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Author Topic: High Volt/ Amp DC Switching hacks.  (Read 1376 times)

glort

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Re: High Volt/ Amp DC Switching hacks.
« Reply #15 on: May 23, 2018, 12:15:56 AM »

I agree with Bruce, a bigger pre heat tank would be better.
You actually don't want the thermostat on the pre tank cutting out because the water is all hot. That's inefficient.  More water even if not as higher temp will have the same energy/ saving imparted to it as less water hotter.
For me, would be better to have 100L @40o than 50L @ the cut out point of 70. 
I would also allow for summer. Your inlet temps will go up ( quite a bit here I noticed) and you will get more concentrated and longer sun radiation.

There is one other thing with using an over sized pre heat tank..... You could do away with all controls completely.
If you know the capacity of the tank is beyond that of the heater and it will never catch up to full temp,  there is no need to have a thermostat and switching at all.
If you go away or stop using the water to the point it may catch up, just switch it off all together. In any case, if it does overheat it will just trip the safety valve and dump some water so not a lot to worry about. Done that a load of times in my oil fired heater.

IF you are using gas, it may be possible to turn that off over summer.  I Imagine a black drum in the sun especially with a reflector behind it and in a transparent fronted and insulated Cabinet would warm significantly.  Put that pre heated water into a tank that was electrically heated and I think you'd be bouncing off the thermostat pretty easily. You'd probably get enough  heat either side of the summer months for a bit to get by on as well.


I'm hoping to get a tank off the neighbour later in the week. He said his element blew in his and it's rusty round the bottom. Thing apparently does not leak  but he wants to install a new one before this one fails.  I asked if he throws it out to throw it over the fence for me which he was more than happy to do.
Pretty sure I saw a service guy in there yesterday but didn't see any new tank in his wagon.  Maybe neighbour just put in a new element.  They certainly aren't expensive.

In any case, I have been thinking about water pre heating only with my gas heater and an oil burner.  Still a couple of weeks away from mid winter but all my solar has just been breaking even. I check my meters several times a day when I'm home.  Morning we are down from electric stove, AC and lack of solar feed.  Water heater has been kicking in mid morning.  Mid afternoon we catch up and when I do my 4pm reads we may be slightly ahead depending on how cold it was the night before and how long Daughter decided to give herself the wrinkled prune treatment.  That said I haven't been exactly quick to jump in and out lately either.  I have the water pretty hot and it makes my aches and pains fell better.
We have had pretty much perfect weather so its been a best case scenario so far.  When the crappy weather hits as it has to do sooner or later, I won't be making the power I need and want.  If I used the gas tank with the oil burner, I'd probably free up another 5 + KWH day on the hot water tank.
Tank I have is only 1/4 size the main one and I do keep a casual eye out for a full size tank.

I'm thinking of just batching the heating every day with a passive updraft burner.  Work out how much fuel it's going to take to raise the water temp where I want it, add the required amount of fuel and let it go.  People think these things have to be drip fed but that is incorrect. The passive pot design works fine in batch mode. I put over 4L in my old one very regularly and it burnt fine from start to finish but it seems to have been lost/ chucked out in the move.
Went to the Charity warehouse shop yesterday and they had every type of pot, pan, tray and cooking vessel imaginable BAR the one I wanted.
Just a big stainless or enameled  Pot with a lid that wasn't glass.

I could do a dedicated Solar fed pre heat as I have more panels but maybe the novelty is wearing off. I know I can do it now, rather than put more panels up just for a water heater I may as well just incorporate them in to the rest of the arrays.
That said, will be OK in winter but in summer I'm starting to get to where I have to be careful.  I'm getting up to the limit on not so much my wiring but the power co's coming into the house.  Likely in summer I'd have to switch a good amount off so maybe dedicated Hot water might be a better idea after all?


veggie

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Re: High Volt/ Amp DC Switching hacks.
« Reply #16 on: March 07, 2019, 11:14:38 PM »
Reviving an old thread here but I came across this idea for switching DC power to water tank heating elements.
For applications where you have 12 or 24 volt panels feeding a DC element in a hot water tank, perhaps these 100 Amp continuous duty DC solenoids would work.

The low amperage trigger circuit of the solenoid could be wired in series with a N/C temperature snap switch which disconnects the power when the tank reaches it's set point.
The heavy load terminals carry the high amperage solar panel current directly to the heating element such as this one..
https://www.ebay.ca/itm/DC-Water-Heater-Element-24-Volt-600-Watt-for-Wind-Generator-Turbine-Solar-Energy/273180576610?hash=item3f9ad4cb62:g:y-gAAOSwAOdbLWF1

If you search ebay with this description you will find many sellers of these DC rated solenoids:
Solenoid Relay Switch Continuous Duty fit for Golf Carts 12V SAS-4201 SAS-4202

- 6/1 GM90 Listeroid - Delco 33si Alternator
- Changfa R175 - Lease/Neville Alternator
- Kubota Z482 - 4kw
- JiangDong R165 Air cooled - 2 kw

mike90045

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Re: High Volt/ Amp DC Switching hacks.
« Reply #17 on: March 08, 2019, 02:01:57 AM »
.... 100 Amp continuous duty DC solenoids would work.

