Wiring toroidal transformer in parallel

[Muzzer]

Often a good idea to do so. However, you may find that every so often you trip the breaker when you switch the power to a transformer. It's not usually due to the capacitive load so much as the primary saturating. But a nice surge current can also stress the rectifiers. If you are suffering from tripping breakers, read on....

When you disconnect the mains, there is usually some residual magnetisation remaining in the cores ("remanence") at the moment of disconnection. The polarity and magnitude of that remanence varies randomly depending on the conditions when you disconnected the supply but that magnetic field persists while the equipment is powered off. When you come along next time and reconnect the supply, if the conditions are right you can further add to that residual field such that the cores saturate and the primary current is then limited only by the low resistance of the wiring and winding. You can prove to yourself that the secondary circuit isn't the cause by completely removing it and finding that the problem persists. The magnitisation flux within the core of a typical mains transformer is often very close to the saturation flux.

You can buy soft starters for transformers but they tend to be industrial ie f expensive. The simple solution is a fixed resistor or NTC with a timer relay (to short it out after it has had time to get the core flux balanced, eg 1/2 second or so) in series with the primary. If you go the fixed resistor route, you have to get the resistance low enough to generate a useful AC flux in the transformer but high enough to avoid the problem you are trying to address. For my 10kW single phase TIG welder, I found 2.2 Ohms was about right. I added a short delay to the primary contactor using a standard timer (delay) relay.