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Motor Starting Study With Industry's Typical Data


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Hello members of LMPforum


I need to validate a modelization of a futur industrial distribution network, especialy typical torque curves, Wk2, moment of inertia, equivalent circuit and soft-starter settings. I would like to know if you could help me to find what I need.


I'm working on mining development project. There will be 575 V motors ranging from 15 HP to 425 HP. Most of them are started by soft-starters, two (2) or three (3) will be DOL started and two (2) are controled by VFD. The applications are compressor, fans, pumps, rock crusher and jumbo drill.


I have to study flickering voltage perturbation amplitude to verify that it respects the flickering limits (being below) of the energy provider (Hydro-Québec, refered as HQ in the rest of the text).


In my case, the starting of the motor are what will created those perturbations. So I need to determined the inrush current (or inrush power) that occured when I start each of the motors.


To date, I evaluate the flickering level once with inrush currents at 200% for every soft-starting motor and once with inrush currents at 400%. With the 200% FLC starting, it's barely below the limits. With the 400% FLC, it's quite over the limits.


The calculation method is simplified. I would be able to refined the inrush current level for each motor using my power system analysis software, CYME PSAF. The objective is to have a model that would be nearer to the projected system.


However, the client have not yet purchased or received all the equipments. Also, most are second hand equipment. So the data for each motor and load are not available and I have to work with typical data.


Right now, I have hard time to find typical data to modelize adequatly the system in my study software. Particularly the torque curves, the Wk2,moment of inertia, soft-staters settings and the equivalent circuit of the motors (i'm quite confused with the PSAF tools for that last)


Thank you




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That's a tough problem. I used to use SKM Power Tools software to do motor and circuit modeling and it came with a wide variety of typical motor data sheets and load profiles. But in practice, I found that these did not necessarily reflect what we would see when we were able to get real data from customers. However, the real data was typically reflective of better performance than what the typical data showed. So in general, if we used typical data to model a project, the real project ended up the same or better off. So for your purposes, if you have any typical data you are probably fairly close to whatever worst-case scenario you might come up against.


That said, if your analysis software is allowing you to start a compressor or a rock crusher with 200% current limit, I'd say it is flawed. I have never seen a crusher be able to fully accelerate with less than 300%, usually 350% or greater. Compressors are similar although occasionally an unloaded screw compressor may start with 250%, but it is rare.


Also to properly evaluate it, I hope you are not attempting to start everything at once. The typical routine would be to start the largest motors first, i.e. the crushers, then feed into them with the smaller systems, then start things like compressors only after everything else is running. The motors that are already on-line can provide a small amount of "spinning reserve" when it comes to supporting other subsequent loads.

"He's not dead, he's just pinin' for the fjords!"
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Thank you for your insight.


CYME PSAF is not as flexible as SKM Power Tools for the purpose of starting motors since I have to simulate start routine "manualy" and by starting motor one at a time. (I used to work with SKM last year for Arc-Flash studies)

I'm still in the process of understanding the way PSAF works on motor starting and so how to modelized it. PSAF give me torque curves according to motor caracteristic I gave and I have to provide "moment of inertia of rotating parts" or "Inertia constant of motor". I'm struggling with these two data.

I hope that I'll figure it out soon cause right now I'm kind of unease to present DOL starting motor with starting peak current 155% FLC.


As for the need of my flicker/sags study, I will stick to my simplified Excel spreadsheet and present a worst-case scenario result.

I would like opinons on these presume starting current limits for soft-starting


425 HP fan 575V 1800 RPM - 200% FLC - Starting twice a day or less

350 HP compressor 575V N/A RPM - 450% FLC - Up to 6 times/day

250 HP rock Crusher 575V (Design B motor) 1800 RPM - 600% FLC - Up to 6 times a day

(I know it might not be the ideal type of motor for the application, but my customer had already purchased the rock-crusher including it's motor)

200 HP shuttle filler 575V N/A RPM - 300% FLC - Starting up to 20 times/day

100 HP fans 575V N/A RPM - 200% FLC - Up to 6 times/day

50 HP and lower pumps 575 N/A RPM - 200% FLC - Up to 15 times a day

Various 15 HP motor loads - 200 % FLC - Up to 6 times a day



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The rock crusher seems excessive at 600%, that should be locked rotor current!


What type of crusher is it?



I don't have more information on the rock crusher, yet. I'm waiting for answers from my customer.

I put 600% cause we had another customer who we could not limit the starting current of his crusher. It might be cause that we started it full of rock. Since I was not on this case, I can't tell you more.

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