Motor soft starting - current limiting

[nkn5]

Hello,

I am using a softstarter for a 6.6kV, 1650kW motor (FLA=172A,LRA=5x172A) by limiting the current to 250% for 14 seconds where as the motor max. stalling time is 20sec at 100% term. voltage.

Although the current is limited to 250% for the 14secs, it reaches a peak of ~340% soon after the 14secs and this is causing a significant voltage drop in the system.
My understanding is that current limiting indirectly limits the voltage on the motor terminals and doesn't allow the motor to reach full voltage. After 14secs, the voltage on the motor terminal shall reach the line voltage and the motor reaches rated conditions.

Can anyone share your knowledge on this kind of scenarios?. I want to avoid starting current more than the set 250%.

Attached the plot from etap

Thank you

 

[waross]

The motor speed is leveling off at about 50%. You need more than 250% current to accelerate that load to close to full speed before going DOL.

After 14secs, the voltage on the motor terminal shall reach the line voltage and the motor reaches rated conditions.

At 250% the motor is not agreeing with this statement.

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

 

[LionelHutz]

The Etap plot is an accurate simulation of what the motor is doing or is it just a simulation of the motor starting?

The starter jumping to line voltage is rather odd these days, but I believe some of the competition still jumps to full voltage when the ramp time expires. We could put a UPS on the control power and run through that voltage dip, but the low voltage might upset other things on the line.

I agree with waross. The 250% current limit appears to be stalling the motor. It appears the motor will need more than 250% current to get to full speed.

 

[nkn5]

Thanks Bill,
Can you suggest on what basis I can estimate the min current limit that I should apply.

For all values upto 340%, I get the peak. If I do a simple voltage ramp, I get the maximum current value at 340%.

I would also like to know if this peak is normal in softstarting.

 

[nkn5]

LioneHutz,
I've seen your reply just now. The plot is a simulation of motor starting from ETAP. Can you give an idea about the selecting the min. limiting current or just I just have to try till I am good ?

 

[LionelHutz]

Go to the motor editor and under the start dev tab change the last step which is "remove" to have a longer time, say 60 seconds so you can properly simulate the motor start with a current limit. Then, just play with the current limit until the motor starts in a reasonable length of time.

 

[jraef]

ETAP is likely telling you that given the load accelerating torque requirements and the limitation on motor torque you are giving it, you will exceed the thermal damage curve of the motor. So it shows it needing to go across the line before that occurs, which causes the current surge. You either must increase the current limit setting or if that cannot happen, a soft starter will not work under those conditions. You don't say the nature of your load, but it would need to be almost non-existent for it to accelerate with only 250% current in my experience. A centrifugal pump with a flow control valve limiting flow as the motor accelerates, maybe. Anything else, increase the current limit to 300-350% minimum.

Often times over zealous sales people will allow you to believe that just because a soft starter is CAPABLE of limiting the current to some low value, it is going to accelerate the load. That is never the case. The load needs what the load needs. ETAP, if properly configured with load data, will tell you that, as it appears to be doing here.

If you have any sort of reasonable load on the motor, you may need to use a VFD to get this accelerated.

"Will work for (the memory of) salami"

 

[nkn5]

Thank you for your sharing your thoughts. That was helpful

 

[LionelHutz]

I'm not sure why 14 seconds was picked since the removal of the current limit at some arbitrary time isn't necessary. The current limit can continue until the motor reaches full speed as long as the current is high enough to accelerate the motor to full speed. I'd also suspect that at least 300% current will be required.

It appears you're just doing simulations at this time. Have you accurately entered the motor data and load data? It amazes me how many people will use the typical data the software comes up with for these simulations, which can easily produce wrong results.

Jeff - Etap isn't that smart. It's simply a setup thing. The current limit type of motor starting has 2 steps. The first step starts at time 0 and allows the current limit to be entered. The second step starts at an entered time and is simply a remove step. Put the remove step at 14 seconds and the current limit gets removed at 14 seconds = motor goes ATL.

 

[jraef]

Lionel,
Thanks, I wasn't aware of that. I'm used to SKM Power Tools, which does that. I've always assumed that ETAP was just a capable. I guess not.

Still, it's highly unlikely that he can start anything but the lightest of load profiles at 250% in my experience, if it continues to accelerate at all, you typically run out of thermal time on the motor damage curve.

"Will work for (the memory of) salami"

 

[unclebob]

This just reminded me a situation where someone tryed to start a 1000 hp 600 V compressor in 10 sec. with a current limited at 2400A...
That 10 sec. is not the current limiting time, it's the delay before starting a 2nd 1000 hp compressor. Voltage dropped to 520V (600 V system) and the soft-starter UV picked-up...

 

[nkn5]

LionelHutz,
I used 14 secs as it motor speed reaches ~50% and doesn't increase any further from 14th second. ( this is when I tried with limiting time of 30).

the load is a centrifugal pump,I selected generic pump curve from ETAP with exact inertia from the manufactuer. For motor, I used the "Parameter Estimation" tool from ETAP with the motor actual Locked rotor / Full load data from manufacturer.

I hope I am as accurate as possible. Let me know if I am missing something

 

[LionelHutz]

Did you match the pump curve to either the closed valve or the open valve the pump actually starts with?

 

[nkn5]

Sorry for very late reply,Stuck up with bad times.
The pump has an option to start with closed valve. I matched the torque to it while modelling in ETAP. It varies from 2.5% during starting to 40% at rated speed. The motor is rated as 1650kW-design & 1490kW-Normal.

Can you let me know if I am missing some thing?