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debequem
03-08-2004, 09:57 AM
I finally got to use my new used Bridgeport this weekend after what seems like years of preparing my new shop.

Anyway, I have a VFD powering my 3-phase motor on the 2J head. When I turn the VFD on and hit RUN, the motor spins up and when the VFD hits around 42 to 43 Hz the motor has a grand mal seizure and starts to visibly shudder. Everything on the mill has been checked and is tight.

I tried this with the gearbox in neutral, so it is the motor that is doing this.

I plan to run the motor set at 60 Hz and just use the variable speed head anyway, but I wonder about the sympathetic vibration that happens at 42 – 43 Hz and what causes it.

Anyone have a clue?

Marv

Forrest Addy
03-08-2004, 10:17 AM
What's the "accel" setting? 2 seconds is a good figure for a BP motor.

debequem
03-08-2004, 11:01 AM
Forrest,

Not only does this do this upon startup, but you can simply dial the VFD frequency to that magic 42-43 Hz zone while it is running and it will shudder and vibrate.

My start up time is probably 3 to 5 seconds, which I think is too slow.

What is a good time for the mill to slow down, too?

I have a 2 HP motor with a 3 HP VFD, but no braking resistor.

Back to the motor. What would cause the vibration? I know that there is a plastic bushing that is up in there that is a wear item.

Marv

JCHannum
03-08-2004, 11:24 AM
It's a good chance that the bearings are less than good. As they start to wear, harmonics will cause vibration in some speed ranges. Try removing the braking, run the motor up to speed, shut off and allow to coast. You will probably get vibration as it coasts through this range.
If you speed past 60 Hz, you will probably encounter another area of vibration.

debequem
03-08-2004, 11:43 AM
Yes, I have noticed it on power down as it passes through that range.

Which bearings are you suspecting? Are you talking about the motor itself?

Are they user replaceable or is this a motor shop job?

Marv

John Stevenson
03-08-2004, 12:29 PM
Marv.
It sounds like the motor bearings but it could also be wear of the variable sheave pullies.
If you can hear it as it drops down on slowing the only way to check which of the two is to drop the belt and lower sheave off, then run it down and see.
Probably just as well to replace the bearings as a matter of course. They aren't hard to do and they are standard off the shelf bearings, not expensive.

Some machinery does vibrate at different speeds, my bridgy doesn't like the range around 1500 as much as 1800.

Many VFD's can be programmed with a skip frequency to overcome just this. Have a read of the book and see if you can't program 42 to 43Hz into it.
It will just skip this band as it goes up and down.

John S.

Paul Gauthier
03-08-2004, 01:09 PM
It could be the moveable pully bearing (actually they are not bearings just a plastic sleeve) on the motor shaft. They are replaceable. I have done this. However in my case it vibrated at all speeds not just a certain range, the wear was so great that the plastic sleeve was completely gone and the motor shaft was badly scored and had to be ground down to clean, I then bored the pulley and made a new bushing of bronze and it still works after 15+ yrs.


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Paul G.

wierdscience
03-08-2004, 10:20 PM
Could be many anoying little things,Had one motor on a Gifford drillpress that shook like no tommorow,took the fan shroud off and there was a giant mud dauber nest between two of the blades,knocked it off and all was well.Another time I had one that was missing a fan blade fin,it only vibrated at a certain speed during startup.Motor bearings would be a source too.