I have a problem on a yankee dryer on a tissue machine. The gearbox is driven by twin motors which operate on a master/slave basis, with one telling the other how much load to take. Last week a high vibration level (approx 17mm/s) was recorded at approx 1125cpm, predominantly around the wet end input shaft. This frequency does not match any of the shaft speeds or other known frequencies. In fact the frequency remains pretty solid (give or take a few cpm) regardless of machine speed. This rules out most mechanical problems. Also, when I freeze the rotation of the motors/input shafts with a strobe I can see them oscillating, as if the shafts are bouncing back and forth.

When we reduced the load on the slave motor to zero, the vibration disappeared completely and the oscillation also stopped when viewed with a strobe. If we reduce the load on the slave by 15-20% the peak decreases to just a couple of mm/s. My theory is that there is a problem with the motor drives causing the motors to fight against one another (hence the oscillation described above). The peak at 1125cpm may well be a natural frequency excited by the constant clashing of the gears. Has anyone seen anything like this before?

The electrical guys have already changed the main control card, the power stage interface card and the pulse generator cards without any improvement. Our next option is to call in the manufacturer to balance the drives.

Al,

I don't have time to look right now, gut if you do a search of my posts, you will find a similar situation that I worked on in August of 06.

I'll be back this afternoon to discuss it further.

Danny

If the motors have a tach generator on them or even just on one of them, I would check the coupling to the generator and the generator itself.

Thanks for the replies.

There has been a development for the better (I hope).

Our electrical engineer called in an engineer from the manufacturer and he's had a play around with the parameters on the drives. I'm still in the dark as to what exactly has happened, but he's made some adjustments and the vibration appears to be under control now.

It's early days and I'm not going to start counting my chickens just yet. Unfortunately the tissue machine goes off tomorrow for a week's maintenance shut, so it may take some time to get some positive results. I'll let you know how it goes and what exactly has been altered.

The manufacturer's engineer altered a parameter on the PID card, which affects the speed of response to any errors on the "A" motor (master). It seems that the "B" motor (slave) was responding quicker than the "A" motor and was bouncing within the backlash of the gears. I still haven't confirmed whether the peak at 1125cpm was a resonance, but it looks likely.

I hope that my explanation is good enough, but as a mechanical guy it's as good as it's going to get.

We've just started up after a week's shut and the vibration appears to still be under control.

Al,

I never posted the data that I was talking about since your problem got fised so quickly.

If you have any, can you post some spectral data so that I can compare it to mine? My case didn't involve an inverter, but I suspected strongly that overdriving was the cause of what I was seeing and if our spectra are similar, that would point me in the right direction.

Thanks,

Danny

Here are a few plots. There's not much to see really apart from a big peak at 1125cpm. I think the key indicator was the oscillation of the input shafts when frozen with a strobe. These are DC motors, not inverter fed AC.

Yankee_Gbx.doc (189 Kb, 14 downloads) Yankee Gbx