PCS Soft Starter 2

[jack02]

I am having a problem with a Sprecher and Schuh PCS-037-480v soft starter. Intermittently it is going into Phase loss/open load fault. There are 5 other pumps within the switchboard all controlled the same way and we have no other issues, I have replaced the starter and still have the same issue, I have swapped both the existing starter and the new starter with other starters in the switchboard and the fault always moves with the starter. Logic says we have a starter issue but I have put 2 new ones in and still it continues to fault.

 

[jraef]

I have replaced the starter and still have the same issue, I have swapped both the existing starter and the new starter with other starters in the switchboard and the fault always moves with the starter.

These statements seem to contradict each other. If you replaced the starter in the same application with a new one, and the problem persists, then the problem is with the application. If you move the starter to a different location and the problem persists, then the problem is in the starter. Which is it?

I'm, not an expert on the A-B / S+S soft starters, but generally this kind of issue is a result of a current detection problem. Some soft starters detect phase loss by detecting a current level in at least one phase that is below a specific threshold, let's say 20% FLA. The nuisance can be that in your case, the motor load is so low that even thought you are not losing one phase, the current in all three phases is below the threshold level and triggers the fault. You8 may need to disable the Phase Loss detection to avoid it.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

That can also be because your phase voltages are unbalanced enough as to cause a big enough current imbalance that the lowest phase is too low.

Is the problem always on the same motor?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jack02]

I have changed the starter with a brand new one from the supplier and the fault has persisted. I have swapped starters in the switchboard and the problem has shifted with the starter. In my mind it appears that there may be an issue with a batch of starters out there??? The low current problem has come up as an option but previously I had the circuit set up a different way bringing the contactor before the starter and never had a trip problem.

 

[waross]

There may be a clue here:

I had the circuit set up a different way bringing the contactor before the starter and never had a trip problem.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

The manufacturer may have changed some internal logic so the new units behaves differently than the older units, even if they are all the same model.

After your contactor comment, it now sounds like you are either closing the contactor too late when you start the pump or you are opening it too soon when you stop the pump.

 

[Shozza]

As suggested by jraef the PCS soft starter does have a current based detection system for the phase loss/open load protection function. Basically if the current falls below 50% of the minimum FLA dial setting then this alarm may occur. For the PCS-037-480V the minimum dial setting is 12.3 Amps so if your actual current is falling below 6.1-6.2 amps then it will trip. It can also be caused by an excessively high current if for some reason the current approached 450% x maximum FLA setting.
There is a couple of technotes on the A-B knowledgebase in regards to this. Try technotes 22373 & 59588. They are based around the A-B equivalent SMC-3 but should help regardless of the label on the front.

 

[jack02]

Thanks Shozza, I am looking at your proposal now, I think the pumps we are running are over rated for the task, I have the overload on the starter set at FLC but looking at it we only we are dropping below that 50% mark on occasions. I was trying to find the technotes you spoke of but can't seem to track them down. Do you have a website where they may be?

 

[Shozza]

Jack02,

Try the following. You may have to register to be able to see the technotes.

http://rockwellautomation.custhelp.com/

 

[jack02]

Thanks Shozza the more I look at it the more it appears to be the low current fault. We have had the overloads set to about 33amps and the motor at stages is dragging approximately 15.5 amps prior to stopping.

 

[Shozza]

If the current is only dropping to 15.5 Amps then this shouldn't be your problem. As mentioned the current must drop below 50% of the minimum dial setting. The PCS-037-480V has a dial range from 12.3 Amps to 37 Amps so 50% of the minimum dial setting is about 6.2 Amps.

Is your installation an inline (3-wire) or inside delta (6-wire) connection as the soft starter may be configurable for both?

I think the more likely problem may be with either your contactor bouncing/dropping out or perhaps a loose connection somewhere on either the line or load side of the soft starter possibly even in the motor terminal box. It may be worth a quick check of all connections. A faulty isolator between the starter and motor may also cause this alarm.

 

[jack02]

Hey Shozza, yeah mate it looks as though it wasn't the problem, it tripped again last night and for the first time I was able to see where in the cycle the fault occurred, it happens on start up. The issue I have is that if I swap the starter with another one in the board which hasn't been tripping the fault acutally moves with the starter which in my mind eliminates the field wiring and line side wiring as the problem. I have had the starter as previously mentioned replaced with a brand new one and the fault is still occuring on the new starter yet not on one that has been proven. It is as though there is a bad batch of starters out or some configuration issue.

 

[Shozza]

I'm pretty sure I know what the problem is now. It may pay to check the nameplate on the side of the starter to ensure you are replacing like for like. They have updated the PCS model with a revision B which can be used for both 3-wire and 6-wire connections where as the revision A could only be used for 3-wire. Because of this the functionality of some of the dip switches has changed. Dip switches 8, 14 and 15 have changed so if you had just set the dip switches by cpoying the old unit this will be your problem. It'll be dipswitch 15 causing you issues as in revision A this was used to change the function of the auxiliary relay. In revision B this changes the configuration between 3-wire and 6-wire. If you've set the dip switched as per rev A then this has selected 6-wire and therefore it has detected an invalid motor attached to the starter. Try confirming this dipswitch is correctly set and let us know of the outcome.

 

[jack02]

Shozza, they are all labelled as revision B, is it possible the starters are labelled incorrectly?

 

[GGOSS]

From personal experience the current does not need to fall below 50% of the min FLC setting for nuisance phase loss/open load trips to occur.

We know that it can and does occur when the run current is less than 50% of the max FLC setting and suspect this is influenced by current imbalance and wave shape.

For the record we more some 1400-1500 PCS starters per annum and on the rare occasions when nuisnace phase loss/open load tripping occur, we have been forced to offer an alternative product/solution.

 

[jack02]

I have changed the way the PCS operates by putting a thermal overload and aux contact block onto the contactor and bringing the contactor in 1st and the aux contact gives the signal to the PCS to start. The PCS is now only used as a soft starter with no integration to the control circuit. This has appeared to eliminate the problem, we still have thermal protection with the overload dropping out the contactor in an over current situation.

 

[jack02]

Still having the occasional trip, the application is pumping fuel from a tank to roadtanker and often prior to the motor starting we are having flow of product due to head of the tank. I was wondering how i should have the dip switches set up? I think in some instances (depending on the tank level) the pump motor is rotating prior to the PCS activating and was wondering what anybody's thoughts are with regards to the PCS being setup as a soft start or current limited start.

 

[GGOSS]

The choice of starting mode really depends on what you want to achieve out of the product. It is true that both will provide reductions in starting current and minimise, but each performs these functions differently.

Soft start will allow the motor to develop and increasing amount of torque as it accelerates through to operational speed and is great for applications where the starting torque requirement is variable ie from one start to the next. Whilst it does reduce starting current,this is also influenced by starting time and the driven machine and can therefore approach levels similar to DOL (Across The line) when the starting time is very short and/or where the driven machine is highly inertial.

Current limit on the other hand holds the current fixed during the starting period and is therefore better in applications where the emphasis is on reducing starting current and voltage drop on the line. If the current is set too low however, the motor will not develop enough torque to accelerate the driven machine to rated speed, so it needs to be set correctly.

I really don't expect changing from Soft Start to Current Limit or vice versa will reduce your nuisance trip issue, but as you have nothing to lose, it's worth a try.

 

[LionelHutz]

Sad to read people are still dealing with soft-starters that can either ramp or current limit when it's simple to make a controller do both...