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Problems with softstarter

Problems with softstarter

Problems with softstarter

(OP)
Dear All

Attached you will find a sketch with the problem description.

This is happening in many  50 kW, low voltage motors. In all windings we can see  on terminal blocks, a phase  over-heating.

What could be the cause?

Thanks for the inputs

Carlos    

RE: Problems with softstarter

Same soft-starter on all motors? Or same type?
Always W winding? I think so.
Balanced currents? Also during start? Any DC component? Anywhere?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Problems with softstarter

The winding is burning because you are applying too much current for too long a period of time. Figure out why and you will know the cause.

The softstarter isn't likely causing this by itself. For example, it could be a combination of a heavy load for the motor and poor soft-starter setup. Too often, people just point fingers before fully understanding the problem.
 

RE: Problems with softstarter

If all the motors are on the same AC mains there may be a Loss of One Phase Problem.

RE: Problems with softstarter

(OP)
Hello Skogs

Yes, it was happened in all motors, not in the same phase but always in one of them.
The starting current is umbalanced in 30% in one phase.

Regards

Carlos

RE: Problems with softstarter

That's it! 30 % unbalance during start. Not good. But I'm afraid you are on your own when it comes to fixing it.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Problems with softstarter

Can you "scope" the output of the soft start? That may reveal some clues.

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

RE: Problems with softstarter

First let us know what soft starter it is and what the motors are connected to. Some of the worst soft starters are those that only ramp 2 phases to the motor, which creates a high DC component if it's not done correctly. They are generally unsuitable for high inertia / slow accelerating loads.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Problems with softstarter

Your drawing does show the soft-starter controlling 2 phases. As already pointed-out, that type of control is only useful to accelerate a very lightly loaded motor.

If you can't accept the high current imbalance during starting because it persists long enough to damage the motor then you will have to switch soft-starters for one that controls all 3 phases.

Personally, I would never use one of the POS cheap 2-phase soft-starters on a 50kW motor.
 

RE: Problems with softstarter

Yes, sorry. I couldn't view the drawing from where I was earlier. That definitely is a 2-phase soft starter, very bad for a motor that has any kind of load on it. I've seen them used successfully on centrifugal pumps and fans, but still, I agree with LionelHutz, I would not use them in my motors, especially 50kW.

Another inherent danger on that design is a shorted SCR. If you have a 6-SCR design soft starter and one SCR shorts, it's not immediately dangerous to the motor because there is no complete circuit path. But in this design, 1/2 of the circuit path is ALWAYS present, so a single shorted SCR immediately results in unrestricted current flow into one winding.

In addition, SCRs can "self commutate" (turn themselves on) if there is a high dv/dt situation, such as high noise spikes on the line. Normally because you would need to have at least TWO of the SCRs self commutate for that to be a complete circuit, it's not much of a problem. But again, because 1/2 of the complete circuit is ALWAYS present here, it can be.

At the very least, I would put isolation contactors ahead of these soft starters so they are truly off-line when not being used. Ideally, junk them and start over with a 6 SCR designed soft starter.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Problems with softstarter

Didn't pay much attention to that sketch. All it shows is a few zener diodes (or shottky diodes). Not a soft starter. But, if you still interpret it as a drawing showing the SS structure, it is clear that a two phase SS isn't a good thing.

Tell us what make and type of motors and SS.  

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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