Problems with softstarter

[petronila]

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

 

[Skogsgurra]

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

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

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.

 

[sreid]

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

 

[petronila]

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

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

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

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

 

[jraef]

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

 

[LionelHutz]

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.

 

[jraef]

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

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.