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Soft Start For Ball Mill Application


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#1 anu_rags

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Posted 25 March 2007 - 10:19 AM

Hello All,

I have a customer who is manufacturing ball mills for chemical industry. He is interested to install Softstarter or VFD for the motor but is apprehensive about whether it can do the job of starting the mill. To tell you about the mill, its driven by a 250hp, 415V 1440rpm motor connected to a SEW eurodrive gear box of 10:1 ratio via a Fluid coupling. The gear box is connected to the pinion of mill by a gear coupling. The pinion is driving a bull gear which makes the ball mill rotate at 14rpm. Customer uses a Star Delta Starter to start the motor and keeps the setting for the Star as minimum 2.5sec only. Current goes upto 1100Amp when the starter goes from Star to Delta.

Can we install a Softstarter/VFD of suitable rating to get the mill started smoothly and eliminate the fluid coupling? Will softstarter/ vfd be able to produce the starting torque? Have you come across or commissioned any such application in past?

Awaiting your reply. Thanks in advance.

Regards,
Anurag

#2 marke

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Posted 25 March 2007 - 11:41 AM

Hello Anurag

I have used many soft starters to start ball mills without problems, but it is important to use the right motor in the first place.
When you start a high inertia load, there is considerable energy dissipated in the rotor of the motor and if the motor is not designed for this, it will fail. There is usually a maximum locked rotor time, or maximum start time or maximum load inertia quoted for the motor and these all basically refer to the ability of the rotor to absorb energy.
The slip energy dissipated during start is approximately equal to the full speed kinetic energy of the driven load. The slip energy is dissipated in the rotor of a directly coupled machine, or in the fluid coupling where one is used.

Some motors have a maximum Locked rotor time of around 5 - 10 seconds, and others can be above 40 seconds. You need to have a motor with a long locked rotor time.
When a fluid coupling is used, the slip losses are in the fluid coupling, not the motor which should get to speed in a very small time. Because the fluid coupling would normally be set up to transfer at least 120% torque, there is no point using a reduced voltage starter unless the fluid coupling used is a delay fill type. Otherwise, the only way to start the motor and get it to full speed is to use a full voltage (DOL) starter.

Another issue, is the ability of the motor to develop torque under high slip conditions. This is indicated by the LRT of the motor, and ideally, you want to use a motor with a high LRT and a low LRC. I take the ratio of LRT/LRC and take the motor with the highest value that has sufficient thermal capacity.

A variable speed controller can also be used in this application, and is of advantage if there is a benefit in operating at different speeds. If the ball mill will always operate at a fixed speed, the VSD is an expensive option.

One major disadvantage of using a VSD or a fuid coupling, is that there is an additional 5% loss all the time that the ball mill is operating. The soft starter has only about 0.5% loss and a much longer expected life.

Best regards,

#3 jraef

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Posted 25 March 2007 - 06:41 PM

I too have done numerous ball mills with soft starters and I agree with everything Marke said. The only thing I would add is that the operating losses on a soft start can be eliminated by using a bypass contactor.

I had one customer use a VFD once on a vertical ball mill because the motor was designed for 50Hz and used in the US at 60Hz, but we had to oversize the VFD by 100% to get enough starting torque from it.
"He's not dead, he's just pinin' for the fjords!"

#4 anu_rags

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Posted 26 March 2007 - 04:34 AM

So in my application since the motor is already driving the ball mill via fluid coupling, is it possible that the motor with softstarter may not be suitable to run the ball mill without fluid coupling. In that case as marke suggested we need to check the LRT and LRC ratings of the motor?

And its a interesting information you have given jraef. Since the motor was 50Hz rated and you increase the speed to 60hz using a VFD, so in that case the torque that a motor can deliver will definately decrease. Why the VFD has to be upsized and how it can help to get more starting torque out of the motor?

Ill be interested to know how to size the softstarter for this application. Is it ok to select the same rating of motor at 100% and with 500% overload current limit?

Regards,
Anurag

#5 marke

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Posted 26 March 2007 - 06:56 AM

Hello Anurag

QUOTE
is it possible that the motor with softstarter may not be suitable to run the ball mill without fluid coupling?

Yes it is possible that the thermal capacity of the rotor may not be high enough to start the ball mill. The first thing that I would do is to find out what the maximum start time or the maximum Locked Rotor Time is. If this time is in the order of 30 - 40 seconds of higher, I would expect that the motor is able to cope with the driven load. smile.gif
If the maximum locked rotor time is in the order of 5 - 10 seconds, I would not even try to start the ball mill without the fluid coupling to absorb the slip losses. mad.gif

Next, determine the start current and start torque torque curves. If the curves are not available, just use the LRC and LRT to give an indication.
You can determine the minimum start current from these curves (figures) for a start torque of around 80%. Use my Electrical Calculations software to do the calculation for you.

If the start current is less than 500% to develop 80% torque, and the rotor is able to absorb the start slip losses, then you can apply a soft starter rated at 500% start current and should have no problems. biggrin.gif

Best regards,

#6 jraef

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Posted 27 March 2007 - 01:25 AM

QUOTE
If this time is in the order of 30 - 40 seconds of higher, I would expect that the motor is able to cope with the driven load. ...
If the maximum locked rotor time is in the order of 5 - 10 seconds, I would not even try to start the ball mill without the fluid coupling to absorb the slip losses.


