vibratory polisher question

[jb_maf]

I've designed and built a ceramic polishing machine that uses two large motors with built in eccentrics. Initially I had two VFD's controlling each motor both with a potentiometer to fine tune the frequency so they run in phase, however I've been advised by someone who works in this industry that one single VFD is the way to go. With this machine if the eccentrics are running against each other then the media isn't moving for that duration of destructive interference. Is it realistic to think that both motors will run in perfect phase ( for the most part) or lock up in phase by running off one VFD? I'm not an electrical guy and buying a 10 HP VFD is an expensive experiment. Let me know if anyone has any experience syncing two motors.

 

[handleman]

As far as I understand it, both motors will run exactly the same RPM as long as they're not overloaded (which I assume they won't be since they're purpose-built vibrators?) I believe the rotor will also lag the stator by an amount proportional to the load (this is from circuits II probably 20 years ago), but that should also be nearly the same for your two motors. What I don't see is any way to sync the vibration part to the motor rotation. If I remember right, there are multiple "poles" of the AC induction motor that the rotor may "lock in" to. This may be totally wrong, and I'd love a proper explanation from a smarter person, but say your motor has six "poles"... While your rotor is getting up to speed, the stator that's driving it is still "pushing" at its full speed, and the rotor is "slipping" or "lagging" until it reaches full speed. But, when it finally hits synchronous speed, there's no way to know or control which "pole" it locked into, so the output shaft may be in any one of 6 positions relative to the driving frequency. And you have two of these motors. So, even if you put them both on the same VFD, I could imagine the possibility that your final performance of the machine varies seemingly randomly from powerup to powerup depending on their shaft position when their speed finally syncs up to the drive.

 

[EdStainless]

Are you running the two VFDs as Master/Slave? I don't see why that would not be the same as a larger VFD.
But to keep them perfectly in sync you need feedback from the motors, rotary encoders or at least a single location specific signal per rotation.

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P.E. Metallurgy

 

[JohnRBaker]

If the goal was to keep the two drives in sync, why did you even consider using two controllers? If it was because there was the chance that they would get out of sync, as handleman has commented on, then what Ed said comes into play, you need a way of getting the feedback required to keep them in sync using separate controllers.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
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[Compositepro]

There are several details involved. Vibrators often run at resonant frequencies as this produces the most vibration for the least power. I have never seen a VFD used on a vibrator, only power controllers to adjust amplitude, not frequency. Multiple vibrators often mechanically self-synchronize because more power is required to run out-of-synch.

When you do not take these things into account in your design, who knows what will happen. My guess is that you will adjust the speed for a resonant frequency and never adjust it again. The load in the basket does not change the frequency, just the damping, and thus the power used.

 

[Strong]

Your machine looks a lot like this one:

https://www.shinysmooth.com/car-wheel-vibratory-polishing-machine

I question whether you have enough strength in the attachment of the two motor drives to the cylindrical case. I hope you have the capability to make vibration and possibly dynamic strain measurements, if this is a commercial product.

Walt