500 HP on soft start trips out intermittently

11bgrunt

Senior Member
Location
TEXAS
What I have gathered so far;
500 HP motor trips out at the soft start with no rhyme or reason.
POCO serves the mill with a 2500kVA pad, 480Y/277 to an 800 amp main. That panel has a breaker feeding an AB soft start. When the soft start does trip out, the only way to get it back on is to kill the circuit and wait 15 minutes. This circuit was built 4 months ago and will run weeks at a time with no problems.
Today we connected a PQA at the soft start with the 500 HP motor running doing his job and recorded for 20 minutes. It did not trip today.
While we were connecting the meter, the drive rattled for a couple seconds like an old contactor failing. Nothing happened and it was just a couple seconds.
The first thing I see in the recording graphs is the currents are not close.
In the currents minimum column, L1-378, L2-464, L3-434.
In the currents maximum column, L1-462, L2-546, L3-514.
Voltage measurements look very good.
In the old days I would lean toward the motor.
Can a soft start do this to the currents? AB has been difficult to work with so far. This is a $20K+ drive.
Looking for a direction.
Thanks,
 

kwired

Electron manager
Location
NE Nebraska
Some questions,

does soft start control just two or all three lines during starting?

Does it go into full bypass (via contacts) after starting or is there still solid state components in the circuit during run time?
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I can tell you exactly what that is. Your motor load is exceeding the rating of the soft starter. On an SMC Flex (assuming that’s what it is), if your motor current exceeds the rating of the bypass contacts, or they over heat, it switches back to the SCRs. Then if the current drops OR the current is too high for the SCRs too, it goes back to the bypass contacts. If you are grossly exceeding the ratings of both, it chatters back and forth, which is that sound you heard.

What is is the nature of the load / machine? The spikes in current may be happening faster than your meter can see them. I’ve seen this happen on a 500HP rock crusher application, where bigger than normal chunks would drop into the crusher and spike the current really high and fast, but it would not show on a standard meter. We could only see it with a scope using a high speed trigger. But the electronics in the Soft Starter could see them and react to them.

There is a parameter in the SMC Flex that lets you adjust the response to this situation. That didn’t work in my situation because the spikes were way too high and it would still trip on OC. But it did help it not to chatter back and forth on the bypass so much.

The other option on this is to add a full blown Across-The-Line rated bypass contactor that closes and stays closed once it’s running. I didn’t have room in the box for that option, I had to put in a 600HP rated Soft Starter.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
You mentioned good voltage measurements but how close were the line voltages? Your current unbalance might be caused by as little as ~2% voltage unbalance.
If practical in this application, shifting the three line inputs to motor and re-checking the currents could help determine whether motor is causing observed unbalance.
​​​​​​
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
You mentioned good voltage measurements but how close were the line voltages? Your measured current unbalance might be caused by as little as ~2% voltage unbalance.
If practical in this application, shifting the three line inputs to the motor and re-checking the currents could help determine whether motor is causing observed unbalance.
​​​​​​
 

11bgrunt

Senior Member
Location
TEXAS
You mentioned good voltage measurements but how close were the line voltages? Your measured current unbalance might be caused by as little as ~2% voltage unbalance.
If practical in this application, shifting the three line inputs to the motor and re-checking the currents could help determine whether motor is causing observed unbalance.
​​​​​​
500 HP on soft start trips out intermittently UPDATE.
In the last 9 weeks, the POCO has monitored the voltage with a PMI recorder and believes they are in the clear.
AB tested everything and agreed to replace the starter. That was done and the 500HP ran with no problems for 3 weeks. Everything from starter to motor was megged by AB.
Now the pattern is the same as before. Some days the soft start will trip, some days no trip.
Today a new complaint in the same mill with the same POCO supply.
Ten motors, computer controlled with starters that have thermal trip heaters. When the mill computer turns those ten motors OFF, the starters trip 30-45 seconds later. The starters have no load.
Voltage was checked at the 500 starter, 277-279, 484 across all three.
When the voltage was checked at the ten little starters, five of ten had a high B phase. The other five did not and had balance voltage.
I plan to be there tomorrow with everything I own.
I have not seen a starter trip with no load before but I haven't been everywhere and seen everything.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Just a thought based on a couple of other experiences I’ve had with soft starters. On the ten “little” starters that turn off and cause the soft start to trip, are they using 480V coils by any chance? I have seen soft starters falsely triggered to fire their SCRs at the wrong time (called “self commutation”) by the inductive kickback of 480V contactor coils even on Size 1 starters. When the SCRs self commutate like that, they fire at the wrong time respective of each other, which causes a spike in current and the soft starter electronics react to it, often even if the soft starter is not running the motor. The solution was simply to add surge suppressors to those other starter coils.
 

Besoeker3

Senior Member
Location
UK
I can tell you exactly what that is. Your motor load is exceeding the rating of the soft starter.
If so, would it run for weeks at a time with no problem? Anyway, at that sort of rating (375kW) I would have expected it to have a bypass contactor.
 

11bgrunt

Senior Member
Location
TEXAS
Just a thought based on a couple of other experiences I’ve had with soft starters. On the ten “little” starters that turn off and cause the soft start to trip, are they using 480V coils by any chance? I have seen soft starters falsely triggered to fire their SCRs at the wrong time (called “self commutation”) by the inductive kickback of 480V contactor coils even on Size 1 starters. When the SCRs self commutate like that, they fire at the wrong time respective of each other, which causes a spike in current and the soft starter electronics react to it, often even if the soft starter is not running the motor. The solution was simply to add surge suppressors to those other starter coils.
All holding coils for these starters are 120 volts from a large dry type delivering 208-120 Y.
The soft start does have a bypass contactor.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
500 HP on soft start trips out intermittently UPDATE.
...
AB tested everything and agreed to replace the starter. That was done and the 500HP ran with no problems for 3 weeks. Everything from starter to motor was megged by AB.
Now the pattern is the same as before. Some days the soft start will trip, some days no trip.
Perhaps the voltage drop across the starter from line-in to load-out on each phase could be measured when it's running to see if there's any noticeable drop from the bypass contactor. Hopefully the contacts can handle the current reliably but it could still be worth checking out.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Ten motors, computer controlled with starters that have thermal trip heaters. When the mill computer turns those ten motors OFF, the starters trip 30-45 seconds later. The starters have no load.
Maybe something else is being turned on or off by the computer these 30-45 seconds later, and that causes a transient on the 480V lines with a high enough dV/dt to trigger the starter SCRs (the self-commutation effect that Jraef mentioned).
 

paulengr

Senior Member
Maybe something else is being turned on or off by the computer these 30-45 seconds later, and that causes a transient on the 480V lines with a high enough dV/dt to trigger the starter SCRs (the self-commutation effect that Jraef mentioned).
Self commutation will cause an SCR to go into conduction regardless of the hate so it shorted out the soft start. It does not trip it out, it blows it up! So it will do far more than just randomly trip.

Hence all soft starts have or should have MOV surge arresters in them somewhere. They usually only have 2 or 4 SCRs per phase, a temperature sensor, the MOV, and some RC snubbers in the power stack side of things. Not uncommon for the snubber cap to fail over time either, You can easily ohm & megger a MOV to test it for failure as well as the caps.
 
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