The low amperage trigger circuit of the solenoid could be wired in series with a N/C temperature snap switch which disconnects the power when the tank reaches it's set point..........

Make sure the trigger coil of the solenoid itself, wont fry the themal snap switch, those solenoids can draw several amps in their electromagnet.

BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #18 on: March 08, 2019, 02:31:03 AM »
 Better test that 12V auto starter relay very carefully; gods only knows how well it will work on 300VDC or whatever your PV string voltage is. For higher voltage DC rated relays different contact metals are used.  It will fail on opening, quite dramatically with smoke and fire, when if fails.  The arc on opening will just keep going someday.

A solid state relay seems a lot easier and more durable. What I do on my analog PV regulator is tap the panel above 0V, and down regulate that 24V to 12V. The 12V can be used for high voltage mosfet gate drive, with the mosfet switching the low side or 0V leg to the load.  The current needed at 24V is tiny, so any small wire will do.  You can use a gate driver IC for control, or an opto-isolated gate driver IC if you want. 









glort

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Re: High Volt/ Amp DC Switching hacks.
« Reply #19 on: March 08, 2019, 10:46:06 AM »

For applications where you have 12 or 24 volt panels feeding a DC element in a hot water tank, perhaps these 100 Amp continuous duty DC solenoids would work.

Yep, for 12/24v I have no doubt they would.

The higher voltage is where the fun begins !   :laugh:

I had a problem with an array a month or so back. I switched my normal AC fuse breakers under load at least a dozen times. They are paired and break both sides of the DC together.  No arcing or problems at all.  I actually thought I may have fried the contacts as causing the problem so I changed them out to new ones, same thing so I put the old ones back.

Switching both sides at once seems to greatly lessen the arcing problem.
If one used TWIN  solenoids it might be very workable.

I have also done more playing around with the  Divert to a dummy load idea and that seems to be sound as well.  it's like the current doesn't ever want to stop flowing but if you send it somewhere else, it's happy and won't burn your relays to a  Crisp.

I put this into practice a few weeks ago with another small array I put up on the back of the house roof.  Due to the array being well under 10A, the 40A cable I used on teh DC side had a bit of latitude in it's current carrying capacity.  I ran it into 2 20A DPDT relays  Paired to give a carrent capacity of 40 A each side.  On the non energised side the things dead short  with a small piece of 6MM cable that was a real bugger to get in the terminals but I managed.

The Upside is that when panels are shorted they efficency curves plummets and they actually put out a fraction of their rating. I measured Current and voltage when shorted and it was way lower( forget the numbers now, just remember thinking I went way beyond what I needed to)  than the capacity of the cable and relays.  Admittedly I might have only Cycled the switching 15 Times or so bit this is a circuit that could very easily never be switched for a year or more. In reality I think it could be switched for ever with no problem to the relays or the wiring.








BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #20 on: March 08, 2019, 05:31:41 PM »
Dual contacts will double the voltage capacity, as in effect it doubles the arc length.  The Midnight Solar breakers for 300V are just two of there 150V in series with a common trip handle.  They are quite affordable.

There are more higher voltage DC rated switches, breakers, and relays on the market every year because of EVs and PV setups.  I think it's a false economy and unnecessary fire risk to use products not rated for your DC application.  20 years ago it was much harder and crude testing of non rated products was sometimes required.  Not so today, thankfully. 



veggie

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Re: High Volt/ Amp DC Switching hacks.
« Reply #21 on: March 08, 2019, 07:15:14 PM »
Better test that 12V auto starter relay very carefully; gods only knows how well it will work on 300VDC or whatever your PV string voltage is.

My Solar array is 24-36 volts depending on load, and my heating element is a 600 watt, 24 Volt unit with the impedance matched to the array output sweetspot.
Max amps is around 27. I don't see any fire risk with the solenoid being this underloaded. And it is a dedicated DC switch.
The application would be different for everyone but for switching of <100 amps at 12 to 24 volts these units have proven themselves for decades.

They are standard equipment on wind turbine and solar array Load Diversion modules.
« Last Edit: March 08, 2019, 07:25:10 PM by veggie »
- 6/1 GM90 Listeroid - Delco 33si Alternator
- Changfa R175 - Lease/Neville Alternator
- Kubota Z482 - 4kw
- JiangDong R165 Air cooled - 2 kw

BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #22 on: March 08, 2019, 09:31:07 PM »
No issues at all for low voltage DC systems using low voltage DC contactors.  They were designed for it, and raising voltage while decreasing amps should be fine. Both voltage an current affect DC arcing so if you are in the voltage range the device was designed for, at lower current, all is well.

It is a different matter for higher voltage DC at moderate currents, the arc length can be quite long, making current interruption a challenge. The relay products designed for it often use permanent magnets to bend the arc to a longer path, and multiple contacts to increase total arc length so the arc on opening will extinguish. 

I've blown lots of switches, breakers and contactors messing with my nominal 120VDC (up to 156V). My home and shop are 120VDC powered, with the shop also having generator or inverter AC.    I used Cooper brand switches that were rated for 150VDC in my house, but those are no longer available.  The Leviton rotary light switches date back to Edison's (110VDC) time in design, and I've tested them for hundreds of cycles switching 4 amps, and only use them for 1 amp.