Pump problem 3

[angryB]

We have a problem with several pumps.
Pump motor P = 90kW, In=166A, 400V connected through 250A adjustable circuit breaker and ABB PSE170 soft starter.
The problem is that when motor is switched off either circuit breaker or soft starter trips quite frequently.
I suspect that at switch off motor starts acting as generator and short circuit occurs. I'm not sure if I'm right and how to solve this issue.

Any suggestions and ides are very appreciated.

 

[electricpete]

Maybe inspect/test the contactor?

=====================================
(2B)+(2B)' ?

 

[ScottyUK]

Are you ramping the pump down or allowing it to coast to a halt?

 

[angryB]

Ramping.

 

[jraef]

You have a "2-phase" soft starter, although interestingly enough, ABB appears to have now taken the tactic from Siemens' book one step further and no longer tells you this! The reason I know is this statement in their literature:

Sophisticated algorithm eliminating the DC-component and thereby providing excellent starting performance.

There is no need to make this statement if it is a full 3 phase soft starter, as there is no "DC Component" to be concerned about. But on a 2-phase soft starter, where there are SCRs controlling the voltage on only 2 of the 3 phases, this is a concern. Certainly, the control algorithms developed by Siemens and now apparently copied by ABB help to mitigate this issue as much as possible, but their claim of "eliminating" it is a stretch.

What this means, ESPECIALLY when trying to control the Decel of the motor, is that the power going to the motor is inherently severely unbalanced while ramping, up or down. Ramping up a pump is usually something you want to do as fast as possible, so nobody sees the problem, but ramping down is done to eliminate water hammer. To do that, you must ramp as SLOWLY as possible to give the kinetic energy in the moving water time to dissipate, so that means finding a 'sweet spot" of torque that you hold at until the check valve closes slowly and gently. If you linger there too long, you are heating up the motor disproportionately to the amount of average current going to it. Fortunately for you, you are using a bi-metal thermal OL in the circuit in the form of your adjustable circuit breaker, and IT is picking up this current imbalance, biasing the trip curve, and tripping early, or at least earlier than the soft starter, which most likely DISABLES the OL protection during Decel to try to hide the inherent problem in using that design.

Another potential pitfall of this design concept is that at all times, you ALREADY have 1/2 of an uncontrolled current flow fault to the motor, because one of your 3 phases, usually the center pole, is live at all times (because it's cheaper that way). In a true 6 SR design, you must have 2 SCRs in opposing phases fail shorted before there is uncontrolled current flow to the motor, which has about the same likelihood of happening as a pair welded contacts. But in a 2-phase (4 SCR) design, the "opposing phase" is ALWAYS already hot, so if even ONE of the remaining SCRs shorts, or more likely in this case, fails to gate off, then you IMMEDIATELY have uncontrolled power flowing to / through one winding of the motor, and that may be getting picked up by your breaker as a short circuit. So in other words, you may be seeing the first of a problem inside of the soft starter and it is not turning off one of the SCRs, which will eventually result in a more catastrophic failure. This means changing out the starter.

Welcome to the world of smaller-cheaper-faster.


"Will work for (the memory of) salami"

 

[angryB]

So my only option is try to make ramp down time as long as possible and turn torque control during stop ramp on?
Currently I have 10 s ramp down time. Torque control is on.

Can you point me to some papers/literature about dc component and when/why it is a problem.

 

[iop995]

At ramp down, try to introduce in motor circuit a resistor or R-L group to limit current; in normal operation this group is shorted by a contactor.

 

[angryB]

PSE soft starter has a current limit function. Default value 4*rated motor current.

 

[ScottyUK]

angryB,

Google 'DC injection brake'. That's what you effectively have if DC is present in your windings. They are common enough, you just happen to have one by accident.

Can you allow the pump to coast down without causing hydraulic problems in the line?


jraef,

Nice explanation.

 

[jraef]

The 'DC component' problem is actually more of a red herring, a distraction from the real issues, but one they can mitigate, so they focus your attention on that issue, then tell you that they took care of it. Magicians use the same technique, it's called 'Misdirection'. It's real, and they do mitigate it, but the REAL problem they are directing you away from is the current imbalance created by using only 4 SCRs instead of all 6. One aspect of the motor harm is associated with the "DC component' of the wave form when you do that, and that is the part that makes the motor sound like crap. So by misdirecting you to focus on that, they hope that you do not pay attention to the motor heating effect of having unbalanced phase currents, which is causing REAL harm to your motor.

If you read pages 52 and 53 of this document from ABB, they mention both issues, but over emphasize the DC Component issue (because they CAN do something about that) and minimize the unbalanced current issue, because they CAN'T do anything about that, other than suggest you NOT use the 2 phase version.

As to recommendations for you:
1) Try LOWERING the Decel time as much as possible without causing water hammer. It will take some trial and error testing, so keep cognizant of the number of starts-per-hour you are subjecting the motor to, keeping in mind that every Decel cycle counts the same as a Start cycle as far as motor heating it concerned.
2) If that doesn't work, assume they might be defective and see if swapping one out for a new one soles the issue. If not, then move on.
3) If that fails, then convince your supplier to loan you a true 3 phase soft starter, and if the problem goes away, then it proves the point that they sold you the wrong type of product for the application and they should allow you to exchange them and only pay the difference.



"Will work for (the memory of) salami"

 

[Marke]

I have had protracted arguments with some sales type people and even design engineers about the merits of this type of design. Some claim to have algorithms to eliminate the current imbalance and therefore the negative sequence currents, but are unable to demonstrate balanced current vectors.
If the magnitudes are equal, but the angles are not, then the problem still exists, just harder to measure with a clamp meter.
The final demonstration of an issue, is to listen to the motor during start, doesn't sound as good.
OK for quick starts on very low inertia loads.

Mark Empson
Advanced Motor Control Ltd