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115v motor vibrating/buzzing only when under power - not when coasting

Fish On

Cast Iron
Joined
Feb 23, 2014
Location
Mobile, Alabama
I've got a vertical bandsaw with a 115/230v 1 hp motor, currently wired for 115v.

Recently, it's started a very loud (probably amplified by the thin gauge sheet metal cabinet) vibration or buzzing when under power. Motor still does it with belt removed, so clearly in the motor, not the rest of the saw. And, only under power, not coasting - goes nearly silent as soon as power is cut. If I bump the start switch to keep it up to speed without the switch locking on, it'll buzz only when the start switch is pressed, and immediately stop when power is removed.

Motor spins freely and smoothly - don't think it's a bearing issue being that it's based on power on, not based on RPM.

I'm surmising it's got something to do with the start capacitor circuit staying engaged when it shouldn't, but I'm not familiar enough with the inner workings of electric motors to know where to start looking for that. The start switch on this is just an on/off switch, not an actual motor starter, so whatever it is would be self contained in the motor.

Any suggestions on what to check?
 

dalmatiangirl61

Titanium
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
While under power the armature is pushing on one of the bearings, its a magnetic push, my guess is that you have a bearing problem. When power is cut, even though its still rotating, its not pushing against bearing, you can probably see the movement when you apply power/cut power, the shaft will pull in, pop out a few hairs.

Centrifugal switch is dead simple to inspect, they come in a few styles depending on mfr, but all pretty simple. Whether its the switch, or bearings, both require disassembly, so get to it.
 

rons

Diamond
Joined
Mar 5, 2009
Location
California, USA
Remove the motor from the cabinet and then inspect.
With the start circuit not disconnecting the buzz is likely due to unbalanced condition. Needed for starting but not for running.
 

mac380abc

Plastic
Joined
Nov 13, 2021
Unless its a brush motor you wouldn't have an armature, or commutator, that said the buzzing noise could be part of the winding, just as
a transformer vibrates when the insulation breaks down between the winding.The motor winding is the same as in a transformer, Also the rotor of the motor is also plates put together with insulators between, and many times when you have a bearing break down, the rotor seats on the bars between the stator winding making a short in the rotor or the winding which like a transformer causes a buzzing sound when it is energized. Hope this helps..
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
A start capacitor issue would be likely to cause a failure of the capacitor in a minute or two. Motor would also get hot, "growl", and if you have a clamp-on ammeter, the current would be higher than specified. Motor would likely run slower than spec, even at no load.

If you do have an ammeter, the current should be high when turned on, then drop, and finally drop suddenly to less than specified current (if at less than full load). The drop would be indicative of the centrifugal switch opening when the motor reaches 60% to 70% of normal speed.

Any of the causes suggested could be "the" cause.

Motors run on "current", so unfortunately, it is necessary to know the motor current to do much diagnosis at log range. It's very basic to motor operation.
 

Just a Sparky

Hot Rolled
Joined
May 2, 2020
Loose tolerance ball bearings can cause this. My lathe motor did this intermittently until the grease warmed up. Since then I deliberately slid the drive pulley an inch or so out of line with the bullwheel and cinched up the belt tension. This keeps enough axial thrust on the bearings to shut them the hell up permanently.

I'd rather change belts a little more often than pull that monstrous 70lb motor back out to replace brand new bearings.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
I'd guess a delaminated stator that's vibrating due to magnetostriction. This is what happens when transformers change from a gentle hum to sounding "angry" without a change in load or performance.

Assuming an induction motor, the simplest check is to measure the frequency of the noise. 60 hz exactly is electrical. A bit less (due to slip) is mechanical.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
Price of a new motor.

You think your time is "free"?

Fair enough.

Send me ten years worth. I'll pay the postage.

It takes an hour to swap out a bearing in a small motor, and even less to change a capacitor. Pretty good chance you'd save money, even accounting for your time. And if it turns out to not be an issue that matters, let it hum away in peace without the risk of downtime.
 

mmurray70

Hot Rolled
Joined
Jan 11, 2003
Single phase motors are extremely bad for vibration anyway. And machines with thin sheet metal definitely make the problem worse. Only does it with power applied like you say.

If its not running hot it could be totally normal. Might have to just try and dampen the vibrating panels.
 

metalmagpie

Titanium
Joined
May 22, 2006
Location
Seattle
I've run into this a couple of times and posted as you have here. These are some of the responses I archived:

"Assuming you have a cast aluminum rotor cage look for a loose/cracked rotor bar or end ring. If it is just loose you might be able to swage it back down. If it is cracked there is no easy repair.

Regarding assembly there is no right or wrong way to assemble the end bells. The bearing seats are not bored in situ to match the centerline of the stator. The airgap uniformity is a function of the manufacturing tolerances only and assembly is not selective.

Ah-ha = the field windings are loose! If needed try wedging some wood shims between the winding mounts and the outside "can" of the motor. Some cheaper motors have the fields spotwelded in place and maybe one of them has broken loose.

--
try opening the motor and disconnecting the start windings then power it up and spin the shaft by hand (or better spin it as fast as you can by hand then apply power and it will lock in and run - if it doesn't hum with the start windings disconnected you have a problem with the windings or the switch."

I am sad to say none of the above did me any good at all. My only solution was to swap out the motor.

metalmagpie
 








 
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