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Ss For Variable Load Conditions


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#1 Jimbo

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Posted 27 March 2008 - 06:04 AM

Gentlemen

I've been browsing through your blog on softs starters here & there appears to be quite a body of knowledge amongst you, which is great !

I am currently looking at an application for a soft starter to ramp a 110kW coal centrifuge into life, and the question i have is this ...

Can a soft starter handle changes in the initial load conditions on a motor, and if so what should i be looking for in terms of an SS?

The centrifuge in question can crash stop at any time (for reasons beyond my control), and it very likely we will need to start the unit up under full load conditions, however typically the unit will start with no load in the basket whatsoever.
My objective is to gain a linear RPM ramp from the motor regardless of the initial load; and I am curious as to weather an SS will be able to provide this?
"torque" & "current ramp" sound like quite sexy feedback control features of the SS, but will they a facilitate the linear RMP output that i need ? the obvious and considerably more expensive and solution is VVVF.

James

#2 jraef

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Posted 28 March 2008 - 11:56 PM

There are still some soft starters out there which have what is called 'Tach Feedback Ramping" as an option. You would need a tachometer on the centrifuge to provide a suitable signal back to the soft starter, which will then control it's output via a PID loop with automatic gain control and ramp to speed linearly.

That said, it is very very difficult to do effectively if the load on the motor varies. The problem is that in a fixed frequency situation, as a soft starter is, the motor slip is staying very high as you are ramping, which means the current draw is high and although you can over size the soft starter to accomodate the extra current, you often run smack into the limits of the motor's thermal design; it just can't take it. IF the centrifuge is using a specially designed motor that was selected to take that kind of abuse, then it can work, but that usually entails knowing what the motor is going to take in terms of load. So your motor would need to be designed for the absolute worst case load scenario AND extended ramping time at high current. By the time you get done with all that, a VFD may have been cheaper.

I have done a number of Alpha-Laval and Bird food oil separators with soft starters, even though the manufacturers said it couldn't be done. I trippled the size of the soft starter so that it could take 300% current continuously, because it took up to 7 minutes to accelerate. But I already knew that the motors were designed for Y-Delta starting and they took 11 minutes to accelerate that way, so thermally it was not a problem. I also did several gold concentraors (similar to centrifuges) with Knelson Concentrators; similar issues. Knelson soon decided however that using VFDs was actually less expensive (but braking was an issue for them).

Neither of them used Tach Feedback however. Why is linearity so important to you by the way?
"He's not dead, he's just pinin' for the fjords!"




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