/240v. next: incoming frequency, all VFD drives I've seen including the vfd I just installed on my mill is setup for 50Hz, Ontario standard = 60Hz, so if you can change that parameter (like in your drive if you have one) that should have a noticeable affect, I've been pondering the idea of installing a capacitor just ahead of the motor, fairly large one, which should clip off any noise coming in, the best way to see the quality of your incoming power is a scope, the smoother the sine wave is, the quieter it will be, so on a noisy feed, a cap will help, places of work that have many electric motors running are required by hydro to install power factor correction systems to combat power imbalance, too much power factor actually results in a new charge to the customer as a penalty for power use imbalance, small use power users like us will never see that, but the affects are still there, just minute, but, it directly affects the frequency coming in, hence, noisy power. sine wave motors are expensive and not used very much in light use equipment, you see a lot of them in industry, their advantage is the number of poles inside that contribute to mag force and torque applied, kinda like the difference between a 100hp 4 cyl car engine, and 100hp v8 engine, the v8 has more pulses per rotation of the crank, which produces smoother power, just like a v12 engine over a v8, same theory, our electric motors most have 4 poles, some high end ones have 6, and the sine wave quite a few, so that is something you can change, guy on here named Rauce installed a servo(sine wave) motor onto his mill, should ask him how his mill runs, likely pretty good as long as his vfd parameters are tuned in. PeterT, you running a vfd drive on your mill? if not, about all you can do is find a guy with a scope and actually view your power wave coming in, and then decide whats up, talk to Rauce and verify with him about his servo motor install, if that runs quiet, it should, but if his power coming in is poor, he might be in the same boat!
 
sometimes, with no load on, a motor will run quiet, its when its under load you hear things.... not saying thats the case, just bear it in mind.
 
grease monkey said:
PeterT, you running a vfd drive on your mill?
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No VFD. Although there is constant pressure from certain 3-phase/VFD 'pushers' to convert LOL. Its a single phase 220V, plate attached.
 

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the nice part about a vfd, is the starting charactistics, with a switch that drops full power on start up, like a contactor or what we call a magnetic starter, your hydro meter flinches a bit, and the lights dim etc, because as a rule of thumb, any current rating on any motor indicates running load current, the motor can and will draw up to 3 times as much current to start because you told it to start and its in a blazing hurry to get there, a vfd however, says no, "thou shalt start in a manner I deem yo shall start" so when you command it to run, using base parameters or values you set, it starts slowly and ramps up, way easier on the hydro meter and the lights and your hydro bill!, and your fuses or breaker can be more inline with your motor rating as a result, instead of going 1.5 times bigger just to get past the starting torque when inrush is maxed. as it stands, there are no parameters you can fenagle with, with a vfd, you can. just saying. my vfd handles a 3hp motor and cost me used $150, they are out there, the newer the better with this stuff. more room for button pushing! my drive also came with an external potentiometer, to allow me to adjust speed on the fly if I wish. for all you lathe guys.....himt hint!
 
I live in windmill country. You wanna see dirty power, comeondown! Drives me crazy!

@PeterT - doesn't one of the guys you've been working with have an oscilloscope?

Regarding strobes, just search for "strobe" on your phones app store. But make sure you get one that you can fine tune so you can find the frequency that matches. Also remember that frequency is usually per second while speed is per minute - it's an easy mistake. Don't ask how I know.

A sound frequency analyser might help too.

Last, but not least, try a VFD that has sensor less vector control mode - it's amazing how much smoother a motor runs using SLV Mode.

Edit - sorry about the VFD, you posted about that after my post.
 
@PeterT the app I played with was a little clunky, there may be better ones now. To use it you marked the pulley with a piece of chalk. Then you guessed the rpm typed it into the app and looked for the mark. You had to keep inputting different rpms until the line stood still.
 
