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Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 05:54pm 17 Oct 2017
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Interesting question- you cant have relays or electronic switches as the Primary switch- but what happens if you leave the existing switch in circuit and add another in series with it? (kinda sidestepping the rules but hey whatever works) You still get the primary will open circuit the heating at max, but if all your switching occurs below this level, then basically it will never open (except if a relay sticks etc) With the cheapness of panels, and the almost pure resistive load, why not angle more to the 360v range for the panel voltages (giving the same effective heating level as 240vac) and by my (admittedly BOTE) calcs should get the current back almost into the peak of the panels as well, then a simple solid state relay doing the switching and a M.M. to do the temp measuring and controlling
Not to disparage your work at all, I like your design, it just seems much simpler (possibly too simple, I guess there is a reason it wouldnt work)
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 06:40pm 17 Oct 2017
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Hi Boppa
Adding another series switch in cct, no problem, its done all the time with timers and time switches.
Cannot use pure DC, if the external control (electronic relay or similar) fails then the AC rated switch contacts in the thermostat and over temp cutout would be destroyed in a few seconds. I just purchased an additional thermostat at $150 trade price for the second element on HWC, so want it to last.
240V DC has the same heating as 240V RMS AC, so no I don't want to go to 360v DC.
There is an element of risk using a pure DC system, and that's possible electrolysis caused by leakages in the earthing and element power contacts, so another reason for not running DC on an element, pin holes etched into the cylinder.
Cheers Mike
hotwater Senior Member Joined: 29/08/2017 Location: United StatesPosts: 120
Posted: 12:03am 18 Oct 2017
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I just don't get this electrolysis at all. If you have electrolysis, then there is a much bigger problem going on with the element. Certainly keeping anything electrical legal in AU is a problem and AC on the element will help maintain switches. I am concerned about the 100KHZ switching. Besides being a real switch killer, you will be inducing a lot of eddy currents. Does the 50Hz side have any dead time to quench the arc?
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 12:54am 18 Oct 2017
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Hi hotwater, yes there is a small dead time window around the switch times. Not sure where you got 100Khz switching from, 300 Hz at best during the 50 hz period.
Possible electrolysis caused by leakage currents in DC systems from earths connected to the PV panel array frames and HWC element.
Cheers Mike
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 01:56am 18 Oct 2017
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I dont get how you got the AC contacts would be destroyed in a few seconds?
They certainly would have a life reduction due to some arcing but fitting a quenching cap across them would soon sort that, but unless there was something seriously wrong, I dont see 15A240vac contacts having an issue with 15A240vDC
In the railways we had 600vdc control circuits in some of the older elcars, and they used 240vac relays at 600vdc with no apparent issues (they originally had 600vdc relays but the manufacturer had closed down, so they tested and passed one particular brand of relay as being ok to use in these control circuits as a replacement, I would normally say that much of an overvoltage would be an issue without testing however)
Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498
Posted: 02:02am 18 Oct 2017
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You have to kill the arc, DC breakers have a large gap and magnets to pull the arc out of the gap. Otherwise it behaves like an arc welder.
A thermostat operating with AC relies on the cycling of the AC and 0 Volts point to extinguish the arc.There are only 10 types of people in the world: those who understand binary, and those who don't.
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 02:10am 18 Oct 2017
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A cap across the contacts should do that ok tho
Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904
Posted: 03:55am 18 Oct 2017
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Not really. With high voltage DC current involved a cap has only one instant to absorb some as the contacts open and its doubtful there is no arc. Once there is an arc established (remember, AC contacts don't open very far at all) its all downhill with the contacts.
But, why don't you try it and let us know. I would test it myself but I have no 240V DC available,only 54V. Edited by Tinker 2017-10-19Klaus
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 11:19am 18 Oct 2017
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I agree with Tinker, with 2Kw available from the panels, once an arc starts, the contacts would be molten metal in seconds. As for experimentation... high voltage DC is bloody dangerous, be careful.
Mike
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 11:31am 18 Oct 2017
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Just a silly idea, but what if the dc voltage was pulsed on and off at a very low frequency, say 1Hz ?
Heating effect would be reduced to whatever the duty cycle was, but it should completely eliminate the possibility of a continuous arc forming.Cheers, Tony.
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 11:55am 18 Oct 2017
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Not so silly, yes that option would also work, if you can get a good match in the resistive HWC element load to the current available from the panels.
