Why does this breaker pop?

Yeah (knowing nothing of AC power stuff) I would guess some sort of inrush current. (as you suggest). I usually think about inrush current charging up some capacitor (w/ DC voltage) perhaps the same type of thing can happen with a transformer. And if you switch it on during the wrong part of the phase, it pops.

George H.

That's a bit of a FAQ on sci.electronics.repair.

If the transformer is turned _off_ near the zero crossing, the core has some remanent magnetization. When you turn it back on, there's a 50:50 chance that the first half cycle is in the same direction. The magnetized core saturates more easily in that direction, so you get a huge current spike when that happens.

How often the breaker blows depends on things like the line voltage and the safety factor built into the transformer--which is usually not a huge number.

People use various soft-start methods, e.g. a resistor and a time delay relay, or an NTC thermistor.

Cheers

Phil Hobbs

Inneresting. Who woulda thunk it?

Do you know if there are commercial starters that'll do this (like from Grangers), or is it something you have to roll your own?

Here in the UK the std/usual breaker is a Type B. There are Type C & then Type D that have time/current curves double of the Type B with the same current breaking capacity. These allow the inrush current to drop before tripping. Motors would usually be on Type C. Of course there are other factors to consider to remain in spec of the circuit.

I presume the USA has something similar.

HTH Mitch

They have about 200 choices. Found that after about 4.4 seconds of looking.

You are using the wrong type of breaker..

Transformers can create back EMF at the worst time when applying power, depending where the phase angle is at the time of contact.

Get youreself a motor starter relay that comes with overloads, they are normally designed to stand off up to 8x the holding current when an event like this happens. Drive the tansformer with it or use a fused disconnect with FRS type fises, assuming that range fits your needs.

Jamie

Thanks eveybody for the replies. I have been away from the internet for a few days so I couldn't reply sooner. I'll get myself a motor starter. How should it be rated? It is a three phase transformer rated at 15KVA. Do I just look for a motor starter rated for 15KVA? Thanks, Eric

Yes, a 35 amp version would most likely work for you just fine..

Motor starters come after fuses or breakers. Unless that is changed first , the starter will not do any good.

It would be obvious there is a higher amp protection somewhere before the starter and the starter comes with overload protection that is interlocked to the coil.

Jamie