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re: cheap choke design
10 mar 2002
n. thornton  wrote:

>> >> >nick, this doesn't add up. look at the figures around 90% lower
>> >> >price elec versus 100% with a relay. the relay is more frugal
>> >> >because it is mroe eficient...
>> >> 
>> >> the on-peak rate is about 15 cents/kwh, vs 5 cents off-peak.
>> 
>> consuming 500 kwh/month costs $900/year on-peak, or $300 off-peak.
>> the breakeven cost of equipment to save $600/year might be $6k at
>> a 10% interest rate, but cheaper equipment saves more money...
>
>your design will not save more money, it will save less. you're an
>intelligent chap, so i don't get why you havent done the figures...

i'm not sure which figures you have in mind.

>the key point is that the relay will use 100% lower rate leccy, whereas
>your device wont. the rlay saves more.

relays could supply the load with 10% more off-peak power, but they would
also cost more to begin with and consume more coil energy. perhaps you are
asking "what's the breakeven ongoing cost of 2 relays vs 2 chokes, assuming
a) a 10% interest rate, b) a continuous 25 w coil power, c) a 90% choke
diversion, and d) electric rates and consumption as above?

with chokes, 90% of the house energy comes from the off-peak source when
it is available, ie 0.9x22h/24hx500kwh = 412.5 kwh/month becomes off- vs
on-peak, saving 412.5(0.15-0.05) = $41.25/month or $495/year. this is
reduced by the 10% yearly interest on the $20 cost of 2 chokes, counting
my time at $0.00/h (learning is expensive), so the net savings is $493/year. 

with relays, 100% of the house energy comes from the off-peak source when
it is available, ie 1.0x22h/24hx500kwh = 458.3 kwh/month becomes off- vs
on-peak, saving 458.3(0.15-0.05) = $45.83/month or $550/year. this is
reduced by the 10% yearly interest on the r $ cost of 2 relays, counting
my time at $0.00/h, so the net savings is 550-0.1r $/year. ignoring the
relay power, these savings are equal when 550-0.1r = 493, ie r = 570, ie
$285 each for 2 two-pole single throw relays... 

if one relay is always on, consuming 25 watts, the relay savings are
further reduced by 25w(0.15x22h+0.05x2h)/1000 = 8.5 cents per day or
$31/year, which makes 519-0.1r = 493, so r = 260, ie $130 each. 

all other things being equal, i'd favor the simpler and more reliable
solution, and the one that uses less energy. the choke solution also
allows more capacity, with 2 sources essentially in parallel, and it
allows more flexibility in tariff interpretation.

>>...where can i buy 2 100 amp dpst relays for "20 notes"? (ious?)
>
>unfortunately i'm not in that field, so can't help with
>suppliers. i would suggest more searching.

please let me know if you run across any relays like this that are
currently being manufactured and cost $20 vs $172.15 each. (i used 
to own a handmade electric car with lots of surplus aircraft relays
with silver contacts worth a lot more than their market price...)

>> >why do you think all the lec enges are saying lose the plastic wire?

almost all. i'm a lec eng too.

>> tradition, lack of imagination, and a need to save space vs money.

some also want higher performance, eg higher qs.

>no, its cos plastic coils are a fire and reliability risk. especially
>on solid cores.

everything is risky.

nick




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