Cooked mppt and breaker problems

[tradrockrat]

Hello all I'm trying to use the search function but I'm doing this on a phone in a harbor in near Guadalupe and it's kind of an emergency. My mppt 100 20 victron cooked yesterday and my replacement is tripping the breaker. The breaker is between the mppt and the batteries. Any immediate advice for me? My setup is two 160w solar panels on an mppt 100 20. one set on starboard one set on Port so that's four panels two controllers each controller running independently

[tradrockrat]

https://photos.app.goo.gl/DzdhaMPSKrTxzYyF8
Pic

[HeywoodJ]

What wires do you have hooked up when the breaker trips? If you just make the battery connection, but not the PV connection, does it still trip? This would tell you if the problem is on the battery cabling side or the PV side. I suspect, from your brief description it is on the PV side. (Unless you also use a load connection, which complicates the troubleshooting, but not by much).

With the 100/20 the battery and PV circuits are not isolated (per the Victron manual for those that would care to debate this ad nauseum). So, it is possible for a short in a PV panel to cause a current drain from the battery, and thus trip the breaker (or cause the burn that you have in the original unit because the breaker did not trip - at least in time). If you can measure the open-circuit voltage and short-circuit current of the offending string(s) in good sun it should give you a decent idea of the state of the PV.

[tradrockrat]

Originally Posted by HeywoodJ

What wires do you have hooked up when the breaker trips? If you just make the battery connection, but not the PV connection, does it still trip? This would tell you if the problem is on the battery cabling side or the PV side. I suspect, from your brief description it is on the PV side. (Unless you also use a load connection, which complicates the troubleshooting, but not by much).

With the 100/20 the battery and PV circuits are not isolated (per the Victron manual for those that would care to debate this ad nauseum). So, it is possible for a short in a PV panel to cause a current drain from the battery, and thus trip the breaker (or cause the burn that you have in the original unit because the breaker did not trip - at least in time). If you can measure the open-circuit voltage and short-circuit current of the offending string(s) in good sun it should give you a decent idea of the state of the PV.

Thank you very much for this. I do not have any load on my mppt just battery and PV so I will test the PV with a multimeter (as soon as I can research the procedure) and see what I can figure out. Thank you again.

[noelex 77]

Private message sent.

[wholybee]

Breakers trip from excessive current. Terminals burn like on your first controller due to excessive current. You need to track down that current source. What is the voltage on the 2 wires you connect to the MPPT, without them being connected to the MPPT? (and the breakers turned on.) Assuming you have a 12v system, connect a 12V lamp (not an LED) to those wires. Does the breaker trip? Bulb burn out? Measure the voltage and current at the lamp. If the lamp works fine, remains at 12V, and somewhere around 1 amp or less (depending on the lamp you used) then the batteries and breakers are ok.

That leaves the MPPT controller. When the breakers tripped, had you connected the PV panels yet? It is possible that if the PV panels exceeded 100V by a large amount, that the MPPT controllers, first the old one and then the new one, have been fried, and are presenting a short to the batteries/breaker. It might be a good idea to bench test the MPPT with a single panel and a single lead-acid 12V battery and confirm it is functioning correctly before proceeding. And measure the PV voltage, and don't connect the PV until the battery is connected.

[Jammer]

Look for physical damage to the wiring between the controller and the solar panel or between the two solar panels. Pinch point where it goes around or into or out of tubing etc.

[smac999]

Originally Posted by wholybee

Breakers trip from excessive current. Terminals burn like on your first controller due to excessive current. .

that burnt terminal is probably from a lose screw or tightening down on the jacket and having lose wires.

[Fishspearit]

Originally Posted by wholybee

Breakers trip from excessive current. Terminals burn like on your first controller due to excessive current.

I know it's counter-intuitive, but terminals don't usually burn from excess current, it's usually low current/high heat from a high resistance connection. Ohm's law, with voltage constant the current goes down as resistance goes up. Heat is created as current goes through that high resistance connection.

It's why bilge pumps burn up when they get a locked rotor and don't have the correct size fuse. Resistance of the motor goes up, current goes down, that 10 amp fuse that you put in for the bilge pump that is rated for a 5 amp fuse never blows.

[bob_h]

Originally Posted by Fishspearit

I know it's counter-intuitive, but terminals don't usually burn from excess current, it's usually low current/high heat from a high resistance connection. Ohm's law, with voltage constant the current goes down as resistance goes up. Heat is created as current goes through that high resistance connection.

It's why bilge pumps burn up when they get a locked rotor and don't have the correct size fuse. Resistance of the motor goes up, current goes down, that 10 amp fuse that you put in for the bilge pump that is rated for a 5 amp fuse never blows.

The conclusion is right, the theory, not quite.
Yes, that burned terminal was probably a loose or oxidized connection, that heated up, developed even more resistance as it the heat accelerated the deterioration, until the resistance got so high it choked off the current enough that something stopped working. And then you went looking for the trouble. But that scenario wouldn't usually blow a fuse. A frayed cable somewhere, resulting in a huge over-current, which the the circuit breaker somehow fails to interrupt? Current flows until _something_ breaks and that weak link in the chain turns out to be this connection?
My quibble is with the bilge pump example. When a motor can't turn, current goes way, way up. A spinning motor generates a back-EMF which opposes the battery voltage, limiting the current flow. It's not resistance, exactly, but accomplishes the same thing. Stall the motor, and it becomes just a coil of copper wire hooked up to 12V, and you'd better have the right sized fuse to protect it. If a stuck motor just had higher resistance, well, then the current would go down, not up, and everything would be fine. Except for the whole water-in-the-bilge thing.

[Fishspearit]

Originally Posted by bob_h

The conclusion is right, the theory, not quite.
Yes, that burned terminal was probably a loose or oxidized connection, that heated up, developed even more resistance as it the heat accelerated the deterioration, until the resistance got so high it choked off the current enough that something stopped working. And then you went looking for the trouble. But that scenario wouldn't usually blow a fuse. A frayed cable somewhere, resulting in a huge over-current, which the the circuit breaker somehow fails to interrupt? Current flows until _something_ breaks and that weak link in the chain turns out to be this connection?
My quibble is with the bilge pump example. When a motor can't turn, current goes way, way up. A spinning motor generates a back-EMF which opposes the battery voltage, limiting the current flow. It's not resistance, exactly, but accomplishes the same thing. Stall the motor, and it becomes just a coil of copper wire hooked up to 12V, and you'd better have the right sized fuse to protect it. If a stuck motor just had higher resistance, well, then the current would go down, not up, and everything would be fine. Except for the whole water-in-the-bilge thing.

This is why I hate posting on here. We're way off topic now and no help to the original poster, but it's a very true and well documented fact that locked rotors cause the current to go down as the windings heat up and resistance in those windings increases. Basic ohms law. You can read all about it from a thousand different sources on the internet, here's just one.
https://www.boats.com/how-to/how-com...ump-is-melted/

[wholybee]

You missed a point in that article. The incorrect fuse was used. Yes, what you described happens, resistance increases with heat. But that heat is caused by excessive current. Only after current rises, generating more heat, does the resistance increases so the current can come back down.

This is why a properly sized fuse works. The motor stalls, current goes up, and the fuse blows before excessive heat builds up . Heat is what causes damage, and what stopping excessive current prevents.

The same is seen with a windlass, which might only draw 50A. But stall the motor, its 500A, and not protected by a fuse or breaker the heat will burn the windings before the current comes back down.

You are right about resistance in the contacts and that usually they burn from a loose connection. However, that would not damage anything else in the controller, or trip the breaker. The poster potentially has 2 mppt controllers that are fried, and is tripping a breaker. He has excessive current, or his breaker would not be tripping.

[Shanachie]

Just in case:


Victron says the solar panels need to be disconnected when connecting the MPPT controller to the batteries. Otherwise, you can fry the controller with the surge of power and no place to go.

[Bycrick]

Except that the temperature coefficient of copper is about 0.4% per degree C. It’s going to have to get REALLY hot before the change in resistance is important. It will likely melt or catch fire first. It doesn’t require a lot of current to make things hot. The filament in a flashlight bulb is white hot with only tens of milliamps.

I use small computer fans for my cabin ventilation. I’ve rigged magnetic reed switches, rated at 2A to control them. The fans normally draw about 0.7 amp. A few days ago, when I woke up, one of the fans was running slowly and fitfully. When I looked closely, the glass-enclosed reed was white-hot, just like a light bulb. The 1 a fuse hadn’t blown. It doesn’t take a lot of current to get things hot.

[Wotname]

Originally Posted by Fishspearit

This is why I hate posting on here. We're way off topic now and no help to the original poster, but it's a very true and well documented fact that locked rotors cause the current to go down as the windings heat up and resistance in those windings increases. Basic ohms law. You can read all about it from a thousand different sources on the internet, here's just one.
https://www.boats.com/how-to/how-com...ump-is-melted/

One of the big downsides of internet sourced 'facts' is that incorrect information gets posted once and then repeated over and over again until everyone assumes it must be true. This is a great instance.

Fishspearit has made an electrical claim about current draw of a locked rotor induction motor. He has read it several times on the 'net and he believes it to be true. There is a link to site that supports his claim. The article linked was written by a guy called Ed Sherman. From the article we know that

Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.

With those qualifications Ed should know, right. Well no, Ed is wrong and a careful reading of the Ed's qualifications tells us the closest Ed has been to an qualified electronics guy is as a journalist (editor). What Ed has written is a fallacy. Probably not really Ed's fault, he is just repeating something someone told him but as a electronics editor, he does have a duty to fact check his source - obviously he didn't in this instance. Don't be like Ed.

Let me say that again - Ed is wrong (and by extension Fishspearit is wrong). Now that is no disrespect to Fishspearit, he is a just victim to the echo effect of the internet. I have been there myself, read stuff that seemed logical and written by people who should know. I have repeated the same stuff only to find the source (and therefore I) was wrong and thankfully others corrected me. In a way, ignorance is a good thing because it can lead to real knowledge.

OK back to the current draw of a locked rotor induction motor. The current increases as the rotor rpm decreases and reaches a maximum when the rotor is locked (0 rpm). There are several ways to understand this but the basic one requires an understanding of ohms law, inductance and electromagnetic fields around conductors. I can explain it but unless you have a sufficient understanding of the basics, the explanation becomes long and tedious. If you have a good understanding of the basics, you will already know I am correct.

So rather than me explaining, a better way to prove it is to try it yourself.

Take a small 12V bilge pump and connect it to a good 12V battery or power supply via the recommended fuse. Turn it ON (out of water) and measure the current flow. The current will be small and the pump rpm will be high. Now place the pump into a bucket of water. As it pumps water, the current will increase a little and the rpm will decrease a little. If you increase the height of the outlet hose (i.e. increase the 'head pressure' of the pump), the pump will have to work harder, the rpm will drop some more and the current will increase some more.

Now jam something into the rotor so the rotor is stationary (locked). The current will increase until the fuse blows (i.e. at a value higher than the fuse rating).

Once you have done this, come back here and recant!

While I could be anyone on a keyboard, I have consistently claimed to have a lifetime of professional electrical experience (practical and theoretical) in aviation, marine and communications trades but really, don't take my word, experiment yourself.