Trying to diagnose induction motor buzzing

brino said:
@graham-xrf
The only part missing is a description of the centrifugal switch.
In the video in post #1 the OP @AndySomogyi says that the motor DOES spin.
One small quibble; in post #25 you say:


Well, yes, kinda....the capacitor stays in circuit until the cetrifugal switch opens, and then it's out of the circuit.
It should never be switched out by an external switch.
I suspect that's what you meant.

-brino
Click to expand...
You are right, and I should have been paying more attention about the centrifugal switch. There are "START" capacitors" and "RUN" capacitors, and often a motor can be provided with both. The "RUN" capacitor, if there, is usually smaller, might be connected internally, or not there at all!

Nearly all the induction motors with a fat capacitor on them, that I have ever had apart, were the way I described, meaning without any centrifugal switch. The motors that did have a speed switch were three-phase, with the switch changing them from a STAR connection for run-up, to a DELTA connection for full torque when at speed.

Andy's motor is a more sophisticated phase lag shifted type which exploits the different amounts of phase lag when at speed compared to when stopped. It is supposed to run smoothly without a start capacitor in circuit once it gets above a threshold RPM. I am thinking there really should be a "RUN" capacitor somewhere, but as you can see from the links, it is not necessarily so.

Dangerous to attempt, but some can be made to run in either direction, sans capacitor, by giving them a "flick" on the pulley. DON'T DO THIS !!!

There are dozens of complete explanations on the net.
Here are a couple.. --> LINK
--> LINK#2

For Andy, the centrifugal switch has to be working properly. I speculate, but if the contacts do not open at speed, or maybe only one leg disconnects properly, then the motor might be running rough because the capacitor remains connected when it should have let go. It sounds as if the switch is operating OK as the motor runs, and Andy has refurbished it, so we trust it is working.

I should mention that some motors which have a permanent capacitor and a separate switched start capacitor, might have a tapping on the S winding. Some motors have to start against a high load, and others are intended only to deliver torque once spinning fast.

Motor science has had decades beyond a century to develop. Getting into the phasor diagrams of motors, especially things like "wave-wound rotating field multipole", or "short-circuit 1-turn shaded pole start",gets incredibly mathematical and motor-nerdy. Let us not even get started on what happens when one adds in VFD drive electronics without knowing absolutely everything about the motor.

Thinking about it, the simple reference to the windings as "M", and "S" would stand for "MOTOR winding" and "START" winding.

Come to that, the whole discussion has so far (reasonably) assumed the motor is an induction type. There are types where the rotor makes itself magnetic with windings of it's own, fed from slip-ring connections.

I apologize for not getting to the centrifugal switch. The time difference had put me into the "wee small hours".
 
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Extremely clear explanations with little circuits showing all the connections is --> HERE
 
I see/hear what you mean about Dayton quality. It sounds to me like the motor is both poorly balanced and also suffering from a bad bearing or the centrifugal switch mechanism is rattling around
One of the windings may also be loose in the stator and is physically vibrating as well
Perhaps, for the sake of your sanity you should consider a replacement?
Mark
 
put a rubber pad under the motor, and try it again, there is something wrong with that! far too loud, is something perhaps way out of balance? are the laminations and coils loose inside the body of the motor, my brother has a sewing machine in his factory with a brand new chinese motor which makes a very similar noise, and the cause is the stator which is loose inside the motor body!
 
I checked the stator windings, they're tight, but very unevenly wound. The rotor is also very poorly machined, but seems to be reasonably balanced. I'm guessing the noise is from the stator windings.

Anyways, it's good enough for now. The motor in general is so incredibly poorly manufactured that I'm going to throw it away once I get the lathe setup and replace it with a better quality motor.

I'm pretty well versed with theory of electric motors, my undergrad degree is electrical engineering, grad degree is physics, however, this is the first time I've had an induction motor apart, and I'm not that familiar with how to balance them and so forth.
 
markba633csi said:
I see/hear what you mean about Dayton quality. It sounds to me like the motor is both poorly balanced and also suffering from a bad bearing or the centrifugal switch mechanism is rattling around
One of the windings may also be loose in the stator and is physically vibrating as well
Perhaps, for the sake of your sanity you should consider a replacement?
Mark
Click to expand...
I completely agree! I would go for a new motor that does not transfer vibrations into everything it touches.
When you apply AC to windings, it makes fields that do 60Hz mechanical forces.
If you get buzzes and vibrations with power applied to windings, without even the rotor in place, you can discover some of the noise. This is not actually sensible to do without some care. Windings need the presence of metal laminations to develop the inductance that limits the current. I tried it, but at lower voltage using a Variac.

Balance is VERY important! You can get a rotor balanced, but if all you are doing is considering a replacement motor, there are better options. From salvaging a variable speed ex-washing machine motor, all the way to getting a Chinese multipole servo-motor. My option for the two South Bend lathes I have stumbled into is to trash the motors (other than for a temporary play time).

