Re: [Emc-users] How come.... --> battery safety due to high current, DC relay

2019-05-20 Thread Erik Christiansen 
On 20.05.19 12:11, Chris Albertson wrote:
> How much DC current are we talking about?Let's say the house is running
> an air conditioning load and you are doing laundry and the 'fridge is
> cycling and you have a few computers on and every light build is turned on
> and you own a 200-gallon saltwater aquarium.Let's call it 20 KW of use
> in a really big house. This a very-worst case scenario.
> 
> Your DC is set up for 480 volts going into the inverter.   20KVA / 480 is
> is 41 amps.

Even there, the fault current is the short-circuit current of the array.
That's not too bad for PV, as it's internally strongly current limited.
But upthread we were mostly talking about DC from the battery, i.e. at
48v in most cases. As I've mentioned upthread, you can expect a fault
current of 10 times AH rating, i.e. 2000 A for a 200 AH battery bank.
That's the strength of the arc we must be able to rupture in the event
of a fault.

Switching normal loads is back to less than 100 A for less than 5 kW, but
still hard on contacts. (The contacts on the starter solenoid for an old
Caterpillar D6 are blocks of copper with a mm of silver on the surface,
to take the several hundred amps at 24v. IIUC, silver oxide conducts.)

> It is not hard to find a good 60 or 80 amp switch.  You can
> buy one on Amazon.com for $120.  Switches that can handle high DC loads are
> very common.   Here is one examle
> amazon.com/Siemens-HNF362-600-volt-...
> 
> These DC disconnects are a code requirement and are installed by the
> million, literally.

All the switches I've come across are stamped with their AC and DC
current breaking capacity. That must cover the potential fault current,
if there's no HRC fuse, or you're building a fire starter. (I.e. you
still need the fuse.)

> No competent engineer will design a high power low voltage system because
> you can no longer us wire, you'd n=be into solid copper busses.

The hundred amps only flows from battery to inverter - and yes, the
cables are about half an inch in diameter, much like starter motor
cables in a car.

I'll be running low voltage DC for LED lighting, but that's at modest
current.

Erik


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Re: [Emc-users] Atomic pi

2019-05-20 Thread Robert Murphy 

Just a quick update.

Installed debian stretch (r13?), all went smooth, installed a backported
kernel (4.19) and ATM am testing various boot options to get latency to
a better value (may even recompile kernel after more research).

So far it looks like it may work with a MESA ethernet card.

Unfortunately it appears the board may have been bought via auction as
excess stock. Another thing the DL website appears to be down when
trying to access OS images & docs. (From Oz anyways)

The Atomic Pi may have been the basis for a robotics project (these
tidbits I got from the Atomic Pi reddit page).


On 20/5/19 10:15 am, Robert Murphy wrote:

My Atomic should be here today.

Ethernet is on the PCIe bus (if that hasn't been mentioned)

The BBB is a great platform for a compact solution but AFAIK machinekit
is the only option for PRU support. I've been running my setup for about
a year now.



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Re: [Emc-users] Semaphores, from real time --> background process

2019-05-20 Thread Nicklas Karlsson 
> > I need a semaphore or similar to signal from periodic task initialization
> > is needed. The concept of semaphores are weel known for me but in Linuxcnc
> > I found several implements, ordinary posix, rt_sem, rcs_sem_t. Any idea
> > which "implementation" is best to use?
> >
> 
> I have no idea, but I did find this (very old) example:
> https://github.com/LinuxCNC/linuxcnc/tree/master/src/rtapi/examples/semaphore

I decided standard posix sem_ini(...) sem_wait(...) sem_getvalue(...) 
sem_post(...) was a good idea ant it works great. As is now it is only used to 
put device in operational mode but it should also be able to handle non real 
time communication then there is a need or good idea to insert into the message 
sent periodically. Do not know how the rtapi_ semaphore is implemented but 
maybe tommorow I get an hour to look at it.


