Possible problem with soft starter

[garciaf]

Here´s my issue.
We were doing a test of automatic transfer switch with a generator
There is a motor of 106KW, with a soft starter
during the test the current rise up to 800 amp when the nominal current in the plate is 222 amp.
The control system started to trip the breaker in order to pretect the system.
what would it be this problem?

 

[waross]

Soft-Start current limit set at 400%?

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

 

[garciaf]

When We started up the electric motor, the analogic indicator show that the currect rised up to 800 amp and then tripper the breaker.
In field I checked the nominal current in plate, that is 222 amp, then I divided 800/222 and the result is 3.6, for me this is a direct start, I supposed that the soft stater didn't work. I'm not sure what is?.

 

[Skogsgurra]

A direct start would normally mean up to eight times FLA in starting current. I think that 800 A at FLA=222 A is reasonable.

What kind of SS is it? Resistor, reactor, transformer, SCR, VFD or other?

Gunnar Englund

http://www.gke.org

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

 

[waross]

rised up to 800 amp and then tripper the breaker.

??????
Which breaker?
What size breaker?
Shunt trip?
Instantaneous or inverse time?
x 2 What is the current limit setting of the soft start?

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

 

[jraef]

I think you do not fully understand the nature of how a soft starter functions. 360% of motor FLA is about average for most types of loads started with a soft starter. Most of them come from the factory pre-set for 350% so either someone already determined that 350% wasn't enough and bumped it up, or there was a momentary overshoot to 360% captured by your meter. Either way if the load accelerates when not on the generator, then that setting is probably correct.

I will guess that most likely your breaker is set too low. You don't however provide enough other information for more than a guess, such as the nature of the load, the size of the generator, the settings of the breaker, or as already mentioned, which breaker you are referring to and actually, whether or not it DOES in fact accelerate when on utility power.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ScottyUK]

Some generators don't behave well with awkward loads like soft starts because of the distorted current waveform. It is possible that your breaker was tripped by something other than plain over-current. It is also possible that the breaker on the generator has an unusually low magnetic trip setting because of the limited capability of the generator to deliver fault current.

 

[garciaf]

Here I let a document with more information and diagrams
I will to explain better, I hope.
When we were doing the test for automatic switch transfer in order to supply electricity to the critical bar, in a event that comercial distribution fail. then the generator set must supply energy to the critical bar, where is the motor of a pump.
That when is started up the current rise up to 800 amp according to the analogic meter amp, so then stripped the automatic interruptor in the 800 amp in order to protect the system, but the ALtistar ATS22, must reduce this started current, and this current must rise up gradually till arrive the rated current.
Please help me.

 

[jraef]

Whatever you were trying to attach didn't work.

The ATS22 does indeed come with a factory current limit setting of 350%, so depending on whether the motor nameplate data was entered correctly, I'd guess that nobody adjusted it. If you have an 800A breaker, it should not be tripping on long time thermal trips though. So unless you can identify exactly what is tripping, it's hard to say.

You can try turning the Current Limit setting down to 300%, but I would not go lower than that.

But really there are other possible settings that could be an issue for you, it's hard to say what the problem might be from afar. Can you call someone trained by Schneider to come out and commission it for you?


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Bar = Bus bar?

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

 

[LionelHutz]

but the ALtistar ATS22, must reduce this started current, and this current must rise up gradually till arrive the rated current.

rated current = the motor nameplate current?

If your answer is yes then what you want is not possible with a soft-starter.

 

[jraef]

Pointing back to my saying he doesn't fully understand the nature of how a soft starter functions...

Garciaf,
A soft starter only reduces the voltage getting to the motor. That in turn reduces the torque, which in turn reduces the current. But until a motor accelerates to about 80% speed, there is a lot of current flowing into it that is not preforming work yet, it is all reactive current. So the current drawn from the line source (mains) is still high, just not AS high as it would have been starting DOL. But at the same time if you reduce the voltage TOO much in an attempt to limit the current to low levels, the motor cannot create enough torque to accelerate its own mass, let alone the load connected to it. In a typical centrifugal (quadratic) pump application, 300% current is about as low as I will go. I have started one or two at 250-280% current limit, but if any conditions change, it becomes unreliable. At 300% current limit, you are allowing the motor to develop roughly 75% of its rated Ffull Load Torque (50% of its Locked Rotor Torque). Once the fluid begins to flow, any less than that runs a risk of stalling.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[djs]

What size is the generator? If it is too small it will not start the load and the generator protection will trip.

 

[Marke]

I agree, to start most motors today, you will typically need 400% - 450% current, so if this current occurs when the motor is accelerating to full speed, then that is not an issue with the motor or the starter.

If the generator trips due to this starting current, then either the protection is too sensitive for the application, or the generator is not suitable for starting this motor.

If the generator has a three phase averaging AVR and external excitation, it should be able to stand a start current of up to three times its rating. If it is self excited and single phase peak reading AVR, then you can not overload the generator. The voltage will drop out once the current exceeds the rated current.

If you use chaser contactors to open transition switch from the generator to main supply, then there could be a major transient which would also trip the breaker.

Mark Empson
Advanced Motor Control Ltd