For me, the absolute best is to seek out a modern small motor with permanent magnet innards (Neodymium?), and go for a low cost multi-phase servomotor electronics. I have tried it in another context for a client. The motor was turning VERY slowly, apparently with unstoppable torque, and could be ramped up to high speed, and brought to a halt in a controlled manner.

As it happened, the motors I was involved with had to turn on, and provide an exact torque to deliver zero motion as the load was "held", as if on a counterbalance, and then accelerate at a known rate to a new position, where the load had "flopped over", so that the force was in the opposite direction.

I was so impressed with that much software control available, without a big budget, that I think this servomotor stuff is ideal for a lathe speed control where you want it to take uncompromising quite heavy cuts at times, and maintain a cut speed, regardless of torque requirement.

You can even program the kit to maintain a cut face speed, compansated for radius (within limits) for a constant smooth facing finish. I still have so much to learn from the CNC script nerds!
 
graham-xrf said:
I completely agree! I would go for a new motor that does not transfer vibrations into everything it touches.
When you apply AC to windings, it makes fields that do 60Hz mechanical forces.
If you get buzzes and vibrations with power applied to windings, without even the rotor in place, you can discover some of the noise. This is not actually sensible to do without some care. Windings need the presence of metal laminations to develop the inductance that limits the current. I tried it, but at lower voltage using a Variac.

Balance is VERY important! You can get a rotor balanced, but if all you are doing is considering a replacement motor, there are better options. From salvaging a variable speed ex-washing machine motor, all the way to getting a Chinese multipole servo-motor. My option for the two South Bend lathes I have stumbled into is to trash the motors (other than for a temporary play time).

For me, the absolute best is to seek out a modern small motor with permanent magnet innards (Neodymium?), and go for a low cost multi-phase servomotor electronics. I have tried it in another context for a client. The motor was turning VERY slowly, apparently with unstoppable torque, and could be ramped up to high speed, and brought to a halt in a controlled manner.

As it happened, the motors I was involved with had to turn on, and provide an exact torque to deliver zero motion as the load was "held", as if on a counterbalance, and then accelerate at a known rate to a new position, where the load had "flopped over", so that the force was in the opposite direction.

I was so impressed with that much software control available, without a big budget, that I think this servomotor stuff is ideal for a lathe speed control where you want it to take uncompromising quite heavy cuts at times, and maintain a cut speed, regardless of torque requirement.

You can even program the kit to maintain a cut face speed, compansated for radius (within limits) for a constant smooth facing finish. I still have so much to learn from the CNC script nerds!
Click to expand...

indeed, electronic controls are cool. The first thing I’m doing with my lathe once I get the mechanicals sorted out is making an electronic lead and cross slide screws.

Then I can do threading, tapers, etc and ditch the whole gear train
 
AndySomogyi said:
indeed, electronic controls are cool. The first thing I’m doing with my lathe once I get the mechanicals sorted out is making an electronic lead and cross slide screws.

Then I can do threading, tapers, etc and ditch the whole gear train
Click to expand...
Andy - when you do go to the point of electronic gearbox, you will find there are always several ways of achieving an end result. For example, most controls are devoted to feedback control of speed first, using encoder pulses from the motor shaft. Provided you have some way to account thread backlash, you can get to position control with software by counting pulses up and down, if started from a known parked reference.

Deriving speed control from rate of change of position is worse than deriving position from integrating speed.
The best way is to let the shaft encoder control speed, acceleration, deceleration, safety stop, torque limit, etc. and use a separate position feedback for CNC style position control. These are just retro-fit digital read-out kits, now much more affordable, modified to allow motor control electronics to use them for position feedback.

The small motor you would use to drive (say) the lathe carriage can then be programmed to (say) cut a thread, in concert with the motor driving the cross slide, to make the thread from start to finish, including taking progressive cuts, and a finish cut.

The really clever nerds sample the tool position at some high frequency, and cut compound curves. I am not actually into CNC projects myself. I just play with the great old machines, but I have seen what can be done with things moving in precise relationship, where previously, they had to be geared together.

P.S. I know the nerds won in the end (American Pie), and these days it's OK to be a geek, but I somehow still don't like the "nerd word". I think I shall go for "enthusiast" or similar.
 
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AndySomogyi said:
I'm pretty well versed with theory of electric motors, my undergrad degree is electrical engineering, grad degree is physics, however, this is the first time I've had an induction motor apart, and I'm not that familiar with how to balance them and so forth.
Click to expand...
You see! We have with us another expert, (though maybe not so much in the finer points of balanced motor bits manufacture). There are folk in this forum with decades of engineering, machining, electronics, maths, and other scientific experience. One can find help here in almost any field of knowledge!
 
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