Softstarter on Pump 2

[powerjunx]

Due to our equipment inventory and operation evaluation, i found out one of 250KW, 460V SCIM uses Softstarter with bypass contactor which i observe is somewhat unusual, that in turn often damages pump assembly.

I was assign to take a root cause failure of the pump but i prefer to investigate first on electrical operation.. i am not well verse of such configuration of this Starter, i know you guys would clear me up.

Here are the following i observed;

1.) At starting, the starting current reaches 4-5 times the FLA then settle down to 75% FLA for about 6 seconds and eventually bypass contactor closes.

2.) Upon Stopping, the SS(Softsatarter)again activated as in "starting sequence" (1.) while bypass contactor open up, it coasts down for about 10 seconds, then machine stops.

I think this action or sequence never satisfy the ramp up/ ramp down feature of Softstarter; that is, not configured to its design feature as for pump application. Anyone would like to share his views on this matter?


Please suggest possible sequence for pump application.



thanks,

bil

 

[jraef]

It sounds fairly typical for a soft starter operation although the 4-5 times FLA seems a bit excessive for a centrifugal pump (you didn't say what kind of pump it is). It also might be that someone has enabled a "kick start" feature, which is not a good idea on a centrifugal either.

In general, the operating sequence that you should use depends upon what your goal is. If you want to reduce the pressure shock wave that can happen with a check valve slamming shut, then the Soft Stop function should be used. If however, you already have a pressure reducing valve that closes slowly in order to do the same thing, then perhaps you do NOT want the soft stop feature enabled. If it is a positive displacement pump, you do NOT want that feature either. Again, it mostly depends on the nature of your application.

Also, if your duty cycle is high, using Soft Stop may become another problem because each stop operation must be counted the same as a start operation when determining the starts-per-hour capability of the equipment. For example if your motor is rated for 6 starts per hour, that becomes only 3 if Soft Stop is used. If you are using it 6 times per hour then you have exceeded the rating of the motor and you will likely experience rapid insulation breakdown.

Describe the entire pump system as best you can and also see if you can tell what your soft starter settings are.

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[powerjunx]

Thanks jRaef, a Star for you! i bet you know exactly about my query. Well, your on the right track!

Upon my thorough seach of PMS and related documents of this unit, i found out that SS is installled against frequent mechanical failures like; check valve damage, twisted shaft,worn out pump bearing, motor burn out, etc but this time the situation is worse in terms of mech'l failures.

I beg an apology to limit you the description of the SS, since i'm not involve in configuring/setting its parameters. I can describe only its operation as posted above.

Nevertheless, here's the entire pump system;

PUMP-MOTOR Assembly:
Vertical Mounted centrifugal pump, 252m^3/hr, 1750 rpm, 4.5 mtrs Head, driven by 250KW, 460V SCI Motor with SS unit (Altistart 48 Telemecanique) with bypass contactor. Upstream pipe equip with Butterfly check valve, pressure build up of 1.7 bars.

Jraef, could you give me a typical configuration for this specific application? And perhaps prove me the Operation sequence as i describe above isn't suitable for such application.

I hate to argue with our maintenance sup't noting that he assure that all configuration(SS), Starting/Stopping sequence are best suited for pump application. I guess his afraid when the root cause is the SS itself.

With regards to number of start, we regulate this strictly.


Big Thanks!


Bill

 

[LionelHutz]

What are the failures you're seeing?

The start sounds OK. The current is going a little high but you would have to trade off a longer starting time for less current.

You say on stopping that it coasts - as in the starter just turns off and there's no power to the motor? Or does the starter provide output power even though the motor seems to just coast to a stop? If it's the first you need to turn-on the decel feature and if it's the second then you need a longer decel time.

The soft-starter should ramp the output to bring the motor to a stop to gently close the check valve. If I understand correctly, the butterfly check valve will open and close by pump flow and will slam shut if the pump is suddenly stopped.

Now, I have never seen an Altistart 48 work. Not all soft-starters are equally capable of decelerating a motor driving a centrifugal water pump. Some cause current and torque transients or oscillations as they try to decel the motor. Some only work on certain applications. I do know the Altistart 48 has a torque control that should work well for water pumps but maybe it just isn't working for your application.

 

[itsmoked]

fbcybil; Please stop with the excess blank lines!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

fbcybil,
I agree with what LionelHutz has said. You still have not described what your pump failures are. Is it the motor that is failing or the pump itself?

Your complete sequence of operation should be as follows:

On a Start command, the SS should energize and ramp the motor to full speed. After reaching full speed, the Bypass Contactor (BC) should close, shunting power around the SCRs.

