Sawmill Motor - LRA, Softstart, etc.
Sawmill Motor - LRA, Softstart, etc.
(OP)
I'm evaluating a customer's request to install a 400 HP motor for de-barking trees at a sawmill on our distribution system. I've calculated the voltage dip due to locked rotor amps of the motor and determined that it would not be appropriate to allow it to start at full voltage (about 5.6% dip on 12.47 kV primary). It may be appropriate to allow the motor if it employs a soft start to reduce starting current.
I am concerned about what will happen should the sawmill become bogged down, or stall, due to excessive load (too much material placed in it at once). My assumption is that the soft starter would not provide any benefit in this case, and the motor current would approach the locked rotor amps value. Is this correct, or does some other factor prevent this from being a problem?
I am concerned about what will happen should the sawmill become bogged down, or stall, due to excessive load (too much material placed in it at once). My assumption is that the soft starter would not provide any benefit in this case, and the motor current would approach the locked rotor amps value. Is this correct, or does some other factor prevent this from being a problem?





RE: Sawmill Motor - LRA, Softstart, etc.
RE: Sawmill Motor - LRA, Softstart, etc.
Just make sure the customer understand that overloading the equipment doesn't help.
RE: Sawmill Motor - LRA, Softstart, etc.
"It will happen unless an MBA makes it his mission to avoid it regardless of effect on production. It will happen less as time goes on an the mill operators find out what they can and cannot get away with."
Debarkers are not so prone to stalling but anything was possible in the mills that I worked in.
From your point of view I would determine the maximum current that you can allow the mill to draw and mandate an instantaneous breaker trip near that level. The mill may avoid outages by using an instantaneous trip breaker on the debarker coordinated with the main trip. With a reduced current start they should be able to find a instantaneous setting that actually protects their motor and your system.
I had a problem with running a 400 HP hog on a soft start due to utility phase angle errors. (Long story). I used a moulded case breaker to bypass the soft start and shut the soft start down once we had a successful start. The breaker was rated at about FLA and gave us a fast motor disconnect on jam-ups.
Bill
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"Why not the best?"
Jimmy Carter
RE: Sawmill Motor - LRA, Softstart, etc.
1) Can you use a soft starter to reduce the starting current on a debarker?
2) If the rest of the sawmill bogs down while the debarker is running, will the soft starter do anything to prevent the debarker from pulling LRA?
I'm not sure why #2 would be important, that's why I'm asking if I got your inference correct. If the entire sawmill is bogging down, the operation of the debarker, which is usually in front of the head rig, seems relatively irrelevant. It's like worrying about the state of the anchor winch as the Titanic is sinking...
Before I embark (pun intended) on one of my usual diatribes, please correct any assumption errors in the above.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Sawmill Motor - LRA, Softstart, etc.
It seems large for a mechanical debarker other than drum types and debarkers don't tend to overload.
Now if this is a hog or chipper the motor size would make sense and jamming is an issue.
Bill
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"Why not the best?"
Jimmy Carter
RE: Sawmill Motor - LRA, Softstart, etc.
In addition, most of the big drum debarkers I've seen with that much HP used multiple small motors, i.e. 8 x 50HP.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Sawmill Motor - LRA, Softstart, etc.
rmw
PS: the google spell checker didn't like the word "debarker".
RE: Sawmill Motor - LRA, Softstart, etc.
Bill
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"Why not the best?"
Jimmy Carter
RE: Sawmill Motor - LRA, Softstart, etc.
The chipper is a standalone unit that has previously been powered by diesel. The owner is considering converting the unit to electric.
Thanks for your comments. Right now, I am proceeding on the assumption that the unit will become bogged down periodically and will see something close to locked rotor amps when it does.
Robert
RE: Sawmill Motor - LRA, Softstart, etc.
No such thing as a chipper that won't jam.
Bill
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"Why not the best?"
Jimmy Carter
RE: Sawmill Motor - LRA, Softstart, etc.
If the soft-starter is a good unit then it will have an over current trip that can be set to shut it down or stop the feed or something like that. Otherwise, the thermal overload will be the only think that would stop it.
RE: Sawmill Motor - LRA, Softstart, etc.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sawmill Motor - LRA, Softstart, etc.
But one thing you do NOT want to do is shut down the chipper if it starts to overload unless it is a real emergency! Modern Chipper knives are designed to work at an optimum speed band. If you shut it down with product in the throat, you can dull the knives and be out of production for longer as the filers or millwrights re-grind them. By the way, the same holds true for stopping chippers by jamming a log into them; any knowledgeable filer will tell you its a great way to shorten the life of the knives.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Sawmill Motor - LRA, Softstart, etc.
Robert
RE: Sawmill Motor - LRA, Softstart, etc.
Looks as though it's using a 375HP John Deere Diesel engine now, if you put a 400HP electric motor on it, I doubt it will ever jam on you. Torque response on an electric motor is far superior to that of an engine.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Sawmill Motor - LRA, Softstart, etc.
Keith Cress
kcress - http://www.flaminsystems.com