Inrush current

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Volta

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Columbus, Ohio
A customer has an occasional problem. In a MCC, a 5kva, single phase, 480:120 transformer sometimes trips a 2 pole 20 amp din rail mounted breaker.

Secondary load is a resistive (oil) heater, 12 amps. Primary reads 3 amps or so when it holds (usually).

I assume this is an inrush issue. What are suggested ways to overcome this problem?

I see in a good post from templdl in 2007 that the smaller transformers can allow a greater percentage of inrush current. Can a time delay fuse handle this where an (inverse time?) breaker doesn't? I am ok with the manufacturer protecting the x-former with a (primary only) 20 breaker, but it wouldn't meet 450.3(B) if I did it.

Any ideas?
 
090119-1425 EST

Volta:

Your load has little to do with the problem.

Transformers have a random initial inrush current that is a function of where in the excitation curve (hysteresis curve) the excitation was turned off, and where in the excitation waveform the transformer is turned on.

If the transformer residual flux was high when current was turned off, and if the polarity of the voltage when the transformer is turned on is such that it forces the flux higher, then it goes further into saturation causing a large input magnetizing current. This easily might be 10 times normal full load peak current.

A negative temperature coefficient thermistor in series with the input is one method to reduce peak inrush current. If not shunted after a time delay it is a power waste. Another is a series resistor shunted by a time delayed contact. And you can use controlled turn off at a current zero crossing (a Triac or back-to-back SCRs). You might be able to use a series inductor. It is a balancing act --- complexity, cost, what you are trying to protect, etc.

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It could also be an issue with the Mini-Circuit Breaker (MCB) that you selected. For transformers, you must select what is called a "D Trip Curve" breaker. Those are specifically designed to allow the high inrush of transformer windings. If you don't what what trip curve your breaker uses, you can find out by writing down the part number and calling the manufacturer or looking it up in their catalog.
 
Thanks. A few good ideas here. It is all existing within the controller. While I didn't put the stuff together, I did run the circuit to it, and would like to help them out.

Next time I'm there, I'll check out the breaker # to get the spec's.

The building is filled with engineers, more educated than I am for sure, and with the neg thermistor / inductor / shuntable resistor ideas maybe we can come up with something.
 
Wow, always wondered how to automatically syncronize the cardio grammeters.

And it uses only six hydrocopic marzal veins!

Unfortunately, I think so far it has only been shown to be useful in milfer treyons.

Thanks Ramsy!
 
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