I like to drive an older, large CNC machine on a smaller power outlet. The machine is made to work by one 32A three phase outlet, but it is servo-driven and only demands a fraction of the maximum current if driven softly or with light loads.

However, the large non-switching power supply will trigger the circuit breakers on a 16A circuit.

So I like to add an soft start device. There are special devices meant for "transformers", but they are hard to get. What is the difference to an ordinary soft-start for motors? Would such a device allow to start the transformer without triggering the breakers too?

  • \$\begingroup\$ Is it servo driven or is there a regular induction motor? If it has a servo, then that should already be controlled by a servo controller, and a soft-start won't help. What would you expect a transformer to do? It sounds like it is the inrush current charging the bridge that is the problem. You might get away with a series power resistor to limit the initial charging current, and then short across it with a contactor or relay after a time delay (typically 3 to 10 seconds). \$\endgroup\$
    – R Drast
    Commented Sep 1, 2015 at 11:59
  • \$\begingroup\$ Alternatively, consider class "C" or even "D" breakers (motor rated) instead of general purpose ones. They should just plug in where the current ones are. \$\endgroup\$
    – user16324
    Commented Sep 1, 2015 at 14:34

1 Answer 1


To clarify what Brian Drummond is talking about with "C" or "D" type circuit breakers - "C" type is for general purpose loads. "D" type is for loads with a high inrush current.

Using a "D" type breaker might allow the circuit breaker to restrain from tripping on inrush curent.

Excerpt from the Clipsal 4-series miniature circuit breaker catalogue:

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With regards to soft starters;

It depends on the type of soft starter.

Some use an inductor or resistor or auto-transformer in series with the load, to reduce the starting voltage and hence the starting current. The inductor/resistor/auto-transformer is bypassed after a few seconds. This type would likely work well for you.

Electronic soft starters use some kind of semiconductor device (usually thyristors) to do pulse-width modulation of the voltage. This is a bit harsh and your machine may not like it.


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