I agree. Here is one I did a while ago for a ceramics mfr. It's only 50HP 460V, but the principals are the same. The motor had a locked rotor time of 40 seconds. With the soft starter it needed 75% initial torque to start moving and 450% current limit to accelerate. I also programmed it for 2 starts per hour and 30 minutes minimum between starts just to be sure. This has been running trouble free for 3 years now, whereas when they used Y-Delta the motors needed to be rebuilt every year.

[attachmentid=58]

By the way, on the one where we used the VFD, we used it so that we could run the motor at 50Hz as it was designed. We needed the VFD because our power was 60Hz. The motor would have over heated at 60Hz in that application.

Attached Files


"He's not dead, he's just pinin' for the fjords!"

#7 anu_rags

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Posted 29 March 2007 - 06:19 AM

Hello,

Thanks for the input till now. I managed to gather the best possible information available. Please find below the motor specifications. As mentioned the Lock Rotor Time or Thermal Withstand time is Hot/Cold : 15/33. So which value shall we consider for our application.

Also torque and speed curves are not available. The Lock Rotor current is 600% of Full load current ( 293 Amp) and Pull out torque is 225% of full load torque ( 117.83 kgm)

Pls guide whether we can remove the fluid coupling and install a softstarter to start the ball mill.

QUANTITY 1
DUTY S1
MANUFACTURER CROMPTON GREAVES LTD.
TYPE 3PHASE, AC SQUIRREL CAGE INDUCTION MOTOR
FRAME DESIGNATION ND315L
OUTPUT ---- KW 180
VOLTAGE ----- VOLT 415.0,+-10%
FULL LOAD CURRENT----- AMP 293
FULL LOAD SPEED ---- RPM 1488
ENCLOSURE TEFC Safe Area
TYPE OF PROTECTION IP55
GAS GROUP Not Applicable
MOUNTING B3
AMB. TEMP/TEMP RISE ---- C 50C/70 Degc (BY RESISTANCE METHOD)
FULL LOAD TORQUE ----- KGM 117.82
APPLICABLE CODE IS:1231,IS:12075,IS:12065,IS:325,
STARTING TORQUE AS --- %FLT 175
%EFF @100% LOAD 95
%EFF @75% LOAD 95
%EFF @50% LOAD 94
ROTATION VIEWED FROM DE Bidirectional
BEARING TYPE --- DE/ODE 6319/6319
TYPE OF LUBRICATION Lithium base grease grade II
NET WEIGHT (APPR.) --- KG 1160
TYPE OF COUPLING NOT SPECIFIED
CABLE SIZE TYPE --- mm. sq STANDARD,STANDARD
PHASE/CONN/NO OF TERMINAL 3/Delta/6
FREQUENCY ---- Hz 50.0,+-5%
INSULATION CLASS F (Temp. rise limited to class B)
No. OF POLES 4
LOCKED ROTOR CURRENT %FLC 600
LR WITHSTD TIME HOT/COLD 15/33
STATOR THERMAL TIME CONST MIN 60
PF @-100% LOAD 0.9
PF @ 75% LOAD 0.88
PF @- 50% LOAD 0.84
PULL OUT TORQUE --- %FLT 225
SPACE HEATERS - WATTS/VOLTS SPACE HEATER 60WX2 (CE280/315)
ACCESSORIES N.A.
PAINT SHADE ACP 631
TB POSITION FROM DE RHS FROM DE
Load GD SQ. (KG-M)
MOTOR GD SQ. (KG-M) 24.97
STARTING TIME @100% V SEC
TYPE OF STARTING Star Delta Starter
NOISE LEVEL As Per IS:12065
VIBRATION As per IS:12075


NOTE : 1) ALL PERFORMANCE FIGURES ARE SUBJECT TO IS 325 - 1996 TOLERANCES .
2) FOR INVERTER DRIVEN MOTORS, MAX. ALLOWED %THD FOR VOLTAGE = 3%
AND TEMPERATURE RISE IS LIMITED TO CLASS F LIMITS
3) FOR INVERTER DRIVEN MOTORS, VOLTAGE SPIKES ARE ALLOWED MAXIMUM 1600 V
AND MIN. RISE TIME OF 0.1 MICROSECOND. CABLE LENGTH TO BE SELECTED ACCORDINGLY.

Regards,
Anurag

#8 marke

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Posted 29 March 2007 - 08:32 AM

Hello Anurag

I believe that the cold start rating should be sufficient, and with a locked rotor torque of 175% and a Locked Rotor Current of 600%, this motor should be able to start a ball mill with out a fluid coupling.
If we design for 80% start torque, that would mean a start current of 400% so I expect that you should be able to start this with a soft starter and a start current of 400 - 450%.

Best regards,

#9 anu_rags

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Posted 29 March 2007 - 08:50 AM

Hello Marke,

But cold starting will come into picture only when the rotor is at ambient temperature. But if incase the mill is stopped in between and again restarted, in that case will it not be hot start?

Can you pls elaborate difference between cold start and hot start?

Regards,
Anurag

#10 marke

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Posted 29 March 2007 - 06:44 PM

Hello Anurag

Yes, you do not want to do lots of starts, or restarts with this machine, but the same applies when using a fluid coupling. When you start the machine, you are dissipating a lot of slip energy somewhere, either in the motor, or in the fluid coupling. In both cases hot restarts will cause overheating and life shortening.

If you require frequent restarts, a VSD is a means of doing this without large slip energy losses.

Best regards,




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