VSD Puzzle

Some years ago I ran into an interesting problem at a fish byproduct processing plant. Basically these guys cooked up byproduct from commercial fishermen and extracted oil and fish meal. (Yes the process STANK like nothing else!!!)

The extraction process was done by feeding the cooked up fish mush into a large (about 2m dia * 3m long) horizontal axis centrifuge. I forget how fast it ran but from memory it had about a 40kW standard induction motor that was originally star-delta started, but someone had retrofitted a Soft-Starter. Unfortunately the SS increased the startup ramp time so the thermal O/L would trip about 20% of the time.

So I suggested a VSD. Customer grumbles at the cost. Month later he rings and asks me to install it in a hurry. Arrive ; S/S has died, centrifuge no starts at all.

A few hours later I have I nice new AB 1336 Classic VSD in place and she starts up and runs like a dream. The owner is especially pleased that he can now fine tune the speed of the drum to suit the type of fish he is processing...and I put really long decel times in to prevent OverVolt trips due to the drum energy being driven back into the VSD Bus. Test it all out and it works fine.

Next day...problems at the fish plant. VSD is tripping out on OverVolt!!! Watch the operator start up; gets to target speed and all is well. About 2 minutes later, with NO operator change of speed the drive trips on OverVolt!

OK dudes...first in with the correct answer wins this weeks' smartypants award!!

Just guessing as I don't have a good feel for the math but the angular momentum of the drum probably changed as the liquid was spun out and the solids tended to collect toward the outside. Would probably not be evenly distributed and as the heavy side came down would feed back. Anywhere close?

Common problem with horizontal axis rotating drums. The product rides up the rising side of the drum and then falls against the falling side. This causes the drum to overhaul the motor for a few seconds causing regeneration energy to flow back to the drive DC bus.

The AB 1336 has very little tolerance for regen energy and will fault on High DC Bus Voltage very easily. The solution is to add a little snubber braking to the drive to manage the cyclical regen energy. You will likely need only about one-quarter of the motor kw rating to regain stability.

Yes you are both close, Bernie has almost hit the nail on the head, but not quite.

The story as told is pretty much the all the info I had. The drum was not out of balance, indeed at the speed it rotated, if it had been out of balance it would have hopped right out of the building.

No, the overvolt trip was SO unexepected that I recall going outside, lying down in a pleasant sunny, grassy spot and pondering the problem for a minute or two, and the answer came in a flash.

Nemo and the other fish ware swiming together increasing load?

Yes the fish were guilty. A real "bottom of the harbour" case. Oily characters all of them.

Can you explain what happened? After reading the first post in tread I tought this is expected...

I couldn't resist mentioning Nemo. I was just watching "Finding Nemo" with my son.
Yesterday I took him to the Zoo. He enjoyed visit until Zoo keeper came to feed polar bears and
my son started screaming and beating glass window in an attempt to defend Nemo.
It was something like "Noooo, no no no, bad bear... Nemoooooo"

Why do they make kids movies where animal character have personalities?
It will only give them nightmares...

I expected that those of you who have worked with liquid centrifuges would be very close to the mark...yes it was the drum regenerating energy back into the drive, but not because there was solids content out of balance.

While lying in the sun pondering the problem I suddenly realised that at the end of the prior run the operator ran water through the system to flush it out. At start up it was still full of water, and it would take some minutes for the oily fish to fill the drum and displace the water. The oil being less dense than the water, changed the mass in the centrifuge, which of course would try to speed up to conserve angular momentum.

In fact the effect was very noticeable and even the variation in oil content between different species of fish would produce a voltage bump on the DC bus.

DickDV is of course right, a small DC Brake module solved the problem.

Interesting!!!

How is this DC brake connected.