Cement Mill Soft Start 3

[Thedroid]

I'm currently working with a ball mill driven by an old GE 1250hp 180rpm synchronous motor through a ring and pinion setup. Lately business has been slower than normal, and the mill sits for a week at a time. After sitting this long, it can't get up to speed and stalls before it can synchronize.

We are currently soft starting this motor with a Motortronics MVC Using the Voltage ramp method.

Initial voltage is set at 50%
Ramp time is 5sec
Current limit is 550%

Does anyone have any ideas to try in order to get the mill started easier. I noticed a "kick start" feature in the MVC parameters, but have no experience with this feature.

The settings are already higher than 4 other mills, that are very similiar, but there is no obvious indication that anything is wrong.

 

[rbulsara]

Initial voltage may be too low to have adequate starting torque. Try 65% V.

There may be some mechanical issues. It has been a while I worked in a cement plant, but aren't the mills fitted with a pony motor to keep it rotating at slow speed (or at some intervals) to keep the bearings in ready conditions?

 

[waross]

Do you put the mill on turning gear for a time before a start?

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

 

[Thedroid]

No we use an old DC spotting cabinet with numerous relays that are closed in a certain sequence by a small motor driven cam. We use this to position the mill for maintenance only. It has too short of a duty cycle to turn the mill every time we start it. Today I increased the current limit to 570% and decreased the ramp to 4sec. My next step will be an increase in initial voltage.

 

[jraef]

The Kick Start feature is intended for applications where "stiction" is a problem. If your lube system is such that there is significant added friction at start-up, it may be useful in overcoming it. But to use it is to essentially render one of the best features of a soft starter null and void. I would hold that as a last refuge.

Increasing the initial voltage is probably best. What version of the MVC is it? Original, MVC2 or MVC3? Does it have a beige Operator Interface panel with a small LCD screen, or a large stainless steel bezel and a 4 line LCD screen?


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[DickDV]

Thedroid, if I understand your original post correctly, after sitting for a week, the motor is able to turn the mill but not accellerate it up to sync speed.

If this is the case, initial starting voltage or even the "kick start" feature will not help since the motor is able to at least start turning.

If I've understood your original post correctly, then you've got some mechanical issues that cause the torque to go so high that the motor simply can't accellerate any further, even at full voltage.

If that's the case, then you need to find the mechanical source of the high torque and deal with that first. At full voltage and 550% overcurrent, I would say that the motor is giving you about all it can give you.

 

[rbulsara]

I also think that reducing the ramp time may not help. You want to give the machine enough time to gather inertia. Trying to bring it up to speed too fast will also stall it.

In fact try increasing the ramp time along with higher voltage tap.

 

[waross]

After sitting this long, it can't get up to speed and stalls before it can synchronize

Does it stall or trip out? The difference may be important!

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

 

[Compositepro]

A ball mill requires higher torque to turn until it reaches a speed where the balls drop onto the down-going side of the mill. Starting torque can also be srongly affected by the position of the ball load in the mill at start and by balls sticking together due to sitting motionless. It may need to run a while to get the operating torque back to normal. Moisture will enter the mill at a certain rate during shutdown. A small amount of cement residue may cure more thoroughly than if there is a large amount of residue.

 

[burnt2x]

How about taking off some balls from the mill? We have had some problems like these and that's what we did. It worked! We returned the balls when the ball mill had turned for some time.
Hope this helps.

 

[Thedroid]

It's the MVC Plus with a stainless bezel and the 4 line lcd.
The motor does not want to make it over the cascade point in soft start mode. When this happens, we bypass the soft start, and the mill starts right up. Inrush current across the line is in the neighborhood of 1300 amps. We don't like to do this often, because it leads to increased failure of the stator windings.

I would say that the motor stalls. The soft start tries starting it until we get a squirrel cage trip on a GE SPM.

When the mill is run daily there are no problems soft starting it with the current settings. It's only when the mill has been sitting for a week or longer. I imagine that the ball charge and powder residue compacts and is harder to break loose on startup. I'm looking for some ideas to try with the soft start in order to give the mill the extra push needed without having to resort to across the line starting.

 

[Thedroid]

Oh and when this happens, the mill is not able to make a complete revolution. It makes it to the cascade point and stalls, tripping the SPM

 

[burnt2x]

" After sitting this long, it can't get up to speed and stalls before it can synchronize.

IMO, if this problem had been identified, something is wrong with your grinding operations! In my previous employment, we followed a "Grinding-out Procedure" before we put off all the mills.
I also understand that the current condition was not considered during the design stages of the milling equipment,a problem of insufficient torque capacity of the mill drive. You can do speed ratio change if money is not a problem.
But I guess you should try operational changes (grind-out process) plus lessening the ball charge prior to laying-in the equipment from long standby periods, that is, if all the softstarter tweaks do not work. The suggested options are cheaper.
My 2¢

 

[jraef]

The MVC Plus has what is called a "Dual Ramp" feature. You can set up a 2nd set of starting parameters and call it up with an input. What I would recommend is setting up "Ramp 2" as a "Hard Start" scenario. If your motor Thermal Register is tripping out before it can accelerate, first check the OL trip curve; make sure that you don't accidentally have it in Class 10 (the factory default).

For the Hard Start ramp profile, set your Current Limit as high as your system can tolerate. 450% is probably a good place to start from. Then set your ramp time to 1 second. What this does is maximize useful torque at the shaft for the limited amount of thermal time you have in the motor curve. If it still trips out on Thjermal Register overload, raise the Current Limit by 20% and try again, keeping cognizant of the Starts-per-hour limitations of your motor. Watch the Thermal Register on the screen and don't restart until you have at least 95% remaining.

The longer the ramp time, the more thermal time you waste without doing useful acceleration. But the start will be harder than when you ramp into it, so that's why you want to program that only for those hard-to-start times. Then wire up a 2 position spring-return selector switch to the input that you program to initialize "Ramp 2", that way an operator has to be holding it to make it ramp that way and he can't forget and walk off leaving it like that. Leave the Ramp 1 settings to provide as gentle of a ramp profile as possible and that will be your "every day" starting method.



"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[LionelHutz]

1300A = what % of FLA current?

There could be 2 problems.

You are limiting the current too much and the motor stalls. If this is the problem then try a higher current limit.

You don't get the motor rotating fast enough and it's inertia isn't enough to give it that little bit extra required to get past the cascade point. If this is the problem then try a lower ramp time.

 

[Thedroid]

1300A is a bout 725% of the motors FLA wich starts the mill up no problem.

We do grind the mill out every time we use it, but I imagine that there is always going to be something left behind, we also leave the bearing lube pumps running 24/7.

This mill is over 50 years old, and so is the motor, wich has had the stator rewound once.

I like the dual ramp idea, and will definately look into this. The thermal register on the MVC never trips, it's alway the Multilin SPM that give a squirrel cage fault. The mill does about a 1/4 turn stall then we get the fault. I want to say that the O/L class is at the default of 10, but I'm not 100% sure. I don't think that I've ever seen the MVC shut down the starting sequence though.

 

[Thedroid]

These mills were originally designed to start across the line, but had soft starts added about 6 yrs ago to reduce the wear and tear on the equipment.

 

[LionelHutz]

OK, you could likely just increase the current limit or remove the current limit. The wear and tear was most likely related to the shock loading from full voltage starting. Things like the ring and pinion banging together. Ramping into the load should remove this even with a higher current limit.

 

[Compositepro]

It may also work to reverse the mill by one quarter turn to get the ball load to the other side before starting in the forward direction. The reversal can be done at very low current.