Nicklas Karlsson


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Re: [Emc-users] How come.... --> battery safety due to high current, DC relay

2019-05-20 Thread Chris Albertson 
How much DC current are we talking about?Let's say the house is running
an air conditioning load and you are doing laundry and the 'fridge is
cycling and you have a few computers on and every light build is turned on
and you own a 200-gallon saltwater aquarium.Let's call it 20 KW of use
in a really big house. This a very-worst case scenario.

Your DC is set up for 480 volts going into the inverter.   20KVA / 480 is
is 41 amps.  It is not hard to find a good 60 or 80 amp switch.  You can
buy one on Amazon.com for $120.  Switches that can handle high DC loads are
very common.   Here is one examle
amazon.com/Siemens-HNF362-600-volt-...

These DC disconnects are a code requirement and are installed by the
million, literally.

No competent engineer will design a high power low voltage system because
you can no longer us wire, you'd n=be into solid copper busses.

On Mon, May 20, 2019 at 3:33 AM Erik Christiansen 
wrote:

> On 19.05.19 09:43, Nicklas Karlsson wrote:
> > For safety I learned relays fail open circuit while MOSFETs fail short
> > circuited, I also experienced MOSFET short circuit failure. Happen to
> > know anything more about this?
>
> I'd only use the MOSFETs to replace relay contacts, and wire a HRC fuse
> in series to rupture the fault current. Otherwise, once a DC arc is
> established, it could bridge a MOSFET package effortlessly, maintaining
> current flow until enough copper flowed away to build a large gap.
>
> Erik
>
>
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-- 

Chris Albertson
Redondo Beach, California

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Re: [Emc-users] How come....

2019-05-20 Thread Ralph Stirling 


> Wikipedia suggests that you still use BTUs for gas?

Unfortunately, yes.  Fortunately, I don't have gas, so
can ignore that.  Guess you can say we are more British
than the Brits, since we still use British Thermal Units.

-- Ralph

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Re: [Emc-users] How come.... --> battery safety due to high current, DC relay

2019-05-20 Thread Erik Christiansen 
On 19.05.19 09:43, Nicklas Karlsson wrote:
> For safety I learned relays fail open circuit while MOSFETs fail short
> circuited, I also experienced MOSFET short circuit failure. Happen to
> know anything more about this?

I'd only use the MOSFETs to replace relay contacts, and wire a HRC fuse
in series to rupture the fault current. Otherwise, once a DC arc is
established, it could bridge a MOSFET package effortlessly, maintaining
current flow until enough copper flowed away to build a large gap.

Erik


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Re: [Emc-users] How come....

2019-05-20 Thread Erik Christiansen 
On 20.05.19 02:02, Chris Albertson wrote:
> Total KWH meant if you have a  10 KHW battery and you chrage and discharge
> it 200 times then you have 2000 "lifetime" KHW
> If you can charge the battery 400 times then you have 4000 "lifetime KWH.
> The second battery as twice as much value.

As a first approximation, yes, but the value is also affected by
loss of utility through degradation of capacity over cycle life.
Looking at:

"In comparison, the Tesla Powerwall is a 6.4 kilowatt-hour battery and
only 5.44 kilowatt-hours of that is actually warranted to be usable.
But after the first 2 years that drops to 4.6 kilowatt-hours.  And 3
years later it drops down to only 3.8 kilowatt-hours of warranted
storage."

from:
https://www.solarquotes.com.au/blog/redflows-zinc-bromide-zcell-battery-may-have-the-edge-over-lithium-ion/

I'm reminded that phone and laptop battery capacity doesn't take long to
nosedive. That's a much bigger problem with house batteries.

One outlier is the ZnBr flow battery - it is claimed to maintain
capacity for its full life, with efficiency falling instead. As that can
be compensated by adding cheap solar panels, it's a very interesting
variation in the cost/energy equation.

OK, reasonable efficiency is still necessary, especially in polar climes
in winter, but in generally sunny Australia, it's just the high initial
cost which detracts.

Erik

P.S. That web page is inordinately folksy, but if you skip the waffle,
 it does cover some of the battery's properties.


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Re: [Emc-users] How come....

2019-05-20 Thread andy pugh 
On Mon, 20 May 2019 at 10:04, Chris Albertson 
wrote:

What you just wrote almost what I wrote.  Except that " big enough" is not
> a number you can put in a calculator.
>
> Cost per "KWh over the lifetime" is a very standard measure to define the
> value of a battery. I did not make it up.


My point is that if you consider only cost/kWh then you could end up
spending many billions on a pumped storage scheme...
https://en.wikipedia.org/wiki/Dinorwig_Power_Station
I am exaggerating, obviously, but my point is that cost/kWh tends to
decrease with increased size, so first you need a system suitable for your
needs, only then does cost/kWh become the right metric.

-- 
atp
"A motorcycle is a bicycle with a pandemonium attachment and is designed
for the especial use of mechanical geniuses, daredevils and lunatics."
— George Fitch, Atlanta Constitution Newspaper, 1916

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Re: [Emc-users] How come....

2019-05-20 Thread Chris Albertson 
On Sun, May 19, 2019 at 3:09 PM andy pugh  wrote:

> On Sun, 19 May 2019 at 22:30, Chris Albertson 
> wrote:
>
> > You just have to do a calculation,  What is the cost per KWH over the
> > remaining
> > life of the battery.
>
>
> I am not sure that that is the correct calculation.
> What really matters is that the battery is big enough for your purposes,
> and then cost-per-year is more important that cost / kWh.
>

What you just wrote almost what I wrote.  Except that " big enough" is not
a number you can put in a calculator.

Cost per "KWh over the lifetime" is a very standard measure to define the
value of a battery. I did not make it up. The figure combines three
things
1) The size of the battery bank
2) The cost of the battery bank and
3) how long it will last, measured in "cycles"
Those three things are what you said, but only with numbers attached

Total KWH meant if you have a  10 KHW battery and you chrage and discharge
it 200 times then you have 2000 "lifetime" KHW
If you can charge the battery 400 times then you have 4000 "lifetime KWH.
The second battery as twice as much value.

Again I'll say what matters is *lifetime* KWH divided by the cost.

"lifetime KWH is the total amount of energy you can get out of a battery
before you have to retire the battery.

Your formula of "big enough then cost per year" does not result in a number
where you and I can compare performance because "big enough" can vary.

Look at the Power Wall and try to apply your formula.  "big enough" means
two units.  Cost for the first year is  $13,700.  Cost for the next 10
years is zero.  Is that a good price?   How would you compare that to a
personwith a smaller house But if you divide by the amount of power
used in ten years and it scales to a more universal number that just
happens to be the same units as the power company uses on their bills.
Makes comparing easy

(And, while we are at it, kWh is a horrible unit. What is wrong with MJ? )
>

The power company does not bill in MJ.   The meter reads KWH and all the
batteries and solar pannels are KWH.  It's an indury standard.

>
> --

Chris Albertson
Redondo Beach, California

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Re: [Emc-users] How come....

2019-05-20 Thread andy pugh 
On Mon, 20 May 2019 at 02:50, Ralph Stirling 
wrote:

>
> > (And, while we are at it, kWh is a horrible unit. What is wrong with MJ?
> )
>
> kWh is ubiquitous in the US.


They use them in the UK too. It doesn't make it any less irritating to a
physicist.

Wikipedia suggests that you still use BTUs for gas?

-- 
atp
"A motorcycle is a bicycle with a pandemonium attachment and is designed
for the especial use of mechanical geniuses, daredevils and lunatics."
— George Fitch, Atlanta Constitution Newspaper, 1916

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