On a Stop command (assuming the Pump Control option is enabled), the BC would immediately open, putting the motor back into the control of the SS, which then begins slowly reducing the output torque by primarilly reducing the voltage. As the voltage is reduced, the motor current will initially increase. Then as torque is lost and the motor slows down, eventually pressure drops and the butterfly valve begins to close. As flow drops with the butterfly valve closing, so eventually does current (because current and flow go hand in hand). When the SS detects that the lowest torque setting has been reached, it will (should) automatically turn itself off.



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[powerjunx]

LionelHutz, thanks.

" as in the starter just turns off and there's no power to the motor? Or does the starter provide output power even though the motor seems to just coast to a stop? "

When Stop pushbutton is pressed, a current raises comparable to starting current as i mention above. With a time delay 10secs, then goes to complete stop. In both cases, Starting and stopping sequence, the check valve slamming badly until it fails, frequently.

Itsmoked, i beg an apolology for such blank lines.

Jraef, upon stopping does the current necessarily reaches as similar to starting current?
Failure often occurs on pump. These are check valve, twisted shaft of pump, severe vibrations and pump bearings.
In comparison to other units (SS other brand), they run with minimal failure.
I'm convince that a fine tuning or refining the configuration of this SS is necessary to attain its specific application, what do you think Jraef?

 

[cbarn24050]

Hi Billy, it sounds like the ramp down is too fast. On a typical pumping station times of 45 to 90 seconds are common, it is dependent on the lenght ,size and slope of the outlet pipe. Try looking at the good units to see whats typical for your operation. During ramp down you should see a gradual rise in current (after an inital fall) untill the motor starts to stall, currnet will quickly rise at this point, shut it off when it reaches 2 times FLC.

 

[DickDV]

Just wondering out loud whether this softstarter might be programmed for DC injection stop. Billybry mentions the initial rise in current when stopping which would not normally be caused by a short decel ramp on a softstarter.

 

[LionelHutz]

It sounds like you need to tune the starter.

However, I don't understand why the check valve would slam on pump starting or why you would see severe vibrations. This certainly sounds abnormal to me, almost like there is a fault with the starter. Are the voltages and currents balanced? Do you get a smooth acceleration from the motor?

If the starter isn't programmed correctly or incapable of properly decelerating the motor I typically see a speed oscillation or speed hunting in the motor and pump, not severe vibrations.

If the starter has a firing or SCR fault the motor will really growl on starting or stopping (like it's angry, which it likely is when you feed it bad voltages). Otherwise, the motor sound should be more of a smooth hum.

 

[jraef]

Good points Lionel. The check valve slamming open still makes me think someone programmed the Voltage Boost feature, something that should almost NEVER be done on a centrifugal pump.

billybry,
The level to which current will rise is completely dependant on the application, there is no hard rule on that. In most soft starters you must manually program the starter for Decel control. If nobody did that it is just allowing the motor to coast to a stop. If all they did was select Decel, they still did not do enough. In the ATS48, you should also manually activate the Torque Control ramping parameters in what they call the Advanced Settings menu if you want the smoothest deceleration profile. That then takes some additional knowledge for setting it up along with some trial and error to fine tune it.

If not, then the default Decel mode is a simplistic voltage decel which is very tricky to set up properly without a lot of experience. The ATS48 also does not provide all of the necessary adjustments to make it as smooth as possible for use with a pump. It only allows programming of the time and final torque levels, which means it always starts at 100% voltage and that is not good for some applications. If the pressure differential at which the check valve closes is very small, by always starting at 100% voltage you often end up with the output pressure passing through that band too quickly, making the valve slam anyway. You can extend the decel ramp time as cbarn24050 mentioned, but then you often run up against the thermal overload curve of the motor and trip off line, which also causes the valve to slam. Also, in this simplistic mode of the ATS48 if someone sets the final torque level too low (below 20%) in an attempt to make it work, it will disable the feature altogether and freewheel stop, which of course ALSO slams the valve!

Either way, the starter settings must be matched individually to each application, leaving it at the factory default settings is not adequate for setup because every pump system is different. Even 2 identical pumps sitting side-by-side will sometimes need different settings.

DickDV,
Good point also. On the ATS48, the difference between setting it up for Decel and Braking is that the display reads "d" or "b", altered by a simple Down keystroke. I have often though that this could easily be misread if done in a hurry, especially by those who's native language doesn't use "d" or "b" in the associated words (although interestingly enough, Braking in French is Freiner).

