[Amps] Sort Start

[Ian White G/GM3SEK]

Will Matney wrote:
>>If you don't have any startup current limiting at all, the current surge
>>is big, but the filter caps are fully charged within the first 4-5 mains
>>cycles (<100ms) and then it's all over. A step-start shouldn't take much
>>more than twice that long, so a nice brisk 'ker-lick' from the switch
>>and the relay (or two relays) is about right... so that's something like
>>0.5s, or 1s tops.
>>
>>Too long a delay will simply overheat the series resistor, but with no
>>advantage.
>>
>
>That's the way I see it too. One takes a chance of burning one or both 
>out, or overheating the relay contacts. Most use around a 25 watt 
>resistor which is really too small to be in line very long.

If you use short step-start, you can protect the resistor with a 
slow-blow fuse. Normally the step-start will be too quick to blow the 
fuse, but if the relay ever fails to close, the fuse will blow inside a 
few seconds.

>The test that was done showed the largest surge within the first 5-6 
>cycles, each cycle falling off sharpley to the next, after this (at the 
>knee), it started leveling out but took as high as 25-26 cycles that I 
>counted. This was a gradual drop though and not a really high current.
>
You only need to let it get to the knee of the capacitor charging curve, 
so the big current pulses are all done. Then it's time to short the 
resistor. The mistake it to let it continue to sweat.


-- 
73 from Ian G/GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek