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]

Freewheeling just means that power is cut off from the motor, so it coasts to a stop based on the shaft load; the same as opening a contactor. As to it being applicable to pumps, it all depends on your application. If you already have what is referred to as a "Pump Control Valve" that slowly chokes off flow, you may not need anything else besides Freewheel stopping.

In the Decel mode, power is maintained on the motor and slowly reduced; this is the OPPOSITE of braking. The intent with Decel is to control the reduction of torque so that the motor does NOT coast to a stop as fast as it normally would. This SS comes with 2 forms of Decel; a "default" mode and also an "advanced" mode. As I said above, the default mode is a simplistic voltage reduction; it just ramps the voltage down linearly. In the an advanced form of Decel the SS estimates the actual motor shaft torque and uses that in a closed-loop fashion to control the Decel better. I suggested that you should probably use that advanced mode because the default mode is somewhat crude compared to other brands and may be difficult to get adjusted correctly.

Braking means that they inject DC power into the AC motor and replace the rotating magnetic field with a stationary one, so the rotor "follows" that field and comes to a stop FASTER than if it were freewheeling. Braking mode is NEVER appropriate for a centrifugal pump. Remember, they sell these starters for all kinds of applications. DickDV and I mentioned braking from the standpoint that someone may have accidentally/ programmed it for Braking instead of Decel; I was pointing out that this would be somewhat easy to do because of the way that unit programs.

As to the settings for the Decel mode, that 20% is just the factory default, any given application may need to be higher or possibly lower. Unfortunately, in the default Decel mode on this particular unit, you CANNOT set it any lower than 20% because at 20% it just automatically shuts off (freewheels) anyway. This means that IF you needed the shutoff point to be 15%, at 20% the starter turns off and may be allowing the check valve to slam anyway.

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[LionelHutz]

Cbarn is somewhat correct - The motor has to be running at a reduced speed to actually draw that much current and it typically doesn't happen with a pump while stopping but it is possible.

There is a setting for the torque control in the Altistart manual(code CLP). Factory setting is torque mode. If it is a voltage ramp that starts at 100% and ramps down then it is totally unsuitable for pump applications. I don't even know if the torque control works on pumps. There are some soft-starter manufacturers who have created quite different control modes and some never worked and others either don't work well or only work on certain applications.

Billy, if you want to try different settings then set the acceleration and deceleration ramps to 30 seconds with the rest of the controls at factory defaults. Possibly try setting the current limit to 300% as well. If these adjustments don't help at all then i'd be highly suspect of proper starter operation. If you have any further descriptions of how the motor starts and stops it may also help.

One last note, I've never heard a complaint about a check valve being damaged on opening before. How is that possible? Won't the check valve even be OK if the motor was started full voltage?

 

[powerjunx]

Thanks, Jraef that's cool and concise definition.

LionelHutz,
"The motor has to be running at a reduced speed to actually draw that much current and it typically doesn't happen with a pump while stopping but it is possible"

As i understand from it, do you mean while pushing the stop button motor draws current as much as 4-5 times FLA? However, it typically doesn't happen when pump is stopping but possible. So you confirm that SS parameter setting is somewhat poorly configured? If possible, what particular setting needs to be reconfigured in order not to draw current as a startting current does.

Well, you pointed out Torque Control (CLP). I suspected that this mode creates incompatiblity with Stop mode (Dec,Braking, Freewheel) when both modes are activated as TC and Decelerate selected.

Guys, for me to consolidate your opininions. The basic setting that affects SS for pump application in order to lessen slamming of check valve and to extinguish current while stopping; Starting Torque, Acceleration ramp time, deceleration ramp time, and Stop(Decel) but i'm doubtful with Torque control mode. Any comment?

Lionelhutz, i agree with your suggestion but ATS has "test small motor" mode feature can i use this mode upon altering such ramp time.
"I've never heard a complaint about a check valve being damaged on opening before. How is that possible? Won't the check valve even be OK if the motor was started full voltage?"
Upon starting, i haven't seen any abnormalities. I can't support you of how it was possible.

Nevertheless, the description of SS operation that would likely to attain is smooth start and smooth stop in order to
reduce valve slamming (lead to damage), to extinguish drawing high current as much as "4-5 x FLA" upon stopping.

My ultimate goal is to define the root cause failure of pump system failures and damages that might rooted from SS.

Thanks,
bill

 

[powerjunx]

Well, here's the setting of SS taken from the field by operation personnel.

BASIC SETTING
Acc. ramp time: 15 sec
Initial Starting torque: 30% of Tn
Stop mode: Deceleration
Decel. ramp time: 15 sec
final decel. torque: 20% of estimated torque
current limit: 430% of In
ADVANCE SETTING
Torque limit: OFF
Voltage Boost: 64% of main voltage for 100msec
Torque control: ON
Decel. gain: 40%
Cascaded function: OFF
INPUT/OUTPUT SETTING
L13: L1A (force freewheel)

I think Force freewheel contradicts with STOP MODE - Deceleration. I presume this input was configured and wired to stop button. How Torque limit affects the torque control where it controls without seeing the torque limit reference? The setting appears that braking is disable so we discard our thought out of it.

On operating manual it says; "Deceleration ramp starting at the "motor load torque point", to maximize the linear deceleration ramp for all pump loads." As qouted, "motor load torque point" At this point, does it something to do with slight high current drawn when stop command is initiated? (Noting SS has Torque Control enable)

Any comments?

 

[LionelHutz]

When you press the stop button while running the motor current should not shoot up to 400% FLA. Especially so if the soft-starter is ramping the output down so quickly that it's unloading the motor and allowing water hammer. The current should increase as the motor slows down and then begin to fall off again as it slows further.

Your voltage boost should not be turned on.

I don't know what the freewheel input does. If you're seeing current after pressing the stop though it's not freewheeling.

I believe you should increase the decel ramp time. As you do so the pump should take a longer time to decelerate (it will not slow down as quickly) and it should also reduce the check valve slamming as you do so. If you get up to 45 seconds or so and this isn't helping then the soft-starter is either bad or not suitable for the application.

HAve you listened to the motor when starting and stopping. Does it have a smooth hum as it accelerates or a angry growl. If it's growling then the starter has a problem.

Can you not get someone who knows what they are doing with the soft-starter to come out and look at it?

 

[jraef]

I was just about to post the growling issue too. That might explain the high starting current and (theoretical) damage on start-up as well. The growling by the way indicates a misfiring of the SCRs, either out of sequence or not firing at all, which either way means a serious problem with the control board(s). If your motor is submersible or for some other reason you are unable to hear it accelerating, connect a small motor to the soft starter as a test. If it is misfiring, the problem will exist at any size.

While I stated (and later had to defend) that the motor current could go up to 400% on Decel, I didn't mean to say that it should; that would be unusual and definitely not desirable. Misfiring could also explain that as well.

I agree with LionelHutz that the Decel time probably needs to be longer and the voltage boost should DEFINITELY be turned off. There is no need for that feature in a pump. of course if the starter is misfiring, changing the Decel time will actually make the problem worse, so check that out first.

That "Freewheel Input" worries me as well. Is there something that is using that input to possibly override the soft starter's Decel routine and force it into freewheel before it is finished? Sometimes an input such as that is tied to the NO aux contact of an external Overload Relay, which interprets the Decel current increase as an overload condition and shuts the motor down immediately, which would cause the valve to slam.

By the way, some Torque Control algorithms cannot coexist with Current Limit, so in order to have a "closed loop" ramp of torque, current must be allowed to go wherever it will, given the desired torque profile. Torque Limit has nothing to do with ramping, it serves as an "electronic shear pin" to trip the starter off-line in the event of a severe load spike so that mechanical components are not damaged. You would rarely (if ever) need that on a pump.

I also second the notion that you need to find someone who is familiar with soft starters to take a look. Your other thread about the damaged soft starter indicates to me that you may have some issues at your site.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
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[powerjunx]

Thanks, Jraef as well LionelHutz!

I am thankful with your expert suggestions. I would then agree about such SCR misfiring. I'll check first the hard wire configuration about the "forced freewheel contact" that might override the deceleration stop.
Anyway, guys im thankful,this would clear up my confusion. I bet there would be great deliberation over my colleagues about this issue. I'm convince myself to track down the root cause failure about pump system and SS with your help.

regards,
bill

 

[powerjunx]

Guys, i would like to hear some comments from you.
Here's our new setting,
SETTING
Acc. ramp time: 20 sec
Initial Starting torque: 30% of Tn
Stop mode: Deceleration
Decel. ramp time: 30 sec
final decel. torque: 20% of estimated torque
current limit: 430% of In
ADVANCE SETTING
Torque limit: OFF
Voltage Boost: disable
Torque control: ON
Decel. gain: 40%
Cascaded function: OFF

Operating parameters:
At start we obtain: 4.7 X FLA
At running: 95%FLA
At Stop (decel): 2.2 x FLA, then ramp down to 95% FLA until it completely stop.
A slight slamming of check valve was notice.I'm not getting any 4-5 X FLA upon stopping.. . Anyone can point what particular setting which reduces this current?

Any comment on this data? will this sounds a smooth tuning?

 

[jraef]

Well, you have current limit set for 430%, yet you claim that the current is reaching 470% at start. Assuming you are reading this off of the soft starter's display (as opposed to a separate meter on the load side), then the timing makes a difference. If you are reading the load current with a separate clamp-on meter this may just be a measurement error.

Is this happening at the very moment you give a start command, and then the current drops? The reason I say this is that most digital soft starters must read current and react to limit it by moving back the phase angle, a process that can sometimes take 1 or 2 seconds. When the initial voltage is enough to cause a rapid surge in current right from the start, the current can often overshoot the limit setting before the starter has a chance to react. If that is the case, the only possible remedy is to lower the initial torque setting. Mind you, 30% is fairly low already, so that may not make enough of a different here, but it may be worth a try.

If your starting current is going above the limit setting sometime AFTER the initial start command, then you may have a problem with your soft starter. It should be able to catch and limit the current to whatever the setting is. One way to find out is to lower the current limit setting, to maybe 350%, and see if the starter still goes to 470%. If it does, there is a problem. If the current is limited, but still over the limit setting by a similar ratio, then it may just be a measurement error inside of the starter.

Another possibility is that, as I said earlier, some Torque Control algorithms are not capable of executing torque control AND current limit at the same time. It might be that the ATS48 is one of them, but Schneider just fails to mention that little detail in their manual (something often done on purpose because it can interfere with marketing efforts). To test that theory, turn the Torque Control feature Off and see if it attains control of the Current Limit setting. If it does, then that was your problem.

Good luck. It will be interesting to see the outcome. Please report back.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[powerjunx]

Thanks, jraef. You really distinguish the obvious. I agree with you, the difference between starting current and current limit is somewhat a measurement error, the fact starting current was taken on uncalibrated (dial)meter on the feeder.
About TC, this is interesting! I would be little experimental on this.