Making a swimming pool electrochlorinator and need some current limiting wisdom. / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Making a swimming pool electrochlorinator and need some current limiting wisdom.

Move Thread

LAN_403

Safrole

Sun Apr 13 2008, 08:31PM

Registered Member #1304 Joined: Sat Feb 09 2008, 09:29PM
Location: Elkhart, Indiana
Posts: 16

Can I add shunts from one MOT to another (or even simply pieces of ferrous metal) to decrease the overall current without wasting power as heat? I have often read here about people shorting MOT secondaries to use the primary as a current limiting device, but does that waste energy as heat?

I am working on my own electrochlorinator for my big (55,000 gallon) pool. I already have 4 - 4"x14" double sided anodes. On hand are 7 MOTs and I am thinking of rewinding one of their secondaries to form a current-limited step down transformer, probably less than 12 volts and maybe 15-20 amps. Is adding shunts from one MOT to another the best approach to do this?

My voltage must be constant at a given electrode spacing, so varying the amperage is the way you vary the overall chlorine output. I thought I could achieve this by running 3-4 low current MOTs in parallel and then selecting different multiples. For example if I had a 2 amp MOT, a 3 amp, a 7 amp and a 10 amp, I could have fine control all the way up to superchlorination. Does that sound feasible? If they're all in parallel feeding from the same supply are the amperages simply additive?

CT2

Mon Apr 14 2008, 12:37AM

Registered Member #180 Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187

One MOT will be more then capable of supplying all the current you need. Maybe instead of paralleling MOT's that are limited you should build a magnetic ballast out of a spare MOT. This way you can vary the amount of current to any amount you want smoothly by increasing the air gap. Non of the energy is wasted as heat in this case, it's like adding an inductor, it only stores and transfers energy, it doesn't use it. About the shunts, I doubt there is enough room on the core to add any more, although if you rewire it then maybe?

...

Mon Apr 14 2008, 01:37AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

A much easier solution would be to simply hook a single mot up to variac. Replace the normal secondary winding with about 15 turns of $12 wire, and wire the primary into a variac. Your power draw is resonably low (400w?) so the variac would shouldn't cost more $20 on ebay or even less at the local hamfest. You could probably also get away with a light dimmer switch in place of the variac, although there is a decent chance that it will die some day.

Safrole

Mon Apr 14 2008, 04:07PM

Registered Member #1304 Joined: Sat Feb 09 2008, 09:29PM
Location: Elkhart, Indiana
Posts: 16

Thanks for the tips. I considered the dimmer, because it appears in an old patent. But I didn't want to undervolt the cell. But how much could that hurt, really? So I think I will go with the dimmer before a single rewound MOT.

I understand shunts, but I don't understand exactly how to implement an air gap when reworking a MOT. Can you just hacksaw a bladewidth out of the center spindle?

Bored Chemist

Mon Apr 14 2008, 05:49PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

How much chlorine do you need?
Typical levels are a few ppm. You have 55000 gal of water that's something like 550,000 lbs of water or about 250 tonnes.
To get 3ppm you need 3g/tonne so about 750g of chlorine
Assuming you replace that every day ( which is a pretty gross overestimate I think- but I've never maintained a pool) you need to add 8.7mg of chlorine a second.
That's about 0.00025 mMoles per second
To liberate a mole of Cl needs roughly 100,000 amp seconds of current so you would be able to chlorinate the whole pool in a day with about 24 Amps.
Can you fill a pool that fast?
Wouldn't a car battery chrarger be an easier solution?

Safrole

Tue Apr 15 2008, 01:39AM

Registered Member #1304 Joined: Sat Feb 09 2008, 09:29PM
Location: Elkhart, Indiana
Posts: 16

I can appreciate someone talking moles! And thanks for the math - you're quite right that I've got WAY more capacity than is practical. The anodes' coatings are gradually consumed through use, so these oversized plates will barely breathe, but in turn last a long time. I paid $300 and was going to cast a few units up to sell, (they cost a fortune retail) until I found out you have to have an EPA number if you make the claim that your product sanitizes the water. I could wiggle around that, but for now I'm just building one mondo unit for myself in a plain PVC pipe.

Patent 4085028 gives some nice equations about plate spacing, amperage, voltage and water flow rates with final ppm free chlorine expected. Its overall design is tragically flawed, trying to pump 120v through plates in series, then catching stray currents with grounding screens. Forget that. They also overamp their plates like crazy. But I set their equations up in a spreadsheet and designed my cell more favorably.

The manufacturer who sold me the coated titanium anodes said not to exceed 400 amps per meter to realize their 10,000hr life expectancy. Calcium carbonate forming on the plates will insulate them and thereby concentrate current flow in the unscaled areas. Then that area overamps and pops the ruthenium off and it's dead. So having a low maximum amperage is important, moreso as the plates become more scaled.

That's where the regulating inductor, or multiple parallel inductors idea comes in. The ability to superchlorinate will require an amperage high enough to be harmful were it always available, so I want to literally have to flip a switch to even enable such a flow.

I do have 18 HID bases, which put out 100 Watts. I think I'll rewind the larger coil and then double up the shunts. I don't realy know how to implement an air gap or even how it would help me here, but I can wrap my mind around shunts. Maybe that will get me down to 5 amps or so.

HID Ballast

[Edit: Image size]

Steve Conner

Tue Apr 15 2008, 11:58AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Why not use the HID ballasts in series with the primary of your rewound MOT. Just hook as many in series, parallel, or whatever, as you need, since you have a pile of them. This way you should be able to get the effect you want without modifying any of the ballasts.

You could use an ammeter to measure the current in your chlorination cell and a bunch of switches to cut ballasts in and out for adjusting the current.

I guess since you live in 120V-land, your HID ballasts will be transformers of some kind. Ours are just plain inductors. That means you have two windings on there, so you have to identify them and choose which one is best.

Dr. Slack

Mon Dec 29 2008, 11:27AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The electricity seems to be the easy bit, what about the chemistry? I've read lots about Ti and coated anodes, but of course the obvious thing to use for a home-build is stainless (or lead, carbon, or anything more accessible). What will happen in a cell with stainless electrodes. I presume that even the exotic electrodes rot, as they are given a lifetime, so I guess SS will as well. This isn't necessarily a problem, as I have an infinite supply of SS scrap sheet, but will using other electrode material create nasties which then end up in your pool(iron posioning, or Ni, or Cr?)

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob