Jump to content

Excess solar to heat water


viper_za

Recommended Posts

5 minutes ago, The Terrible Triplett said:

1) BMV's relay can handle 60v, so it is designed to operate on 12/24/48v systems. If it was connected to a 220v AC current the BMV's relay will fry on the spot. Right?

Let's go back to basics. When you open a switch, the action is not instantaneous. The switch action represents a very rapid increase in impedance, but in slow motion you have an air gap that widens from zero to however far the particular switch has its contacts apart.

Now, a small air gap does not have a zero resistance. If the voltage is high enough, it will jump across. In addition, when current is jumping across it ionizes the air and ionized air has a lower resistance.

So back to your switch in slow motion. As it starts to turn off, it creates a small air gap. The current continues to flow because the resistance is too small. In addition, the air gap is now ionized, lowering the resistance further. Next high-speed frame. Now the contacts are even further apart, but the ionized air plus the extra air gap still has too low an impedance, so more air is ionized and the current continues to flow. While all this is happening, things get hot, copper contacts are damaged, and if you have not extinguished the spark by the time the switch reaches its widest position... well... those videos about burning fuses, it looks a bit like that.

This does depend somewhat on the current going through there. Breaking the arc depends on the total impedance that is introduced, so driving a small load (such as a relay coil) means a much smaller arc to start with. The problem is if something goes wrong with that load.

AC, as mentioned, has less of an arcing problem. The ionisation of air is somewhat hindered by passing through zero 100 times a second. DC on the other hand... that becomes an involuntary welding machine rather easily.

So my gut feeling is that you're fine with 250VAC on there, because I seem to recall from somewhere that the ratio is about 1:10 for AC/DC arcing, but I may be wrong. I would just, to be safe, fuse it to 1 ampere. If anything goes wrong with your external load (the other relay), then your internal relay doesn't have to break 250V at a large current, that is, it would protect you from having to break a low-impedance situation.

I'm not an electrical engineer though :-)

Link to comment
Share on other sites

9 minutes ago, The Terrible Triplett said:

So help me out here for a lot has been said so far, way above my simpleton head:
1) BMV's relay can handle 60v, so it is designed to operate on 12/24/48v systems. If it was connected to a 220v AC current the BMV's relay will fry on the spot. Right?
2) The BMV's relay can handle max 1amp yet the Finder's relay is rated at 10.5mA. Is the Founders relay not at risk being milli amps?

1) No, it will not fry on the spot. The relay contacts will just not last as long as they would at the rated voltage and current. Higher voltages might also cause arcs between the relay contacts and / or the pads, tracks and components on the pc board. Do you remember what I've said about the gap between the relay contacts and arcing and fuses also having a voltage rating at which the arc can safely be distinguished.

2) The BMV's relay can switch up to 1A, the Finder only draws 10.5mA (not rated at 10.5mA, draws 10.5mA when operating) - the Finder draws much less than what the relay contact can handle in terms of current. (Forget about the voltage for a moment) If it was the other way around and the current drawn / switched was more than what the relay contact could handle it would fail sooner or later depending on the amount of over-current.

Link to comment
Share on other sites

4 minutes ago, superdiy said:

1) No, it will not fry on the spot. The relay contacts will just not last as long as they would at the rated voltage and current. Higher voltages might also cause arcs between the relay contacts and / or the pads, tracks and components on the pc board. Do you remember what I've said about the gap between the relay contacts and arcing and fuses also having a voltage rating at which the arc can safely be distinguished.

Yup, as I said in my post. While the arc is going (before the gap extinguishes it), the copper (or whatever the contact face is dipped in) is actually damaged a little bit. It makes little pit marks with every switch iteration. That's what the whole switching life is about. Run it outside the specs, it might be perfectly safe, but the arc is going to take a couple microseconds longer to extinguish on each switch and it will reduce the life of the relay. Which might not be a biggie... I mean it might bring it down from 10 million to 1 million, and switching twice a day, that's 500 000 days or a thousand years... :-)

Link to comment
Share on other sites

My kop is nou seer. Ek gaan nou huistoe. :D

Plonkster, good write up how relays work. Yes, the little I know about this thing called relays (is there a word in Pedi for relays?) are the pitting and what happens if you operate them outside design specs. I also know the cheaper the relay is ... 

Nor am I going to test the theory of connecting a BMV's relay direct to a 220v AC circuit. :P
I may be safe for 500 000 days - so that may, just may, outlast the BMV unit.
The Finder draws much less than what the BMV relay contact can handle in terms of current, so it is fine also then.

End result: Not going to be replacing the Finders relays based on our chat here.
I will if an electrical engineer or Victron says it is wise to do so, after having provided me with an approved alternative relay, to avert any further discussion. :D

 

This was fun though. I like it even more that none of us got heated as can happen so easily on these forums. Bells all around.

Link to comment
Share on other sites

Hi TTT

I hope to explain this in everyday terms

Think of the coil of your Finder relay as the load (equivalent to the missus's hairdryer)  and the BMV as the conductor (wiring in your house). The Finder relay draws 10.5 mA so this is well within spec for the BMV which can handle 1A. The problem is you are supplying 220 V AC through a conductor that is rated @ 60V probably DC. What is the hassle with higher voltages? The insulation in conductors  may not be rated for the higher voltage. In the BMV I don't think there is any hassle with components popping as it is just in and out and you have demonstrated that it works. So is it really a problem? I know it is an extreme case but with HT transformers the monkeypality chaps have about a 4m insulated pole to switch the transformers off and on since the 22kV current can jump a 1 m gap as some cable thieves have found out. Inside the BMV there is a risk of  a surge on your  220V supply  on it way to the finder relay could make a detour through your BMV circuitry and Victron would be quick to point out the 60V limit. The current your relay draws is minimal its the voltage that has us a little concerned. Show the solar engineers who designed the system page 36 in the manual and get back to us. It may be that we are just being old women but I am sure you would be hesitant to wire up a stove to 380 V when it is rated for 240V. (Don't laugh I've seen it done - farmer common sense is in short supply at times. A lightning strike burnt off the wires at a supply pole and one of the phases and neutral was swapped in the just before sunset repair.) Supper was beans on toast which I declined and carried on with the beers.

 

 

Link to comment
Share on other sites

16 minutes ago, viper_za said:

But will it last

I cannot guarantee that it will last. I've used the relay from Mantech (http://www.mantech.co.za/ProductInfo.aspx?Item=72M0796) on my battery bank and it was energized 24/7 and worked fine for a few months until I scrapped the project. Yours are only going to be energized a few hours per day. If the ambient temperature is not extremely high the relay should be fine - they can handle quite a bit of heat.  What will the battery voltage be at the time the relay is energized? Fully charged at mid day?

You can always add a few 1N4007 diodes in the circuit to drop the voltage by about 0.7V - 0.8V per diode to get closer to 48V on the coil. That is a cheap way of solving the issue -  18c / diode. (http://www.mantech.co.za/ProductInfo.aspx?Item=64M0027)

OK, stand corrected, R1 each at RS: http://za.rs-online.com/web/p/rectifier-schottky-diodes/6289546/

 

Link to comment
Share on other sites

1 minute ago, superdiy said:

I cannot guarantee that it will last. I've used the relay from Mantech (http://www.mantech.co.za/ProductInfo.aspx?Item=72M0796) on my battery bank and it was energized 24/7 and worked fine for a few months until I scrapped the project. Yours are only going to be energized a few hours per day. If the ambient temperature is not extremely high the relay should be fine - they can handle quite a bit of heat.  What will the battery voltage be at the time the relay is energized? Fully charged at mid day?

You can always add a few 1N4007 diodes in the circuit to drop the voltage by about 0.7V - 0.8V per diode to get closer to 48V on the coil. That is a cheap way of solving the issue -  18c / diode. (http://www.mantech.co.za/ProductInfo.aspx?Item=64M0027)

OK, stand corrected, R1 each at RS: http://za.rs-online.com/web/p/rectifier-schottky-diodes/6289546/

 

The relay will energize starting from +- 52.8 volts all the way up to 56.4 for until absorb phase drops down to the float volts of 52.8 (that's the plan anyway)

Link to comment
Share on other sites

56 minutes ago, viper_za said:

The relay will energize starting from +- 52.8 volts all the way up to 56.4 for until absorb phase drops down to the float volts of 52.8 (that's the plan anyway)

Add diodes in series with the relay coil - 10 should do the trick - 10 diodes should give you a voltage drop of between 7 and 8 volts.

Link to comment
Share on other sites

4 minutes ago, jdp said:

So why not use a solid state rely. It has no moving parts and switches so rapid you don't even see a drop en current.

https://www.robotics.org.za/index.php?route=product/product&search=relay&description=true&product_id=1334

That is an option - just not the one in the link, it won't work. It can only switch DC and only up to 60V - Viper wants to control the geyser element - 230VAC. You need a lower voltage to control the SSR as well - not 55V odd volt, but I guess Viper can tap that somewhere in the bank, either at 12V or 24V.

Link to comment
Share on other sites

1 minute ago, viper_za said:

See it needs to run from the BMV's relay as the BMV will be used to determine when it should energize.

Yes, it will be able to work like that - the SSR is just an "electronic relay" - the control side is similar to the coil on a mechanical relay. BUT, the SSR linked above will not work, you need to get one with a "switch side" capable of switching 230 / 250V AC.

Link to comment
Share on other sites

@Chris @Plonkster @SuperDIY and any other interested parties: 

From Victron:
Hello Jaco,

The BMV's relay is potential free.

The SOC / volt settings will activate the internal relay and you’re relay (that is my Finder relay) will also be activated when connected on the relay contacts of the BMV.
The external relay does not have any effect on the BMV. 
The internal relay can handle 5 amps maximum at 230 volts, as long as the current is below 5amps.

Regards,

 

Very clever of them for a combustible atmosphere ... maybe like where batteries are charged. :D

I quote from the web - for other dummies like me:
Potential free contacts are also sometimes called volt free or "dry" contacts, and simply refers to relay contacts, or switch or sensor contacts, which haven't been wetted with voltage by the machine or device in which they exist. So if you were interfacing to a machine with dry contact outputs for interlocks, you would have to provide the circuit voltage source as well as the relay coil or PLC input.

Potential free contacts are used in applications where intrinsically safe electrical classification is required, or in hazardous areas where a sparks may ignite a potentially combustible atmosphere.

 

 

 

 

Link to comment
Share on other sites

2 minutes ago, The Terrible Triplett said:

@Chris @Plonkster @SuperDIY and any other interested parties: 

From Victron:
Hello Jaco,

The BMV's relay is potential free.

The SOC / volt settings will activate the internal relay and you’re relay (that is my Finder relay) will also be activated when connected on the relay contacts of the BMV.
The external relay does not have any effect on the BMV. 
The internal relay can handle 5 amps maximum at 230 volts, as long as the current is below 5amps.

Regards,

OK, so they've confirmed that you can use your relay with 230V coil.

2 minutes ago, The Terrible Triplett said:

So, it seem to me that it is a very good idea to have potential free relay connecting to a BMV.

You don't get anything like a potential free relay.  The relay contacts are potential free meaning that they are electrically isolated from the BMV (in this case) and you can compare it to a stock standard light switch on your wall, where, in this case, the BMV is the "person" switching the switch on and off. Hope that makes sense.:)

Link to comment
Share on other sites

Hahaha sorry I thought this was the tread where somebody wanted to disconnect the solar panels. Thats why I posted the DC one. My bad.

 I use 480v 40-100amp 3 phase SSR all the time. Works like a charm to drive big industrial motors from small controllers.

 

ssr3.JPG.199e202612c495ec6ae92d6db5e1cfbssr2.JPG.5c76bda48adb2985cd7a57b8878ed55ssr1.JPG.90e66f0c0286eb60065fac80b844580

 

 

 

 

 

Link to comment
Share on other sites

JDP, that is too small. I want bigger! :D

 

Seriously, can we get consensus on what is the best relay to use on a BMV to switch on geysers, stoves, poolpumps and anything else you want, that is smaller? :D

https://www.robotics.org.za/index.php?route=product/product&search=Solid+State+Relay+&description=true&product_id=1335

And if I find ONE person using a Finder ... :D

Link to comment
Share on other sites

1 minute ago, The Terrible Triplett said:

Typo fixed in my post.

 

24 minutes ago, The Terrible Triplett said:

So, it seem to me that it is a very good idea to have a relay that has potential free contacts, connecting to a BMV.

No, this sentence does not make sense at all, rather ditch it. The BMV's internal relay has a potential free contact already - no use in trying to connect another relay with potential free contacts to it. :D

Link to comment
Share on other sites

1 minute ago, superdiy said:

 

No, this sentence does not make sense at all, rather ditch it. The BMV's internal relay has a potential free contact already - no use in trying to connect another relay with potential free contacts to it. :D

What are you talking about? I never posted that.  :P

Link to comment
Share on other sites

9 minutes ago, jdp said:

Hahaha sorry I thought this was the tread where somebody wanted to disconnect the solar panels. Thats why I posted the DC one. My bad.

 I use 480v 40-100amp 3 phase SSR all the time. Works like a charm to drive big industrial motors from small controllers.

ssr3.JPG.199e202612c495ec6ae92d6db5e1cfb

I LOVE that heatsink, but how many watts does that thing dissipate? More than your daily PV generation?:D:D

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...