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Simple Inrush Current Limiting


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  • MVP 2023

Hi all,

I thought I'd share a little trick I've had for many years...in case I get hit by a bus.  Hate to see it wasted.

Much of my stuff is machines that plug into normal General Purpose 10A outlets (Aus = 240V).  As such, years ago I had a lot of trouble with DC motor drives pulling way too much current when they were powered up, and they would routinely trip the breaker on that circuit.  A normal solution would be a different curve breaker, but in these locations this was not possible.  Most Inrush limiting devices are designed to be on the load's PCB, but back then there was little around for me to use anyway.  I had to come up with a bullet-proof but simple way of doing it.  As usual the brain got the washing machine going and out came this idea at the end of the cycle.

I ended up with 2 variations of the theme on the attached pdf.

1).  The pdf version uses a relay and normal light globe.  The globe reduces the inrush to minimal levels because initially it is running at it's "normal" conditions.  But there is enough voltage still getting through to the load that it charges up it's capacitors with hardly any higher current at all and the globe's brilliance decreases as this happens. It's essentially working as a simple resistor.  Once voltage at the load reaches the relay's trip threshold, the relay turns on and supplies full power until the main source is switched off.  You vary the globe's wattage to find the ideal compromise time, in most instances I work on 0.5 to 1 second.  You will be surprised how well it works if you do an experimental lashup.

2).  In many cases I have multiple drives to power and I use the PLC to control the initial powerup.  They all go through a master safety contactor when running directly, with a further individual relay/contactor downstream for each.  In this case I initially run them sequentially through a link to a globe in the same manner.  ie the globe "neutral" is on the downstream side of the master contactor but before the individual switching.  The PLC runs them all through a sequential power up via the globe and then closes the main contactor once all the routines finish. Thus I have a single globe that glows brightly, dims, next drive turned on and globe brightens again, dims etc etc with all drives eventually being powered direct.  Once it is all finished, I then do the motor control necessary.

I know that most of the time this is not necessary.  But I still encounter moments when this method has proven itself time and time again.  The beauty is the inherent simplicity and also the ease of replacing the restrictive mechanism...the globe.  Not that I have had to do it. I first implemented it over 20 years ago and on ALL devices it is still working on the same drives....and the same globe!  

Globes must, of course, be nasty "environmentally unfriendly" old fashioned filament types.  Not modern, "full of mercury and electronics" flourescents, or "specific frequency hard on the eyes" LEDS, which are all sooooooo much better for the environment...ha!  But that's another discussion along the lines of a Prius vs a Hummer!





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