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[powerjunx]

LionelHutz,
"This certainly sounds abnormal to me, almost like there is a fault with the starter. Are the voltages and currents balanced? Do you get a smooth acceleration from the motor?"
Operating parameters like voltage and current were normal. Acceleration.. i'm hesitant to confirm i'll still verify this issue soon when i came back in the field.

cbarn24050, i think your right ramp down is too short. During stopping, i never notice a "gradual increase of current" but rather it is comparable to starting current about 4-6 times the full load.

DickDV, i dont think if dc injection is configured, the fact i never seen any record on its setting.

Jraef:
Upon my investigation, i restrict myself on check valve issue.
I bet you're right, Voltage Boost feature - if i'm right, it is similar to pre-select voltage level starting of 30, 60, 80, 100% voltage starting or other way around. But in my case, it starts around 80 to 100% voltage where starting current raises to 4-6 times FLA. While stopping, it goes as in Starting Mode no difference on current profile.
With this scenario, i think, this causes the "severe slamming" of check valve that lead to its failure that is; quick open(at starting) and quick closing (at stop). Give me your views in this.
Voltage boost or kick start are interchageable?

I get through the details of all your advices on tuning and
re-configuration issues on this matter. I urged my colleague to check it.

Well, thank for your helpful and witty advice.



Regards,

bil

 

[cbarn24050]

Hi Billy, if your getting4-6 times FLA during the ramp down that means your motor has allready stopped unless you have injection braking enabled. It's begining to sound like you have some machanical problem.

 

[powerjunx]

cbarn24050, at this point of current level motor still running, the machine slows down proportionate with the current until it completely stops after 10 secs. Maybe, a braking is enabled.

 

[cbarn24050]

Yes Billy I think it must be after all if the motor was taking 4-6 fla on all 3 lines it would be starting not stopping.

 

[jraef]

Current can raise to the same level on Decel as it would on Accel, the torque/speed/current curves are a mirror image. But on Decel, the current would not rise until the voltage drops significantly, and from what I can see on that ATS48, it always begins at 100% voltage, then drops at a straight line towards the final voltage setting, with the slope set by the Decel time setting. So current should take at least a few seconds, maybe 3-5, before it would reach LRC on Decel. If your current is IMMEDIATELY going to 400-600% on a stop command, then I would suspect it is in Braking mode.

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[cbarn24050]

Sorry Jref but you are seriously confused here. Firstly to get LRC on a motor it has to be stopped and full line voltage applied so it cant happen on ramp down. Secondly if you have FLC or above on a motor it will accelerate load permitting, up to full speed. Thirdly the tourque/speed/current curves are not a mirror image, there is only 1 set of curves. Fourthly you keep talking about decel, a soft starter is not a speed controller, it does not decel the motor the load does that, in a typical pumping station there can be several tons of water moving through the pipe which keeps the motor turning for some time hence the need for a long rampdown time.

 

[jraef]

Semantics.
OK, a Soft Starter can't "decelerate" a motor, it slowly chokes off the voltage, which slowly reduces the torque. Yes, it technically is the load that is "decelerating" the motor but that is referred to by everyone in the industry as "decel" in a soft starter.

The torque speed curve of that motor, as the voltage is being reduced, looks exactly like the torque speed curve does as it is being increased during acceleration. At some point in that curve, the torque is reduced enough to drop the slip back to where the motor is producing Breakdown Torque again and at that time the current can, depending on conditions, reach 400-500% FLC, which is damned near LRC. So OK, I took a shortcut and overstated it a smidgen. I was addressing his question as to why he was seeing 4-5X FLC by pointing out that this could be entirely explained as an expected phenomenon if his adjustments were such that it made it happen. I have witnessed this on countless occasions.

Misunderstanding of this set of facts has led to a lot of motors being abused and damaged during deceleration with soft starters. Current rises as the Decel function is activated.

[Reposted with minor correction]

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[cbarn24050]

Well Jref when Billy get his system running hopefuly he will post what happens for you.

 

[powerjunx]

cbarn24050, i'm getting closer to clear up as i'm getting more confused.

Jraef, seeing the manual it has "3 type of stop mode" namely
Decceleration, Braking, and Freewheel. As i found, noted on manual, Deceleration mode is suitable for pump application, i think the ramp time should be set to specific time until torque is at 20%, then a short freewheeling will follow and eventually motor completely stops. I'm also doubtful with the BRAKING enable. Could this be also applicable to pumps? How about Freewheeling mode?

i would rather furnished a final setting of the SS controller.

Thanks,
bill