so I have a transformer (3kva 120vac -> 240vac) that runs great but blows the breaker occasionally on startup. So I'd like to add a soft start. This is an industrial control environment so I could build a circuit myself, eventually, but an off the shelf component would be better.
The only soft starts I see online are specifically for motors. Would a motor soft start work here, or am I going to have to build one myself?
Here's the problem - excessive inrush current is when the transformer is activated close to the AC cycle passing thru zero. Consider the steady-state case: -
Current lags at about 90 degrees and if you applied the supply voltage close to when the voltage peaks, current will naturally not have the problematic inrush state: -
When powering the transformer with a supply voltage passing thru zero a much larger flux density occurs and this saturates the core causing (sometimes) a massive over-current.
If you can find a motor protection device that explicitly states it performs the above function then it should be OK.
I'm really not sure how to make this happen for your application, but large power transformers utilize a technique called harmonic suppression.
It's not what it sounds like exactly - the basic idea is to use relaying to detect the presence of very large 5th and 7th harmonic currents, coincidental with closing the input breaker. There's also a timer so that if the harmonics overstay their welcome, the protection system stops ignoring them and opens the breaker.
Another method commonly used with industrial circuit breakers is to simply employ a bypass timer. Shortly following a closing operation, it simply blocks any calls for trip. If the timer runs out and the transformer is still overloaded, the protection circuitry doesn't have to start it's timing over - it can trip immediately. Note that this method requires more consideration about exactly how overloaded the transformer could be.
You need to do a short circuit study before you arbitrarily suppress protection, but these methods will probably give you what you want better than a motor controller.
