small three phase transformers inrush current

[fab1961]

Gentlemen
we use a plasma cutting equipment that may be seen from the supply side as a three-phase transfomer. For safety reasons it might be switched on and off quite frequently, say every ten minutes in worst cases.
The primary voltage is 400 V, the size of transformer is 50 kVA.
Is there a proven and cost-effective solution to limit the inrush current?
Will a set of three resistors by-passed after a short time do the job?
Not a real problem, I am trying to improve and reduce potential causes of trouble since we have a lot of delicate equipment around.
thanks for your time

 

[l3city]

Hi there!
doesn't plasma welders work with very high inrush currents? I dont see how by avoiding the inrush current, you will be able to run your welder. This is the way I understand your commnent, if you can be more specific please state so...
Regards

 

[l3city]

well...by plasma welder I also meant plasma cutting equip. as you stated.
Regards

 

[itsmoked]

I3city; fab will suffer regular large inrushes switching on his 50kva transformer that has nothing to do with cutting and will, in fact, be much larger. It has to do with when the transformer was turned off in the power cycle and when it is turned back on.

fab; Certainly a switched in/out resistance will do the trick. Might be interesting too to just turn the unit on and off at just the correct point in the power cycle. You'd only need to monitor one phase obviously. I wonder if anyone has tried that..

Otherwise you'd just run an "ON DELAY" relay to cut in a shorting contactor around your resistance 1 second after the ON switch is turned on.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[LionelHutz]

I wonder if a capacitor contactor with the pre-charge block would work in this application. When they close they first connect through resistors or resistor wire then connect the main contacts. I have no idea how well they work but they are available.

 

[fab1961]

thanks to everybody for your prompt replies and advice

 

[l3city]

itsmoked,....thanks for clarifying:
"It has to do with when the transformer was turned off in the power cycle and when it is turned back on"
I kept thinking of just the plasma welder not the whole picture (engineering basic mistake, jajaja!)
Regards