Question Regarding Soft-Start

[metalsculptor]

so how does this work?? is it that when the coil is sucking huge power at startup, the relay does not activate, sending the current through a resistor, then when everything settles down the relay gets current, activates and removes the resistor from the circuit. ?

Pretty much, the voltage across the filter capacitors gets reflected on all the other transformer windings so until these charge there is not enough voltage to pull the relay in and short out the series resistor. Try it yourself, short out the secondary of a transformer with a mains rated lamp in series with the primary, There will be very little voltage on any winding until the short is removed.

 

[ernsttt]

it is such a simple circuit.....really nice

 

[agdr]

I'm new to the forum - joined a couple of weeks ago after buying new Shure headphones and quickly realizing that I needed a headphone amp.

This thread caught my attention. A few years ago I was working with a company that sold lighting power regulators in the range of 30kW to 90kW that would power lighting in multi story parking garages, car dealer lot lighting, street lighting, etc. Average unit was about 70kW. These were three phase 277 and 480 volt units with three large toroids in each, several contactors, and some control logic.

We were getting random breaker trips for awhile. Not just the breaker in the unit, often the breaker in the feed (circuit breaker) panel. The solution turned out to be inrush current limiting. The designer added the resistor with contactor short, as described in this thread, and the problem went away.

I'm really enjoying reading the forum! A mountain of useful information on the site.