Have you eliminated any issues with the start circuit and wiring inside the motor? I would look at that long before, looking in to the power. Are any other motors exhibiting issues. Dryer for example. You can call the utility and they should come out and test the line for you. They will use a power analyzer, which covers everything pretty much all at once. I’ve done that with rural properties, where they had issues. Most of the time it was bad overhead issues. As a home owner in an urban setting your utility should come out and fix the issue for you.
 
djberta said:
Have you eliminated any issues with the start circuit and wiring inside the motor? I would look at that long before, looking in to the power. Are any other motors exhibiting issues. Dryer for example.
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~2019 when machine was new I had what I thought were centrifugal switch issues. After about 500 starts it would go into what sounded like heavy cogging mode. I fiddled with CS position, confirmed the springs & counterweights were operating correctly, everything locked down, new switch, cleaned pads. After some interweb digging I learned there was potential of a factory issue encompassing my year where the start/run capacitors were somehow not up to snuff. I learned of this through some discussion on Hobby Machinist Precision Mathews sub-forum & even spoke with Matt directly which was nice of him because i was not a PM customer. Back to my Calgary vendor with this information, they pursued & confirmed same story. I was provided a new set of caps & new CS at no cost. Delivery took a while & maybe even a pandemic in there somewhere. I'd have to consult my notes but I want to say the main suspect culprit was start cap, but received both just for good measure. I installed them to confirm they worked & they sounded identical to a normal start/run. So (maybe dumb-ly) decided to save my dry ammo & run the original ones until 'the next time'. I must be getting close now. Its a good point you've raised. Maybe if the original run cap is off spec it can adversely affect a motor? I'm quite sure I had this discussion with the electrician at the time the noise started to enter the picture & he thought 'probably not' but its becoming a bit fuzzy now.

The mill is in the garage on its own 40A circuit subpanel coming from house. Mill never runs same time as another machine. No other electrical issues to house appliances. My only other 220V machine is 2HP (15A SP) lathe & no surging issues there.

I'm in a relatively quiet residential neighborhood. No heavy welders (or I'd be over there). No known basement grow op heaters or fans coming on (that I'm aware of).
 
Johnwa said:
@PeterT the app I played with was a little clunky, there may be better ones now. To use it you marked the pulley with a piece of chalk. Then you guessed the rpm typed it into the app and looked for the mark. You had to keep inputting different rpms until the line stood still.
Click to expand...

Removing the vent panel exposes some of the variable speed Vee belt & some of the drive cog belt beneath that. Not the greatest visibility but it would be interesting to watch the belt in 'freeze frame' to see if it did anything funky coincidental to the resonance noise.
You are saying I would set the rpm in the app manually to motor RPM a guestimate, tweak it +/- until it matched & then be able to to view things in stop motion?
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This compression spring area under motor shaft belt sheave has been on my suspect radar, but I may be barking up a tree. The ends of springs are ground flat but due to the coil helix, only partial circumference contact to the parts above & below. Its under tremendous force so should not be going anywhere. But metal to metal on both ends could be buzz source? I don't thing the spring ID is close to the bolts or shaft to buzz. My only clue is doing the screwdriver handle magnified listening test on this plate. You can hear nice smooth whirring of bearing balls & then kind of like an extra audio track, the much slower harmonic in & out resonant hum mystery noise. But... this exercise is like being in a pitch black cave where somebody says a word & echoes bounce everywhere before getting to your ears. You hear the noise alright but guessing its Jim at coordinate XYZ is difficult. This would be another great spot to focus a freeze frame strobe though.

But step-1 was detached motor from mill, now sitting on floor, same spring, same power setup, it purrs like a kitten. Pulley is tight, running true, keyway is sharp. No sense of resonance I could detect. Its the combination of motor + head where the resonance comes in. Again, its much better now with the new 'slightly loose' bearing, but resonant noise is still quiet in the background.

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Susquatch said:
@PeterT - doesn't one of the guys you've been working with have an oscilloscope?

Regarding strobes, just search for "strobe" on your phones app store. But make sure you get one that you can fine tune so you can find the frequency that matches. Also remember that frequency is usually per second while speed is per minute - it's an easy mistake. Don't ask how I know.
Click to expand...
I haven't got that far, hence this post wondering out loud about source electrical. My gut says mechanical but ignorance can lead one down all kinds of paths LOL
I'll check out some apps, that also sounds useful for some other of Life's Puzzles
 
@PeterT I can come over with my Tec scope, but I think the supply will be fine. we need to check the resistance of each of the windings -- a weak one can cause vibration under load.
 
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