One reason for using capacitor discharge, is that when it gets charged to the correct panel mppt voltage, then discharges supplying the required current into the load, where the panels due to possible low light conditions may not be able to supply that directly and would pull their voltage down, making the power transfer inefficient.
Cheers Mike
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 12:22pm 18 Oct 2017
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grr bloomin cat just wiped my reply I learned on 1200vdc @ 20KA so I am very wary of its power I have a pic (in storage atm with all my other gear grr) of the remains of a spanner that dropped onto a live busbar and breached its rubber cover- pretty little silver splatterballs all over the (asbestos lol) backing board
We had oil quenched contactors on the high amp stuff, but the low amp lines were just standard airgapped with cap quenching, seemed to work ok
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 01:01pm 18 Oct 2017
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Very low frequency pwm could be implemented with a hysteric voltage comparator across your capacitor bank. As an example, turn on the load when it reaches 240v, turn off the load when the capacitor voltage falls to 230v (or whatever). Discharge times should always be the same, because the load resistance is the same.
But charging up times will be highly variable. If discharge times are kept fairly short by capacitor bank sizing, and the allowable voltage swing, arcing can only continue on during the discharge time. There will then always be a definite full pause of variable length, for capacitor bank recharging.
Discharge times could be kept short enough (tens or hundreds of milliseconds?) so that arc damage would be absolutely minimal. It could make the whole thing very simple, just a suitable voltage comparator and a mosfet may be all that it needs.Edited by Warpspeed 2017-10-19Cheers, Tony.
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 02:55pm 18 Oct 2017
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Yes I agree, could be simplified to a more basic approach, I have just added 3 more mosfets a cheap cpu and 1-wire temp sensors to ensure a DC polarity change across the element, possible LCD readout and HWC temp sensing, wouldn't want to make it any more complex though...considering I have yet to fully test this idea in practice and failure is always an option.
Mike
Ralph2k6 Senior Member Joined: 24/09/2017 Location: AustraliaPosts: 129
Posted: 03:23pm 20 Oct 2017
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Tony, I like the cap mosfet idea. But there would still need to be some type of 'flip flop' to reverse the polarity on the element to prevent electrolysis, and some sort of protection to prevent shoot through of the oppositely driven MOSFETs. Controller makes adding dead time etc easier or more controlled?Ralph
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 04:10pm 20 Oct 2017
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The electrical part of the heating element is not in galvanic contact with the water. Its fitted inside a metal sleeve or jacket, that is insulated from the energized electrical part of the actual element. That screws into the metal shell of the HWS and is grounded.
Its exactly like the heating element inside a metallic electric kettle or an electric stove.
Something more crude like an electric jug DOES have a fully exposed heating element in direct electrical contact with water, but the jug itself must then be made of an insulating material such as plastic or ceramic.
Electric HWS are all grounded through the copper pipework and must have a fully insulated and isolated dry running heating element, so electrolysis is not a problem.
Dc for heating will be fine.
The only problem with using dc is the probable destructive arcing of the contacts in the absolutely mandatory thermal over temperature cutout. The heating element itself will be fine with dc.
Cheers, Tony.
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 04:12pm 20 Oct 2017
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Still confused about this electrolysis point as well, where and how do you get it? eta Warpspeed beat me in posting)Edited by Boppa 2017-10-22
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 04:56pm 20 Oct 2017
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Normally not an issue, there is little chance of this ocurring, providing everything is bonded and earths are up to spec, however when bonding is not 100% or the element gets some leakage in it due to a fault or other moisture causing tracking and differences in potentials especially at high voltage 250 vdc then thats when you may have an issue, more expecially if a copper cylinder is used. So alternating the DC voltages applied to the element will help alleviate this.
People have been using HWC elements as dump loads (albeit at < 50 volts) etc for years and there doesnt seem to be any issues. So perhaps I'm just unduly cautious.
Cheers Mike
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 05:21pm 20 Oct 2017
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Moisture is only likely to accumulate within the element if its been switched off for a a very long time. The heat from a normal temperature cycling element is going to drive out any moisture pretty quickly.
Cheers, Tony.
Ralph2k6 Senior Member Joined: 24/09/2017 Location: AustraliaPosts: 129
Posted: 08:50pm 20 Oct 2017
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Thanks for clarifying Tony and Mike, I am sparky by trade not e.e I only dabble. Would love to do much more given time. Pulsing DC should be the way to go, even though its a square wave, as mentioned the gaps in pulses would certainly help quench arcs right?Edited by Ralph2k6 2017-10-22Ralph
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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