Cement Mill Soft Start
Cement Mill Soft Start
(OP)
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.
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.





RE: Cement Mill Soft Start
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?
RE: Cement Mill Soft Start
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cement Mill Soft Start
RE: Cement Mill Soft Start
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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RE: Cement Mill Soft Start
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.
RE: Cement Mill Soft Start
In fact try increasing the ramp time along with higher voltage tap.
RE: Cement Mill Soft Start
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cement Mill Soft Start
RE: Cement Mill Soft Start
Hope this helps.
RE: Cement Mill Soft Start
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.
RE: Cement Mill Soft Start
RE: Cement Mill Soft Start
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¢
RE: Cement Mill Soft Start
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.
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RE: Cement Mill Soft Start
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.
RE: Cement Mill Soft Start
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.
RE: Cement Mill Soft Start
RE: Cement Mill Soft Start
RE: Cement Mill Soft Start