Sawmill Motor - LRA, Softstart, etc.

[Robert789]

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?

 

[magoo2]

I don't believe the soft starter will be of any help when the motor stalls. You'll draw locked rotor current for a longer period of time (than just motor starting) which will make the flicker more aggravating to your customers. we installed a series capacitor on a distribution circuit about 10 years ago to deal with this type of problem. It wasn't a cheap solution, but it was effective in reducing the flicker. You might explore some of the more modern techniques like mini SVCs.

 

[burnt2x]

FWIW, soft starters help you if you have voltage quality restrictions by the Poco, large voltage drop prevents your equipment from starting or you are restricted by your service transformer capacity.
Just make sure the customer understand that overloading the equipment doesn't help.

 

[waross]

If this was a hog or chipper I would say
"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
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

As I see it there are two separate issues here:
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
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Is this a drum debarker? 400 HP seems a lot for most types of debarkers. I've seen hydraulic debarkers with 1500 HP of pumps but overloading doesn't affect the pumps.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Good point Bill, I didn't notice that. I used to do work on Nicholson A6 debarkers and even with the largest 44" ring, I think they used a 75HP motor.

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

 

[rmw]

Well he/she said "debarker" which I didn't interpret as hog-a totally different animal. Most sawmills that I messed with had horrendous instantaneous load problems but the debarker wasn't typically one of the culprits. But they were usually drum type debarkers. Kind of hard to stuff them too full all at once as I remember it.

rmw

PS: the google spell checker didn't like the word "debarker".

 

[waross]

Thanks for the info on drum debarkers, rmw. I am aware of them but have never seen one up close and personal.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Robert789]

I have received further information about the planned installation and it is indeed a chipper, not a debarker, that has the 400 HP motor proposed for it.
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

 

[waross]

I remember one chipper that was supposedly jam proof by virtue of an over powered motor. Then came the day that the discharge became plugged. The chips backed up in the housing and brought the chipper to a stop.
No such thing as a chipper that won't jam.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Yes, the current will still rise as the chipper is overloaded, to the point of reaching LRA if it is severely overloaded.

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.

 

[waross]

The unknown that affects the design is how often will the chipper overload. It could be daily, weekly, monthly, or every few years. A chipper that is prone to stalling may require better and more sophisticated overload scheme than one which runs years between jam ups. Another protection option is a speed switch set at about the pullout point (or point of no return) on the torque curve.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

The best system design, as Lionelhutz alluded to, is to control the chipper infeed based on monitoring the chipper motor loading. But this depends on the chipper task. If it is a waste reduction chipper where it is accepting random loads of scrap from other processes within the mill, then this model doesn't work because you end up with jams in the infeed system. But if it is a production chipper being fed by a controllable source, then you can slow down or interrupt the source flow to avoid overloading, or under loading, the chipper by watching the motor current (or better yet, the power).

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

 

[Robert789]

The system (Bandit 3680) is separate from the rest of the mill. An operator is using a front end loader to deliver the material that will be chipped. The unit presently is set up to retract the infeed conveyor when the speed of the unit drops by 300 rpm. Upon conversion from diesel to electric, the mill operator says that the infeed conveyor will retract when the current flowing into the motor reaches 175% of full load amps.

Robert

 

[jraef]

Sounds like a decent plan to avoid jams (although I wouldn't want to be the poor sucker who has to clean out under the infeed after it retracts).

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

 

[itsmoked]

I suspect you could use a 350hp motor and still have improvement over the i.c. engine. A 400HP motor may actually break things.

Keith Cress
kcress -

http://www.flaminsystems.com